THE TRIM SYSTEMS . . .

SPORTPLANE BUILDER

FIXED AND CONTROLLABLE

By Antoni (Tony) Bingelis EAA Designee Program Advisor

TRIM TABS

8509 Greenflint Lane Austin, Texas 78759

A

AIRCRAFT S

LOT OF people have conceived a lot of clever ways to relieve the pilot of much of the work of holding continuous control pressures, during long flights in particular. This labor saving magic can be b u i l t into any homebuilt aircraft simply by adding some form of trim system. We all know the homebuilder constructing a small, light, low-powered plane for p u t t - p u t t i n g around the local pea patch is not one to bother himself with trim devices as long as his center of gravity is within reasonable 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 t r i m m i n g 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 sensitive to conditions affecting their C.G. . . . to the point that there are those who claim they have to retrim after each sneeze. (An exaggeration, no doubt.) The real reason most builders install trim tabs, especially cockpit controllable trim tabs, is because they are basically good pilots and appreciate the finer things in life. (Listen for the drum roll and a flourish of trumpets.) As a builder, you have several options to choose 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) 5. Stabilizer jacks (adjustable stabilizers) 6. Movable tail section ( l i k e Mooney aircraft) 7. Automatic flap trim systems Because of space limitations, only the fixed trim, cockpit controllable trim, and the automatic flap t r i m systems will receive our attention at this time.

TO

FIGURE

ATTACH

THE

NOSE

CORRECTS

LEFT

I.

WITH

SHEET METAL SCREWS OR POP

RIVETS ADJUST TAB

TRIM FOR

BENT (MORE) AS

NORMAL CRUISE.

AS A T T A C H E D

(T)

TO

(LEFT

IN (J)

OBTAIN TRIM

MUST BE

SAME RESULT

NEED

ILLUSTRATED)

KNOB - O P T I O N A L ^*LEVER FRICTION

I/S" - 3/16" ALUM

WASHER

1/8* PLYWOOD OR COMPOSITION

035

PIANO WIRE

FIXED TRIM TABS (GROUND ADJUSTABLE)

Fixed trim tabs are most often bent strips of aluminum fastened to the trailing edges of control surfaces with sheet metal screws or rivets. While this is OK for wood and all-metal structures, steel tube and fabric construction does not lend itself to fixed tab installation unless some provision is made for its attachment prior to covering. The reason, of course, is that it would 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 sort of thing, you have, undoubtedly noticed at some time

METAL

WASHER SMALL

( I 1/2")

(ANY BOLT

LEVER

TO

TURNBUCKLE

VARIETY)

STRUCTURE

0« USE

MOUNTING

BRACKET

FIGURE 2.

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 accomplish the job of trimming without an excessive deflection, fine! If not, try a larger one. 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 consideration illustrated in Figure 1. We often forget which way to bend the tab for the trim effect wanted. Remember, bending the tab in one direction causes the control surface to be deflected aerodynamically in the opposite direction. The plane then reacts directionally to the aircraft's deflected control surface. After installation, you will probably have to make several test flights to obtain the trim results you want as ground adjustable tabs are at best a bend-and-try operation.

Bend the tabs by increments until the aircraft is trimmed to maintain a level attitude at its normal cruise power setting (without the necessity of manually holding continuous control pressures). Be careful in trying to bend a tab attached to a control surface as you may twist it off. To lessen this risk, clamp small boards on either side of the control surface and ask someone to grip the trailing edge area as you rebend the trim tab. It is usually better to remove the tab, if you can, before bending it. In the final analysis, fixed trim tabs can trim the

aircraft for one flight condition only. Add a passenger, consume fuel, change the power setting or cruise speed, and the aircraft's trim degenerates. You might say this much for fixed trim tabs. They can always be added after the airplane is b u i l t and flown and, they are a darn sight better than no trim at all . . .

FIGURE 3.

OR

ALUM

TUBING

transfer its action from a movable control surface (elevator in this example) to a stationary structure (stabilizer/fuselage). The simplest way to accomplish this is to use a flexible sheathed push-pull wire running from the tab, on the movable surface, to the stabilizer. This permits you to cross at some point close to the elevator hinge axis rather than exactly through it as would be necessary with any other nonflexing mechanical linkage. The sheathed portion of the push-pull wire does not have to run all the way to the cockpit except in a single wire installation. Figure 4b. Since the conduit sheathing of the ordinary push-pull control is heavy, its use in

long lengths is to be avoided. Some builders prefer to switch to a l u m i n u m tubing for the long run to the cockpit. The cockpit trim control may be a wheel, lever, crank or switch. Whatever it is, it should have an irreversible characteristic, or at least have sufficient friction builtin to prevent the creeping of the control, in flight, once it has been set. Mechanical linkage should provide positive control without slack or sloppiness and without excessive sensitivity to tiny movements of the trim. Make a determined effort to eliminate play in the linkage and tab connections as it is believed that such a condition invites the possibility of flutter.

CONTROLLABLE TRIM TABS

AUTOMATIC FLAP TRIM

Controllable trim tabs have all of the characteristics inherent in the ground adjustable variety plus the beneficial aspects of being adjustable in flight from the cockpit. In some aircraft, the controllable elevator trim system consists of an unbelievable number of components and a complex routing of cables from the tab to where they terminate in the cockpit in an equally complex assembly of sprockets, chains, pulleys, brackets, wheels, levers or cranks, friction devices, and irreversible gears. To illustrate the point, one four seater inspected had 37 separate components including 12 pulleys (priced any

Last year a major aircraft manufacturer proudly announced that one of their models would have an automatic trimming feature integral to the flap operation. That is, when the flaps are deployed, the automatic

pulleys lately?)! This parts tabulation did not include the numerous bolts, nuts and washers used to hold that trim system creation together. Is all this essential to

the operation of a simple slab trim tab affixed to the

elevator?

Fortunately, most homebuilders find ways to obtain effective controllable elevator trim without such complexity and expense. See Figure 4 for reassurance that this is, indeed, possible. One difficulty in installing controllable trim tabs, p a r t i c u l a r l y on elevators is in i n s t a l l i n g the control linkage so that the position of the trim tab remains in the same relative angle to the control surface, no matter how much the elevator is moved. This, of course,

creates a bit of a challenge as the control linkage must

20 MARCH 1976

trim would relieve control pressures by compensating for the pitching tendency that the use of flaps produces.

Heck, homebuilts like the Emeraudes have had this provision incorporated (and simply too) for the past 20 years. In the Emeraude, for example, two separate trim tabs are attached to the trailing edges of the elevators. The tab on the left side is a conventional trim tab actuated by two wires r u n n i n g to a cockpit lever. This

controllable trim tab takes care of in-flight nose-heavy or tail-heavy conditions developing from the using of

fuel, changes in power setting, changes in airspeed, and of course, passenger and baggage variations. The other tab, on the right elevator, works only in one direction. It is connected by a piano wire to the flap

control arm or bell crank. As flaps are deployed, the wire simultaneously pulls the trim tab down causing the elevator to respond correctly to the sudden nose heavy unbalance produced by dropping flaps.

As the flaps are raised, a spring loaded wire simultaneously pulls the tab back to its neutral position. See

Figure 5. It is a dandy device and can be installed very easily in any flap equipped homebuilt. Trim tabs built into the elevators, or rudder or ailer-

ons, are a bit more difficult to construct than the externally affixed tabs as in the Emeraude design. However, either design works equally well. It could be though that tabs b u i l t into the control surfaces might have to be made a bit larger to achieve the same degree of trim

effectiveness as those add-on types.

WHICH WAY DOES IT GO?

Make sure the trim tab moves in the correct direction when you move the cockpit lever, crank, or wheel. A nose down trim indication inside the cockpit, for example, corresponds to an "up tab" position on the elevator surface. Correct? If you want the nose to turn to the right . . . the rudcer tab must be deflected to the left. Right? (Sorry, couldn't resist.) How about a right wing heavy condition where there is only one fixed tab mounted on the left wing? Do you bend the tab down? Right again! Remember a deflected tab, with the help of the slipstream, forces the control surface in the opposite direction. The tab influenced control surface causes the aircraft to respond to it as though the rotated surface was actuated by the pilot.

COCKPIT

TRIM

Cockpit trim controls are labeled to indicate correct movement for "Nose Down", "Nose Up", etc. However, you may not consider t h i s as important a need when only an elevator trim lever is mounted parallel to the longitudinal axis . . . and the direction of movement is obvious. If a t r i m control is mounted overhead or elsewhere when its direction of movement or rotation may be in doubt, it should be marked, as it may (really) rotate in a direction contrary to what another pilot might expect. Except possibly for a lever type trim, which by its design literally shouts out its trim position, other types

of trim control, such as wheels and cranks do need a

positive indicator that affords some clue as to t h e i r exact relative position with t h a t of the remote tab. COCKPIT CHECK LIST

"Trim Tabs" or "Set Trim" should be added to your printed cockpit check list (you do have one, of course?). The bigger and faster the airplane, the more important this reminder as to forget to reset the trim could be an unexpected invitation to a traumatic take-off.

CONTROL

AUTOMATIC

FLAP

TRIM

SYSTEM

(SCHEMATIC)

On the left elevator is the normal controllable trim tab. On the right is the automatic flap trim. Notice that the wing flap and trim tab are both down

A close view of the deflected flap trim which indicates that the flaps are deployed. Note the shape of the top horn. It stops the trim tab in the neutral position when flaps are raised. SPORT AVIATION 21