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Trim Systems Fixed and Control THE TRIM SYSTEMS . SPORTPLANE FIXED AND BUILDER CONTROLLABLE By Antoni (Tony) Bingelis EAA Designee Program Advisor TRIM TABS 8509 Greenflint Lane Austin, Texas 78759 AIRCRAFT S NOSE CORRECTS A LOT OF people have conceived a lot of clever ways TO THE LEFT to relieve the pilot of much of the work of holding con- tinuous control pressures, during long flights in par- ticular. This labor saving magic can be built into any homebuilt aircraft simply by adding some form of trim FIGURE I. system. We all know the homebuilder constructing a small, light, low-powered plane for putt-putting around the local pea patch is not one to bother himself with trim devices as long as his center of gravity is within rea- sonable limits. Those of us with larger or faster aircraft, however, can't dismiss the problem of trim as readily, because the effects of changing flight conditions are more apparent . more obnoxious. Constant changes in the C.G. caused by different power settings, airspeeds, and fuel consumption, all point to the need for a means of trimming or adjusting longitudinal balance in flight. Suddenly, the persistent requirement for holding considerable control pressures gets to be very tiring. Remember? Homebuilts have a reputation of being rather sensi- tive to conditions affecting their C.G. ... to the point ATTACH WITH that there are those who claim they have to retrim after SHEET METAL SCREWS each sneeze. (An exaggeration, no doubt.) The real rea- OR son most builders install trim tabs, especially cockpit POP RIVETS controllable trim tabs, is because they are basically ADJUST TRIM FOR NORMAL CRUISE. good pilots and appreciate the finer things in life. (Lis- TAB AS ATTACHED IN (J) MUST BE ten for the drum roll and a flourish of trumpets.) BENT (MORE) TO OBTAIN SAME RESULT As a builder, you have several options to choose AS (T) (LEFT TRIM NEED ILLUSTRATED) from in the trim device department: 1. Fixed tabs (ground adjustable) 2. Cockpit controllable trim tabs 3. Bungee/Spring trim systems 4. Anti-Servo trim (cockpit controllable) KNOB - OPTIONAL 5. Stabilizer jacks (adjustable stabilizers) ^*- 6. Movable tail section (like Mooney aircraft) LEVER - I/S" - 3/16" ALUM FRICTION WASHER 7. Automatic flap trim systems 1/8* PLYWOOD Because of space limitations, only the fixed trim, OR cockpit controllable trim, and the automatic flap trim COMPOSITION systems will receive our attention at this time. 035 PIANO WIRE FIXED TRIM TABS (GROUND ADJUSTABLE) Fixed trim tabs are most often bent strips of alumi- num fastened to the trailing edges of control surfaces with sheet metal screws or rivets. While this is OK for METAL WASHER ( I 1/2") SMALL TURNBUCKLE wood and all-metal structures, steel tube and fabric (ANY VARIETY) construction does not lend itself to fixed tab installa- BOLT LEVER TO STRUCTURE tion unless some provision is made for its attachment 0« prior to covering. The reason, of course, is that it would USE MOUNTING BRACKET not be prudent to drill holes in the small trailing edge tubing just to attach a fixed tab. Certainly, not without first reinforcing that area. Although you may not have been looking for that FIGURE 2. sort of thing, you have, undoubtedly noticed at some time SPORT AVIATION 19 or another, that some trim tabs are bent (deflected) severely. As a casual observer, you might have thought that the airplane simply needed a lot of corrective trim. However, more probably it was an indication that the trim tab is too small. How big should a trim tab be? Who can say accurately? Why not start with one about 3" x 7" (made of 2024-T3 aluminum about .040" thick). If that size tab can ac- complish the job of trimming without an excessive de- flection, fine! If not, try a larger one. FIGURE 3. Fixed tabs are attached to the bottom side of the aileron or elevator trailing edges for appearances sake more than anything else. As for rudders, the side you select is also optional except, perhaps, for the considera- tion illustrated in Figure 1. OR ALUM TUBING We often forget which way to bend the tab for the trim effect wanted. Remember, bending the tab in one transfer its action from a movable control surface (ele- direction causes the control surface to be deflected aero- vator in this example) to a stationary structure (sta- dynamically in the opposite direction. The plane then bilizer/fuselage). The simplest way to accomplish this reacts directionally to the aircraft's deflected control is to use a flexible sheathed push-pull wire running surface. from the tab, on the movable surface, to the stabilizer. After installation, you will probably have to make This permits you to cross at some point close to the ele- several test flights to obtain the trim results you want vator hinge axis rather than exactly through it as would as ground adjustable tabs are at best a bend-and-try be necessary with any other nonflexing mechanical operation. linkage. Bend the tabs by increments until the aircraft is The sheathed portion of the push-pull wire does not trimmed to maintain a level attitude at its normal cruise have to run all the way to the cockpit except in a sin- power setting (without the necessity of manually hold- gle wire installation. Figure 4b. Since the conduit sheath- ing continuous control pressures). ing of the ordinary push-pull control is heavy, its use in Be careful in trying to bend a tab attached to a con- long lengths is to be avoided. Some builders prefer to trol surface as you may twist it off. To lessen this risk, switch to aluminum tubing for the long run to the cock- clamp small boards on either side of the control surface pit. and ask someone to grip the trailing edge area as you The cockpit trim control may be a wheel, lever, crank rebend the trim tab. It is usually better to remove the or switch. Whatever it is, it should have an irreversible tab, if you can, before bending it. characteristic, or at least have sufficient friction built- In the final analysis, fixed trim tabs can trim the in to prevent the creeping of the control, in flight, once aircraft for one flight condition only. Add a passenger, it has been set. consume fuel, change the power setting or cruise speed, Mechanical linkage should provide positive control and the aircraft's trim degenerates. without slack or sloppiness and without excessive sen- You might say this much for fixed trim tabs. They sitivity to tiny movements of the trim. can always be added after the airplane is built and Make a determined effort to eliminate play in the flown and, they are a darn sight better than no trim at linkage and tab connections as it is believed that such all ... a condition invites the possibility of flutter. CONTROLLABLE TRIM TABS AUTOMATIC FLAP TRIM Controllable trim tabs have all of the characteris- Last year a major aircraft manufacturer proudly tics inherent in the ground adjustable variety plus the announced that one of their models would have an auto- beneficial aspects of being adjustable in flight from the matic trimming feature integral to the flap operation. cockpit. That is, when the flaps are deployed, the automatic In some aircraft, the controllable elevator trim sys- trim would relieve control pressures by compensating tem consists of an unbelievable number of components for the pitching tendency that the use of flaps produces. and a complex routing of cables from the tab to where Heck, homebuilts like the Emeraudes have had this pro- they terminate in the cockpit in an equally complex vision incorporated (and simply too) for the past 20 assembly of sprockets, chains, pulleys, brackets, wheels, years. In the Emeraude, for example, two separate trim levers or cranks, friction devices, and irreversible gears. tabs are attached to the trailing edges of the elevators. To illustrate the point, one four seater inspected had 37 The tab on the left side is a conventional trim tab ac- separate components including 12 pulleys (priced any tuated by two wires running to a cockpit lever. This pulleys lately?)! This parts tabulation did not include controllable trim tab takes care of in-flight nose-heavy the numerous bolts, nuts and washers used to hold that or tail-heavy conditions developing from the using of trim system creation together. Is all this essential to fuel, changes in power setting, changes in airspeed, the operation of a simple slab trim tab affixed to the and of course, passenger and baggage variations. elevator? The other tab, on the right elevator, works only in Fortunately, most homebuilders find ways to obtain one direction. It is connected by a piano wire to the flap effective controllable elevator trim without such com- control arm or bell crank. As flaps are deployed, the wire plexity and expense. See Figure 4 for reassurance that simultaneously pulls the trim tab down causing the ele- this is, indeed, possible. vator to respond correctly to the sudden nose heavy un- One difficulty in installing controllable trim tabs, balance produced by dropping flaps. particularly on elevators is in installing the control As the flaps are raised, a spring loaded wire simul- linkage so that the position of the trim tab remains in taneously pulls the tab back to its neutral position. See the same relative angle to the control surface, no mat- Figure 5. It is a dandy device and can be installed very ter how much the elevator is moved. This, of course, easily in any flap equipped homebuilt.
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