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