Weight And Balance-Why

he was advanced to the position of manager of commercial engineering. During his 22 years with Beechcraft he has intennittently taught night classes at the University of Wichita in ael'odynamics and flight testing. He holds a comme1'cial pilot license with single and multi-engine land and insh'ument ratings. • • • • • • planes than others. We have loaded airplanes experimen- tally far enough aft to demonstrate this adverse situation. All Beech airplanes are approved at an aft cg that is well forward of this reversal point. How- ever, in practice, it would be possible to load our airplane, as well as most other commonly used airplanes, far enough aft that the stick force char- acteristics could be very unpleasant. An airplane loaded with extremely heavy cargo in the baggage compartment, with no consideration given to the ap- proved limit, would require a high de- gree of skill by the pilot for takeoff and landing. In good weather under good conditions, an experienced pilot might get away with this. However, add turbulent air and short fields, per- haps at night or in instrument weather, and the combination becomes extremely critical. One normal reaction would be, "Why not just design the airplane with the cg • • • • (ContinI/cd fl'olll p"ge :13) airplane and lean on the horizontal tail, you will find that it requires considerable effort to lighten the nose wheel. In Figure 4 we have a Bonanza loaded on the ground in a typical real' cg condition. Here again, you will no- tice that the main struts and tires are deflected, the nose gear strut is extended more than the normal ground unloaded attitude, and the airplane sits on the ground at a higher angle. If a person leaned on the tail, it would be possible to lighten the nose gear without too great an effort. As mentioned earlier, there seems to be a general view that performance is better when the airplane is loaded at forward cg. As a matter of fact, a good friend of mine made quite a point of explaining that even when he was by himself, he considered taking his suit- case out of the baggage compartment and putting it in the front seat to get the best possible performance out of the airplane. Recently we took a new Bonanza, loaded it to Station 80.4 at 3,000 pounds and made a flight test of the cruising speed for a rang'e of power settings. We then reloaded the airplane to the same gross weight to a cg of 85.7 inches' and 3,000 pounds. Test points indicated no appreciable difference be- tween the two conditions except at lower speeds, where real' cg is better. During World War II this false im- pression reached such magnitude that the military found it necessary to run actual flight tests on a number of bombers in order t9 prove that cg was not an important factor for long-range cruise. Perhaps one of the most important effects of cg position on airplane char- acteristics is the change in longitudinal stability. In Figure 5 we have plotted the change in stick force with change in air speed for two different cg positions . From this diagram it is obvious that the stick force-that is, the force which the pilot feels as he changes from a given trim speed-varies greatly with cg position. In the case illustrated, which is for power-off landing config- uration, the stability at forward cg is approximately twice as great as it is for rear cg. From a practical standpoint this is important, because it gives the pilot warning of a change in speed. From an FAA standpoint, it is necessary that a change in force be clearly perceptible to the pilot with a change in airspeed. As the cg goes further and further aft, this change of force becomes less and less. In an extreme aft cg, it is possible that the force will go to zero, then actu- ally reverse. Stick travel will do the same thing. In this extreme aft cg con- dition, it would be necessary for the pilot to push for takeoff and push to make a normal landing. Naturally, this required change in forces would be con- fusing and dangerous. This condition would be easier to reach on some air- THE AOPA PILOT r -- I -~ It:. -- ..- • FLAPS UP I -- -- -- ••••• ••••• •••••GHT· APPROVE GROSS WE -- -- - -- • FLAPS DOWN I - - --._~ ••••• ••••• ••••• jII •• l APPROVE GROSS WErHT -. u FORWARD C.G 86 REAR C.G 85 on stalling 84 83 * * Circle no. 19 on reader senice card ~W1J ~ ~Pu'1f}at CLUB PEACE & PLENTY Civil Air Regu]ations went into effect, trim requirements were very severe. Many four-place airplanes were de- signed which had difficulty in meeting- trim requirements in a glide. It was found that by installing the stabilizer at a negative angle it was easier to pro- vide sufficient up-elevator (down tab) to meet the trim requirements in glide. For this reason many of the early post- wal· airplanes, including the early Bo- nallZa8, had the elevator down anywhere from 40 to 80 in cruise flight. Later, FAA trim requirements were relaxed and this down angle has been elimi- 79 78 77 50 80 81 82 C.G,.-FUSELAGE STATIONS FIGURE 6 With an aircraft gross weight of 3,125 pounds, this drawing shows the effect of cg speed, power off Ability to trim the airplane in a hands-off landing glide is usually a critical design characteristic. F]ight tests are conducted to determine the most forward cg position at which it is possible to trim at a speed of 1.5 times the stall speed in a power off glide. This requirement generally limits the most forward cg at which we are able to meet FAA requirements. From a pilot's standpoint this is very apparent. In the forward cg condition on most four-place airplanes, it is not possible to reduce the elevator forces to zero fOI· landing, even with all elevator trim power wound in. At rear cg it is pos- sible to increase the tab setting to where elevator forces in landing can be re- duced pretty much to zero. This is another good indication to the pilot as to the location of the airplane cg. Where the trim speed in a glide is high, or wh'.!n the forces required to land are high, he knows that he is loaded well forward. In cruise flight the ease with which the airplane can be trimmed is di- rect]y related to the cg position. When the airplane is loaded well forward, the tab is less sensitive and easier to set for any desired flight condition. The pilot will also be made aware of cg position by the characteristics of the elevator. At forward cg he will find that it takes more force and more travel for landing. On many four-place airplanes it is difficult, if not impossible, to stall the airplane when it is loaded forward. At rear cg the elevator becomes much more sensitive and requires much less travel, because more than enough e]e- vator power is available to stall the airplane. This is shown in our diagram of basic forces and moments in Figure 2, where the down load required on the tail is much greater, requiring much more elevator travel. In addition to changes in elevator power, there is a difference in the posi- tion that the elevator assumes in flight. In cruise flight at forward cg the e]eva- tor tends to trail up. As the cg is moved aft, the elevator angle tends to decrease and could actually, in an extreme case, assume a position below neutral. In the late 1940's, after Part 3 of the current 80 :I: ~ 70 §"- </)-'~ 60 </) always so far forward that the extreme real' condition cannot be reached?" The practical limitation of such an ap- proach is obvious: as the cg goes for- ward, the stick forces increase. It then becomes difficult to design a longitudinal control system so that the stick forces do not become uncomfortable 01' unrea- sonab]y high. FAA requirements put a limit of 40 pounds on the effort required to reduce the speed from trim speed to 10'1, above stall. Also, from Figure 2 it can be seen that as the eg- goes forward, the down load on the horizontal tail becomes pro- gressive]y higher. As this down load in- creases, the load carried by the wing must increase, decreasing- the total effi- ciency of the airplane. The structural loading on the wing increases and im- poses a penalty on the weig-ht of the structure. Another interesting and important flight characteristic is the effect of cg on maneuvering stick forces. This is simply the situation where the pilot chooses to execute a banked turn out of level flight. A 60° bank imposes a 2g load on the airplane. For the two con- ditions we have chosen to study, at forward cg- if the pilot chooses to per- form a 60° bank out of level flight at a speed of 150 to 200 m.p.h., an additional force on the control column of 30 pounds will be required. At rear cg, a force of only 10 pounds is required to execute this same 60° bank. This knowledge, recognized by the pilot, will give him a good indication of the flight attitudes and cg of his air- plane from the feel of the controls. Under extreme conditions, a pilot who has been flying the airplane loaded to forward cg could get into trouble if he then flew the airplane loaded to real' cg without realizing that the same ma- neuver could be accomplished with one- third of the force. It is my opinion that these forces should be reasonably high so that the pilot has better feel and is less likely to overstress the airplane. We have made a concentrated effort to increase these maneuvering stick forces in our airplanes over the past few years. We feel this contributes to safety and therefore is a desirable characteristic. The stall speed of the airplane is also affected by cg position. In general, the range of cg travel at gross weight is so small that the pilot is rarely aware that the stall speed varies with cg position. In Figure 6 stall speeds are plotted for the full range of cg posi- tions on the Bonanza. A]though it is not possible to load the airplane to gross weight at the forward limit, the curves have been extended to show this effect. Total change in stall speed is not great -only a matter of a couple of miles per hour. However, it does tend to illustrate that as the cg moves forward the down load on the tail increases. This makes it necessary for the wing to carry addi- tiona] load. In the average pilot's expe- rience, the airplane is at light weight at the forward cg condition, so the stalling speed appears to be less. This again leads to misconception as to the true effect of cg position. JULY, ]962 47 FIGURE 7 Typical loading for Beechcraft Bonanza. L indicates the distance of each load item from a reference line. From this example you may compute the weight and balance of your airplane I 84.4 263,952 3T25 "w •. w WEIGHT - LBS. MOMENT IN.-LB. X 19= 494 X 348 = 26,100 X 1981 = 153,738 X 170= 14,450 X 170= 14,450 X 340 = 41,140 X 97= 13,580 3125 263,952 ~w •. w would be useful for the pilot to study such data and become familiar with effects of loading on the cg of his par- ticular aircraft. Also, he may have op- tional equipment installed which has not properly been entered in the weight and balance of the basic airplane. Figure 7 shows a typical loading for a current Beechcraft Bonanza. Using this same procedure you may readily compute the weight and balance of your airplane for each condition in which you are interested. First, determine the dis- tance of each load item from a refer- ence line. This is shown in Figure 7 as L, or length of each of the load items. The weight and balance data supplied with your airplane will give this di- mension. In engineering language it is known as an arm. This distance multi- plied by the weight of the item produces a "moment"-force times distance. In this case it is the weight of the load item times the distance the item is from a reference line. All these moments, including the empty airplane moment, are totaled. This total is divided by total weight of the same items to find the cg 01 the plane. Many people share a view that a little knowledge can be a dangerous thing. In the case of weight and balance, how- ever, a study of all the factors involved should enable the pilot to get greater utility out of his airplane-more safely and efficiently. END TOTAL MOMENT TOTAL WEIGHT ~W LENGTH -INCHES (ARM) 26 75 77.6 85 85 121 140 TOTAL C.G. (IN INCHES) L FUEL L AIRPLANE EMPTY. L PILOT & PASS L REAR PASS L BAGGAGE OIL FUEL AIRPLANE BASIC EMPTY WEIGHT PILOT FRONT PASSENGER REAR PASSENGERS BAGGAGE ITEM istics are conspicuously better at for- ward cg than they are at rear cg. As a matter of fact, it is often difficult if not impossible to spin an airplane at for- ward cg, whereas the same airplane at rear cg can be spun easily. The cg posi- tion affects altitude lost in a turn, the attitude which the airplane tends to assume in a spin, and the rate of recov- ery or number of turns required once recovery is initiated. It is not uncom- mon to require twice as much time for recovery at rear cg as at forward cg. For some designs of four-place air- planes, spin recovery at rear cg is the limiting factor as to how far back the airplane cg can be approved. In addition to aerodynamic character- istics, there are a number of important structural characteristics affected by cg position. Probably the most conspicuous of these is that when the weight is lo- cated forward the nose wheel loads are increased. It is considered good practice to use a little more care in ground ma- neuvering at forward cg to keep from overloading the nose gear. Sometimes it is desirable to hold the wheel back to provide an aerodynamic relieving mo- ment on the nose gear. The forward fuselage loads are increased by forward fuselage and the nose gear. Tail loads are also increased, which affects not only the horizontal tail, but the aft fuselage. All manufacturers supply weight and balance data with their aircraft. It BAY AIR SERVICE Albert Whitted Airport St. Petersburg, Florida Circle no. 163 on reader service card "Where can you fly" and taxi your airplane to your lakeside cottage? It's. *' MINNESOTA'S : OTIS LODGE American Plan Resort recommend- ed by Duncan Hines. Private golf course, water skiing, fishing. Write for folder. Mr. & Mrs. Arthur R. Otis, Grand Rapids, Minnesota. Telephone FA 6-3511. DULUTH SECTIONAL. nated. Airplane longitudinal stability is comprised of two major elements- static stability, which we have already discussed, and dynamic stability. Prob- ably most interesting from a pilot's standpoint is dynamic stability, of which the pilot is aware by the tendency for the airplane to return to equilibrium after it is displaced. To the pilot, this is apparent in how quickly the airplane wants to come back to the original trim speed in cruising flight when the speed is changed. This tendency to return to the original trim speed results in what is called phugoid oscillation. From in- side the airplane it is nothing more than how many trips on a roller coaster are necessary to return to the original atti- tude and air speed. In the general case, as the cg moves forward the airplane's tendency to return to its original trim speed becomes more positive. Airplanes can be stable, neutrally stable, or unstable. Some well known airplanes will not tend to return to their trim speed when displaced or unstable, especially at rear cg. For example, if the speed is reduced 20% from cruise flight and the controls released, the air- plane will continue to slow down until it approaches a stall. Conversely, if the speed is increased 20%, the speed will continue to increase until the red line is reached. This is an extreme case that would occur on most airplanes only if the airplane were loaded far beyond the approved rear cg limit. Although there is no FAA requirement on dy- namic longitudinal stability, most four- place airplanes will not diverge at a rate fast enough to be of any concern to the pilot. The period for one cycle is usually long-approximately 20 to 30 seconds- therefore the pilot has ample time to initiate recovery. This characteristic of an airplane, if known by the pilot, gives him another clue as to how his airplane is loaded. The cg condition also has a marked effect on spin characteristics. For a typi- cal four-place airplane, spin character- 48 THE AOPA PILOT Forced Landing (Continued from page 38) the ground, I knew we were going too fast so I reached for the brakes and couldn't find anything to get my feet on. Then I remembered that Duffy had them, so I yelled at him. After two or three tries, I managed to rouse him enough to get him to stop the ship. We were about 15 feet from the biggest road culvert I have ever seen. I crawled out of the cockpit after cutting the switch. A minute or two later I started to shake all over. The fear that I hadn't had time to feel had caught up with me. Duffy wasn't in any better shape. When the gas station attendant from across the road came running up with his fire ex- tinguisher, he found two very sick sad sacks standing by a very sick airplane! After we recovered, we returned the T-Craft to the field. For a long while the mechanic there couldn't find any- thing wrong. The motor seemed to run perfectly; he couldn't even get it to act up at full throttle. I'm sure the me- chanic thought we had misused the plane somehow, but we were insistent so he kept looking. The ignition was checked but nothing was found. After about an hour's work, he decided to check the gas line. This action hit the jackpot. It was full of leaves. That's right, common ordinary tree leaves. I'd heard of all kinds of things forcing down airplanes but not leaves. After draining the gas tank and removing it, the mechanic dis- covered that the filtering screen inside the tank had come loose and, during all the years of standing outside, somehow or other leaves had gotten into the tank and down into the gas line. At full throttle there was enough pull to close off the flow of gas. END •••••••••• THE AUTHOR Donald L. Hauck, author of "Forced Landing," has been flying since 1945 (when he was a high school sopho- more) and has owned all or part of five different airplanes since then. He presently owns a Cessna 140 and flies it whenever his duties as manager of a department store at Madison, Minn., permit. A graduate of the Univer- sity of Minnesota, Hauck is now 32 years of age. He is married and the father of thee children. ••••••••••