Aerodynamic RefinementOf Light Aircraft

By Raymon H. Parker (Photos by Peter M. Bowers,The "Tiny Mite" sailplane, designed and built by RaymonParker, was thoroughly tested out by the AerophysicsDepartment of Mississippi State University. The "TinyMite" is shown in its original configuration with the darkpaint trim, and in its later modified form which includedmany improvements brought about by the tests.

INTRODUCTIONThe material in this article was presented at a Los

Angeles, Calif. Chapter 11 meeting. The subject is ap-plicable to every airplane owned by EAA members.Whether the aircraft is a biplane or a fast cross-countrytype, these results of aerodynamic refinement are de-sirable: shorter take-off run, greater rate of climb, highercruise and top speed, increased gliding capabilities, andlower stall speed.

A noted designer, builder and pilot of sailplanes.Raymon H. Parker, contributes his knowledge of the sub-ject. Mr. Parker, a World War // graduate of the CentralInstructors School at Randolph Field, was Chief Pilot atthe Twenty-Nine Palms Air Academy for Glider Pilots,and technician and pilot at Mississippi State with Dr. Ras-pet. He designed and built the "Tiny Mite" sailplane,placed second in the 1954 National Soaring Meet, andwas a member of the 1958 U.S. Soaring team. RaymonParker designed and built the "T" Bird, and placed eighthin the 1962 National Soaring Meet.

I N MY OPENING statement I would like to point outthat the homebuilder has opportunities to obtain aero-

dynamic refinements and aesthetic values not enjoyedby the manufacturers. We are not too greatly concernedwith man-hour cost and short-cuts to make productioneasy. A little extra effort on our part, and we can pro-duce a better than average aircraft.

Those of us who have designed and built our ownaircraft have done so for varying reasons. Some of thesereasons are based on the extent of interest by the indi-vidual in aerodynamics or the desire for an artistic out-let, or to fly in competitive sports. In the sailplanes thatI have built, the purpose has been a combination of thesethings. Most of the rules governing the design of a ClassI sailplane and the techniques for its construction can beapplied to a powered aircraft as well. In the competitiveclass sailplane, it is important that any and all methodsto improve the aircraft must not be overlooked if thepilot desires to be a potential winner. To fly furtherthan your competitor in a sailplane, your task will bemuch easier if you have built in the best aerodynamicprinciples that are readily available. The sailplane de-signer has only one approach to maximum performance,and that is to keep drag to a minimum by taking fulladvantage of all drag reduction techniques.

In the field of drag reduction, I have had the oppor-tunity to work on several projects where a sailplane wasused as a test bed. The sailplane was modified to suit24 JANUARY 1964

the requirements of the test, instrumented and thenflown in the early morning hours to obtain as stable airas possible. The aircraft was usually towed to an altitudeof 14,000 ft. and flown at the prescribed air speeds, plusor minus y2 mph. With this technique, the atmospheremakes a reasonably good wind tunnel. This method wasapplied by Dr. Raspet to many powered aircraft. Afterremoving their propellers, they were tested in very muchthe same manner as a sailplane and for the same pur-pose. The first test would be to establish the existingcondition of the aircraft and to decide its potential, thenmodifications were made and the aircraft retested. Everysuccessful refinement to the aircraft meant that it couldbe flown a greater distance for a given amount of fueland/or fly at a greater speed.

In aircraft design, the area where the most oftenrepeated mistake is made is in the wing root and fuse-lage junction. Even though extensive improvement pro-grams have been made on sailplanes such as the "TinyMite", and papers published giving the results of correct-ing poor design in the aforementioned area, productionsailplanes in some areas of Europe are still ignoring thisinformation. The poorly designed intersection is mosteasily avoided by building a high-wing aircraft, butsome of us prefer shoulder-wings and possibly low-wings. It is in these latter configurations that the prob-

(Leo J. Kohn Photo)The simple form of drag reduction through the use oftape to seal the gaps, as referred to by the author, wasalso used to great advantage in the National Air Races.The light lines shown on the fuselage and tail of thisNorth American P-51C-10-NT are cellulose tape seals overvarious joints and gaps. This racer, NX-1204, was flownby Thomas Mayson to 6th place in the 1947 BendixTrophy Race.

lem of correct air flow is most acute. For illustration, aSchweizer TG-3 war surplus sailplane was used. The air-craft description briefly was a low-wing with no fillets.

The fuselage sides were parallel to each other to a pointadjacent to the trailing edge. On the right side only, a"quick and dirty" fillet was formed using cardboard andmasking tape. The fillet extended higher on the fuselagethan it did horizontally, the purpose being to preventthe air from decelerating at a greater rate than that inthe immediate wing area. In all cases, this rate shouldto kept the same. Flown in this condition, the stallingspeed was reduced by 3 mph, and it stalled away fromthe filleted wing. Wool tufts were also applied so obser-vations could be made of the flow at various speeds.The method of studying the wool tufts will be discussedlater in this article. The above mentioned "Tiny Mite",at the time of its original flights, was extremely poordue to incorrect flow path over the root. This particularaircraft is a good example of an old error in design, theone often repeated at the present time: the maximumwidth of the fuselage was forward of the wing at thepilot's compartment.

At the leading edge the fuselage was already taper-ing rapidly in all planes towards the rear of the sailplane.The aircraft was a shoulder-wing with an oval fuselagecross-section. The original glide angle was 19. The majormodification was to remove that section of fuselageabove the wing, basically making the ship into a high-wing. The only protuberance was the bubble type canopy.This was well forward of the wing. With this modification,the glide angle went up to 23. After several additional mod-ifications, maximum efficiency was reached with the re-sultant glide angle of 31:1. To design a perfect wing rootjunction, the formula for area rule would have to be ap-plied. It is doubtful if many individuals would go to thismuch trouble but very good results can be obtained by, ifyou will, the educated eyeball. (Note: Reference drawing).

It is unfortunate that an improperly designed wingroot junction makes a contribution towards early stallwarning (the disturbed air flow over the empennage).Because of this stall warning, many designers have feltit unwise to work for increased efficiency. In fact, state-ments have been made in favor of this poor design tech-nique for its contribution to stall warning. It is felt bythe author that the reduction in drag by the use of cor-rect design and the resultant lower stalling speed is farmore desirable, a thought worth investigating further.The reduced stalling speed is a result of the clean air-craft being able to reach a higher angle of attack. Atthe higher angle of attack the stall can have a some-what more violent pitching action. It can also have anincreased tendency for roll prior to pitch. To prevent this

-rr.it."A thing called 'drag' blasted all my dreams."

Pnolo by Peter M. Bowers;"Tiny Mite" old configuration. Don't knowwhether it's pre-war or early post-war.

rolling off on a wing brings us to a point of improvedwing tip design.

If you are seeking the ultimate in low drag in yourdesign, you are naturally going to want to reduce thewing tip vortices to a minimum. Some of the ideas todate have been the wing tip plates, streamlined bodies orspecial contoured wing tips such as the Hoerner design.The wing tip plates are known to improve the effectiveaspect ratio and contribute to better tip stall character-istics, but this is true of all the above mentioned ap-proaches. The point is to select a technique that producesthe greatest advantage. Compared to other low-drag wingtips, the tip plates are considered to be making minimumcontribution. This statement is made on the fact that theend plate has the 90 deg. intersection with the profile.An investigation into comparisons was performed atMississippi State, using a medium performance sailplane,aspect ratio of 15. One wing tip was modified to supporta tip body. The other wing tip was not modified and re-mained somewhat rounded or elliptical in plan form. Inhigh speed flight, the aircraft tended to turn into theunmodified wing tip. At the stall, the tendency was toroll in the same direction. These in-flight tests wouldindicate that the tip body was a definite improvement,mostly in high speed low drag, and in a lesser degreeat the stall.

The wing tip design becoming more common eachyear is the so-called Hoerner design. There are so manyvariations of this design it would be difficult to find anillustration of the true Hoerner wing tip. Two of themajor aircraft producers, Cessna and Beechcraft, havebrought out various modifications to Hoerner's ideas, andmany of the modern competition class sailplanes areusing this wing tip because of its low drag. Anticipatingthe question of the Hoerner wing tip versus the tipbody, I need only to point out that one has far lesswetted area than the other, yet appears to be giving thedesired effect. It should be pointed out that the manyvariations in the Hoerner wing tip are often called the"raked tip", implying that the greatest wing span is ad-jacent to the trailing edge.

We now come to the point where statements shouldbe made pertaining to the construction of a wing that willdo the greatest amount of work with the least amountof drag. To construct a wing with a high degree of ef-ficiency is to constantly strive, during its period of de-sign, to retain the contour of the selected profile to ahigh degree of accuracy. The fabric covered wing is

(Continued on next page)

SPORT AVIATION 25

, Leo J. Kohn Photo)The extensively modified Ryan L-17B assigned to theMississippi State University has most of its surface care-fully filled in for experiments in geometric boundarylayer control. This smoothing of the aircraft's surface alsoresulted in a substantial increase in performance.

AERODYNAMIC REFINEMENT . . .(Continued from preceding page)

probably the poorest method of construction from thestandpoint of low drag, but it is not meant to imply thatthe use of fabric cannot be properly controlled. Themost common errors are too widely spaced ribs or notcarrying the leading edge skins far enough aft. Refer-ence is made to the modern lightplane.

If a careful observation is made of a station betweentwo ribs, it would be found that this cross section hadvery little resemblance to the adjacent ribs. One of thefinest examples of a good fabric covered wing can be ob-served on the old stagger wing Beeches. The ribs areclosely spaced and the leading edge plywood skins arecarried aft to near the maximum depth. To obtain the ul-timate in low drag, it would be best to avoid this typeof construction and turn to either all metal or all ply-wood. In both these cases, it is still extremely difficultto produce a true laminar flow wing. To obtain this typeof flow, the contours must be held within .002 in. ofbeing wave free. The contours can vary in actual profiles,but there must be no possibility of waviness. It wouldbe unwise to go into this subject any deeper at this timeexcept to illustrate the work that has been done.

A large percentage of all competitive sailplanes arecontoured each year prior to competition (ref: filling lowspots, sanding off high spots). Experiments on geometricboundary layer control were carried out at Starkville onthe Ryan "Navion" which has an overlapping of theskins at the spar line. Behind this overlap, the contourwas carefully filled in and all waviness was correctedforward of the spar line. If my memory serves me cor-rectly, there was a 15 mph increase in speed. Using thesame technique, several Beechcraft "Bonanzas" weremodified and realized an increase of 11 mph. This in-crease in speed was never intended to be a selling fea-ture for this process, but was encouraged on the basisthat a large increase in fuel range was available if thepilot was content to fly on previous manifold pressuresettings.

One of the more simple forms of drag reduction usedby the sailplane pilots is the sealing of gaps with mask-ing tape. In the area where wing panels are bolted to-gether, the use of masking tape is very common. The useof sheet metal gap covers pulled tightly over the junctionof two wing panels is common on most types of smallaircraft. Because these areas must be easily accessiblefor inspection or dismantling, the design approach tothe problem is usually a strip of metal with varyingmethods of holding this cover in place. Masking tape isnot the solution where a long period of assembly is nec-essary, but not enough importance can be placed upongetting a true air seal in these areas. Even a well fittingaluminum strip held on tightly does not prevent crossflow between the upper and lower surfaces of the wing.It is only the first step in the right direction. There aremany ways to obtain a perfect seal but I do not feel that I

The Schweizer TG-3A, a war surplus type sailplane asmentioned by the author, has no fillets at the fuselage-wing junction. Early experiments involved the simplefilleting of this area, which reduced the stalling speedby several miles per hour.

could make a suggestion that would be both aerodynamic-ally proper and compatible with inspection and service.The one technique I did use was fabric and dope over themetal inspection strips. Another approach would be todesign a one-piece wing.

In the belief that many individuals will be curiousas to the actual performance of their aircraft, I wouldlike to suggest the two test techniques common to thesailplane pilot. The use of wool tufts is very good, butused to the best advantage involves in-flight photography.Large placards of the speed to be flown are carried inthe test aircraft. The selected speed will be identifiedin the photographs by the placard displayed during thespeed run. It is wise to know the abilities of the pilotof the photographic airplane because of the close prox-imity of the two aircraft. Telephoto lenses increase thechances of success. The less costly type of test is theflight of two identical or similar aircraft in close prox-imity. In the case of the powered aircraft, manifold pres-sure settings would be established prior to take-off, therate of climb timed to a given altitude.

The various drag reduction techniques mentioned inthis article could, each in itself, be subject matter forseveral thousand words. Therefore, it is hoped that thereader will realize the important point, and that is: neverbs satisfied with your aircraft unless you are positivethere is nothing more that you can do to improve its ef-ficiency factor. A

CROSS HATCHING IS ADDEDBODY VOLUME FOR WING

TO FUSELAGE AIR FLOW

CONTROL

26 JANUARY 1964