By Don Hewes (EAA 32101) 12 Meadow Drive Newport News, VA 23606 Overview

This three part article concerns the effects of rain and bugs on the flight behavior of canard and tandem-wing airplanes which have been of concern to a number of homebuilders for quite some time. In the past few months a number of rather harrowing inflight experiences have been related in some of the aviation magazines and the apparent causes for them have been discussed to some extent. This article covers the subject in considerably more technical detail and addresses some aspects that have not been treated elsewhere. Some recent NASA data pertinent to the subject are presented along with some information supplied by designers of these airplanes. An analysis and interpretation of the available information is included along with a number of suggestions and recommendations for interested designers and homebuilders. There are some areas which are not yet documented and some which are still not clearly understood. However, based on available information, it is judged that this behavior is not necessarily dangerous BUT proper attention must be given to it in the design, building and operation of a tail-first airplane. Because of the complex interaction of the aerodynamic characteristics involved, there are different forms of behavior that can be encountered with airplanes of different design. Moreover, it appears that the behavior of airplanes of the same design may differ depending on the specific design features, construction techniques and workmanship details. Selection of the airfoils and accuracy in duplicating the shape, smoothness and alignment of the lifting surfaces are considered to be of prime importance. Because each homebuilt aiplane is more or less unique in the aerodynamic sense due to the variables introduced during construction, the builder must exercise his responsible role in assuring the airworthiness of his own airplane. The builders and designers can be of great mutual benefit by conducting specific flight tests and exchanging detailed information in those areas not yet well documented and understood. Introduction

Recent issues of SPORT AVIATION, Homebuilt Aircraft and Aviation Consumer have highlighted a number of instances in which there have been extreme changes in the longitudinal flight behavior of different homebuilt tail-first airplanes with particular emphasis on the tandem-wing configuration. Accumulation of

rain, bugs or some other material that disrupts the laminar airflow over the lifting surfaces has been attributed as the primary

cause for these changes that, in some cases, have resulted in forced landings and injuries. Although it is important to understand that this is the primary cause, it is equally important to understand many of the other factors that are involved and how these can be controlled by the designer and builder so as to avoid or 36 MAY 1983

minimize problems in the future. This article, therefore, is intended to be an indepth assessment of the phenomenon to help the designer and builder gain a better overall understanding. In general, the several instances of extreme flight behavior referred to in these magazine articles represent the extreme or most severe cases of a phenomenon which I've chosen to call "Flight Behavior Change" or "FBC" for convenience in this article and for lack of a proper term. To say that these cases represent the norm and apply equally to all current designs would be grossly incorrect and would be a great injustice to the designers who have made very sincere efforts to minimize or eliminate this type of behavior. It is important, however, to relate to these cases because they tend to "show us the way", but bear in mind that there are many tail-first airplanes flying which show only minimal changes in behavior or none at all. Although FBC does occur with conventional airplanes, it is generally of little concern for them. Sailplane enthusiasts have long recognized and dealt with rain and bugs but the concern there apparently has been primarily with the effects on performance at cruise conditions. Now, however, the phenomenon has become something of a problem with the new canard and tandem wing designs not only from the standpoint of performance but, more importantly, from that of control and maneuvering. Because both canard and tandem-wing configurations are generically similar, I'll generally use the term "tail-first" hereafter for convenience to refer to either a canard or a tandem-wing design. By "generically similar", I mean that they both have two lifting surfaces each carrying a significant portion of the airplane's weight and both have the elevator located on the forward surface. It is understood, of course, that the area and load ratios of the surfaces are different and this may be important from the standpoint of severity of the FBC, but this is not important for purposes of classification. The difference between pusher and tractor configuration is believed to be of relatively little significance for this phenomenon. FBC can be triggered by any one or more of several agents including rain, bugs, frost, snow, ice or any other material that disrupts the boundary layer airflow of the lifting surfaces. Therefore, for convenience, I am simply using the term "contamination" to refer to the condition brought on by any of these agents. Contamination may produce either a pitchdown or pitchup trim change and may be apparent to some degree over the complete flight envelope. In some cases, it may become very serious as airspeed is reduced and landing conditions are established or as a takeoff is attempted. Furthermore, the nature and seriousness of the problem can vary from one copy to another of the same design. We will cover this subject in three installments with the first presenting an analysis of some of the aerodynamic factors involved. The second installment will discuss some pertinent wind tunnel and flight test data, and the last will discuss a number of related subjects including suggestions and recommendations. Although considerable technical details and data are involved, we have tried to include definitions and explanations of those items which may not be readily understood by non-technical readers.

Unfortunately, this led to a fairly lengthy text that many may find tedious to read, but I encourage those who are interested in tail-first airplanes to read it all the way to the end. Background

I first took note of the phenomenon several years ago at one

of the forums at Oshkosh where a description was given of a very mild change in trimmed airspeed along with a gradual change in altitude when rain was encountered. Over the intervening years, I had been hearing of an increasing number of similar experiences and noted that some of the designers cautioned about flying in

rain or with dirty wings. Finally, when I started to build my Dragonfly airplane over a year ago, I decided to took further into the problem myself. I've been a pilot since college days with essentially all my time in lightplanes but I have briefly flown several larger airplanes

including helicopers and heavy twins. I have briefly flown four different tail-first airplanes, the prototype VariEze, Dick Rutan's Long-EZ, the RAF Long-EZ and the prototype Drangonfly This experience certainly does not qualify me as a test pilot by any stretch of the imagination but I felt that my past 33 years experience as an aero research engineer specializing in dynamic stability and control at NASA's Langley Research Center provided me with the background to approach the problem on a fairly sound technical basis. (I am now retired and have no direct connection with NASA other than as a retiree.) Fortunately, some of my former associates at NASA had just completed a wind tunnel study using a full-scale model of the VariEze tested in the Langley 30x60 foot wind tunnel and some of the data were published recently (see Reference 1). The model had been made in the Langley model shops and was a very

accurate replica of the VariEze design as depicted by the plans. Since the tests were also run at speeds very close to the approach and landing speeds for the VariEze, the wind tunnel data could be considered as being directly applicable to the actual airplane without worry about scale effects. The canard had been mounted to the fuselage with a force measuring balance so that the direct lift, drag and pitch contributions of the canard were recorded along with total aerodynamic contributions of the complete airframe as measured by the primary balance system. Some special tests were made in which water was sprayed on the airplane so as to represent the conditions which appeared to cause the pitch-change problems. Others were made with the leading edges coated with coarse grit to act as a boundary layer tripping device.

The researchers also had conducted some flight tests using a VariEze built by EAA member Bob Woodall from Baltimore and flown by him at the request of the NASA researchers. Other

related flight tests were made at Mojave, CA, through the cooperation of Bun Rutan using a Long-EZ. These flight tests provided some very important verification of the wind tunnel test results (see Reference 2). The VariEze was selected for testing because it incorporated a number of unique and interesting advanced design features and

NOT because it had any significant problem. I have used these

data because they are my only convenient source of information directly applicable to the subject of this article. By examining

these data it was possible to observe some trends which help

explain certain aspects of the FBC phenomenon.

At this point, I want to comment about the reasons for writing

this article. First, there is very little technical data available on this subject and very little of this has been presented in the current

literature available to most homebuilders. Some of the canard

designers have spent much effort to evaluate the behavior of their

respective designs and have devoted considerable space in their newsletter to this subject. However, there appear to be areas which some have not fully evaluated.

Next, I'm sure that there is a very large group of potential designers, builders and pilots who do not have access to the information made available by the designers through their newsletters and direct contacts with their builders. This group needs information from some source to help in making decisions and taking actions which could affect their safety and their satisfaction with the airplane. Consequently, it appeared appropriate that the subject be reviewed in an open manner so as to create an avenue for greater interchange of information. My background is that of a research engineer working with the primary objective of conducting analytical, wind tunnel and flight studies to identify unusual behavior

of airplanes and attempting to find the cause for and solutions to the problems encountered. The end product of such work was a report of this effort written in an objective and unbiased manner, insofar as possible, and distributed to appropriate interested persons and organizations. With this background, it was only natural for me to apply the same approach to this particular subject as an extension of my interests in the homebuilt airplane movement

and my desire to help advance the principles of safe flight. I'll conclude this discussion by noting that in the past few

months indirect references have been made in two separate newsletters to statements that I have made concerning the behavior of tail-first airplanes as well as to the material presented in this article and in both cases I have been incorrectly quoted. I hope

that those who may have noted those references will reassess the

contents of those newsletters in the light of the material presented in this article. Please bear in mind, of course, that there are

several areas in which personal opinion or judgment has to be used because there is insufficient information on which to base firm conclusions. In these cases, you must allow room for honest differences of opinion. Definition of Problem Areas Our first concern is to identify just what flight behavior prob-

lem areas we are going to be concerned with. In general, there

appear to be seven areas in which contamination effects can play a significant role; that is, 1) takeoff distance and speed, 2) rate of climb, 3) landing speed and distance, 4) stick forces and pitch trim travel, 51 maneuvers at takeoff and approach speeds, 6) cruise

efficiency, and 7) maximum speed. Of these, the last two are not considered pertinent to the concerns of this article although they may be of general interest. Contamination effects should increase the distance and airspeed required to takeoff and, in an extreme case, could actually prevent takeoff altogether. Although takeoff may not be prevented, the rate of climb after takeoff could be reduced to the point that avoidance of ground obstructions could be difficult. The same contamination effects that come into play at takeoff also are involved in landing and, as a result, landing speed and distance can be increased markedly. Perhaps of equal concern is the fact that the ability to maneuver effectively at the takeoff and landing

speeds may be somewhat limited. Whether or not these areas become real problems depends, of course, on the magnitudes of the various contamination effects. Small changes of a few ounces to a couple pounds in stick force can be uncomfortable and disturbing for sustained periods of

cruising flight, especially if the stick trim system is inadequate, but these should not be much of a bother during landing or takeoff. However, if the force changes are in the order of several pounds or more, the impact on the pilot may be significant. While the effects themselves in some cases may not be too severe, just the distraction of having to cope with unknown or unfamiliar behavior at a critical moment could be serious, especcially in the case of an inexperienced pilot. Thus, seemingly small changes in the flight behavior due to contamination should not be regarded lightly.

Discussion of Aerodynamic Factors Which Influcence Flight Behavior

Two basic conditions under which an airplane is flown are 1) with pilot holding the controls, and 2) with the controls free. Although the controls can be easily moved by the pilot when he

is holding them, we will refer to this as the STICK-FIXED condition because he does hold the stick in a more or less fixed position in normal steady flight. We will refer to the latter condition as

STICK-FREE. Two aspects of the stick-fixed condition will be

discussed as follows: 1) the aerodynamics of the airplane without

consideration of the forces that the pilot must apply to hold the

controls fixed, and 2) specific reference to the control forces and the trim system used to balance the aerodynamic hinge moments. In the following discussions, we will be considering the possible effects of contamination on various aerodynamic characteristics and the resulting changes in the flight path of the airplane that would be induced by the aerodynamic changes. It is recognized that we will be discussing these characteristics relative to more or less steady level flight conditions, however, the results have a direct bearing on the dynamic or non-steady cases of landing and takeoff which are of more concern. We will not be concerned at

this time with the magnitude of the effects but with the possible trends or changes involved. SPORT AVIATION 37

Stick-Fixed Aerodynamic Characteristics We need to concern ourselves with three of the basic aerodynamic characteristics of a tail-first airplane which regulate the longitudinal motion of the airplane, that is, lift L, drag D and pitching moment M. We use the stick-fixed case so that we don't have to worry at this point about the effects of a free floating control surface on the pitching moments of the airplane. Actually, we will be concerned not only with the TOTAL lift, drag and pitching moment but, also, with the contributions to each of these made mainly by the TAIL and the WING. For this part of the discussion, DEFLECTION and TRIM of the elevator and POWER or THRUST T remain CONSTANT unless otherwise noted. In general, the effects of contamination of the wing and canard normally would be expected to INCREASE DRAG and to DECREASE LIFT at a given airspeed. Although the pitching moments of the individual components might not be altered significantly, these drag and lift changes of the wing and tail result in significant changes in the contributions of the components to the total PITCHING MOMENT of the airplane. Furthermore, the resultant pitch change, if there is one, MAY BE EITHER IN THE NOSE-UP OR NOSE-DOWN direction depending on the location of each component relative to the center of gravity. To understand how these effects can influence the behavior of the airplane, we'll first introduce the familiar simple force diagram in Figure 1 which shows the TOTAL forces acting on an airplane TRIMMED for STRAIGHT and LEVEL FLIGHT at some arbitrary airspeed. The flight path is aligned with the airspeed vector VI and is at a zero angle relative to the horizon. We'll assume that the airplane, in this case, is clear of contamination (no rain, bugs or what have you). The main point of this diagram is to remind you that, in level flight, the weight W of the airplane is balanced by the total lift L and the thrust T from the propeller is balanced by the total drag D for level flight, (for simplification, we are ignoring the fact that the thrust may not be aligned exactly opposite to the drag. Also, the simplified equations for the assumed steady flight conditions are included in this and the following two figures for those of you who are more technically oriented.) Because the airplane is assumed to be in trimmed flight, the pitching moment is necessarily zero. Next, in Figure 2, we show the diagram and equations for the case where the airplane has been subjected to the assumed DRAG and LIFT changes produced by the contamination. Taking the effects one at a time, we first note that the direct effect of the

total DRAG INCREASE is to cause the airplane to PITCHDOWN

somewhat and establish a RATE OF DESCENT denoted by the inclined flight path. Note that although the pitch ATTITUDE has changed, the ANGLE OF ATTACK HAS NOT CHANGED. (A pilot cannot determine angle of attack unless he has a suitable sensor and indicator available.) The additional thrust required to balance the higher drag was provided by a component of the weight which now acts along the descending flight path. If there had been no change in lift, there would have been NO SIGNIFICANT CHANGE IN AIRSPEED. However, since the total LIFT has to remain essentially equal to the weight of the airplane for our assumed condition of STEADY FLIGHT, the effect of the LIFT DECREASE caused by contamination was to INCREASE THE AIRSPEED so as to restore the full amount of lift required for balance. (Note that, at a given angle of attack, lift is a function of both airspeed and lift coefficient. When we say here that lift is decreased, we actually mean that lift coefficient is decreased, thus airspeed must increase to regain the original total lift.) This, in turn, resulted in additional DRAG and, since both drag and airspeed were further increased, the FLIGHT PATH and RATE OF DESCENT INCREASED further also. Thus, we can see that a lift change tends to have a more complex influence on the behavior of the airplane than a drag change. If ONLY the lift had changed, the airspeed would have changed just as much and the rate of descent would have changed also because of the added induced drag. Up to this point, we have not changed the pitching moment or permitted the pitch control surface to move. So, we have the airplane at the SAME ANGLE OF ATTACK to the airstream as in the original situation. However, the PITCH ATTITUDE in Figure 2 has changed to a slight nose down condition because of the descending flight path angle. We now have the elements of a pitchdown behavior, that is, INCREASED AIRSPEED and RATE OF DESCENT with a NOSEDOWN ATTITUDE change even though the pitching moment of the airplane has not been changed. If we had started originally with a climbing condition rather than 38 MAY 1983

FIG. I - LEVEL FLIGHT BALANCE OF FORCES.

APPROX. EQUATIONS

L - W= O

FLIGHT PATH ANGLE (%)FIG. 2 -

0

EFFECTS OF LIFT AND

DRAG CHANGES (*L AND L.

'

J *\ "^^^*^^

^r=^

__ — •——

l

\ \ \^W cosX2

W L 2-W cos

^

'?... *-"'/7TIZ

RATE OF DESCENT =

level flight, the trends would have been the same, that is, the airspeed would have increased and the rate of climb and noseup attitude would have been reduced. The fundamental effect of a PITCHING MOMENT CHANGE for steady flight conditions is to cause the airplane to CHANGE TO A NEW ANGLE OF ATTACK where the total moment is returned to zero. Of course, when this happens, BOTH the lift

FIG. 3 - EFFECTS OF PITCH TRIM CHANGE

and drag characteristics are altered EVEN FURTHER due to the combined effects of angle of attack and airspeed. The nature of the changes will depend on both the magnitude and direction of the moment change, as well as the original angle of attack or airspeed. If it is a noseup moment change, then the airplane will

tend to slow down and climb when operating close to cruise

conditions or slow down and descend when near landing condi-

tions. (The crossover point is the speed for best rate of climb, that is, maximum lift/drag ratio.) The opposite tendencies will happen if the pitching moment changes in the nosedown direction. It is quite evident now that there is a rather large range of possible combinations of effects for the STICK-FIXED condition we asumed. In some cases, the pitching moment change may tend to cancel the direct pitchdown effects of the contamination on lift and drag, but in others, the moment change will further increase those effects The MOST CRITICAL combination from the standpoint of the pitchdown behavior appears to be a NOSEDOWN MOMENT CHANGE ENCOUNTERED AT LOW AIRSPEEDS This case is depicted in Figure 3 As we noted previously, the pitching moment changes are dependent on the location of the individual components of the airplane, so we'll now refer you to Figure 4 where we have a diagram for a TAIL-FIRST arrangement. Here we are considering only the lift contributions of the wing and tail to the pitching moment of the airplane as these are major factors involved. One of the features of current tail-first designs is that the tail carries a significant share of the airplane's weight. For instance, the VariEze tail carries about 30 percent of the weight and the Quick ie, Q2 and Dragonfly about 60 percent. The amount depends on the location of the center of gravity which is dictated by stability considerations. Obviously, any changes in the lift of either the wing or tail are going to have a significant effect on the pitching moment of the airplane. It is evident that a pitchdown moment change will result if there is a decrease in the tail lift and that an opposite moment change will result from a loss in wing lift. There will be no net moment change if both lifts change by the same percentage. Thus, the actual net moment change will depend to a large extent on the degree to which the tail and wing are each affected by the contamination. This is an important point which will be referred to later. Stability and control considerations dictate that the FORWARD LIFTING SURFACE MUST STALL BEFORE THE AFT SURFACE, that is, that tail must stall before the wing. All current tail-first designs have the primary elevator surface mounted on the forward tail surface; consequently, at low speeds the elevator TENDS TO OPERATE IN STALLED FLOW CONDITIONS AND LOOSE EFFECTIVENESS. The term "elevator effectivenss" refers to the ability of the elevator to generate a pitching moment change with a change in elevator deflection. This tendency to loose effectveness has been used in some, but not necessarily all, tail-first designs to provide stall-proof characteristics for the airplane as a whole. "Stall-proof means that the wing of the airplane cannot be stalled and that adverse behavior associated with a partically stalled wing is avoided by LIMITING THE MAXIMUM ANGLE OF ATTACK that can be reached. Very careful attention to design details is required to provide this feature and, if the design is compromised by poor workmanship or other factors, the benefits of this feature may be lost. Whether the airplane is or is not stall-proofed, the tail group is subject to partial or full-stalled conditions near the minimum airspeed of the airplane. Any factors which can influence pitch trim of the airplane AND stall of the lifting surfaces very likely will influence the MINIMUM AIRSPEED at which the airplane can be flown and possibly lead to some serious stability and control problems. If contamination by itself or in combination with factors associated with poor workmanship can cause the tail to lose lift and stall at an angle of attack lower than intended by the designer, it is possible that the minimum speed will not be low enough to permit a safe takeoff or landing. On the other hand, if these factors have a strong influence on the wing, it is possible that the minimum speed will be reduced to the point where other undesirable effects, such as wing rock or divergence, might be encountered.

L3- W cos &3 = O W

=0

AT

CURVE SLOPE RATE OF DESCENT

V3si*33

FIG. 4- SIMPLIFIED DIAGRAM OF

PITCHING MOMENTS FOR TAIL-FIRST DESIGN.

L.

, , '-w

W

Lc+ LW-W = O (LcxXc)

- CLW* Xw)= SPORT AVIATION 3«

From the several points covered in the last few paragraphs, it appears that the aerodynamic characteristics of the tail can be very important in determining the nature and magnitude of the FBC. Up to this point, we have assumed that the throttle, elevator and pitch trim controls have not been moved by the pilot and have looked at what the resulting behavior of the airplane might be. Rather than let the airplane respond in these ways, the pilot most likely will move the controls to try to restore the airplane to its original trimmed flight condition. The movement of the controls from their original trimmed positions to the final positions will be direct indications of the effects of contamination on the basic aerodynamic characteristics of the airplane. The subject of measurements of these movements in a series of flight tests will be addressed later in this article. Control Forces For the Stick-Fixed Conditions Now we need to consider the forces that the pilot must exert to hold the stick fixed or to move it because they provide the pilot's "feel" of the airplane and influence the manner in which he controls its motions. Obviously, the forces must not exceed the force capabilities of the pilot's arm, wrist and hand. But of equal importance, they must stay within rather narrow limits to be considered acceptable and they should be adjustable by the pilot for reasons of comfort. The pitch control forces come from four different forces — 1) the aerodynamic hinge moment of the elevator, 2) the pitch trim system, 3) the control system friction, and 4) the mass or inertial moments of the system. We will not worry about the last two because they are not pertinent to this discussion. Please note that the hinge moment of a FIXED elevator has no connection with the pitching moment of the airplane and will not influence the motion of the airplane in anyway whatsoever. Of course, if the elevator is deflected as a result of the hinge moment, then a pitching moment will be produced and the airplane's motion will be affected. But in this situation, it is the DEFLECTION not the HINGE MOMENT that causes the motion. That is not the case we are discussing at this time. The hinge moment of the fixed elevator generally increases with both angle of attack of the tail and deflection of the elevator but the actual variation is dependent on the specific shape of the elevator and location of the hinge line. We won't worry about the details other than to note that at larger angles of attack (lower airspeed) the elevator generally will have a moment in the trailing-edge-up direction. Consequently, the pilot would have to produce a continuous rearward-pull force to hold the elevator fixed at lower speed unless some form of a pitch trim system is provided. Although some trim systems may balance part of the hinge moment using some aerodynamic means, such as a tab, there usually

are some mechanical springs included to facilitate adjustment of

the stick forces. Once the control force is balanced to zero for a given airspeed, any attempt by the pilot to move the stick from the trimmed position will be resisted by the aerodynamic hinge moments and the springs of the trim system. These forces provide the "feel" for the stick. The aerodynamic hinge moment is the direct result of essentially the same pressure forces that act on the elevator to produce it's contributions to lift of the complete tail surface. However, the pressure forces acting at the trailing edge are more effective in producing a moment about the hinge line than those close to the hinge line, consequently, the hinge moment is very sensitive to small surface or airflow changes at the elevator that may have very little effect on the lift generated by the tail. Let's assume then that contamination causes ONLY the aerodynamic hinge moment of the elevator to change without changing any other aerodynamic characteristic of the airplane. Now, if the pilot had the stick force originally trimmed to zero while in steady flight, a force will be exerted at the stick trying to move it to a new position. If the pilot wants to maintain the original airspeed, he must resist that force with his hand, at least until he can readjust the trim system. As long as the surface does

not move, the change in only the aerodynamic hinge moment will

not produce an effect on the motion of the airplane BUT the pilot will be feeling an effect through his stick. If the change in the hinge moment may be exceeded and the pilot will have to apply the extra force needed to completely balance the stick force. Take note that either the change in the stick force or the trim control position will be an indication or measure of the hinge moment change. If some of the other aerodynamic characteristics 40 MAY 1983

also had been changed by contamination, as discussed in the previous section, the stick force or trim control change would reflect these other changes as well as the hinge moment changes. More will be said about this later. You probably are aware by this time that we have been talking about two different kinds of "trim" relative to pitch behavior of the airplane. One has to do with pitching moments about the CG of the airplane, and I prefer to call this PITCH TRIM. The other has to do with the pitch stick forces felt by the pilot and I prefer to distinguish this from the other by calling it STICK TRIM. In affect, pitch trim has to do with the POSITION of the pitch control, that is, the position required to adjust the pitching moment to zero at the desired airspeed. Also, stick trim has to do with the FORCE exerted by the pilot to maintain pitch trim. If a pitch trim system is used, then the stick force can be reduced to zero in which case you can refer to stick trim in terms of trim-system

POSITION. When we say the plane is FULLY TRIMMED, then we should

mean that both PITCH and STICK trim have been achieved under steady flight conditions. In this case, the controls can be released without disturbing the airplane. I have found that a great deal of confusion and misunderstanding often arises when a clear distinction is not made between these different trim conditions. THIS DISTINCTION IS IMPORTANT in our discussions of FBC.

Stick-Free Condition Basically, if an elevator is completely free to float, it will float at the specific deflection where the net hinge moment is zero for any given flight condition. As long as the net hinge moment is zero at the desired speed, then the airplane will be fully trimmed and there will be no tendency for the airspeed to change. However, if the aerodynamic hinge moment or the trim springs are altered, then the elevator will tend to change to another angle thereby causing the airspeed to change usless the stick is restrained from moving. Let's assume for the moment that contamination of the airplane affects ONLY the hinge moments of the elevator, just as we did initially in the last section. Also, let's assume that the airplane has been fully trimmed for level flight and the stick released before encountering contamination. Obviously, the elevator will move and the airplane will respond to the elevator motion in some manner as soon as the hinge moment changes with contamination. For instance, if the elevator tends to float further downward when contamination is encountered, the airplane will pitch up and slow down to some lower airspeed. It will also be climbing if the final speed is above that for best rate of climb, or it will be descending if the speed is lower. Naturally, if the elevator tends to float further upward, the airplane will pitch down and the airspeed will increase. If the floating angle is changed too much, it will be necessary for the pilot to intercede either by readjusting the trim control or holding the stick if the trim control is not sufficient. If the phugoid motion of the airplane (the long-period oscillatory pitching motion) is very lightly damped, it is possible that a sudden encounter with the contamination will tend to initiate an oscillatory response. However, this should impose no serious problem unless the speed change is significant, in which case, the pilot most likely will want to grab the stick while he retrims the airplane. If the other aerodynamic characteristics are also changed with contamination, the resultant behavior of the airplane with controls free would not necessarily be any more severe but the behavior could be completely different depending on which characteristic was affected the most and which was the most influential. Because of the many variables involved for the stick-free case, not much useful information can be learned about the sources of the changes produced by contamination.

Use of the Controls As Indicators of Contamination Effects

In the previous section, we have been discussing primarily the possible effects of contamination on the aerodynamics of the airplane and the resulting types of flight path responses without regard to the magnitude of these possible effects and how to document their impact on behavior of the airplane. The answer is to utilize wind tunnel data, if available, for some of the information and to extract some of it from actual flight tests. Fortunately, we have some pertinent wind tunnel data available and will discuss that in the next installment. There also is a limited

amount of flight data and that will be discussed later as well. At

this point, however, we need to discuss just what type of flight data is needed and how best to use it. To obtain all of the desired information from flight tests, we would need a complete set of instrumentation that will record accurately all of the flight path variables involved along with the positions of all the flight controls. This would permit us to calculate all the changes in lift, drag and pitching moment and relate them to the actual motions that resulted from the effects of contamination. Of course, this would be a very costly and time consuming process and is not suitable for our purposes. Therefore, we must rely on some simpler flight test data that relate directly to the behavior of the airplane but can be obtained with very simple instrumentation. The use of the control movements required to reestablish the original flight conditions, as mentioned briefly in the prior section, provides us with a suitable method which can be easily implemented. All that is required for much of the information are calibrated scales mounted on the appropriate controls so that the pilot can observe the before and after control settings and a notebook and pencil to record the measurements. Even if measurements are not taken, some useful qualitative information can be obtained from the pilot's impressions of the control movements

as long as he is properly prepared to make the observations.

To use these observations or measurements it is important to understand the significance of the various control movements involved. For instance, if ONLY the trim position or stick force changed, then it is known that the hinge moment was the only aerodynamic characteristic that was influenced by contamination. It is apparent then that only a modification of the elevator will be effective if the FBC is serious enough to require a fix. On the other hand, if movements of the stick or throttle were also involved, then other aerodynamic characteristics were affected by the contamination and other types of modifications might be dictated. As long as the airspeed and flight path are returned to their original conditions, the change in throttle position reflects the TOTAL drag change caused by the contamination assuming, of course, that operation of the engine and propeller have not also

been influenced. I've emphasized the word "total" because we cannot obtain separate indications of the drag contributions of

the individual components of the total airplane with this very simple method. The change in the stick position reflects a NET change in lift, pitching moment and elevator effectiveness. To separate the combined change, it is necessary to add an angle of attack sensor and indicator so as to determine if the angle of attack had changed or not. If there had been no angle of attack change, then there

would have been no change in the total lift. In this case, the stick

position reflects only the net changes in pitching moment and elevator effectiveness.

The change in angle of attack indicates a change in total lift

of the airplane but does not differentiate between the wing and tail lift contributions. This differentiation, however, can be inferred from the indicated change in pitching moment changes derived from the stick position measurements. For instance, a pitchup

change would suggest that the wing lost more lift than the tail, and vice versa. As you have no doubt realized, this kind of analysis is not

exactly easy and a fair amount of judgment is involved because the measurements involved are only INDICATIONS of the contamination effects and not direct accurate measurements. Even though they are relatively crude, they should help to document the type of behavior involved and to isolate the possible source

of the major aerodynamic changes that cause that behavior. Such information is helpful in comparing various airplanes and in trying to develop fixes for major problems.

Tail-Aft Versus Tail-First Pitch Behavior The question as to why FBC is not as apparent with conventional tail-aft designs as with tail-first designs can be answered to a large extent by noting that, in the case of the TAIL-AFT design, the wing is intended to carry essentially all of the weight of the airplane. The CG is therefore located very close to the lift of the wing (near the wing quarter-chord location). Although a noticeable lift change may result from contamination of the wing, its effect on the pitching moment will be small because of the negligible moment arm. Furthermore, the design requirements for the aft-tail are different than previously and an airfoil shape

less affected by contamination is normally used. Also, the elevator in its tail-aft location tends to maintain sufficient power at low speeds so that the pilot can override any serious moment changes induced by the contamination Discussion Summary and Comments

The two physical features that make most current canard and tandem wing airplanes generically similar also make their pitch behavior susceptible to the effects of contamination. These features are 11 carrying of a significant portion of the weight by each of two lifting surfaces, and 2) location of the elevator or pitch control on the forward lifting surface. Because of these features, the aerodynamics of the forward surface or the tail can have a major influence on the behavior of the airplane when contamina— tion is encountered. However, there is a large number aerodynamic characteristics and their combinations for the airplane as a whole that can be altered by contamination and produce numerous types of pitch behavior. Specific areas to be concerned with are 11 takeoff distance and speed, 2) rate of climb following takeoff. 3i landing speed and distance, 4( stick force and trim control travel, and 51 maneuvers during takeoffand approach. Knowledge of the specific responses of the airplane to encounters with contamination under carefully controlled conditions will help identify the specific aerodynamic factors involved and should aid in finding methods for correcting

serious problems in these areas of concern. The following is a summary of the responses to changes in specific aerodynamic forces: 1) An increase in ONLY drag caused by contamination will cause an increase in the rate of descent (decrease rate of climb)

and cause the airplane's pitch attitude to change to some extent in the nose down direction. Angle of attack and airspeed less than maximum will not be affected to any noticeable degree. 2) A decrease on ONLY lift will cause a direct increase in

airspeed which, in turn, causes the drag to increase further. Consequently, the rate of descent and pitch attitude will change also, BUT there will be no angle of attack change.

3) A change in ONLY pitching moment will cause the angle

of attack to change which in turn will cause changes in the pitch attitude, airspeed and rate of descent. The direction of the change will be determined primarily by the loss of lift of the wing and tail. A pitch-down change will be produced by a lift loss at the tail and a pitch-up change by a loss at the wing. The changes in rate of descent will depend on the trimmed airspeed relative to the speed for best rate of climb (maximum lift drag ratio) of the airplane. The most critical condition for a pitch-down behavior

appears to be the loss of tail lift at low airspeeds.

4)A change in ONLY the elevator hinge moment will cause the airspeed to change ONLY if the stick is free. If the stick is held fixed, the change in only the hinge moment will cause the

force required to hold the stick fixed to change, or the pitch trim control to be changed by the pilot so as to reduce the force back to zero.

Movements of the controls, that is, the changes in elevator

stick position (pitch trim and trim control position (stick trim) as well as the throttle, required to return the airplane to the original

trimmed flight condition, can be used to document the FBC and to aid in finding the possible causes of the behavior. In this first installment, we have been concentrating on basic

aerodynamic and flight dynamic principles without regard to specific details of a particular tail-first configuration. Next month

we will cover pertinent NASA wind tunnel and flight tests of the

VariEze airplane and discuss those construction and operational factors which appear to contribute to FBC. We will also discuss

the relation of these findings to the several other current tail-first designs.

References

1. Yip, Long P. and Coy, Paul F.: Wind-Tunnel Investigation of a Full-Scale Canard-Configured General Aviation Aircraft. 13th ICAS Congress/AIAA Aircraft Systems and Technology Conference, Seattle, Washington, Aug. 22-27, 1982. ICAS Paper Number 82-6.8.2. 2. Holmes, Dr. B. J. Obara, C. J.: Observations and Implications of Natural Flow on Practical Airplane Surfaces. 13th ICAS Congress/AIAA Aircraft Systems and Technology Conference, Seattle, Washington. August 22-27, 1982. ICAS Paper Number 82-5.1.1. SPORT AVIATION 41

By Don Hewes (EAA 32101) 12 Meadow Drive Newport News, VA 23606

I N THE FIRST installment of this 3 part article, we discussed primarily aerodynamic characteristics that could influence the flight behavior of tail-first airplanes when they encounter some form of contamination; that is, rain, snow, frost, bugs, dirt or what have you. The term "Flight Behavior Change" or "FBC" was coined to identify this influence. This month we will look specifically at some pertinent wind tunnel and flight test data obtained by NASA using the VariEze airplane and relate these findings to our previous discussions of FBC and to the behavior of several other current tail-first designs. We will also discuss various design, construction and operational factors that can have some significant effect on FBC. Before proceeding, however, I would like to restate what I said previously: that the VariEze was selected by NASA because it incorporated a number of unique and interesting advanced design features and NOT because it has some particular problem. The

data were used for this article because they were the only data available pertinent to the subject of this article. The specific data applies only to the VariEze design and should not be applied to the other designs without proper consideration being given to the physical differences. However, some of the trends and effects noted can be applied in a more or less straight forward manner as will

be discussed in this installment. Wind Tunnel Data For VariEze Tail Surface

In the first installment we focused our attention on the critical nature of the tail surface, so let's examine this further by looking at the NASA wind tunnel data obtained from Reference 1 for the VariEze's tail surface. Figure 5 shows how the lift of the tail and the complete airplane varies in the wind tunnel as the angle of attack of the airplane is varied with the elevator set at several

deflections for the clean or uncontaminated condition. (These data for the tail are given in terms of the area of the main wing and, consequently, have smaller values than you would normally expect to see. The difference between the two curves represents the lift contribution of the wing and the fuselage combination.) Note that the increments of tail lift produced by deflection of the elevator (the spacing between the tail lift curves) decrease as the elevator is deflected further and further downward. This is especially evident for angles of attack above about 10°. Also, note that the canard begins to stall at about 14° with zero deflection BUT it begins at about 8° with maximum down deflection. Thus, DOWNWARD deflection of the elevator REDUCES THE LIFT EFFECTIVENESS OF THE ELEVATOR AND CAUSES EARLIER STALL of the complete canard surface. Bear in mind that this surface uses a slotted flap as the elevator which generally is more effective in generating lift increments with deflection than 48 JUNE 1983

the simpler plain flap used in other tail-first designs. The general trends in the data as illustrated here should be fairly similar, however. In Figure 6, three sets of data are shown, one for the tail with a dry smooth surface (FREE), one with the leading edges artificially roughened with a small amount of coarse grit glued to both left and right tail panels (FIXED), and the last with both panels smooth but with one panel wetted with water sprayed from nozzles to simulate rain (FREE, WATER ON). The elevator deflection is zero for all cases. (The terms "free" and "fixed" refer to the types of air flow transition associated with the smooth and roughened leading edges.) There is a LOSS OF ABOUT 27% of the TAIL LIFT throughout the normal flight range of angles of attack for the second set of data compared with the first set showing that the application of the coarse grit, representing a "bad case of the bugs", had a very pronounced effect on the aerodynamic characteristics of the canard. The loss of lift produced by the WATER

SPRAY IS ROUGHLY EQUIVALENT TO THAT OF THE GRIT when you consider that only half of the canard was sprayed with water. There are also corresponding increases in the drag of the canard associated with these losses of life. These are very large changes in the aerodynamic characteristics and certainly indicate a significant effect of contamination on the characteristics of the tail surface. (Remember, however, we are looking only at the tail contribution and not the total aero characteristics of the airplane.) Of course, the influence of surface contamination on the drag of sailplanes has been recognized for many years and is associated with the premature TRANSITION from laminar to turbulent flow which causes the drag to increase significantly but does not alter the lift to any extent at cruise conditions. The somewhat unusual aspect of these data is the relatively large loss of lift at the low angles of attack due to the roughness but this type of loss can be found with some other airfoils that are unusually thick. The loss is attributed to premature turbulent flow SEPARATION near the trailing edge occurring at the lower angles of attack. The trends noted in the data do correspond to those assumed in our earlier discussions, however, we are missing elevator hinge moment data which unfortunately were not available. Discussion of Tail Sensitivity to Contamination

What is it that makes the tail aerodynamics so sensitive to the effect of rain and bugs? The quick answer to this question is that it is the particular type of airfoil that is being used for the tail surface and is not the location of the surface on the airplane. But this needs explanation. As air flows past an airfoil, the molecules of air next to the surface are slowed by the frictional forces acting between the molecules and the airfoil surface as well as the molecules themselves. This action forms what is referred to as a boundary layer on both the top and lower surfaces. We will concern ourselves with the layer over the upper surface. This boundary layer and what happens to it are the key factors to the FBC phenomenon.

FIG. 5 - LIFT CHARACTERISTICS OF

•8f .6

VARIEZE (REF I.)

Q-«.

m

- 8 - 4 0 4

8

12

16 20 24 28

32 36 40 44

WING STALL

CANARD STALL

12

16

20 24

a, cleg

28

32

36 40 44 SPORT AVIATION 49

CANARD TRANSITION

.4

O FREE D FIXED O FREE, WATER ON

.3 .2 .1 0

,01 .02 .03

.04

.05 .06 .07

- 4 0 4

8

12

FIG. 6 -

16 20 24 28 32 36 40 44 a,deg EFFECTS OF CONTAMINATION ON CANARD.

The boundary layer grows in thickness from just a few molecules at the nose to several tenths of an inch as the flow progresses toward the trailing edge. If we were to measure the velocity distribution across the boundary layer with a very small velocity probe and plot the variation of the velocity as a function of the distance from the airfoil surface, we would be looking at what is called a "velocity profile" as shown in Sketch A. As you can see, the actual outer edge of the layer where the velocity becomes constant is very difficult to locate, so for convenience we actually refer to the thickness as that point where the velocity has reached some arbitrary percentage of the free stream velocity above the surface. We are not really interested in the actual thickness for this discussion but rather in the manner in which it changes and the character of the flow within this layer. The flow in the boundary layer near the nose of the airfoil is

very orderly as the molecules slide smoothly, one over the other, and we have what is called "laminar" flow as long as the surface itself is relatively smooth. This results in a low drag or shearing force on the airfoil. The velocity profile for laminar flow shows a very rapid increase in velocity in a distance of only a few molecules. The shape of the profile is depicted in Sketch B. Although the boundary layer gets progressively thicker as the flow moves aft from the nose, the shape of the profile does not change. The significance of this is that the molecules within the boundary

layer maintain most of their energy and are able to continue moving in a uniform manner following the contour of the airfoil for some distance. The greater the distance the laminar flow continues, the lower the drag will be. At some point along the airfoil, however, the molecules are going to start loosing their uniform motion and begin to tumble over each other. This action is referred to as "transition" and the 50 JUNE 1983

point where this occurs is referred to as the "transition point". At

this point, the layer thickens, as illustrated in Sketch C, and the flow from here on to the trailing edge and beyond is very random in nature and referred to as "turbulent". The velocity profile for this condition, also shown in Sketch B, indicates that not only has thickness increased but the velocity of the molecules at a given distance has decreased greatly. This condition creates much more drag on the airfoil and the flow finds it much more difficult to follow the contour of the airfoil exactly. Although the lift being generated by the airfoil is not altered to any significant degree by the transition to turbulent flow, the POTENTIAL FOR SEPARATION of the flow from the airfoil altogether is increased. If separation does occur, the result is then a marked loss of lift over those portions of the airfoil aft of the "separation point". In effect, the flow at the separation point starts to reverse direction and

the air from the lower surface starts to spill over the trailing edge. Two of the key factors affecting the SEPARATION POINT are the THICKNESS of the boundary layer and the LOCATION OF THE TRANSITION POINT. If the transition point is located FORWARD on the airfoil where the boundary layer is quite thin, then the boundary layer becomes thicker immediately behind the transition point, as indicated in Sketch C, but the overall velocity or energy level within the turbulent boundary layer is relatively high. The potential for separation in this case is RELATIVELY LOW. If, on the other hand, the transition point is located much further AFT where the laminar boundary layer has thickened considerably and become "aged", that is, the molecules close to the surface have lost most of their energy, then the turbulent layer that now forms is very much thicker, as indicated in Sketch D, and its energy level is greatly reduced. The potential for separation is now RELATIVELY HIGH.

The expanding shape of the forward portions of all airfoils

creates a positive pressure gradient which helps the air molecules

move out of the way of the wing as it passes through the air. This favorable gradient also helps to damp or stabilize the small amounts of airflow turbulence that tend to build up in the boundary layer. This causes the TRANSITION POINT to be further aft on the airfoil than it would be if there were no favorable gradient. The tapering shape of the rear portion of the airfoil creates a negative or adverse pressure gradient which tends to cause the

airflow to become turbulent sooner than it would if there were no adverse gradient. Thus the TRANSITION POINT tends to move forward in the presence of such a gradient. Not only this but also the negative gradient, if strong enough, will cause the

SEPARATION TO OCCUR PREMATURELY. Increasing angle of attack to produce greater lift increases the

pressure gradients over both portions of the airfoil. This increase in the negative gradients over the aft portion is what causes the airfoil to stall ultimately. If one airfoil at zero lift has a greater negative gradient than another airfoil due to greater trailing-edge

taper, then that airfoil will stall at a lower angle of attack and develop less lift than the other. The first of the two airfoils shown in Figure 7 is known as the

GU 25-5(11)8 and is the one used on the VariEze tail. The GU

25 and variations of it are used on most all of the other canard designs currently flying as well. It is a laminar-flow airfoil developed to give high lift at quite low Reynolds numbers (low

airspeed) and also to have some unique stall characteristics which

make it very useful for the highly loaded condition unique to the tail-first designs. The GU airfoil obtains its special characteristics by having a rather large thickness (20%) and having this thickness located quite far aft. These features cause the boundary layer to remain laminar over about 55% of the upper surface and much further aft on the lower surface under normal conditions thereby produc-

ing very low drag. Comparison with the modified NASA GAW-1 airfoil used for the less heavily loaded main wing of the VariEze,

also given in Figure 7, shows that the maximum thickness of the GU airfoil is greater than that of the GAW airfoil. For the GU airfoil, as long as the flow is laminar before reaching the negative-gradient region and the angle of attack is not too large, the flow will continue to stay attached to the surface so that the full chord length of the airfoil can produce lift until the normal stall angle is reached. If, however, the flow has become turbulent due to roughness on the leading edge surface before reaching the region of adverse pressure gradient, then premature separation or stalling may occur even though the angle of attack is far below that for the normal stall, perhaps at zero angle of attack or less. Consequently, the GU airfoil will experience PREMATURE SEPARATION when there is sufficient accumulation of any type of material near the leading edge that will induce turbulence. A small amount of roughness or waviness existing in

the airplane flying in steady flight. The other points represent

non-steady flight, but the distinction is usually not made in this type of figure by the engineer because of normal practice. As pointed out earlier, the "clean" canard stalled in the range of about 8 to 14 degrees angle of attack but it can be clearly seen in Figure 5 that the complete airplane is not fully stalled until about 24° is reached. Furthermore, the figure shows that the

ELEVATOR CANNOT TRIM THE AIRPLANE TO ANGLES OF

ATTACK HIGHER THAN ABOUT 17°. There appears to be a

wide safety margin for stall of the wing built into this design, however, the margin actually is not as wide as it appears because

there are aerodynamic problems associated with a partially stalled wing which must be considered. Departure of the lift curves from

a straight line at about 17° identifies the initiation of the partially

stalled condition. Thus, the margin actually is very small.

The loss in elevator effectiveness associated with the stallproofing feature discussed earlier is evidenced by the bunching of the curves for the higher elevator deflections which occurs in

the range of 15 to 17° angle of attack. (Refer to the portions of the pitching moment curves close to the CM = 0 axis in the upper figure.) The significance of this loss can be seen with the aid of the solid curve of Figure 8 which shows the ability of the elevator to trim the airplane. This curve is a cross-plot of the data given in Figure 5 for the "clean" condition. It shows that the elevator cannot trim the airplane to a trimmed lift coefficient greater than

about 1.5 which corresponds to the 17° angle of attack limit just mentioned. (An airspeed scale in knots has been added to the original figure for reference purposes. These airspeeds refer to non-maneuvering level flight for the aft CG position at gross weight at sea level and may not agree directly with the airspeeds indicated in flight.) This indicates that a speed lower than about

61 knots could not be reached even though the elevator was

deflected to relatively large downward (positive) angles. Changes in weight and center of gravity position will alter the curve

somewhat but the general trend will still be evident. This figure also shows a dashed curve for the "dirty" condition in which the flow was tripped by grit at the leading edges of the

wing and tail. Comparison of the two curves shows that the effect of contamination was to cause a noticeable downward shift in the elevator deflection required to trim at a given lift coefficient or

airspeed and that an increase of about 8 to 10 knots for the

minimum airspeed due to contamination is indicated. This result

can be traced directly to the loss of tail lift illustrated in Figure

CANARD

the surface may not, by itself, cause the separation because of the stabilizing effect of the positive pressure gradients along the forward portions of the airfoil. But this should tend to make the

tail susceptible to much smaller amounts of contamination than

otherwise. It should be noted that other types of airfoils, such as that used for the wing, are not as critical from the standpoint of premature separation because 1) the shape of the forward position tends to cause transition to occur further forward, 2) the adverse pressure gradients due to the aft shape are not as severe as that for the GU airfoil, or 3) there is a combination of both. However, these other airfoils are not necessarily immune to the problem. It is possible that there are a number of other airfoils, including modifications of the GU section less critical as far as contamination is concerned, that would be better suited for use on tail-first designs. Complete-Airplane Wind Tunnel Data The pitching-moment data for several elevator settings as measured for the "clean" condition of the complete airplane are also presented in Figure 5 along with the previously mentioned lift data. The pitch data are given in coefficient form based on the wing area and mean chord of the main wing with the center of gravity located at the design aft limit. For those of you who are not familiar with wind tunnel data, I'll point out that only the data points for lift and drag for which the pitching moment coefficient is zero, that is, trimmed lift and drag, correspond to

GU 25-5(11)8

WING

MODIFIED NASA

GAW-I

FIG. 7-AIRFOIL COMPARISON. SPORT AVIATION 51

CANARD TRANSITION FREE ——— FIXED

15

max

10

0

-5

150 106

87

75

67

61

58

53

AIRSPEED, KNOTS

-10

-0.5

0

1.0

0.5

1.5

2.0

L

trim

FIG. 8 -

ELEVATOR TRIM CHARACTERISTICS FOR AFT C.G.CREF. 0

6 and its associated effect on the pitching moment produced by the tail relative to the CG of the airplane.

The results of the wind tunnel tests are consistent with the trends that were discussed in the previous installment. Some of you who recall that an airfoil normally stalls at about 14 to 16 degrees may wonder about the rather high 24° angle of attack at which the VariEze main wing stalls as shown in Figure 5. Bear in mind that the VariEze main wing is quite highly swept and this has the effect of reducing the slope of the lift curve and extending the stall to the higher angles of attack. You will find that the straight or nearly straight wings used in the other

tail-first designs will stall at the lower angles.

Discussion On Use Of Excessive Elevator Deflections

The marked upswing of the solid curve shown in Figure 8 is attributed partly to the significant stable break (nose down) in the pitching moment curve as angle of attack is increased beyond about 14° with zero elevator deflection. This break is associated with the initial stalling of the canard. However, the upswing of the curve is also attributed to the loss of effectiveness of the elevator as the elevator is deflected further and further from its zero position, noted earlier. It is especially important that sufficient elevator effectiveness be retained at speeds just above minimum so that the airplane can be safely maneuvered for take-off or landing. This being the case, then it is important to not "use up" the elevator for purposes of trimming the airplane

in cruise and high speed flight. In other words, it is important to adjust the relative incidence of the tail and wing so that the elevator will be trimmed close to zero deflection or, perhaps, slightly up for cruise conditions. If flight tests reveal that excessive amounts of down elevator are being used to maintain steady conditions in the normal flight envelope, there are problems with the construction and/or rigging of the airplane. These problems can be traced to improper airfoil 52 JUNE 1983

shape, incidence or twist distribution of the lifting surfaces and

should be corrected before considering the airplane to be fully airworthy. We will say more on these items a little bit later, but the reason for mentioning them here is to make the point

that the amount of ELEVATOR DEFLECTION required for a

given airspeed can be used as a PARAMETER in evaluating how closely a given copy of a particular design matches the original or other copies. If the deflections match within 2 or 3 degrees for the same airspeed, weight and CG location, then there is some assurance that the airplane is properly constructed and rigged. Comparison With Flight Data

Now, let's look at some VariEze flight test data to see if they

agree with the wind tunnel data. Figure 9 shows a plot of the variations of elevator deflection with airspeed for the conditions of level flight with both "clean" and "dirty" leading edges. These data were obtained from a series of unpublished NASA tests of the VariEze that were related to those reported in Reference 2 for the Long-EZ. The figure reveals that the airspeed was reduced, the minimum airspeed was increased and more down elevator deflection was required to maintain a given speed within the speed band when the leading edges were contaminated. Note that very little additional deflection was required at the high speed end whereas added deflections of 3 to 5 degrees were required at the low speed end. Aside from an offset of about 3° at the high speed end of the curves, there is reasonably good agreement between the curves given in Figure 8 for the wind tunnel tests and those of Figure 9 for the flight tests. Thus, it appears that there is good qualitative agreement between the wind tunnel results and those obtained from flight test. (There are a number of technical reasons for the data from these two different types of testing to differ to some extent, consequently engineers never expect to get exact agreement.)

14

12

Transition

o

V MIN.

free

fixed

10

ELEVATOR DEFLECTION DE6.

8

6

V

MAX.

I_ 60

80

100

120

140

160

INDICATEDAIRSPEED, KNOTS

FIG. 9 -FLIGHT TEST DATA

A reasonable question to raise at this point is, "Well, did the flight tests reveal any significant FBC as far as the pilot was concerned?" The answer to this is a qualified "Perhaps". It was observed that the landing and take-off speeds with the "dirty" edges were noticeably higher than normal. This might present a problem to a pilot unfamiliar with the airplane, especially if he tried to operate from a short field. However, it should be noted that the tests really were inconclusive relative to FBC behavior because they were not tailored to study this phenomenon and the test pilot was not specifically asked to address this problem. The tests were made primarily to obtain the performance data for straight and level flight with power required to sustain constant altitude. No tests were made to determine effects on rate of climb and on the stick forces or trim. Also, banked and accelerated maneuvers at low airspeeds, such as might be performed in abnormal landings and take-offs, and forward CG loading conditions were not performed because they were not considered pertinent to the primary objective of these particular tests. Inasmuch as these maneuvers and conditions require additional down-elevator deflections from those required for normal level flight, they are pertinent to the FBC problem and need to be performed. More on this subject will be said later. An additional comparison of flight data can be made by exa-

mining the Long-EZ test data given in Reference 2. We should note that the Long-EZ appears to be similar to the VariEze but does differ in several respects. The wing is considerably larger and carries more of the total weight of the airplane. It also has less sweepback and uses a different airfoil section. The canard, however, appears to be quite similar. The data, shown in Figure 10, were obtained for the same conditions as those for the VariEze and indicate somewhat similar results. Some later data provided by Burt Rutan for the Long-EZ show essentially the same trends as in Figure 10 but indicate that the minimum airspeed was affected only slightly by loss of the laminar flow.

FOR VARIEZE.

Just as I was completing this article Burt invited me to come to Mojave and fly the Long-EZ to check on its flight behavior. As it turned out, I arrived there in the midst of the series of heavy rain storms that hit California at the end of January and had an excellent opportunity to fly in both rain and dry air. Dick Rutan provided me with a very effective demonstration of aerobatics with his plane in the rain and then I explored low speed behavior in both rain and dry air and found no significant deterioration in the flight characteristics. I essentially repeated the same thing with Mike Melvill in the factory Long-EZ because Dick's bird is not a stock Long-EZ, but there was very little difference between the two as far as rain effects were concerned. Inasmuch as I was riding in the back seat, I did not make the take-offs and landings but these did appear to be reasonable even though it was raining at the time. Although these flights do not represent exhaustive testing under all conditions they did serve to demonstrate to me that the Long-EZ design and these particular copies of it have no significant FBC piloting problems even though the flight tests do show some influence on the elevator characteristics. Comparison of FBC Characteristics For Copies of the Same Design There are no specific data available to me for comparing the FBC characteristics of various copies of the VariEze design. However, in recent discussions, Burt Rutan has stated that there have been noticeable differences in these characteristics observed for the large group of VariEze airplanes currently flying. In the case of a few of these airplanes, very definite pitchdown tendencies have been encountered while, for the majority, only relatively mild tendencies have occurred having no significant effects on flight behavior. Surprisingly, in a few instances, a very mild pitchup has been encountered. It would appear that the results obtained from the NASA tests were consistent with those obtained SPORT AVIATION 53

for the majority of these airplanes currently in use, but these tests do not account for the extremes noted in a limited number of cases. As I'm sure you're already aware, no two of these homebuilt copies can be built EXACTLY alike and none can be exactly like the original version because of the building tolerances involved and the small changes introduced by each builder to suit his individual requirements. It, therefore, should be no surprise to anyone in learning that the flight behavior of these do differ to some extent. The real problem is the large range of this variability. While some of these extreme cases may be attributed directly to rather extensive modifications to the original design or very poor workmanship, there apparently are some where there are no readily apparent differences to explain the behavior. In our earlier discussions, we spent much time showing that different forms of FBC could be produced by each of the basic aerodynamic forces and moments. Also we showed that the exact form would depend on the extent that the aerodynamic contributions of the components of the airplane were influenced by contamination. Discussions of the tail and its airfoil, which have been identified as being a primary source for the FBC phenomenon, revealed that the tail's contributions were very sensitive to small irregularities on the surface of the tail. It is only reasonable, therefore, to conclude that some cases of extreme behavior may have resulted from SMALL DIFFERENCES in various features of the airplane or other factors that are not necessarily readily apparent. If so, then, we need to evaluate the effects of small variations of a number of design, construction and operational factors. Unfortunately, there are no technical data to show us the relative effects of the numerous factors that might be involved. Consequently, we can only list those that we think might be involved and discuss their possible impact. Obviously, this involves a high degree of personal opinion and I freely admit that I am merely making educated guesses at this stage. The reader is invited to make his own assessment after reviewing this article, and the author will be glad to hear from anyone who has some constructive information to offer.

Discussion of Possible Factors

We will discuss the following: 1) airfoil shape and surface skin conditions, 2) elevator hinge gap and trailing edge shape, 3) incidence and twist, 4) center of gravity location, and 5) weight. First, we should consider how close the wing and tail surfaces can be duplicated by the many different homebuilders, a larger number of them building their very first airplane. Good workmanship is vital in building to a high degree of accuracy and an excellent finish is always a very good index of the quality of workmanship. But a smooth and glossy finish and good workmanship do not necessarily guarantee that the airfoil itself is made to the required accuracy. Much can depend on the construction techniques involved in addition to the skill and experience of the builder. Airfoil designers will tell you that departures of only a few thousandths to hundredths of an inch in critical areas of the airfoil can alter basic characteristics very markedly. The leading edge and upper surface are particularly critical. If the construction techniques do not permit the builder to work to required accuracies in these areas, then inconsistent results may be obtained even by very careful and experienced builders. Prior to working with the fiberglass-foam type of construction myself, I thought that it should be a very accurate way of reproducing a given airfoil surface. Now that I have been involved to some extent in building several of them, I have changed my opinion, at least concerning the current homebuilding techniques I have used or observed. There are several steps and working conditions in the process that can influence the final shape. Some of these are 1) copying template shape, 2) excessive melting due to cutting the cores too slowly or with the wire too hot, 3) under cutting or scalloping due to wire lag (wire too slack, wire too cold or cutting speed too fast), 4) wire bowing due to variable density at core joints, 5) foam core bowing (after wire cutting) due to internal and surface stresses, 6) assembling final structure from many core sections without accurate jigs, 7) surface irregularities caused by core defects and cloth texture, overlap and joints.

20 18 16

V

TRANS ITION OFR D FIXED

min trim

14 ELEVATOR DEFLECTION 6

e'

de

9

12 10

8 6 V

4

max

2 0

60

80

100

120

140

160

INDICATED AIRSPEED, V ( , knots

FIG. 10 -FLIGHT 54 JUNE 1983

TEST DATA FOR LONG-EZ. CFtEF. 2)

180

Some of these items are beyond the control of even a very careful builder. Furthermore, there are no procedures using templates for checking the final airfoil shape after completion.

VariEze or modified versions of it. The first three designs employ Eppler airfoils for the main wing which I believe is similar to that used with the Long-EZ. The Retro uses a NACA 74-series

he goes to a great deal of extra work. But if he does, he has no way of knowing that the final shape is acceptable. Based on my own experience and observations, I believe that

employed by the others. There are numerous copies of both the Quickie and Q2

The builder has no way of knowing what the exact shape is unless

deviations of 1/16 to 1/8 inch or more from the design shapes for both the tail and wing are possible and that deviations of this size are sufficient to account for some of the extremes in FBC that

have been noted. There is little doubt in my mind that the CONSTRUCTION TECHNIQUE is responsible for these deviations to

a large extent. Aside from producing significant differences in the shape of

the airfoil, the fiberglass-foam construction methods also result

in roughness and waviness of the surface skin which requires extensive filling and sanding to achieve an acceptable condition.

Of the two, WAVINESS probably is the more critical factor from the aerodynamic standpoint and probably is the more difficult to

detect and eliminate. I know that at least one of the designers has gone to great lengths to get the builders to pay attention to the waviness problem, but I expect that there is a wide variation in these two factors for the final surfaces from one plane to the

next.

Another surface factor to consider is that of the actual SURFACE FINISH. If the surface is covered with a very glossy and slick finish, the rain will tend to adhere in the form of raised drops which will cause significant turbulence. On the other hand, if the finish is dull or satiny, the rain may tend to spread out in a thin smooth layer with little influence on the airflow. Also, the quantity of bugs, ice or other materials that adhere to the surface may depend to a large extent on the type of surface treatment. Closely related to the airfoil "shape" problem are those of the ELEVATOR GAP and TRAILING EDGE. The shape and width of the gap between the elevator and the fixed portion of the tail has a very strong influence on the flow separation behavior over the elevator. Consequently, small differences here can cause two otherwise identical tails to have significant lift and elevator control power differences. The manner in which the elevator is mounted with the external hinges does lend itself to possible variations in the gap, but I really have no feel for how much of a problem this may be. The shape of the trailing edge has a very powerful effect on the hinge moments of the elevator although the lift is not influenced to any appreciable degree. Here again I do not have any information relative to the variability of these factors in the field, but I have a feeling that little attention is paid to the trailing edge shape. As a matter of fact, I believe that some of the early VariEze airplanes had a rather blunt trailing edge and that this was later changed to a sharp edge. This certainly could account for noticeable differences in the stick trim characteristics between some of the airplanes. As noted earlier, variations in INCIDENCE and spanwise TWIST in both the tail and the main wing can be a source of trim differences from one airplane to another. Furthermore, inasmuch as LOCATION of the CG has a direct effect on trim of the airplane, we can see that differences in CG location as well as lifting surface incidence will lead directly to differences in elevator deflection required to maintain a given airspeed. Also, differences in WEIGHT of the airplane will cause the elevator to be trimmed differently for a given airspeed. Discussions with two of the designers have indicated that there have been cases where the elevator deflections required for cruising flight have been down from the normal settings by several degrees, indicating some significant and unusual variations in the rigging and loading of the various airplanes. I strongly suspect that these particular airplanes have significantly different FBC than those that were rigged and loaded uniformly. Discussion of Other Current Tail-First Designs

Now, let's try to relate our information on some of the other current tail-first designs to our discussions of the VariEze and Long-EZ. We are going to talk about the Quickie, Q2, Dragonfly and a new single-place design called the "Retro" by its designer and builder, Gion Bezzola of Estavazer Le Lac, Switzerland (see February 1982 SPORT AVIATION). Unfortunately, we have only limited and very sketchy information on these designs. While these designs differ considerably from each other and from the VariEze configuration, they all have heavily loaded front lifting surfaces which employ the GU 25 airfoil used by the

section for the wing. The first three designs also use a plain flap design for the elevator rather than the slotted flap design

airplanes and there are several reported encounters with significant pitchdown. As a matter of fact, most all of the reported cases covered in the previously mentioned magazines have involved one or the other of these two designs. In reviewing the literature that Gene Sheehan, current president of Quickie Aircraft Co.,, very kindly sent me, I found that he and the late Tom Jewett have devoted a significant amount of effort studying the pitchdown behavior experienced with their designs. Gene indicated that his company had provided the builders with considerable information on the subject warning against flying in rain or taking off with wet or dirty wing and tail surfaces. Also he mentioned that the design modification permitting the ailerons to be reflexed to provide additional pitch control power has proved to be very effective in minimizing or eliminating the pitchdown behavior. He also mentioned the design modification developed by Carry LeGare, a former associate, was also effective and was available to builders. This modification consists of a small horizontal trimming surface mounted on the vertical tail. In my recent visit to Mojave, I also visited Gene and saw the Q2 prototype which has been fitted with the NASA LS(1>0417MOD airfoil for the front lifting surface. He reported that rain has been found to have very little effect on the behavior of the modified Q2 and that the company plans to make the new surface available for retrofitting on current Q2 airplanes. He also said that they would be evaluating this airfoil for the Quickie

also but he was not sure that it would be as effective because of the lower Reynolds number invovled.

Bob Walters, designer of the Dragonfly, states that there is

only a mild pitchdown with his airplane and claims that it is not a significant problem. He does caution builders about flying in rain and warns not to take-off with wet or dirty surfaces. It should be pointed out that, up until very recently, his airplane has been the only Dragonfly flying, consequently he does not know yet how typical the behavior is for the numerous copies that are now approaching flight status.

One of the first builders to fly his Dragonfly, Terry Nichols,

reported in the recent Dragonfly newsletter that he performed stalls in and out of rain and noted a 10 mph increase in stall speed and apparently a more pronounced stall break due to the rain. This result is consistent with Bob's recent comment that he

found the stall speed of his prototype was increased about 8 to 10

mph. While on my recent trip, I was able to discuss this further with Terry as well as with Rex Taylor who has taken over the Dragonfly company. Rex has had similar experiences in rain but indicated he has not tested the airplane extensively in the rain. None of these people feel there is any significant problem with the Dragonfly. Although I was able to fly the prototype Dragonfly through some extreme maneuvers at low speed with no problem,

I was not able to do this with contaminated surfaces and, therefore,

cannot verify their opinions. Examination of the Dragonfly templates given in the plans

revealed that the tail airfoil is different from the standard GU

25 airfoil in that it is slightly thinner and the maximum thickness

appears to be moved slightly forward. When I asked Bob about

this, he stated that he was concerned about the abrupt contour change of the GU 25 section and made the modification to help eliminate the separation tendencies encountered with the VariEze tail. (He is one of the early builders of a VariEze.) He did not explain whether this was merely to reduce drag for better cruise

performance or to minimize the effects of rain, but regardless of which was his intention, it appears that his modification to the airfoil may have been effective to some degree in alleviating FBC

characteristics which otherwise might have been more evident. It is pertinent to note here that an airfoil developed independently for another application by airfoil designer John Roncz, whom I have consulted, matches closely the contours of the Dragonfly airfoil. John's study shows that his airfoil is not as sensitive to flow separation problems as the original GU 25 section. Thus, since the two airfoils appear to be quite similar, it is probable that the Dragonfly airfoil truly is effective in minimizing the pitchdown problem as Bob's results have indicated. But until more Dragonflys have been test flown, this supposition will not have been proven. SPORT AVIATION 55

DISTANCE REFERENCE THICKNESS VELOCITY

LAMINAR

f

SKETCH A

SKETCH B

TRANSITION

SKETCH C

SKETCH D

56 JUNE 1983

SEPARATION

On a recent personal trip to Switzerland, I was fortunate to have the opportunity to talk with Gion Bezzola and observe him flying his prototype version of the Retro. He stated that it is subject to pitchdown much the same as the VariEze but, unfortunately, we did not have time to discuss the subject in depth. He is a pilot for the Swiss Air Force and has many hours in numerous airplanes of all types, including the VariEze. His views on the factors involved in the pitchdown behavior were consistent with those I have expressed in this article. Let's summarize our discussion at this point by observing that

we have covered a total of six different designs and have found

that each does demonstrate some form of FBC but the type and severity are not necessarily the same for all designs. Certainly,

there are basic design differences between the various designs that are important and we should not expect that the behavior of all would be necessarily the same.

I would like to point out that most of the FBC encounters that I have heard about for any of these designs have been isolated incidents with the airplane flying into rain while in NORMAL

CRUISING flight and were NOT SPECIFIC TEST flights designed

to isolate and identify the behavior. In some cases, these encounters have been fairly mild and the persons reporting these events seem to be left with the impression that the phenomenon is of little or no concern. Inasmuch as FBC may be much more critical for low speed flight conditions, there is the POSSIBILITY that

some airplanes that have been judged as having no problem will, in fact, demonstrate undesirable or unacceptable behavior. The "bottom line" for this discussion is that, regardless of the

specific design, the builder or pilot must be aware of the POSSIBILITY that his particular airplane may have some unusual

characteristics. Therefore, the only way to know for sure is to conduct proper flight test with his own airplane.

The process of evaluating the acceptable behavior of a

homebuilt airplane is strictly a qualitative process utilizing the

judgment of one or more knowledgeable and experienced pilots. Strictly speaking there are no firm quantitative parameters that must be met, however, the pilot should make his judgment of FBC based on the extent that the "numbers" are changed for the following items: 1) take-off distance and speed, 2) rate of climb following take-off, 3) landing speed and distance, 4) stick force

and trim travel, and 5) maneuvers at take-off and approach speeds.. Closing Remarks

The discussion covered in this article leads to the following

statements: 1) FBC is a premature-stall phenomenon associated with tailfirst airplanes in which a significant portion of the weight is

carried by the two lifting surfaces and the elevator is located on the forward surface. Because thick laminar-flow airfoil sections such as the GU 25 section, which are quite sensitive to surface roughness or contamination have been used for the forward tail

surface, the tail can have a major influence on the behavior. However, there are a number of aerodynamic and physical factors

which influence the nature of this phenomenon and these factors can vary significantly from one copy to the next of a given design.

Use of other airfoils less sensitive to the effects of contamination

should minimize or eliminate this behavior. 2) Builders should be particularly aware of the critical nature

of the various factors involved and are cautioned to follow the designers instructions and design details as closely as possible. They should avoid making any changes in the airfoil shape or control surface configuration without thorough knowledge of the influence of these changes on the flight behavior of their airplane.

Such changes may cause the airplane to fly completely different

from the original design with severe degradation in performance, Discussion On Potential Dangerous Behavior

We have been discussing at great lengths "FBC", "potential problems", and "undesirable or unacceptable behavior". But what do we mean when we use these words when applied to airplanes? Do we mean that their behavior is dangerous, and what is it that actually makes it dangerous?

flight behavior and safety. 3) FBC appears to be more critical for low speed flight conditions than for cruise and high speed because of loss of pitch control

dangerous because the skill level of the pilot in command must be taken into account. At this point, we must be a little bit careful about assuming what the term "pilot skill" actually means. Many pilots consider themselves highly skilled because they have several thousand hours in many different type airplanes, and they certainly are justified to do so. However, in the case of flying an airplane that is not familiar to them and that has an unusual

power and lifting capability along with increased drag. Maneuvering at low speed may aggravate the behavior. Pilot control inputs in response to the FBC encounter will depend on the type of behavior. Pilots who are unfamiliar with this phenomenon may not be able to recognize aggravated behavior and may apply wrong recovery inputs. 4) Although this phenomenon is not necessarily a serious problem for a particular design, FBC of each copy should be evaluated completely with flight tests in much the same manner as stalls and other flight behavior. These tests should be made during the initial airworthiness fly-off period required for certification of the airplane in the experimental category. 5) Care should be exercised by the pilot when flying a tail-first airplane for the first time and he should check with others who may have flown it previously to learn about its specific behavior. He should be aware of the potential for encountering some unusual behavior and the proper technique for recovering. The statements made here and throughout this article have been based on an analysis of a number of basic aerodynamic facts and well established principles of flight dynamics and pilot behavior, as well as a very limited amount of experimental wind tunnel and flight test data. However, there are areas of personal judgment and speculation, and consequently, these statements should not be treated as firm conclusions until a much broader base of experimental data can be gathered and a more thorough analysis made. Next month we will conclude with a number of suggestions and recommendations pertaining to flight testing and operation of tail-first airplanes.

happening or the situation does not permit them the time to "feel out" the problem. If they were provided the information about the nature of the problem beforehand, then it is highly likely that all of them would be able to cope. Thus, we can see the importance of being ADEQUATELY INFORMED, as well as SKILLED, in cases of dealing with airplanes having UNUSUAL CHARACTERISTICS. There should be little confusion in dealing with the term "undesirable". You can consider it as referring to nuisance traits. There are a number of airplanes that have one or more characteristics that can be termed "undesirable" yet the airplanes are considered acceptable on the basis of their overall behavior.

References 1. Yip, Long P. and Coy, Paul F.: Wind-Tunnel Investigation of a Full-Scale Canard-Configured General Aviation Aircraft. 13th ICAS Congress; AIAA Aircraft Systems and Technology Conference, Seattle, Washington, Aug. 22-27, 1982. ICAS Paper Number 82-6.8.2. 2. Holmes, Dr. B. J.., Obara, C. J.: Observations and Implications of Natural Flow on Practical Airplane Surfaces. 13th ICAS Congress—AIAA Aircraft Systems and Technology Conference, Seattle, Washington. August 22-27, 1982. ICAS Paper Number 82-5.1.1.

To answer these questions, let's first define "dangerous behavior" as being "motions of the airplane, either controlled or

uncontrolled, that expose the occupants to the threat of immediate injury or loss of life". Next, "unacceptable behavior" will be defined as "motions that require extraordinary or exceptional pilot skill for safe controlled flight". Finally, we will define "undesirable behavior" as "motions that require normal pilot skill but impose an extra or disconcerting workload". I prefer not to use the term "dangerous" to describe the behavior of an airplane because it is imprecise and can be misunderstood by many people. For instance, the act of getting out of bed is "dangerous" if it results in falling and breaking your leg. Of course, many people also consider that flying in any airplane is "dangerous" but many others do not. Actually, it is more appropriate to use "dangerous" to describe the total situation; that is, the airplane PLUS the specific flight conditions or environment. By these definitions, then, we can say that, if the behavior of

an airplane is judged to be "unacceptable", it may or may not be

behavior under some flight condition, some of these pilots may not be able to cope because they may not understand what is

SPORT AVIATION 57

By Don Hewes (EAA 32101) 12 Meadow Drive Newport News, VA 23606

_L HIS IS THE final installment of a three-part article on the subject dealing with the responses of tail-first airplanes to rain, snow, bugs or whatever. The term "Flight Behavior Change", or "FBC" for short, has been coined to refer to this type behavior. This behavior results primarily from changes in the aerodynamic characteristics of the lifting surfaces. This part covers several recommendations and suggestions for people who are designing, building or flying this type airplane.

Recommendations and Suggestions The keyword for conducting FBC tests, as in the case of any flight testing, is BE PREPARED. Initial flight testing of any homebuilt airplane, even if it is the 500th copy of a well proven design, should always be treated as a truly experimental flight test operation with all appropriate precautions taken. After all, what is on the sign that you had to place on the door? If you don't remember, it is

spelled E-X-P-E-R-I-M-E-N-T-A-L. It is suggested that the critical portions of the tests be delayed until all others are completed. However, you can perform the preliminary portions of the tests while doing the other normal tests but be sure that the lifting surfaces are dry and clean and avoid flying in the rain. Most of the following tests should be done using a mid-location for the CG to begin with and then repeated with the CG moved progressively forward and rearward. The test pilot may elect to eliminate some of the intermediate steps depending on the observed behavior from the previous tests. Although there are many tail-first airplanes currently flying which show no significant problems when they encounter rain, bugs or whatever, there are enough cases of FBC problems to indicate that caution should be exercised when flying an airplane of this type with unknown characteristics. Because of the many variables associated with current techniques used in constructing homebuilt fiberglass-foam airplanes, I believe that the only way a

builder of a tail-first airplane can be certain of the airplane's flight behavior with contaminated surfaces is to conduct flight tests designed specifically to evaluate this condition. I believe that such testing is in keeping with the builder's responsibility for the airworthiness of his airplane. Furthermore, it is a matter of the builder being fair not only with himself but his family and anyone who rides in or flies his airplane that this be done. This recommendation applies to any canard or tandem wing airplane whether it be the first copy of a new design or the umpteenth copy of a proven design. In making the decision to conduct the flight tests, it is recommended that the builder contact the designer to discuss the tests and obtain his advice. The builder should be familiar with conducting such experimental test flights and have some recent flight time. As is true for initial flight testing of any homebuilt airplane, if the builder does not fit the role of test pilot, he should find some qualified person to do the flying for him. It is suggested that the builder prepare a flight report of the tests and submit them to the designer. The report should cover the effects of contamination on at least the following items: 1) take-off distance, 2) rate of climb following take-off, 3) landing speed and distance, 4) stick force and trim control travel, and 5) maneuvers at approach speed. Other factors such as elevator deflections and angle of attack changes should also be included. The designer can then correlate this information with his own tests and those of the other builders as a way of isolating those factors which are most important in affecting the FBC. Included in the builder's report on an airplane with a significant FBC problem should be detailed measurements of the various critical parts of the airplane. Concerning the accuracy required to provide repeatable aerodynamic characteristics for these airplanes, it is suggested that designers 1) evaluate the need for greater accuracy, and 2) provide information on how to achieve the greater accuracy, if it is required, taking into account the wide spectrum of builder skill involved in the current

homebuilding movement. Incorporation of final contour templates and alignment jigs should be considered. In the following sections, a series of flight tests to evaluate FBC is suggested for use in lieu of any other specific information supplied by the designer. The actual procedures will need to be developed by the builder and pilot following these suggestions. Also, suggestions are made regarding pilot preparedness and possible cures for

severe FBC problems. SPORT AVIATION 61

Flight Testing Before commencing these tests, the airplane should be equipped with accurate airspeed and rate of climb indicators and inflight calibrations of the airspeed installation should be made to insure reliable airspeed measurements within a couple MPH or better. An outside air temperature indicator should be available for airspeed corrections. Remember that these measurements need to be accurate so that reasonable comparisons can be made of other measured quantities obtained from various different airplanes using airspeed as the basis for comparison. If airspeed is in error, then the comparisons will not be reliable. Note that airspeed probes located in different positions of the airplane may produce significantly different readings because of local pressure differences (position error). An angle of attack indicator will be quite useful but is not an absolute necessity. A relatively simple vane mounted on a boom extending about a foot or two forward from the front surface near the tip can be used but it must be calibrated to account for flow upwash. To do this, fly the airplane at constant altitude for a series of speeds over the speed range and compare the vane reading with an inclinometer mounted in the cockpit. You will be dealing with only a fairly small angular range of about 15° and you should obtain each reading with an accuracy of about 1 A° or better. You can use a small electrical potentiometer attached to the vane and a meter hooked up in simple Wheatstone- bridge circuit to obtain the vane readings. Because the existence of a laminar boundary layer is the key to the FBC phenomenon, it is important to determine the amount of laminar flow that exists on your particular airplane for the various flight conditions. Therefore, it is recommended that the first test should be one to obtain a visual indication of the boundary layer flow conditions on the wing and tail surfaces. This can be done with either of two fairly simple techniques, the first is the one described in Reference 3 using sublimating chemicals. This has been used very successfully by NASA and is highly recommended. The second technique is similar but uses plain motor oil in place of the chemicals. It has been used extensively in wind tunnels but I have not had any personal experience with it. Also, I don't have any convenient reference for the steps involved, so a bit of experimenting will be involved. You should be able to find a couple quarts of old used motor oil that are heavily loaded with soot so that the oil is very black. Wipe this uniformly over the lifting surfaces to provide a thin layer of oil which will tend to migrate to the region just aft of the TRANSITION POINT. Be sure that the oil does not tend to form drops that linger otherwise you are creating the same effect as rain or bugs. You may have to adjust the viscosity of the oil by thinning with kerosene or thickening with heavier weight oil to get the proper effect. You will probably need someone to help observe the flow patterns which will undoubtedly change with the different flight conditions. You should expect to see flow transition point somewhere near the mid-chord position on the tail and probably further forward on the wing. If this test shows that the transition point is near the leading edge, then there is relatively little laminar flow and the subsequent tests will probably show only a mild FBC if any. On the other hand, if the boundary layer is laminar near or past the mid-chord position, it is possible the behavior will be much more

evident. It should be necessary to conduct this test for only one loading condition but at least three speeds from landing to cruise conditions should be covered. Remove the flow visualization material at end of test.

During this test, observe the position of the elevator

required to maintain straight and level flight for a given 62 JULY 1983

loading condition. Normally the elevator should be very

close to zero deflection or whatever was specified by the designer. If there is greater than a couple degrees from the desired setting, carefully inspect the airplane for improper rigging or some of the other factors covered in the previous sections. Following this test, conduct a series of baseline data tests to measure take-off speed and distance as well as the rates of climb at take-off power in the range from take-off speed to somewhat greater than that for maximum rate of climb using increments of about 5 mph or so. The measurements should be taken as you climb through the same altitude for each different speed so a series of saw-tooth climbs and descents will have to be made. If you wish, at the same time you can conduct idle-power tests which should also be made to measure the rates of descent for the same speed range. (You may have to be concerned with excessive cooling and heating cycles involved.) Be sure that you have a well stabilized climb or descent established before reaching the specific altitude for taking the measurement. You will probably need several hundred feet to do so. It takes practice . . . and a passenger to take the readings is very handy. A small hand held tape recorder is also useful in place of the passenger. Note touchdown speed and landing distance. The next step is to make a complete elevator vs. airspeed calibration (similar to that shown in Figures 9 and 10) for each of the loading conditions at power required for straight and level flight as part of the baseline data for the subsequent special tests. All that is required is a scale on the elevator control calibrated in terms of elevator position and on the trim control calibrated in terms of any convenient scale appropriate for the type of control handle used. If the trim control is a crank type, then you will need to keep track of the number of turns. The elevator scale should be accurate to within about one-quarter of a degree and should be read to at least one-half a degree. The trim system scale should be accurate to about 1 or 2 percent of the full travel. It is recommended that all these tests be flown at safe altitude of at least three thousand feet above

the local terrain.

The next step is to repeat the last step taking data for coordinated banked turns of 30 and 60 degrees. Then the following step is to perform a series of banked coordinated turns at the normal approach and touchdown speeds using slow and then rapid control inputs. Do this with power to maintain essentially constant altitude and then with normal reduced power for landing. Start with shallow turns and observe any increased tendency to pitch down or up. Carefully note any additional aileron/rudder deflections required to correct for possible roll/yaw tendencies caused by partial flow separation on only one side of the tail or wing. If a pitch break occurs, hold controls steady if possible to observe the motions of the airplane. Note rates of descent. Attempt to recover by pulling further aft on the control. This may result in an aggravated FBC with the airspeed increasing significantly. In this case, you may need to PUSH THE CONTROL FORWARD as you would in the case of the stall of a conventional airplane. .'Remember that the tail is in a stalled condition with the elevator deflected downward. Raising the elevator will allow the tail to become unstalled.) Note altitude lost in recovering. Increase the bank angle in small increments and observe any further tendencies for FBC, roll or yaw. Note the elevator deflections and airspeed at which they occur. Bank angles in excess of the limits normally observed need not be reached. CAUTION: The objective of the following steps is to fly the airplane with an artificially induced turbulent boundary layer on portions of the tail so as to simulate the effects of contamination. These steps pose some addi-

the last step checking to see that the elevator deflections and airplane flight behavior are still within acceptable limits. If the incremental changes experienced with the two different lengths of tape are insignificant, then you can proceed repeating the tests with the tape applied full span. Otherwise, proceed with smaller increments until the results indicate that you should proceed no further. Repeat these tests with the CG moved to the mid-point

ELEVATOR DEFLECTION DE6.

V UAX.

•0

100

110

and then the forward locations. If the full-aft limit for the

CG was not tested previously, it should be done also unless prior tests indicate otherwise. Be aware that these tests will probably be more critical so proceed with caution.

140

INDICATEDAinSPfCO, KNOTS

20 18

FIG. 9 -FLIGHT TEST DATA FOR VARIE2E.

16

min trim

TRANSITION

OFREE D FIXED

14

tional hazard and should not be performed by anyone who is not fully qualified and prepared to handle the airplane under conditions requiring emergency actions. When you are ready to start the final series of tests, carefully clean off the first 3 inches of the WING and TAIL leading edges using a cleaning agent to remove any traces of dust or oil so that a strip of masking tape will stay firmly attached in flight. Apply a double thickness strip (about .008 to .010 in.) of'/»to 3A inch masking tape about 2 inches from the leading edge, both top and bottom, starting inboard and extending out to 1A the span of each tail panel. Be sure that the tape used has very good adhesive qualities and press it firmly onto the surface. It is necessary that the tape be applied in short sections of about 12 inches each so as to eliminate the possibility of one loose end causing the whole length to peel off. Tape is applied inboard only for the first flight to minimize the possibility of large rolling moments caused by unsymmetrical separation, and to approach the most severe condition (full span trip) in a careful manner. With the tape in place, conduct a series of high speed taxi runs checking to see that there is sufficient elevator power to lift off within the first quarter of the runway. Use the mid CG loading condition for these tests. If it is determined that a satisfactory liftoff can be made without reaching excessive airspeeds, apply a small amount of the boundary layer flow visualization material used previously so as to check to see that the tape is tripping the flow. Place it in a location where you can easily see it, and then proceed with the take-off. Carefully note and record takeoff distance and airspeed. If the flow appears not to be tripped, land and add another layer of tape. Otherwise, proceed with repeating the previous rate of climb, rate of descent and elevator vs. airspeed tests for comparison with the initial data. Make a careful note of the minimum trim speed of the airplane for the landing condition. Then repeat the coordinated-turn maneuvers. Note any unusual characteristics that may be associated with the tripped flow condition. If any unsatisfactory or unsafe effects are noted, terminate the testing immediately. When landing, make only gradual turns and maintain an approach speed about 1.3 times the noted minimum trim speed. Note and record touchdown speed and landing distance. Compare the data and check to see that the elevator deflections required with the tripped flow condition are not excessive. There should be only very small changes from the original data of 1 or 2 degrees at the most. If the results are judged to be acceptable, add more

tape span wise and extend it to the Vz-span location. Repeat

ELEVATOR DEFLECTION

6

12 10

deq

60

80

100

120

140

160

180

INDICATED AIRSPEED, V ( . knots FIG. 10 -FLIGHT

TEST DATA FOP LONG-EZ

Cfi£F. 2)

Having performed all of these steps, you should have fairly well defined the operating characteristics of your airplane FOR THE MOST SEVERE CASE OF RAIN OR BUGS. If you were unable to complete the full series, then you know that some operational limitations should be placed on the airplane for conditions where the airplane might become contaminated. Be sure to remove the masking tape from the surfaces after a few days of testing to avoid possible damage to your finish due to the "curing" of the adhesive over a longer period of time.

Pilot Preparedness

If you are flying a tail-first airplane that has not been thoroughly tested for its FBC, you should be aware that you may encounter a FBC problem unexpectedly and you should be prepared to take the proper corrective action immediately. Do not attempt a flight if rain or snow are threatening or if the field is heavily infested with flying bugs. If possible, load the airplane so that the CG is in the mid to aft portion of design range.

Inspect the lifting surfaces and remove any surface

contamination that could cause flow turbulence. Inspect

the elevator travel for proper down travel limits. The travel stop should be positive with no "spongy" tendencies. Do not take-off from a field with long grass or weeds which can strike the leading edge. On take-off, check to see that excessive speed is not required to reach liftoff.

Abort the take-off if in doubt. If there is sudden power loss, avoid abrupt elevator inputs and banked turns.

SPORT AVIATION 63

If rain, drizzle, mist or bugs are encountered during flight, avoid slow flight and steeply banked turns. Make a SHALLOW STRAIGHT-IN approach with airspeed 10 to 15 knots higher than normal. Be prepared to ADD POWER if the plane suddenly starts to pitchdown and pick up speed. Avoid making abrupt aft stick inputs and allow speed to increase significantly before attempting to apply any aft stick. Allow for a longer than normal landing runout. In the event of a go around, remember that the airplane has higher drag and lower lift than normal and will not climb as rapidly as normal. Expect a shallow climbout and avoid any abrupt turns.

Curing Severe FBC Problems

Finally, we will address the question of what to do to the airplane if it demonstrates UNACCEPTABLE behavior. The first thing to do is review all information supplied by the designer (owner's manual, newsletters, etc.) and then contact him if you do not find an obvious source of the problem. If this is an original design, review the earlier discussions in this article. Check for misalignment. Check the tail for insufficient incidence and the wing for excessive incidence. (Both will cause excessive down elevator settings.) Shim the surfaces or make new attachment fittings. Carefully check the airfoil shapes using external templates to see that they conform to the design airfoils. Unless the designer can provide specific data for the desired contours of the airfoil, you will have to develop them from the normal construction templates. In this case, you will have to make allowances for the added thicknesses of fiberglass, resin and surface filler. An alternate method to making templates from the drawings is to make a series of exact half-templates (upper and lower) of the actual surface contour. These are then compared with a drawing of the desired airfoil. A simple procedure is to mask the chordwise section of the upper surface to be checked with a narrow strip of Saran Wrap or wax paper and then lay down a thin strip of "Bondo" plastic body filler about Vz inch wide. Before the filler hardens, press in a piece of plywood or hardboard previously rough cut to the approximate contour of the upper surface. The piece should extend below the upper surface at the leading and trailing edges. After the plastic hardens, carefully mark on the template the position of the trailing edge and a reference mark placed on the leading edge of the surface. Repeat the process for the bottom surface and then match the two parts of the template using the leading and trailing edge marks on the two parts. This process is quite easy and fairly quick but requires rework of the template whenever the surface is reworked. Some contour errors can be corrected by filling with microballoon slurry and refinishing. Others may require

building a complete new surface. If possible, the CG forward limit could be set further aft so as to avoid the higher loading of the tail. A possible redesign solution is an aileron reflex mechanism such as the system developed for the Quickie airplanes. Also, a small moveable horizontal surface located at the aft end of the fuselage to provide an inflight adjustable pitch trim moment could be installed in some

cases. Perhaps the most drastic but most satisfactory solution would be to build a new tail surface with an airfoil less susceptible to the flow separation problem. Unfortunately, there is relatively little data available on which to base a decision for selecting an alternate airfoil section. However, some of the latest airfoil computer design 64 JULY 1983

techniques now offer some hope for obtaining a suitable section. These steps represent major redesign effort and should not be taken unless all else fails, and then should be taken only after consultation with the designer. Of course, any modifications to the airplane should be checked with the FAA and thoroughly flight tested for all flight conditions.

Closing Comments We have specifically aimed this article at the problem of a more or less symmetrical stalling phenomenon in which there are no significant rolling or yawing moments present. However, it is quite possible that an unsymmetrical condition of the airplane exists so that the stall itself will be unsymmetrical. The pilot, therefore, should also think in terms of the potential for a ROLLOFF or SLEWING behavior associated with the FBC. This article has been presented to the reader for the purpose of exchanging information. Because of the highly experimental nature of both the information of this article and the flight activity associated with homebuilt airplanes, the author cannot assume responsibility for actions taken as the result of using this information, the suggestions or the recommendations presented herein.

Acknowledgements I would like to thank all of the following persons for their help in obtaining some of the information and data presented herein, and some of them for the advise and comments pertaining to this presentation: Dr. Bruce Holmes (Flight Research Engineer), Joseph L. Johnson (Head, Dynamic Stability Branch — full scale wind tunnel), Long Yip (Wind Tunnel Research Engineer) and Dan Somers (Airfoil Research Engineer), all of NASA's Langley Research Center. Also John Roncz (Airfoil Designer), 1510 E. Colfax Ave., South Bend, IN; Burt Rutan (President, Rutan Aircraft Factory); Gene Sheehan (President, Quickie Aircraft Co.); Bob Walters (Dragonfly Designer) and Rex Taylor (Viking Aircraft Ltd.). This acknowledgement should not necessarily be construed as representing an endorsement on the part of any one of the individuals or organizations mentioned.

References 1. Yip, Long P. and Coy, Paul F.: Wind-Tunnel Investigation of a Full-Scale Canard-Configured General Aviation Aircraft. 13th ICAS Congress/AIAA Aircraft Systems and Technology Conference, Seattle, Washington, Aug. 22-27, 1982. ICAS Paper Number 82-6.8.2. 2. Holmes, Dr. B. J., Obara, C. J.: Observations and Implications of Natural Flow on Practical Airplane Surfaces. 13th ICAS Congress/AIAA Aircraft Systems and Technology Conference, Seattle, Washington.

Aug. 22-27, 1982. ICAS Paper Number 82-5.1.1. 3. Holmes, Dr. B. J., Croom, C. C., Obara, C. J.: Sublimating Chemical Method for Detecting Laminar Boundary-Layer Transition. Handout available from

Dr. Holmes, Mail Stop 286, Langley Research Center, Hampton, VA 23665.