CRAFTSMAN'S CORNER
Ben Owen
AIRCRAFT RIGGING AIDS The Curtis Pitts Wing
Incidence Tool This "wing incidence tool" is designed for fabric covered biplanes. The 5-1/4" dimension represented rests against the leading edge; the other vertical dimensions rests right over the spars. It is suggested that a wing incidence tool of this type could be fabricated for practically every aircraft. I do not recommend that you use an airfoil outline as a rigging aid. Too often there are humps and bumps in airfoils and you are better off having the wing incidence tool rest on parts that you know are fairly straight, such as spars or spar projections, on any kind of wing. If you have a taper wing or a wing that has a non-uniform airfoil section, you could make a series of such tools very easily and quickly so that you can compare the incidence on both wings at different spanwise locations, one side with the other. On composite wings and other wings that are braced by wing skin, it is suggested that you check before covering as you certainly can't adjust them after assembly. This would include metal covered wings, composite wings and wooden wings covered with plywood sheets such as on Tailwinds, etc. Fabric covered wings and wire braced biplanes have the advantage in that they can be adjusted after cover. There is no reason why other wings could not have their ribs and spars aligned properly prior to closure. If you have done all you can to make things straight and you are still getting wing drop, in spite of adjustments on the wings and rudder and if you had tried trim tabs on the ailerons, etc., the last resort would be to rebuild the fittings on the offending side, redrilling the holes as needed. In addition to flying better, properly adjusted wing and tail surfaces
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Author's Note - I have to compliment both Dr. Roger P. Gordon, EAA
Chapter 73 of New Jersey, and Alan Perrone, Mobile, AL for finding an er-
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With this tool the incidence along the wing (or wash-in and wash-out along the wing) can be checked to eliminate any twist. The dimensions shown here are for either Pitts or Acro Sport I. The tool is useful for any aircraft but dimensions may need to be changed. The better the level used - the more accurate the tool.
108 OCTOBER 1993
Wing Incidence Aid
your friends' airplanes. It makes a very handy tool as four of them fit Johnny Livingston Wing conveniently in a pants pocket. AdIncidence Aid mittedly, they work only on low/mid I lived in Florida quite a few years wings but if you raise the tail, upper ago and chanced to make the ac- wings can be checked also. It might quaintance of a very fine gentleman, help if you are to buy an airplane and Johnny Livingston, the famous rac- the owner declares that it flies ing pilot. Johnny was a perfectionist straight but won't let you test it. and a practical engineer, par excel- Check it out with this Livingston tool. Johnny's suction cups were fitted lence, in addition to being a highly-skilled aviator. If you loaned with screws so that the dowels could him your Pitts for a week it would be adjusted up and down, making come back, usually, flying better them all perfectly level. I bought my than you ever imagined. By simply suction cups at Wal-Mart in the picrigging the wing and tail surfaces ture hanging department. I took off properly, airplanes would climb bet- the picture hooks and glued the suction cups to the dowels with intant ter, fly faster and even stall slower. One of the aids he used was a set glue. You then sand the tops slightly of four suction cups, about one inch to bring them all into exact level. in diameter, screwed to 1/4" round Quite frankly, if you can find suction dowels. When pressed down on a cups with screws on them this will be surface these round dowels would be easier - just drill the dowels for the about 1-1/2" above the surface. By screws. If you get the height of the placing them on the trailing edges dowels just right, when you stand near the wing root and on the trail- back about 20' from the nose with the ing edge of the wing tip on the lower suction cups stuck near the trailing wings of a biplane, he could stand edge, all dowel tops should just appear back and tell if there was wash in or above the thickest part of the wing wash out in the wings. Then, by hopefully, all the same, with the tips adding a washer here and a washer just showing. If one looks higher than there, or tightening a wire here or the other you may have wing twist. Johnny cared for airplanes. He there, he would improve the flying brought them back safely. On his characteristics of the airplane. You can use his "Livingston" inci- last day he test flew a Pitts Special, dence meter to check out yours and landed and stopped with the prop just ticking over and parked on the runway side. His friends went out to WING INCIDENCE TOOL the airplane and shut off the engine. 1/2"x3/4", Johnny died shortly thereafter.
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51/4"
will not have to have the occasional trim tab at the trailing edge or a "spoiler" on the control surface or small triangle sections glued to ailerons on one side. There are many other devices to cure wing drop. When it comes time to judge your aircraft for competition the absence of such devices will go a long way toward putting you in the winner's circle. This tool is just one of many contributions the great Curtis Pitts has made to aviation.
ror in the auto-fuel article in this column. The formula for grain alcohol or ethanol is CH3OH. The formula I used is in Webster, 1984, defining ethanol but it is quite obviously wrong.
