TOWARD BETTER PERFORMANCE By Al Backstrom (EAA 1162) Rt. 1 Frisco, Texas 75034 Article IV EPILOGUE

-L HE PREVIOUS ARTICLES in this series have presented data and suggestions on aerodynamic improvement of airplanes. However, no general discussion can cover all the information that would be useable in aerodynamic cleanup and each individual design will have different areas of primary importance. As an exercise let's look at what can be done for aerodynamic improvement of a couple of good basic airplanes. No structural changes will be considered, so that the changes could be made to existing airplanes without taking them out of flying status for long periods. The Evans VP-1 Volksplane and the Jensen VJ-22 Sportsman would be good airplanes for review. I have not attempted to calculate performance changes because a reliable estimate of original airplane drag is

FULL COWL FOR VW

FIGURE 1

16 FEBRUARY 1975

extremely difficult. Also, if you were checking the changes on an actual airplane the results could be affected by the order in which the changes are made. One point that should be made is to not look at top speed as the only measure of performance gain. The basic reason for this is that a 10% reduction in drag will produce approximately a 3% increase in top speed. On the other side of the coin, a 3% increase in top speed without a drag reduction will require a 10% increase in power. At least you can't lose them all because a 10% reduction in drag will give a 10% increase in range at the same speed. Also a reduction in drag will leave more power available for cruise-climb conditions at the same speed. Now let's look at the VP-1. The greatest drag reduction can be made by adding a canopy and a good engine cowling. The canopy design selected is essentially the NASA X-l (Report WR-L-642 and Turner T-40). This canopy has very low drag and can be built without having to use compound formed plexiglass. Cowling a VW installation like the Volksplane is a real problem. This is one of those areas where the inability to do an excellent job is not an excuse. The best way out of this problem is to start with a large spinner. In this case a 12" spinner is assumed and lines sketched out from this. The exhaust system uses a 180° bend on the front stacks outside the cowling and carburetor air heat taken from muffs on the rear stacks. The cooling air exit is along the side of the fuselage.

SPINNER-LIKE COWLING

STRUT FAIRING

LANDING GEAR FUSELAGE FAIRING FOR

UP POSITION

FIGURE 2

The wing root and strut fairings are based on NASA TR 482 and the information in Article 3 (Sport Aviation, January 1974) of this series. The elevator and wheel fairings complete the major rework; from here on you could happily proceed with fairing minor areas. The resulting

appearance is shown in Figure 1. Well, on to the VJ-22. The primary items here are fairing of the landing gear in the up position, smooth canopy, fillets, and strut fairings. The engine cowling used on most Sportsmen could be better. The gear fairings recommended fair behind the wheel and cover the strut in a manner similar to the fairings shown in Sport Aviation December 1974. A piano hinged lower section allows for gear retraction. A free blown canopy might be used, or one built using a compound formed Fiber-glass top with flat wrap sides. The best choice would depend on the method of hinging desired. A fillet around the wing leading edge and underside at the hull should be used. Fairing of the wingstrut intersection and tip floats completes the major fairing rework. An improved engine cowling could be based on a spinner forward and an approximately 14" spinner aft. The middle area bulk will depend on the engine carburetor air heat and muffling requirements. A shape like the Osprey-Air Skimmer is good if you don't need carb heat and can live with the noise of racing type stacks.

Heating Plexiglas From Robert E. Richards (EAA 7221) 302 W. 4th Ave. Barrington, NJ. 08007

Do not use a household oven for heating Plexiglas to bend it. Heating this plastic in an oven produces monomer bleed-off, and these gases are highly combustible. If the gases are ignited they can cause a severe explosion. Kitchen ovens and other heating devices that do not circulate air to prevent accumulation of gases, and which are not equipped with proper temperature controls and safety devices for this kind of work, should not be used.

For safety, use only a strip heater, plans for which are available. Assembled heaters also can be purchased where Plexiglas" is sold. For a series of brochures on using this plastic send 50c to Rohm and Haas Co., Dept. WN, P.O. Box 9730, Philadelphia, Pa. 19140.

The resulting appearance to these changes is shown in

Figure 2. I hope that these suggestions will encourage you to make your own airplane look and perform better.

SPORT AVIATION 17