SPORTPLANE BUILDER Tony Bingelis

PART 1 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 as- semble your airplane you are, at the same time, also rigging it. Neverthe- less, even after you have completed the final assembly, other, smaller, rigging jobs will still have to be completed. Rigging Is Not Just For Biplanes

Mention rigging an aircraft to You kit builders may find, to your surprise, that the engine mount furnished is offset most any pilot and he will probably a few degrees to the right. This is not a manufacturing error but is intended to re- assume you are talking about bi- duce the amount of rudder input required - especially at high power settings and planes and things like the assembly, low speeds. 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 con- This drill is rigged to accurately drill the holes for the drag and anti-drag wires in a struction phase. wood wing ... just one of the many alignment challenges which may confront a builder. b. Verifying the alignment of each of the major components with each stabilizer, and vertical fin) to which move these surfaces, are properly in- other as you assembled them. it is attached. stalled and move the control surfaces 2. The Control System Checks in- b. Setting and verifying the mini- in the direction they should move. cluding: mum travel (angular deflection) of d. The correct adjustment and a. The correct alignment of each each control surface. safetying of the push-pull tubes, bell movable control surface with the ad- c. Visually proving to yourself that cranks, control cables and control jacent fixed component (wing, each of the cockpit controls, which stops.

SPORT AVIATION 111 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 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 reac- tion 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!" Although landing gear alignment may not affect the aircraft's in-flight characteristics, Well, that may not be necessarily wheel misalignment can result in excessive tire wear and skittish runway behavior. 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 uncom- mon for a first test flight to be proudly declared a "hands off," trou- ble-free perfect event, when it is only close to it. It is an understand- able assumption. However, it is not until after a few more test flights are made that Make a ground check of your turn and bank instrument for mounting alignment. Its the average builder/pilot is no longer inclinometer ball should be perfectly centered when the aircraft is leveled laterally. clutching the control stick as hard, and has become a bit more relaxed. So relaxed, in fact, that the eu- phoria 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 ad- Make a ground check of your turn and bank instrument for mounting alignment. Its mits to himself that the controls are inclinometer ball should be perfectly centered when the aircraft is leveled later- a bit stiff and, maybe, just maybe, ally.By standing about 30' in front of an airplane, you can sight along the wing and the oil temperature is running kind easily detect a misaligned or warped installation. Apparently, this group attending of low (low?). . . and so it goes. the fly-in did a fine job of rigging and alignment as they all looked right. I'll admit, there are perfect, gripe-

112 JULY 1994 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 home- components are not properly aligned of the aircraft will be increased, and built will need a slight adjustment, due to inaccurate construction, or fi- its performance will suffer because here and there, to fine tune its per- nal assembly. trim tabs have to be installed. Of- formance. Minor rigging and ten, when trim tabs are necessary, trimming adjustments, therefore, The Not So Perfect Rigging Job they may have to be severely de- are to be expected. flected to achieve that elusive hands When we refer to an aircraft as The consequence of a serious rig- off condition in flight. being "out-of-rig," we assume its ging deficiency is that the total drag There is another concern. In some All Epoxies Are Not Created Equal!

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SPORT AVIATION 113 Those Final Rigging Checks 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. Some- times 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: Do not try to adjust wing incidence and alignment until the aircraft is set in a level a. Climb in the airplane. If the attitude. In this airplane (RV-3) the longerons in the cockpit area are used as a level lateral level bubble is now slightly reference. off with you in the pilot's seat, have someone let a little air out of the high side tire (remember to reinflate installations, the amount of control conformance with the designer's it after you have finished). travel available may be insufficient specifications for: angles of inci- b. Next, check your turn and to effect the correct response in dence, dihedral, wash-in or bank instrument. Its inclinometer flight. wash-out, engine thrust line align- ball should be perfectly centered. Be In other instances, the amount of ment, horizontal tail incidence, the sure you are looking at the instru- control travel may be too great, and amount of vertical fin offset (if any), ment square-on (no parallax error, a sudden deflection of the control and the amount of control travel. thank you). surface by the pilot could fail the Needless to say, your wheels If the ball is skewed off to one structure due to the excessive loads should also be aligned, or you may side, however slightly, loosen the in- (stresses) imposed. experience excessive tire wear and, strument mounting screws and twist It is very important, for this rea- possibly, runway control problems the instrument a bit to get the ball son, that your airplane be rigged in during landing or takeoff. 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 ac- complish 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 With flaps in an up position, confirm that both flaps are streamlined with the trailing the very two instruments that can- edges of the ailerons. If for any reason you have the flaps rigged slightly up, or not tolerate any lateral deviation. down, be sure both are symmetrically adjusted. The slightest misalignment of ei-

114 JULY 1994 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 posi- tions 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. During construction, a close check on alignment is essential. The builder here is Move the control stick to the using two airfoil templates to align his aileron with the wing. right. The right aileron should move up and the left one down. Move the stick to the left and the binding or rubbing noises through- g. Check elevator travel. Pull left aileron must go up and the right out the control stick travel. Aileron back on the stick and visually verify side down. stops must effectively limit the that the external surfaces actually There should be absolutely no amount of control stick travel. move up. Feel out the elevator stops.

AIRCRAFT UALITY FINISHES

SPORT AVIATION 115 You should get full stick and control surface travel in both directions. ^______B^^^^^^^^^^^^^ h. Make similar rudder checks. •r *&, ^^^IH -Sportplane Builders (Air- Left rudder pressure must move Lt -A^S- ^^8 craft Construction Methods, the rudder to the left. And, as with •fc. iflH^E: . » ^90 nanpc'i

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116 JULY 1994 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? If your airplane did, indeed, fly straight and level "hands-off" during the initial flight, you are to be con- Here is a rudder trim tab type popular with some builders. It is a triangular wedge gratulated for the fine assembly and fastened to the rudder. In trying to establish its ideal size and shape the wedge can rigging job you did. be taped on temporarily for testing. 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. The vertical stabilizer (fin) on this Mustang II is offset to the left to compensate for Naturally, if you have a three-axis 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. trim capability, you should be able to re-trim the airplane anytime you A less than discriminating pilot apply and hold the necessary pressure want, in order to relieve whatever may not know or notice that his air- to whatever control requires it to manually applied control pressures plane is out of trim . . . however maintain coordinated flight. you are holding. severe that condition might be. Of course, if the airplane is This capability is especially appre- Most of us, fortunately, immedi- equipped with a turn and bank indi- ciated during level flight cruise ately sense when some flight cator, a directional gyro, and an conditions. condition isn't quite right . . . even artificial horizon, even the most Without cockpit controllable trim when all we may have aboard in the oblivious among us should notice tabs it is a different story. At best, way of a sophisticated flight system is when an airplane is not tracking like the airplane, if it does fly hands off, a whiskey compass, needle/ball and it should . . . that it is out of trim. will do so only for one specific power airspeed. Last month, in Part 1, considerable setting, airspeed, altitude, and, yes, For the most part, we instinctively attention was devoted to the impor- cockpit loading.

98 AUGUST 1994 FUSELAGE CENTERLINE

VERTICAL FIN OFFSET TO THE LEFT

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• Lean Find Mode automatically finds the first EGT to peak, avoiding cylinder damage due to over leaning. • Identifies unhealthy cylinders RUDDER DEFLECTED TO using greatest span method, THE RIGHT • Shock Cooling: First time ever available as an actual value in SLIPSTREAM SLIPSTREAM degrees/minute. • Identifies cylinder that may fail due FIXED METAL to shock cooling. TRIM TAB WEDGE ACTS LIKE A FIXED • One degree precision resolution TRIM TAB with selectable C or F. • Detects voltage loss due to alterna- tor failure which is a FIGURE 1 must for IFR. RUDDER TRIM OPTIONS • Supplied with JPI's (NON-CONTROLLABLE) well recognized grounded fast How To Trim For Level Flight pedal. Now, when you put your feet on the floor, the T&B ball response probes. Here is the correct way to trim should remain centered. for level flight when you have a 4. Finally, concentrate on the • Normalize Mode for instantly deter- cockpit controllable three-axis trim wings. Is one wing heavy and you ring engine trends while in flight. Ten capability: have been holding aileron control 1. Accelerate to your intended pressure to keep that wing up? If degrees per bar. level flight cruising speed. so, adjust the aileron trim to re- • TSO quality, FAA STC and TSO. 2. Adjust the elevator trim to lieve the stick pressure you are maintain altitude. holding. Approved by all insurance companies. 3. Take your feet off the rudder That's it. Simple as one, two, • Only analog EGT/CHT made in pedals, but use whatever aileron three . . . simple, that is, if you have USA. 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 indica- JP INSTRUMENTS apply rudder pressure to center it. tions are actually neutral when the Then adjust the rudder trim to re- external trim tabs are streamlined 1-800-345-4574 lieve your foot pressure on the with the control surfaces. (714) 557-5434 Fax:(714) 557-9840 Box 7033, Huntinglon Beach CA 92615 Ground Adjustable Tabs 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 ad- 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 justable trim tabs can relieve the pilot sumo wrestler as a rear seat passenger. 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 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 This airplane had just taxied in. Looking at his elevator trim setting you can tell that may also have had a trim tab on one the pilot had the aircraft trimmed slightly nose up for landing ... a normal practice aileron . . . and, in rare instances, one for most low wing aircraft. on each aileron. I doubt if you have ever seen a fixed trim tab on an elevator. As previously stated, in my opin- ion 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 re- sembles 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 des- perate attempt to compensate for a An electric (Aero-Trim) cockpit controllable rudder trim was installed by this builder. rigging problem ... a misaligned or I think the elevator trim tab would look better had he installed the piano hinge on warped wing, perhaps? the bottom. Some wing heavy conditions are so

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SPORT AVIATION 101 measures about 3" x 6". About 1" of its width will be used in attaching the SLIPSTREAM tab, leaving a 2" width to do the work. STABILIZER ELEVATOR 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 se- FORCES ELEVATOR DOWN curing the trim tab to loosen. (CAUSING AIRCRAFT TO NOSE DOWN) Use a pair of pliers, or a duck bill hand seamer to assist in making the bend. Your trim tab adjustments (bends) STABILIZER ELEVATOR should be made in small increments followed by a check flight to deter- mine 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. FORCES ELEVATOR UP The Offset Vertical Fin (CAUSING AIRCRAFT TO NOSE UP) FIGURE 3 The slipstream from the propeller, in conventional aircraft, does not AILERON TRIM ACTION 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 "prop- sharply deflected trim tabs (in oppo- 1. If the nose of the aircraft is wash" (slipstream) to hit the vertical site directions). yawing to the left, bend the rudder fin and rudder on one side at some- Here is the bad news about trim trim tab to the LEFT. what of an angle thereby aggravating tabs . . . any kind of trim tab. In 2. If the aircraft has a nose heavy the effects of torque. short, the more you must deflect a condition, bend a fixed elevator trim Therefore, by rigging the fin off- trim tab to overcome an unbalanced tab DOWN. center a bit, the tail surface will more flight condition, the more drag you 3. If one wing is heavy, bend the nearly be parallel to the airflow. create . . . and since drag is a parasite trim tab on that wing UP. Thus, the side loads on the tail sur- that greeds on performance your air- You can make your fixed trim tabs faces are reduced as would be the speed 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. ing your wings is so important. stiff enough for the job. Of course, this is a rather simpli- That raises another question. How fied cause and effect explanation but Adjusting Fixed Trim Tabs big should a trim tab be? it is adequate for now. Well, I know of no particular stan- The fin offset, when used, is to the These tabs, naturally, can only be dard size, shape, or placement for LEFT for conventional aircraft en- re-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 little gadget, when propeller). tab can be confusing if they don't deflected, exerts a powerful correc- How much should this offset be? stop and think about it a bit. tive force against the control surface. 5/8"? More? Less? Guess wrong 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 diffi- the opposite direction from which snap roll when you let go of the cult as the fin-to-stabilizer fairing the tab is bent. stick?), you might try a trim tab that would also have to be reworked.

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 cor- rective 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. 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 Other Considerations the flaps are lowered. The tab on the left is pilot controllable. In general, the larger the trim tab control surface flutter. more than 2-1/2% of the chord. A the smaller its deflection will have to be The amount of free play at the tab with a 4" chord would have a to obtain the desired trimming effect. trailing edge of the trim should be maximum amount of play of 4" x Cockpit controllable tabs that are minimal. FAA's AC 43.13-1 gives a .025 (0.100 inches total motion up inset in the elevator surface are struc- simple formula for establishing the and down). turally better and are as effective as maximum permissible free play at the For all practical purposes you should trim tabs attached to the trailing edge trailing edge of the tab. They recom- strive to keep the total amount of free of the elevator. The Emeraude is a mend limiting this movement to not play in your trim tabs under 1/8". typical example. Controllable tabs should be as light as possible to avoid adding too much weight behind a control surface's hinge axis. Both are ways to spend summer, 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 be- low 75% of the movable control surface angular deflection to prevent one is clearly better. an overbalance effect. With part-span trim tabs, an out- board location is a slightly better position, aerodynamically. However, an inboard location for the trim tab is considered to be a better choice struc- turally. 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

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104 AUGUST 1994