SPORTPLANE BUILDER Tony Bingelis

Here is a rudder trim tab type popular with some builders. It is a triangular wedgefastened to the rudder. In trying to establish its ideal size and shape the wedge canbe taped on temporarily for testing.

The vertical stabilizer (fin) on this Mustang II is offset to the left to compensate fortorque and the spiral slipstream effect. It reduces the need for a lot of right rudderinput during takeoff. If properly set, it should also be about right for cruise speeds.

A less than discriminating pilotmay not know or notice that his air-plane is out of trim . . . howeversevere that condition might be.

Most of us, fortunately, immedi-ately sense when some f l ightcondition isn't quite right . . . evenwhen all we may have aboard in theway of a sophisticated flight system isa whiskey compass, needle/ball andairspeed.

For the most part, we instinctively

apply and hold the necessary pressureto whatever control requires it tomaintain coordinated flight.

Of course, if the airplane isequipped with a turn and bank indi-cator, a directional gyro, and anar t i f ic ia l hor izon, even the mostoblivious among us should noticewhen an airplane is not tracking likeit should . . . that it is out of trim.

Last month, in Part 1, considerableattention was devoted to the impor-

tance of maintaining accurate align-ment during the construction of themany parts, and later, in joining thoseparts to each other in final assembly.Included was the advice to installyour turn and bank indicator so itsball is perfectly centered when theaircraft is level laterally.

However, even after you have done thebest you can to get the wing incidenceperfectly aligned with a Smart Level*(buy, beg, or borrow one) and the airplaneaccurately rigged and ready for flight, youstill won't know how effective an assem-bly and alignment job you did until theairplane is flown.

You Say It Hew Hands-Off?

If your airplane did, indeed, flystraight and level "hands-off" duringthe initial flight, you are to be con-gratulated for the fine assembly andrigging job you did.

However, I am sure you under-stand that a "hands-off" condition, ifrealized, was noted during a particu-lar period of the flight. And, withouta doubt, this feat was accomplishedwith a slight tweaking of a cockpitcontrollable elevator trim.

NOTE: I believe every real air-plane (make that read "every airplanewith a cross country capability")should have a cockpit controllable el-evator trim.

Think back. Exactly when did itfly hands off? During the takeoff?While orbiting the airport in levelf l ight at cruise rpm? At reducedpower? For most of the flight?

As perfectly rigged as your air-plane may be, you will learn later ifyou aren't aware of it already thatyour airplane does not always flyhands off - not without a little helpon your part.

Naturally, if you have a three-axistrim capability, you should be able tore-tr im the airplane anyt ime youwant, in order to relieve whatevermanually applied control pressuresyou are holding.

This capability is especially appre-ciated dur ing level f l ight cruiseconditions.

Without cockpit controllable trimtabs it is a different story. At best,the airplane, if it does fly hands off,will do so only for one specific powersetting, airspeed, altitude, and, yes,cockpit loading.

98 AUGUST 1994

FUSELAGE CENTERLINE

VERTICAL FIN OFFSETTO THE LEFT

SLIPSTREAM

RUDDERDEFLECTED TO

THE RIGHT

SLIPSTREAM

FIXED METALTRIM TAB WEDGE ACTS

LIKE A FIXEDTRIM TAB

FIGURE 1RUDDER TRIM OPTIONS

(NON-CONTROLLABLE)

How To Trim For Level Flight

Here is the correct way to trimfor level f l i gh t when you have acockpit controllable three-axis trimcapability:

1. Accelerate to your intendedlevel flight cruising speed.

2. Adjus t the elevator trim tomaintain altitude.

3. Take your feet off the rudderpedals, but use whatever aileronpressure is necessary to keep thewings level.

Look at the ball in your T&B. Isthe ball centered in its race? If not,apply rudder pressure to center it.Then adjust the rudder trim to re-lieve your foot pressure on the

pedal. Now, when you put yourfeet on the f loor , the T&B ballshould remain centered.

4. Finally, concentrate on thewings. Is one wing heavy and youhave been holding aileron controlpressure to keep that wing up? Ifso, adjust the aileron trim to re-lieve the stick pressure you areholding.

That's it. Simple as one, two,three . . . simple, that is, if you havecockpit controllable trim tabs.

One final note. If you have trimposition indicators in the cockpit,check to see that the neutral indica-tions are actually neutral when theexternal trim tabs are streamlinedwith the control surfaces.

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Here is another airplane just after landing. Apparently, a lot of nose down trim hadto be cranked in for landing. Either the airplane has a tail heavy condition or had asumo wrestler as a rear seat passenger.

This airplane had just taxied in. Looking at his elevator trim setting you can tell thatthe pilot had the aircraft trimmed slightly nose up for landing . . . a normal practicefor most low wing aircraft.

An electric (Aero-Trim) cockpit controllable rudder trim was installed by this builder.I think the elevator trim tab would look better had he installed the piano hinge onthe bottom.

Ground Adjustable Tabs

But, what about the homebuiltthat has no cockpit adjustable trimcontrols except maybe for an elevatortrim?

Well, the trimming sequence willbe the same when fixed ground ad-justable trim tabs are attached to therudder and aileron(s).

There is, however, this difference.Your airplane trimming exercise isgoing to be a drawn out process be-cause every time you re-adjust a fixedtrim tab by bending it, you will haveto fly the airplane to see how well youguessed.

Actually, you may have to makeseveral test flights before you get theresults you want.

Proper ly ad jus ted , ground ad-justable trim tabs can relieve the pilotof some in-flight control pressures -but can do nothing, for example, tocompensate for the unbalance createdby the uneven use of wing tank fuel,taking on a passenger, or changingyour altitude and/or power setting.

Adjusting Fixed Trim Tabs

Trim tabs, as small as they are,exert a surprising amount of aerody-namic force against the controlsurfaces to which they are affixed.Moving (bending) a trim tab in onedirection causes a control surface tobe deflected aerodynamically in theopposite direction.

I'm sure most of you have seenparked aircraft with ground ad-justable metal trim tabs installed.Sometimes there was only a ruddertrim tab. However, the same airplanemay also have had a trim tab on oneaileron . . . and, in rare instances, oneon each aileron.

I doubt if you have ever seen afixed trim tab on an elevator.

As previously stated, in my opin-ion even the simplest aircraft needs acockpit controllable elevator trim tab.

Fixed (ground adjustable) trimtabs are quite common on many lightpersonal aircraft because they aresimpler and cheaper to install thancockpit controllable tabs.

Such trim tabs are so common thatyou will hardly notice them until yousee one so severely bent that it re-sembles a deployed flap more than itdoes a simple trim tab.

When you see a tab like that, youwouldn't be wrong in assuming a tabthat badly bent is indicative of a des-perate attempt to compensate for arigging problem . . . a misaligned orwarped wing, perhaps?

Some wing heavy conditions are so

100 AUGUST 1994

AILERON IS FORCED UP

TRIM TAB DOWN(LOOKS BETTER)

AILERON IS FORCED DOWN

TRIM TAB ON HEAVY WING TRIM TAB UP

AILERON IS FORCED UP

TRIM TAB ON LIGHT WING TRIM TAB DOWN

FIGURE 2AILERON TRIM ACTION

A typical fixed rudder trim tab. Its size and degree of deflection to help compen-sate for torque is also typical.

That's a lot of deflection for an aileron trim tab. It could indicate a wing alignmentproblem or a warped wing .. . especially if the left wing also has to have a similartrim tab bent in the opposite direction.

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SPORT AVIATION 101

SLIPSTREAMSTABILIZER ELEVATOR

STABILIZER

FORCES ELEVATOR DOWN(CAUSING AIRCRAFT TO NOSE DOWN)

ELEVATOR

FORCES ELEVATOR UP(CAUSING AIRCRAFT TO NOSE UP)

FIGURE 3AILERON TRIM ACTION

severe that both ailerons may requiresharply deflected trim tabs (in oppo-site directions).

Here is the bad news about trimtabs . . . any kind of t r im tab. Inshort, the more you must deflect atrim tab to overcome an unbalancedflight condition, the more drag youcreate . . . and since drag is a parasitethat greeds on performance your air-speed suffers because of it. That'swhy a good job of rigging and align-ing your wings is so important.

Adjusting Fixed Trim Tabs

These tabs, naturally, can only bere-adjusted on the ground, and thatintroduces a dilemma for some folks.The direction in which to bend thetab can be confusing if they don'tstop and think about it a bit.

All you have to remember is that atrim tab forces the control surface inthe opposite direction from whichthe tab is bent.

For Example:1. If the nose of the aircraft is

yawing to the left, bend the ruddertrim tab to the LEFT.

2. If the aircraft has a nose heavycondition, bend a fixed elevator trimtab DOWN.

3. If one wing is heavy, bend thetrim tab on that wing UP.

You can make your fixed trim tabsof 2024 T3 aluminum. A tab made of.025" or .032" aluminum should bestiff enough for the job.

That raises another question. Howbig should a trim tab be?

Well, I know of no particular stan-dard size, shape, or placement fortrim tabs. Actually, the tab may nothave to be as large as you think be-cause that l i t t le gadget, whendeflected, exerts a powerful correc-tive force against the control surface.

If your unbalance condition isn'tsevere (how severe is severe . . . asnap roll when you let go of thestick?), you might try a trim tab that

measures about 3" x 6". About 1" ofits width will be used in attaching thetab, leaving a 2" width to do the work.

If that size tab proves to be exces-sive, you can always tr im it downwith tinsnips. Anyhow, that hasproved to be a good starting point.

Attach your trim tab to the trailingedge of the control surface with riv-ets or screws, or by whatever meanswill work best for the type of con-struction used in the control surface.

When you have to re-bend a trimtab that is already installed, try notto exert a twisting pressure on thetrailing edge of the control surface asyou try to bend the tab, because youmay damage it or cause the rivets se-curing the trim tab to loosen.

Use a pair of pliers, or a duck billhand seamer to assist in making thebend.

Your trim tab adjustments (bends)should be made in small incrementsfollowed by a check flight to deter-mine the result.

Designers have long realized theneed to compensate for the varyingeffects of engine torque in their air-craft and often incorporate designfeatures which are intended to reducethe need for drag producing t r imtabs. The two most common of these"fixes" are:

1. An offset vertical fin.2. An offset engine centerline.

The Offset Vertical Fin

The slipstream from the propeller,in conventional aircraft , does notflow straight back but takes a spiralor helical path around the fuselage.This, in effect , causes the "prop-wash" (slipstream) to hit the verticalfin and rudder on one side at some-what of an angle thereby aggravatingthe effects of torque.

Therefore, by rigging the fin off-center a bit, the tail surface will morenear ly be para l le l to the a i r f low.Thus, the side loads on the tail sur-faces are reduced as would be theneed for a substantial ly deflectedtrim tab.

Of course, this is a rather simpli-fied cause and effect explanation butit is adequate for now.

The fin offset, when used, is to theLEFT for conventional aircraft en-gines having a clockwise propellerrotation (as viewed from behind thepropeller).

How much should this offset be?5/8"? More? Less? Guess wrongand once incorporated, realigning thefin to a neutral setting would be diffi-cult as the fin-to-stabilizer fairingwould also have to be reworked.

102 AUGUST 1994

Incidentally, the fin offset method,if used, would be to the RIGHT forVW engines, and others that rotateopposite from standard U.S. aircraftengines.

The Offset Engine Centerline

As with the offset fin, the designermay have determined that offsettingthe engine thrust line, say 3 degrees,would help reduce the need for cor-rective rudder pressure in flight.

Many aircraft designs utilize eitheran engine offset or an off-center ver-tical s tabi l izer as a rigging andtrimming ploy.

Here again, however, even if allthe calculations work out, the aerody-namic trim achieved will be mosteffective, hopefully, during normalcruise conditions.

Other Considerations

In general, the larger the trim tabthe smaller its deflection will have to beto obtain the desired trimming effect.

Cockpit controllable tabs that areinset in the elevator surface are struc-turally better and are as effective astrim tabs attached to the trailing edgeof the elevator. The Emeraude is atypical example.

Control lable tabs should be aslight as possible to avoid adding toomuch weight behind a control sur-face's hinge axis.

An aileron's tab chord is more ef-fective when it is well over 5% of theaileron chord because a chord lessthan 5% would be too ineffective athigher deflection angles.

In general, keep tab aileron deflec-tions to 20% maximum.

According to early NACA studies,the tab deflection angle should be be-low 75% of the movable controlsurface angular deflection to preventan overbalance effect.

With part-span trim tabs, an out-board location is a sl ightly betterposition, aerodynamically. However,an inboard location for the trim tab isconsidered to be a better choice struc-turally.

Although tabs are effective at allspeeds, they are less effect ive athigher angles of attack (low speeds).

In the final analysis, trim tabs onrudders and elevators are similar tothose on ailerons, therefore, maxi-mum deflections relat ive to theattached control surface are best lim-ited to plus or minus 20 degrees.

It is also advisable to keep as muchfree-play out of your trim tab instal-lations as possible because floppytrim tabs have been known to induce

The Emeraude features two externally attached cockpit controllable elevator trimtabs. Notice the one on the right is deflected downward slightly more than the lefttab. It is connected directly to the flap bell crank and automatically is deflected asthe flaps are lowered. The tab on the left is pilot controllable.

control surface flutter.The amount of free play at the

trailing edge of the trim should beminimal. FAA's AC 43.13-1 gives asimple formula for establishing themaximum permissible free play at thetrailing edge of the tab. They recom-mend limiting this movement to not

more than 2-1/2% of the chord. Atab with a 4" chord would have amaximum amount of play of 4" x.025 (0.100 inches total motion upand down).

For all practical purposes you shouldstrive to keep the total amount of freeplay in your trim tabs under 1/8".

Both are ways to spend summer,

one is clearly better.

SPORT AVIATION 103

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104 AUGUST 1994

And, finally, if you have a highperformance aircraft that has ex-tremely effective and sensitive trimtabs, you should also add a trim tabcheck to your Cockpit Check List.

Useful Information

SMART Level® in format ionsource: T&L Investment Protec-tion, Inc., Hangar 6, Wiley PostAirport, Bethany, OK 73008

FAA's Acceptable Methods,AC43.13-1A (PAR 55): (Tech-niques and Practices Aircraf tAlterations)

NACA Report No. 528 (1035):Reduction of Hinge Moments ofAirplane Control Surfaces by Tabs

If you wish to contact the authorof this column for additional infor-mation, please send a SASE to:

Tony Bingelis8509 Greenflint Ln.Austin, TX 78759BOOKS BY TONY

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