SPORTPLANE BUILDER

Tony Bingelis

tance of maintaining accurate alignment during the construction of the many parts, and later, in joining those parts to each other in final assembly. Included was the advice to install your turn and bank indicator so its ball is perfectly centered when the aircraft is level laterally. However, even after you have done the best you can to get the wing incidence perfectly aligned with a Smart Level* (buy, beg, or borrow one) and the airplane accurately rigged and ready for flight, you still won't know how effective an assembly and alignment job you did until the airplane is flown. You Say It Hew Hands-Off?

Here is a rudder trim tab type popular with some builders. It is a triangular wedge fastened to the rudder. In trying to establish its ideal size and shape the wedge can

be taped on temporarily for testing.

The vertical stabilizer (fin) on this Mustang II is offset to the left to compensate for torque and the spiral slipstream effect. It reduces the need for a lot of right rudder

input during takeoff. If properly set, it should also be about right for cruise speeds.

A less than discriminating pilot may not know or notice that his airplane is out of trim . . . however severe that condition might be. Most of us, fortunately, immediately sense when some flight condition isn't quite right . . . even when all we may have aboard in the way of a sophisticated flight system is a whiskey compass, needle/ball and airspeed. For the most part, we instinctively 98 AUGUST 1994

apply and hold the necessary pressure to whatever control requires it to maintain coordinated flight. Of course, if the a i r p l a n e is

equipped with a turn and bank indicator, a directional gyro, and an a r t i f i c i a l h o r i z o n , even the most oblivious among us should notice when an airplane is not tracking like it should . . . that it is out of trim. Last month, in Part 1, considerable attention was devoted to the impor-

If your airplane did, indeed, fly straight and level "hands-off" during the initial flight, you are to be congratulated for the fine assembly and rigging job you did. However, I am sure you understand that a "hands-off" condition, if realized, was noted during a particular period of the flight. And, without a doubt, this feat was accomplished with a slight tweaking of a cockpit controllable elevator trim. NOTE: I believe every real airplane (make that read "every airplane with a cross country capability") should have a cockpit controllable elevator trim. Think back. Exactly when did it fly hands off? During the takeoff? While orbiting the airport in level flight at cruise rpm? At reduced power? For most of the flight? As perfectly rigged as your airplane may be, you will learn later if you aren't aware of it already that your airplane does not always fly hands off - not without a little help on your part. Naturally, if you have a three-axis trim capability, you should be able to re-trim the airplane a n y t i m e you want, in order to relieve whatever manually applied control pressures you are holding. This capability is especially appreciated d u r i n g level flight cruise conditions. Without cockpit controllable trim tabs it is a different story. At best, the airplane, if it does fly hands off, will do so only for one specific power setting, airspeed, altitude, and, yes, cockpit loading.

FUSELAGE CENTERLINE

VERTICAL FIN OFFSET

TO THE LEFT

Consider The EGT/CHT Advantages: • Lean Find Mode automatically finds the first EGT to peak, avoiding cylinder damage due to over leaning. RUDDER DEFLECTED TO THE RIGHT

• Identifies unhealthy cylinders using greatest span method,

• Shock Cooling: First time ever available as an actual value in

SLIPSTREAM

SLIPSTREAM

degrees/minute.

• Identifies cylinder that may fail due FIXED METAL

TRIM TAB

WEDGE ACTS LIKE A FIXED TRIM TAB

to shock cooling. • One degree precision resolution

with selectable C or F. • Detects voltage loss due to alterna-

tor failure which is a

FIGURE 1 RUDDER TRIM OPTIONS (NON-CONTROLLABLE)

How To Trim For Level Flight

pedal. N o w , when you put your feet on the floor, the T&B ball Here is the correct way to trim should remain centered. for level f l i g h t w h e n you have a 4. Finally, concentrate on the cockpit controllable three-axis trim wings. Is one wing heavy and you capability: have been holding aileron control 1. Accelerate to your intended pressure to keep that wing up? If level flight cruising speed. so, adjust the aileron trim to re2. A d j u s t the elevator trim to lieve the stick pressure you are maintain altitude. holding. 3. Take your feet off the rudder That's it. Simple as one, two, pedals, but use whatever aileron three . . . simple, that is, if you have pressure is necessary to keep the cockpit controllable trim tabs. wings level. One final note. If you have trim Look at the ball in your T&B. Is position indicators in the cockpit, the ball centered in its race? If not, check to see that the neutral indicaapply rudder pressure to center it. tions are actually neutral when the Then adjust the rudder trim to re- external trim tabs are streamlined lieve your foot pressure on the with the control surfaces.

must for IFR. • Supplied with JPI's

well recognized grounded fast response probes. • Normalize Mode for instantly deterring engine trends while in flight. Ten degrees per bar. • TSO quality, FAA STC and TSO. Approved by all insurance companies. • Only analog EGT/CHT made in

USA.

JP INSTRUMENTS 1-800-345-4574 (714) 557-5434

Fax:(714) 557-9840

Box 7033, H u n t i n g l o n Beach CA 92615

Ground Adjustable Tabs

But, what about the homebuilt that has no cockpit adjustable trim

Here is another airplane just after landing. Apparently, a lot of nose down trim had

to be cranked in for landing. Either the airplane has a tail heavy condition or had a

sumo wrestler as a rear seat passenger.

controls except maybe for an elevator trim? Well, the trimming sequence will be the same when fixed ground adjustable trim tabs are attached to the rudder and aileron(s). There is, however, this difference. Your airplane trimming exercise is going to be a drawn out process because every time you re-adjust a fixed trim tab by bending it, you will have to fly the airplane to see how well you guessed. Actually, you may have to make several test flights before you get the results you want. Properly a d j u s t e d , ground adjustable trim tabs can relieve the pilot of some in-flight control pressures but can do nothing, for example, to compensate for the unbalance created by the uneven use of wing tank fuel, taking on a passenger, or changing your altitude and/or power setting. Adjusting Fixed Trim Tabs

This airplane had just taxied in. Looking at his elevator trim setting you can tell that the pilot had the aircraft trimmed slightly nose up for landing . . . a normal practice for 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 on the bottom. 100 AUGUST 1994

Trim tabs, as small as they are, exert a surprising amount of aerodynamic force against the control surfaces to which they are affixed. Moving (bending) a trim tab in one direction causes a control surface to be deflected aerodynamically in the opposite direction. I'm sure most of you have seen parked aircraft with ground adjustable metal trim tabs installed. Sometimes there was only a rudder trim tab. However, the same airplane may also have had a trim tab on one aileron . . . and, in rare instances, one on each aileron. I doubt if you have ever seen a fixed trim tab on an elevator. As previously stated, in my opinion even the simplest aircraft needs a cockpit controllable elevator trim tab. Fixed (ground adjustable) trim tabs are quite common on many light personal aircraft because they are simpler and cheaper to install than cockpit controllable tabs. Such trim tabs are so common that you will hardly notice them until you see one so severely bent that it resembles a deployed flap more than it does a simple trim tab. When you see a tab like that, you wouldn't be wrong in assuming a tab that badly bent is indicative of a desperate attempt to compensate for a rigging problem . . . a misaligned or warped wing, perhaps? Some wing heavy conditions are so

AILERON IS FORCED UP

wind

TRIM TAB DOWN (LOOKS BETTER) AILERON IS FORCED DOWN

PILOT AVIONICS

CFI EDITION PA 11-20 PA-9 Electret MIC Telescoping Flex Boom Air-Gel Earseals

Brass Molded Plugs

TRIM TAB UP

TRIM TAB ON HEAVY WING

Delent Volume Control

5-Year Warranty Super-Cushion Headpad Weight 15902

AILERON IS FORCED UP

TRIM TAB ON LIGHT WING

ZEPHYR PHC-31 PADDED HEADSET CASE

TRIM TAB DOWN

FIGURE 2 AILERON TRIM ACTION

•Made of 1000 Denier

Cordura • Molds headset & Intercom in Main Compartment • Side Pocket Holds Calculators • Reinforced Top Handle • Reinforced Shoulder Strap

• Measurements: 10-1/2" x 9-1/2' x 5"

• Larger Style Available 13' x 17' x 12' $24.95 (LPHC-32)

All Headset Purchases Include: (S24.95 Value)

FREE PHC-31 Padded Cordura Case FREE Shipping in USA

FREE MIC Windscreen FREE Cloth Earseal Covers

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

$ ZEPHYR PDFC-35

42.95

Padded Deluxe Flight Case • Made of 1000 Donior Cordura

•Adjustable Padded Center Dividers

• Two End Pockets For Hoadsulb/Radios etc

• Two Front Pockets

• Side Slit Pocket For Charts

• Easily Holds 2 JEPPS, 2 Headsets, Intercom • Measurements 17-1/2' x 10-1/2' x 9-1/2"

WEST WIND

• FREE Catalog Available • UPS Blue (2nd day) $5 00/UPS Red $10.00

1-800-456-1665 That's a lot of deflection for an aileron trim tab. It could indicate a wing alignment problem or a warped wing .. . especially if the left wing also has to have a similar trim tab bent in the opposite direction.

Order Desk Open 24 Hours

k

1850 Union St. #1633, San Francisco

California 94123

SPORT AVIATION 101

SLIPSTREAM STABILIZER

ELEVATOR

FORCES ELEVATOR DOWN (CAUSING AIRCRAFT TO NOSE DOWN)

STABILIZER

ELEVATOR

FORCES ELEVATOR UP (CAUSING AIRCRAFT TO NOSE UP)

measures about 3" x 6". About 1" of its width will be used in attaching the tab, leaving a 2" width to do the work. If that size tab proves to be excessive, you can always trim it down with tinsnips. A n y h o w , that has proved to be a good starting point. Attach your trim tab to the trailing edge of the control surface with rivets or screws, or by whatever means will work best for the type of construction used in the control surface. When you have to re-bend a trim tab that is already installed, try not to exert a twisting pressure on the trailing edge of the control surface as you try to bend the tab, because you may damage it or cause the rivets securing the trim tab to loosen. Use a pair of pliers, or a duck bill hand seamer to assist in making the bend. Your trim tab adjustments (bends) should be made in small increments followed by a check flight to determine the result. Designers have long realized the need to compensate for the varying effects of engine torque in their aircraft and often incorporate design features which are intended to reduce the need for drag producing trim tabs. 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 not flow straight back but takes a spiral or helical path around the fuselage. severe that both ailerons may require For Example: This, in effect, causes the "prop1. If the nose of the aircraft is wash" (slipstream) to hit the vertical sharply deflected trim tabs (in opposite directions). yawing to the left, bend the rudder fin and rudder on one side at someHere is the bad news about trim trim tab to the LEFT. what of an angle thereby aggravating 2. If the aircraft has a nose heavy the effects of torque. tabs . . . any kind of trim tab. In Therefore, by rigging the fin offshort, the more you must deflect a condition, bend a fixed elevator trim center a bit, the tail surface will more trim tab to overcome an unbalanced tab DOWN. 3. If one wing is heavy, bend the nearly be parallel to the a i r f l o w . flight condition, the more drag you create . . . and since drag is a parasite trim tab on that wing UP. Thus, the side loads on the tail surYou can make your fixed trim tabs faces are reduced as would be the that greeds on performance your airspeed suffers because of it. That's of 2024 T3 aluminum. A tab made of need for a substantially deflected why a good job of rigging and align- .025" or .032" aluminum should be trim tab. stiff enough for the job. Of course, this is a rather simpliing your wings is so important. That raises another question. How fied cause and effect explanation but it is adequate for now. Adjusting Fixed Trim Tabs big should a trim tab be? The fin offset, when used, is to the Well, I know of no particular stanThese tabs, naturally, can only be dard size, shape, or placement for LEFT for conventional aircraft enre-adjusted on the ground, and that trim tabs. Actually, the tab may not gines having a clockwise propeller introduces a dilemma for some folks. have to be as large as you think be- rotation (as viewed from behind the The direction in which to bend the cause that l i t t l e gadget, when propeller). How much should this offset be? tab can be confusing if they don't deflected, exerts a powerful correc5/8"? More? Less? Guess wrong stop and think about it a bit. tive force against the control surface. All you have to remember is that a If your unbalance condition isn't and once incorporated, realigning the trim tab forces the control surface in severe (how severe is severe . . . a fin to a neutral setting would be diffithe opposite direction from which snap roll when you let go of the cult as the fin-to-stabilizer fairing stick?), you might try a trim tab that would also have to be reworked. the tab is bent.

FIGURE 3 AILERON TRIM ACTION

102 AUGUST 1994

Incidentally, the fin offset method, if used, would be to the RIGHT for VW engines, and others that rotate opposite from standard U.S. aircraft engines. The Offset Engine Centerline

As with the offset fin, the designer may have determined that offsetting the engine thrust line, say 3 degrees, would help reduce the need for corrective rudder pressure in flight. Many aircraft designs utilize either an engine offset or an off-center vertical stabilizer as a rigging and trimming ploy. Here again, however, even if all the calculations work out, the aerodynamic trim achieved will be most effective, hopefully, during normal cruise conditions. Other Considerations

In general, the larger the trim tab the smaller its deflection will have to be to obtain the desired trimming effect. Cockpit controllable tabs that are inset in the elevator surface are structurally better and are as effective as trim tabs attached to the trailing edge of the elevator. The Emeraude is a typical example. Controllable tabs should be as light as possible to avoid adding too much weight behind a control surface's hinge axis. An aileron's tab chord is more effective when it is well over 5% of the aileron chord because a chord less than 5% would be too ineffective at higher deflection angles. In general, keep tab aileron deflections to 20% maximum. According to early NACA studies, the tab deflection angle should be below 75% of the movable control surface angular deflection to prevent an overbalance effect. With part-span trim tabs, an outboard location is a slightly better position, aerodynamically. However, an inboard location for the trim tab is considered to be a better choice structurally. Although tabs are effective at all speeds, they are less effective at higher angles of attack (low speeds). In the final analysis, trim tabs on rudders and elevators are similar to those on ailerons, therefore, maximum deflections relative to the attached control surface are best limited to plus or minus 20 degrees. It is also advisable to keep as much free-play out of your trim tab installations as possible because floppy trim tabs have been known to induce

The Emeraude features two externally attached cockpit controllable elevator trim tabs. Notice the one on the right is deflected downward slightly more than the left tab. It is connected directly to the flap bell crank and automatically is deflected as

the flaps are lowered. The tab on the left is pilot controllable.

control surface flutter. The a m o u n t of free play at the trailing edge of the trim should be minimal. FAA's AC 43.13-1 gives a simple formula for establishing the maximum permissible free play at the trailing edge of the tab. They recommend limiting this movement to not

more than 2-1/2% of the chord. A tab with a 4" chord would have a m a x i m u m amount of play of 4" x .025 (0.100 inches total motion up and down). For all practical purposes you should strive to keep the total amount of free play in your trim tabs under 1/8".

Both are ways to spend summer,

one is clearly better.

SPORT AVIATION 103

'\ Books ft Construction A Videos Materials ^

Builders' k Workshops

YOU CAN BUILD THAT AIRPLANE? Well Help You.

Builders and restorers know us as the one-call source for all their fabrics, dopes, hardware, tools, resins, instructional materials... and we conduct 2-day handson Aeroplane Builders' Workshops all over the country. We're here to help.

And, finally, if you have a high performance aircraft that has extremely effective and sensitive trim tabs, you should also add a trim tab check to your Cockpit Check List. Useful Information SMART Level® i n f o r m a t i o n source: T&L Investment Protection, Inc., Hangar 6, Wiley Post Airport, Bethany, OK 73008 FAA's Acceptable Methods, AC43.13-1A (PAR 55): (Techniques and Practices Aircraft Alterations) NACA Report No. 528 (1035): Reduction of Hinge Moments of Airplane Control Surfaces by Tabs

If you wish to contact the author of this column for additional information, please send a SASE to: Tony Bingelis

8509 Greenflint Ln. Austin, TX 78759

Fabric Covering

Composites

Welding

You'll see how easy This workshop gets you It's easy to master the

today's methods and ready to build a sleek, basics of oxy-acetylene materials can make it.

fast' glass" airplane.

gas welding.

BUILDERS HELPING BUILDERS Workshop Schedule Update August 27-28 • Trenton, New Jersey

BOOKS BY TONY

The following books by Tony Bingelis are available from the EAA Aviation Foundation, EAA Aviation Center, Box 3086, Oshkosh, WI 54903-3086, 1-800/843-3612. Major credit cards accepted.

-Sportplane Builders (Aircraft

Construction Methods, 320 pages) - $19.95

September 10-11 • Bloomington, Illinois September 24-25 • Griffin, Georgia

-Firewall Forward (Engine Installation Methods, 304

8OO-83f-2949

-Sportplane Construction Techniques (A Builder's

CALL FOR YOUR FREE CATALOG? Call Us For Workshop Info,

Orders, Even Free Advice!

Weekdays 8:30 to 7, Saturday 9 to 1 Customer Service • 800-358-5228 FAX Line • 404-229-2329

104 AUGUST 1994

LEXANDER EROPLANE COMPANY, INC. Box 909, GRIFFIN, GEORGIA 30224

pages) - $19.95.

Handbook, 350 pages) - $20.95. Add $2.40 postage and handling for each publication ordered . . . or order all three for $52.97 plus $6.95 postage and handling. Wisconsin residents add

5% sales tax.