CONSTRUCTION OF A SIMPLE LOADING CHART FOR YOUR AIRPLANE By Don Hewes (EAA 32101) Aero Research Engineer 12 Meadow Dr.

Newport News, VA 23606 L ERFORMING WEIGHT AND balance calculations for an airplane has always been a rather boring chore for me. I began doing them years ago in 1945 when I was a fledgling aeronautical engineer and worked at the Engineering and Research Corporation plant in Riverdale, Maryland. There I was given the job of preparing weight and balance forms for the production Ercoupes as they rolled out of the factory. Years later, as I was completing my first homebuilt airplane, the BD-4, and preparing it for flight testing, I was reminded of this tiresome task, but, being by this time, a specialist in stability and control with 30 years experience under my belt, I was much more aware of the very critical nature of maintaining the location of the center of gravity of the airplane within very specific limits. Because I also tended to be a man who likes to take short cuts, I resolved to develop a simplified weight and balance method that would be convenient to use and be accurate enough to ensure that the inflight loading conditions will be legal and safe. I am convinced that most pilots don't perform weight and balance checks when they should because the standard method is rather time consuming. The solution was to spend one evening performing a series of calculations that would cover the normal range of loading conditions for my BD-4 and then put them in a simple chart that could be interpreted at a glance. I won't discuss the actual calculations because you probably are familiar with the method already. (If you are not, refer to any book on learning to fly. You have to know the method if you want to get your pilot's license.) Instead, I'll discuss the selection of appropriate loading conditions and the presentation of these on the chart and then comment on how to use the resultant chart for conditions other than those selected. Before doing this, however, I'd like to take a moment to stress the importance of knowing the weight and balance conditions of your airplane before you take-off and throughout the flight. The penalty for not knowing or not heeding can be very drastic. For example, I quote from a news brief in a recent national flying magazine: "A Cessna 150 crashed in Florida last fall killing all three passengers and seriously injuring the pilot . . ." The practice of cramming four people into a two-place airplane is, of course, very uncommon; however, the result of overloaded conditions beyond the aft center of gravity limit is not at all uncommon. Most airplanes can be loaded beyond their limits when carrying what appears to be a normal load of passengers, baggage and fuel, and accident statistics reveal this to be a common situation in many fatal accidents. The hazards of exceeding the specified limits of your airplane are illustrated in Figure 1, which shows a typical 32 JULY 1978

diagram of the loading limits for an airplane and lists those deteriorated flight characteristics encountered when exceeding the limits in a particular manner. Reasonable and safe flight characteristics will be experienced as long as the weight and center of gravity (C.G.) fall within the specified region. If the aft C.G. limit is exceeded, the airplane will become longitudinally unstable with very light control force gradients. This can result in over rotation of the airplane at any time in flight and lead to stalls and spins at slow speeds and to structural failure at cruise speeds. Even if these hazards are not encountered, the pilot will become very fatigued from the constant vigilance required to maintain stabilized flight. If the forward C.G. limit is exceeded, the airplane will become too stable and the control forces will be very high which, once again, will lead to pilot fatigue. Furthermore, the load on the nosewheel of the tricycle landing gear may cause it to fail. The takeoff and landing speeds will become excessive due to inadequate elevator power. The pitching response of the airplane due to turbulence is likely to become annoying and contribute to pilot fatigue. Exceeding the gross weight limit will produce insufficient rates of climb and excessive landing and takeoff speeds and distances. Operation in gusty conditions, even at speeds below normal maneuver speed, can lead to structural failure of the wings or tail surfaces. Furthermore, the landing gear is likely to collapse in the event of only a moderately rough landing. In the case of the Cessna pilot, it is obvious that he did not perform a weight and balance check, or that he paid no attention to the answer. Otherwise, he would not have taken off to an almost certain rendezvous with death. But, what about you? Do you know for sure whether or not your plane is within reasonable limits each time you take off? What about your next flight? I bet you won't, unless you happen to be going up for your private pilot's check ride, or you just got one of those new handy-dandy calculators, or you sit down now and work out a loading chart similar to the one I use. Okay, so let's get on with it and discuss the appropriate loading conditions and the preparation of the chart. There is no mystery here, I merely use a sequence of typical loads that I would normally expect to carry in the airplane. The starting point is the empty weight of the airplane and the C.G. locations are calculated for each new load. The results are plotted on the loading diagram which is shown in Figure 2. This loading diagram is based on Jim Bede's design specifications and the results of my own flight tests. You will note that for my purposes I have indicated three weight limits. The lower limit represents maximum weight for short field and high altitude takeoff conditions dictated by use of the 150 horsepower engine with fixed pitch propeller combination; this limit is more of a reminder of possible takeoff problems than an absolute limit. The second lim-

•EXCESS CONTROL FORCE •PILOT FATIGUE

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C.G. LOCATION, INCHES FROM DATUM

FIGURE 1. TYPICAL LOADING LIMITS AND RELATED FLIGHT CHARACTERISTICS it is the normal gross weight limit for all flight conditions. The third or upper limit is a special gross weight limit for takeoff from long fields and for long distance flight in which landings w i l l be made at under normal gross weight due to fuel consumption. If turbulence is encountered with these over-normal gross weights, the airspeed

must be reduced to a maneuver speed less than that for

the normal gross weight. The empty weight of the airplane serves as a reference point in the diagram and is represented by the circular symbol at the point corresponding to a weight of 1220 pounds and 32.00 inches aft of the datum. For

convenience, the dry weight value includes the weight of the engine oil inasmuch as the oil is always maintained close to normal capacity whenever the plane is flown and the small variations have a negligible effect on loading

diagram. It is wise to place the date on which this weight

man. This condition is represented by the square symbol connected directly to the circular symbol by a straight line.

In order to fly, of course, I must take on some fuel. The tanks will hold a maximum of 55 gallons as represented by the second square symbol connected to the first symbol by another straight line. The next load to consider is the passenger in the other front seat. Inasmuch as the chances of my carrying an-

other person as big or bigger than me is very slight, I have used the standard 170 pound weight. This has been added to both the empty-fuel and full-fuel weights of the previous case. Notice that the shift in C.G. due to adding the passenger is less than that of adding the pilot. This is due not

only to the fact that the passenger was assumed to be lighter than the pilot but also to the fact that the C.G.

and balance was completed as a reminder to keep this condition current. If equipment is added or removed, this

was closer to the front seat for the second loading condition.

fact that although the C.G. position is calculated to the closest .01 inch, and weight is given to the closest 1 pound, the true values probably differed from these tol-

in the rear seat rather than the front seat. Consequently we skip this condition and proceed directly to the case of

point should be recalculated. Note should be made of the

erances by one or two orders of magnitude due to the accuracies of the basic measurements. This uncertainty

of measurement is represented by the size of the symbol. That is to say, the actual weight and C.G. location probably falls someplace within the area covered by the circle and not necessarily at the exact center. This applies to all other conditions as well. The first loading condition corresponds to that for the pilot. Inasmuch as I am the only person to fly the plane from the pilot's seat, I use my own 'gross' weight which is considerably greater than FAA's standard 170 pound

It is highly unlikely that a single passenger would ride

a second passenger who obviously must ride in the rear seat, and then to the third passenger who also must ride

in the rear seat: Inasmuch as it is generally standard practice to put heavier passengers in front and the lighter ones in the rear, the weights for the rear passengers therefore have been set at 140 pounds each. The loading conditions for these two cases are represented by the two dif-

ferent sets of triangles.

At this point we observe that the loading condition for the last 140 pound passenger falls outside the loading diagram. Consequently this is an unsafe condition and must be avoided. The symbols for this condition have therefore been filled in to highlight this fact. SPORT AVIATION 33

Up to now, I have not considered the effect of baggage because I often take passengers for short rides with-

out carrying any baggage. But now, let's take it into account.

After checking the weight of several pieces of luggage, I have found that the more-or-less standard size suitcase weights about 18 pounds when packed full of clothes. Also, an attache case loaded with charts and flight planning equipment weighs about 15 pounds. Whenever I take a cross country trip I always carry a set of tools which adds another 20 pounds. Therefore, when I take a trip with one passenger I can figure on at least two suitcases plus the other luggage for a total of about 70 pounds. Since I already know that I can carry twice this weight in the back seat in the form of passengers, I don't worry about checking this case. I just load the luggage on to the back seat where it is much more accessible than in the luggage compartment aft of the rear seat. There is no point in worrying about the case of luggage for the pilot and 3 passengers since this is already critical without luggage. Consequently, the only case to be really concerned about is that for the three occupants.

Because we already can see that the C.G. is quite far aft without luggage, it is obvious that the luggage must be limited; therefore, I have taken the case of 70 pounds stowed in the aft baggage compartment. This case is represented by the pie-shaped symbols which are darkened because they fall outside the safe operating limits. At this point we have essentially finished the chart except for discussing the lines connecting various symbols and the markings on these lines. The purpose of the lines is to represent the approximate variation of the loading conditions for loads different from those used in the calculations. For instance, the line between the circle and first square represents all the loading conditions for pilot weights less than the 230 pounds. Correspondingly, the line that continues on to the diamond represents those conditions for combined weights of the pilot and passenger between 250 and 400 pounds. Putting it in a slightly different way, we can say that the two lines between the circle and the diamond approximate the variation produced by the total weight in the front seat. Thus, the diamond symbol could represent one occupant

weighing 390 pounds. (WOW! I don't think he'd fit.) The dotted lines passing through the square symbols show the exact variations for the front-seat loading conditions and illustrate that use of the straight lines between the points results in reasonable approximations for the range of loads considered. Thus the lines can be used to quickly determine loading conditions different from those represented by the symbols. The marks on the lines are used for convenient reference marks to approximate intermediate loading conditions. Those on the near-vertical fuel lines correspond to 10 gallon increments in fuel for each of the other loading

conditions. Notice that the full fuel load of 55 gallons corresponds to solid lines. The marks on the other lines correspond to 70 pound increments in loads in the back

seat and 35 pound increments in the baggage compartment. These latter marks fall halfway between the sym-

bols. The dotted lines indicate the effect of 50 pound increments in the front seat for each of the conditions rep-

resented by the symbols. It is not necessary to draw additional lines through these points. To do so would make the chart very complex and confusing. Merely use the appropriate dots to visualize how the straight lines would shift, if a front seat load, different from that given in the chart grid, were used. The grid lines provide a graphic picture of the sensitivity of your airplane to the various loading conditions. This

sensitivity is indicated by the slopes of the various lines.

34 JULY 1978

The steeper the line, the less sensitive the airplane is to loading, that is, a given load will produce less change in the C.G. position. Thus it can be seen that rear seat and baggage compartment loadings are more sensitive and critical than fuel and front seat loadings. It is very helpful to study the diagram for your air-

plane and fix it in your mind. You will probably find that after awhile you will even be able to perform your weight and balance check mentally without looking at the diagram. However, you should put the diagram in a convenient place in the airplane where you can make reference to it when you know you are getting close to a critical loading. Put it in the glove compartment, on the back of the sun visor, or in one of your log books. There are several ways to use the chart. First, I have already indicated how to account for different amounts of fuel. Merely select the appropriate gallon-reference marks on the lines between the loading symbol. Notice how the fuel-load lines can be used to visualize the change in loading conditions as fuel is used. With aft loading, fuel consumption has very little effect on C.G. position. Another way to look at this is that taking on fuel will not produce a balance problem regardless of how much the rest of the load is, that is, for my airplane — this may not be true for yours. You better check it! Taking on fuel, of course, does directly affect the weight, and the diagram shows that fueling may become more critical whenever more than about 70 pounds is carried aft of the front seat. It may become critical even without the 70 pounds if the takeoff is to be made from a very short field. Second, the maximum total load to be carried in the back seat can be determined by the intersection of either the aft-limit line or the weight-limit line and the loading line between the two different triangular symbols for whatever fuel load is being considered. For instance, with 30 gallons of fuel on board, the maximum rear seat load is about 140 Ibs. plus 60 pounds equals 200 pounds as limited by the normal gross weight (follow the line through the 30 gal. reference marks). An additional 30 Ibs. can be carried without exceeding the aft limit. This load can be split any way you wish between passengers and luggage as long as it all is placed on the rear seat or floor. Third, the diagram helps to determine how best to distribute the load aft of the front seat. Notice that if we elect to place 140 Ibs. of the total load in the back seat and the rest in the baggage compartment, we can put there only about 60 Ibs. for a total of 200 Ibs. This is 30 Ibs. less than what could be carried if it were all placed on the rear seat. Of course, you may not be able to get it all on the rear seat due to the volume of the load. The best solution, in these critical cases, is to try to place some of this load on the front seat passenger's lap, under the seat or his legs, but be sure it cannot possibly restrict full movement of the controls, especially if it should shift position. Fourth, if the total load in the front seat is less than the 400 Ibs. assumed for the conditions with loads in the rear, the loading lines are merely shifted in the direction and by the amount dictated by the front-seat-load dotted lines as discussed previously. Notice that by selecting some loads which produce critical loading conditions outside the specified limits, you are able to interpolate rather than extrapolate so as to determine the actual limited loads. In the way of warnings, I'd like to make the point that this method of determining the loading condition produces approximate answers. The accuracy of the method become worse as the actual loads differ from the assumed loads. It is suggested that if the estimated loading condition comes within 0.25 inches aft of the aft limit, an actual calculation could be made as the C.G. position

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C.G LOCATION, INCHES FROM DATUM FIGURE 2

may not be as critical as indicated by the chart. It should be remembered, however, that there can be fairly significant errors in the measurement of the empty-weight location of the C.G. due to inaccuracies in the original measurements. Consequently, some allowances should

be made for this type of uncertainty. The C.G. may ac-

tually be further aft than the position determined by the original calculations. In no case should you try to operate the airplane when the chart or subsequent calculations indicate that the C.G. will be aft of the specified aft limit. The logic used in developing this chart may not apply to other type aircraft. The use of additional seats, fuel tanks and baggage compartments makes the development of a single chart quite difficult. However, use of multiple charts to account for all of the various combinations may still be much more effective than resorting to the conventional methods. The following is a summary of a few simple rules or guidelines to follow when using this method to prepare a chart for your particular airplane: 1. Always prepare a new chart and date it whenever the empty weight of the airplane is changed or equipment is shifted fore or aft in the airplane. 2. Select a logical set of loads and apply them in a logical set of loads and apply them in a logical sequence. 3. Include some loads in the chart (not the airplane) that exceed the limits by a small amount. 4. Connect the individual loading conditions by straight lines and subdivide them into segments representing logical load increments.

5. Keep it simple. The simpler it is, the easier it is to remember and use. 6. Establish your own rules about using the chart and

stick to them.

7. Never use a chart belonging to someone else even

if it applies to the same design. No two airplanes are alike. 8. Check and recheck your calculations. Remember you will have to live or die with them. 9. Always refer to the chart before taking off. ABOUT THE AUTHOR

Don Hewes graduated from Rensselaer Polytechnic Institute in 1945 with a degree in aeronautical engineering. He worked for a year and a half at Erco of Riverdale, Maryland as a production design engineer . . . under Fred Weick. Joining NACA in 1947, Don specialized in stability and control, stall/spinning and handling qualities, utilizing wind

tunnel, flight testing, simulation and analytical techniques. He has researched lunar landing prob-

lems and helped train all of the Apollo astronauts

for the landing mission. Later, Don was the principal investigator for the space maneuvering unit experiment on Skylab missions. He is now branch head of Flight Dynamics Branch at NASA's Langley Research Center in Virginia. A pilot since 1944, Don has also been a long time radio control model airplane designer and builder. His BD-4 (see April SPORT AVIATION) is his first homebuilt. SPORT AVIATION 35