PART 1. stabilizer, and vertical fin) to which it is attached. b. Setting and verifying the minimum

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1 SPORTPLANE BUILDER Tony Bingelis Very few homebuilders know or understand what the term "rig" ("rigging") has to do with airplanes. Most of us do know it is used in reference to things nautical done aboard ships and boats. However, the term is equally applicable to aircraft and the realm of flight. According to my dictionary, when you rig anything, you "construct," "adjust" or "arrange" (it). As a homebuilder you are probably more familiar with your construction manual rigging words like "jigging" and "alignment." Technically speaking, as you assemble your airplane you are, at the same time, also rigging it. Nevertheless, even after you have completed the final assembly, other, smaller, rigging jobs will still have to be completed. PART 1 Rigging Is Not Just For Biplanes Mention rigging an aircraft to most any pilot and he will probably assume you are talking about biplanes and things like the assembly, installation, and adjustment of their landing and flying wires. Could be, but the term "rigging" has broader application than that. All aircraft, not just biplanes, must be rigged properly to fly right... that is, to fly both straight and level... hands off. Let's assume you have just completed the construction of your airplane and are making the final preparations to launch it soon on its first flight. Here is a brief summary of the rigging essentials you have probably had to cope with up to this point. 1. The Structural Checks including: a. The completion of the precise rigging and alignment of each of the aircraft's major fixed structural components (engine, wings, fuselage, landing gear, and tail surfaces) during that long challenging construction phase. b. Verifying the alignment of each of the major components with each other as you assembled them. 2. The Control System Checks including: a. The correct alignment of each movable control surface with the adjacent fixed component (wing, You kit builders may find, to your surprise, that the engine mount furnished is offset a few degrees to the right. This is not a manufacturing error but is intended to reduce the amount of rudder input required - especially at high power settings and low speeds. This drill is rigged to accurately drill the holes for the drag and anti-drag wires in a wood wing... just one of the many alignment challenges which may confront a builder. stabilizer, and vertical fin) to which it is attached. b. Setting and verifying the minimum travel (angular deflection) of each control surface. c. Visually proving to yourself that each of the cockpit controls, which move these surfaces, are properly installed and move the control surfaces in the direction they should move. d. The correct adjustment and safetying of the push-pull tubes, bell cranks, control cables and control stops. SPORT AVIATION 111

2 After applying careful attention to these rigging essentials you should now have a perfectly rigged airplane at your disposal. What Perfect Rigging Will Get You Although landing gear alignment may not affect the aircraft's in-flight characteristics, wheel misalignment can result in excessive tire wear and skittish runway behavior. Make a ground check of your turn and bank instrument for mounting alignment. Its inclinometer ball should be perfectly centered when the aircraft is leveled laterally. Make a ground check of your turn and bank instrument for mounting alignment. Its inclinometer ball should be perfectly centered when the aircraft is leveled laterally.by standing about 30' in front of an airplane, you can sight along the wing and easily detect a misaligned or warped installation. Apparently, this group attending the fly-in did a fine job of rigging and alignment as they all looked right. A perfectly rigged airplane is in its best aerodynamic configuration and will provide you with the most efficient flight characteristics possible... and it will require the minimum or no additional trim to fly straight and level, hands off, at normal cruise. I can visualize your gleeful reaction after your first couple of flights. "My airplane flew hands off right from the first flight," you say. "How about that? I must have it rigged just right!" Well, that may not be necessarily so, amigo. It is possible for an airplane to be somewhat "out of rig" and still fly hands off. Let's see how that can happen. Even though your wings might be level in flight, the airplane may not be flying (tracking) straight. That is, its nose may be drifting slowly off to the left, or to the right... and yet, there it is flying hands off. If this is so with your airplane, there is apparently a slight rigging problem, a trimming problem... or both. I don't mean to be picky, but I must point out it is not at all uncommon for a first test flight to be proudly declared a "hands off," trouble-free perfect event, when it is only close to it. It is an understandable assumption. However, it is not until after a few more test flights are made that the average builder/pilot is no longer clutching the control stick as hard, and has become a bit more relaxed. So relaxed, in fact, that the euphoria is wearing off and he is becoming more discerning... and, yes, picky. For example, now he may notice that the ball in the turn/bank indicator is skewed off a bit, or that the airplane tends to drift off, ever so slowly into a steepening turn when the control stick is turned loose, or maybe, that the airplane's behavior in some stalls is quite breathtaking because the bird wants to tuck a wing under. After a while, he reluctantly admits to himself that the controls are a bit stiff and, maybe, just maybe, the oil temperature is running kind of low (low?)... and so it goes. I'll admit, there are perfect, gripe- 112 JULY 1994

3 All aircraft, not just biplanes, must be rigged properly to fly right... that is to fly both straight and level - hands off. free, test flights but most any homebuilt will need a slight adjustment, here and there, to fine tune its performance. Minor rigging and trimming adjustments, therefore, are to be expected. When we refer to an aircraft as being "out-of-rig," we assume its components are not properly aligned due to inaccurate construction, or final assembly. The Not So Perfect Rigging Job The consequence of a serious rigging deficiency is that the total drag of the aircraft will be increased, and its performance will suffer because trim tabs have to be installed. Often, when trim tabs are necessary, they may have to be severely deflected to achieve that elusive hands off condition in flight. There is another concern. In some All Epoxies Are Not Created Equal! PTM&VV Industries introduces AKKOI'OXY. the first epoxy laminating resin lo combine modern non-toxic hardener leclmologx with superior high temperature cured strength. Easy to use. AKKOI'OXY is specially formulated for engine cowlings, wing spars, landing gear legs, and other very high performance applications. Quality Materials For High Performance Composite Parts Manufactured by ITMcUV Industries. Inc. Santa Fc Springs. CA Call SPORT AVIATION 113

4 Those Final Rigging Checks Do not try to adjust wing incidence and alignment until the aircraft is set in a level attitude. In this airplane (RV-3) the longerons in the cockpit area are used as a level reference. installations, the amount of control travel available may be insufficient to effect the correct response in flight. In other instances, the amount of control travel may be too great, and a sudden deflection of the control surface by the pilot could fail the structure due to the excessive loads (stresses) imposed. It is very important, for this reason, that your airplane be rigged in conformance with the designer's specifications for: angles of incidence, dihedral, wash-in or wash-out, engine thrust line alignment, horizontal tail incidence, the amount of vertical fin offset (if any), and the amount of control travel. Needless to say, your wheels should also be aligned, or you may experience excessive tire wear and, possibly, runway control problems during landing or takeoff. With flaps in an up position, confirm that both flaps are streamlined with the trailing edges of the ailerons. If for any reason you have the flaps rigged slightly up, or down, be sure both are symmetrically adjusted. Let's go back to square one and review some of the last minute rigging checks and adjustments that will help get top performance out of your airplane. Here's what you can do: 1. First, level the airplane laterally and longitudinally. In most homebuilts you can simply lay a long level across the top longerons to level the airplane laterally. Sometimes a level cockpit floor can be used. At any rate, utilize the official leveling point for your aircraft and level it as accurately as you can laterally. Do not attempt to make your final rigging checks until the airplane is in a level attitude. 2. Next perform the following cockpit rigging checks: a. Climb in the airplane. If the lateral level bubble is now slightly off with you in the pilot's seat, have someone let a little air out of the high side tire (remember to reinflate it after you have finished). b. Next, check your turn and bank instrument. Its inclinometer ball should be perfectly centered. Be sure you are looking at the instrument square-on (no parallax error, thank you). If the ball is skewed off to one side, however slightly, loosen the instrument mounting screws and twist the instrument a bit to get the ball perfectly centered. Re-tighten the mounting screws. c. Make the same check for your artificial horizon... if you are so equipped. Note: Most turn and bank instruments and artificial horizons have slightly elongated mounting screw holes which will allow you to twist the instrument slightly to help get the ball centered. If your instrument doesn't have the elongated holes you can elongate the holes in your instrument panel instead with a small round file to accomplish the same thing. However, you had better think twice before you start filing or you might elongate the holes in the wrong direction. No matter how careful you may have been, it is quite possible to have inadvertently mounted one or two instruments in your panel slightly tilted. Naturally, if that happened, it probably happened to the very two instruments that cannot tolerate any lateral deviation. The slightest misalignment of ei- 114 JULY 1994

5 ther the turn and bank or the artificial horizon can result in a pesky in-flight trim problem. d. Center the control stick and rudder pedals. Verify that the ailerons, rudder, and elevator are streamlined with the fixed surfaces. e. With the flaps in their up positions confirm that both flaps are streamlined with the trailing edges of the ailerons. If you have the flaps rigged so that their up-position is slightly above the trailing edges of the ailerons, be sure both flaps are so, and are symmetrically rigged. e. Lower the flaps, and visually check that both are going down at the same time and to the same degree. If you have a cockpit flap position indicator check it for accuracy. Readjust it if necessary. Do the same for any external flap position indicator. f. Check aileron operation. Move the control stick to the right. The right aileron should move up and the left one down. Move the stick to the left and the left aileron must go up and the right side down. There should be absolutely no During construction, a close check on alignment is essential. The builder here is using two airfoil templates to align his aileron with the wing. binding or rubbing noises throughout the control stick travel. Aileron stops must effectively limit the amount of control stick travel. g. Check elevator travel. Pull back on the stick and visually verify that the external surfaces actually move up. Feel out the elevator stops. UALITY AIRCRAFT FINISHES SPORT AVIATION 115

6 You should get full stick and control surface travel in both directions. h. Make similar rudder checks. Left rudder pressure must move the rudder to the left. And, as with the other control surfaces, check out the rudder stops. i. Be sure your cockpit adjustable trim control(s) works in the direction you expect it to work, and that you can obtain the desired amount of trim tab travel. There must be absolutely no binding or rubbing noises throughout the limits of the control stick travel and the same for the rudder pedals. Finally, have somebody hold each of the control surfaces while you wiggle and work the control stick and rudder. There should be little or no play in the controls. Excessive play in any of the control surfaces or trim tabs can result in control surface flutter in flight... a frightening experience nobody needs to experience. More next month. Additional reading - "Biplane Assembly and Rigging Procedures," Sport Aviation, February 1988, page 27. ^ r *&, ^^^IH B^^^^^^^^^^^^^ ^^K H^B^v "jl Lt -A^S- ^^8 fc. iflh^e:.» ^M*^ ^Hnf rim ^9 Hi JI ^^^^^K j^^^^k SH^ * 4^^^^^M Li - Jfi^B^H BOOKS BY TONY -Sportplane Builders (Aircraft Construction Methods, ^90 nanpc'i <K1Q <K JirU pdgcaj - ijl7.7j -Firewall Forward (Engine Installation Methods, 304 pages) -$ Sportplane Construction Techniques (A Builder's Handbook, 350 pages) - $ 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. The following books by Tony Bingelis are available If you wish to contact the from the EAA Aviation author of this column for ad- Foundation, EAA Aviation ditional information, please Center, Box 3086, Oshkosh, send a SASE to: Wl , 1-800/843- Tony Bingelis Major credit cards 8509 Greenflint Ln. accepted. Austin, TX MEMUt ttt-ut'tom-hhi'niu NEVER 4 SURCHARGE WORLDWIDE SHIPPING 116 JULY 1994 THE KIT BUILDER'S "PANEL PACKAGE" VFR / P A R T I A L IFR / FULL IFR Call the Experts In Kit Plane Panel Build-Up. m aam 10 Gulf Coast Avionics offers complete panel build-up and prewiring for kit plane builders. With our experience, knowledge, and technical expertise, we can work with you to deliver a built-up panel to your specifications, ready to install. And we can pre-wire for additional equipment that you might want to add later. Call us today for complete details on the "Panel Package". CALL FAX O N. Westshore Blvd Tampo International Airport " OUR F S t^j? Tampa, Florida USA 83o l FAX - 5:3 p r ^T* Se Hoblo Espoml E.S.T. CORROSION & AIRCRAFT DON'TMIX! Did you know that many over-thecounter cleaning products actually promote corrosion on aluminum surfaces? There's no need to worry anymore. Give your aircraft what it deserves: ZI-400 AIRCRAFT CLEANING COMPOUND! ZI-4 00 has been proven most recently by the Royal Australian Air Force which deemed it the best of the best cleaners for their P-3 Orion ocean patrol aircraft, and chose it for both its cleaning power and its anti-corrosion action. On aircraft ranging from fighters to corporate jets, ZI-400 concentrate continues to be proven as a safe and cost-effective way toclean all common aircraft surf aces ~ exteriors and interiors machine shop tools, bare metals, painted surfaces, ceramics, plastics, composites, glass, vinyl and various fabrics. Try a gallon! Send check or money order for $25.00 including UPS (CA residents add $1.65 sales tax), or call us for more information: Zelman International Limited th St.. Suite 303-SA07 Santa Monica. CA Tel.: (310) 451-SAFE (7233) Fax: [310)

7 SPORTPLANE BUILDER Tony Bingelis apply and hold the necessary pressure to whatever control requires it to maintain coordinated flight. Of course, if the airplane is equipped with a turn and bank indicator, a directional gyro, and an artificial horizon, 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 importance 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 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 anytime you want, in order to relieve whatever manually applied control pressures you are holding. This capability is especially appreciated during 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. 98 AUGUST 1994

8 FUSELAGE CENTERLINE VERTICAL FIN OFFSET TO THE LEFT Consider The EGT/CHT Advantages: SLIPSTREAM How To Trim For Level Flight RUDDER DEFLECTED TO THE RIGHT SLIPSTREAM FIXED METAL TRIM TAB WEDGE ACTS LIKE A FIXED TRIM TAB Here is the correct way to trim for level flight when you have a cockpit controllable three-axis trim capability: 1. Accelerate to your intended level flight cruising speed. 2. Adjust the elevator trim to maintain altitude. 3. Take your feet off the rudder pedals, but use whatever aileron pressure is necessary to keep the wings level. Look at the ball in your T&B. Is the ball centered in its race? If not, apply rudder pressure to center it. Then adjust the rudder trim to relieve your foot pressure on the FIGURE 1 RUDDER TRIM OPTIONS (NON-CONTROLLABLE) pedal. Now, when you put your feet on the floor, the T&B ball should remain centered. 4. Finally, concentrate on the wings. Is one wing heavy and you have been holding aileron control pressure to keep that wing up? If so, adjust the aileron trim to relieve the stick pressure you are holding. That's it. Simple as one, two, three... simple, that is, if you have cockpit controllable trim tabs. One final note. If you have trim position indicators in the cockpit, check to see that the neutral indications are actually neutral when the external trim tabs are streamlined with the control surfaces. Lean Find Mode automatically finds the first EGT to peak, avoiding cylinder damage due to over leaning. Identifies unhealthy cylinders using greatest span method, Shock Cooling: First time ever available as an actual value in degrees/minute. Identifies cylinder that may fail due to shock cooling. One degree precision resolution with selectable C or F. Detects voltage loss due to alternator failure which is a 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 (714) Fax:(714) Box 7033, Huntinglon Beach CA 92615

9 Ground Adjustable Tabs 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. But, what about the homebuilt that has no cockpit adjustable trim 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 adjusted, 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. 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 100 AUGUST 1994

10 TRIM TAB ON HEAVY WING AILERON IS FORCED UP TRIM TAB DOWN (LOOKS BETTER) AILERON IS FORCED DOWN AILERON IS FORCED UP TRIM TAB UP wind PILOT AVIONICS CFI EDITION PA PA-9 Electret MIC Telescoping Flex Boom Air-Gel Earseals Brass Molded Plugs Delent Volume Control 5-Year Warranty Super-Cushion Headpad Weight TRIM TAB ON LIGHT WING FIGURE 2 AILERON TRIM ACTION TRIM TAB DOWN ZEPHYR PHC-31 PADDED HEADSET CASE 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 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 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 1850 Union St. #1633, San Francisco k California SPORT AVIATION 101

11 STABILIZER STABILIZER ELEVATOR SLIPSTREAM FORCES ELEVATOR DOWN (CAUSING AIRCRAFT TO NOSE DOWN) ELEVATOR 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. Anyhow, 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. severe that both ailerons may require sharply deflected trim tabs (in opposite directions). Here is the bad news about trim tabs... any kind of trim tab. In short, the more you must deflect a trim tab to overcome an unbalanced flight condition, the more drag you create... and since drag is a parasite that greeds on performance your airspeed suffers because of it. That's why a good job of rigging and aligning your wings is so important. Adjusting Fixed Trim Tabs These tabs, naturally, can only be re-adjusted on the ground, and that introduces a dilemma for some folks. The direction in which to bend the tab can be confusing if they don't stop and think about it a bit. All you have to remember is that a trim tab forces the control surface in the opposite direction from which the tab is bent. FORCES ELEVATOR UP (CAUSING AIRCRAFT TO NOSE UP) FIGURE 3 AILERON TRIM ACTION For Example: 1. If the nose of the aircraft is yawing to the left, bend the rudder trim tab to the LEFT. 2. If the aircraft has a nose heavy condition, bend a fixed elevator trim tab DOWN. 3. If one wing is heavy, bend the trim tab on that wing UP. You can make your fixed trim tabs of 2024 T3 aluminum. A tab made of.025" or.032" aluminum should be stiff enough for the job. That raises another question. How big should a trim tab be? Well, I know of no particular standard size, shape, or placement for trim tabs. Actually, the tab may not have to be as large as you think because that little gadget, when deflected, exerts a powerful corrective force against the control surface. If your unbalance condition isn't severe (how severe is severe... a snap roll when you let go of the stick?), you might try a trim tab that 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. This, in effect, causes the "propwash" (slipstream) to hit the vertical fin and rudder on one side at somewhat of an angle thereby aggravating the effects of torque. Therefore, by rigging the fin offcenter a bit, the tail surface will more nearly be parallel to the airflow. Thus, the side loads on the tail surfaces are reduced as would be the need for a substantially deflected trim tab. Of course, this is a rather simplified cause and effect explanation but it is adequate for now. The fin offset, when used, is to the LEFT for conventional aircraft engines having a clockwise propeller rotation (as viewed from behind the propeller). How much should this offset be? 5/8"? More? Less? Guess wrong and once incorporated, realigning the fin to a neutral setting would be difficult as the fin-to-stabilizer fairing would also have to be reworked. 102 AUGUST 1994

12 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 amount of free play at the trailing edge of the trim should be minimal. FAA's AC 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 maximum 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

13 '\ Books ft Construction A Videos Materials ^ Builders' k Workshops 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. 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. Useful Information SMART Level information source: T&L Investment Protection, Inc., Hangar 6, Wiley Post Airport, Bethany, OK FAA's Acceptable Methods, AC A (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 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 Trenton, New Jersey September Bloomington, Illinois September Griffin, Georgia 8OO-83f-2949 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 FAX Line LEXANDER EROPLANE COMPANY, INC. Box 909, GRIFFIN, GEORGIA BOOKS BY TONY The following books by Tony Bingelis are available from the EAA Aviation Foundation, EAA Aviation Center, Box 3086, Oshkosh, WI , 1-800/ Major credit cards accepted. -Sportplane Builders (Aircraft Construction Methods, 320 pages) - $ Firewall Forward (Engine Installation Methods, 304 pages) - $ Sportplane Construction Techniques (A Builder's Handbook, 350 pages) - $ 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. 104 AUGUST 1994