PART 1

Airfoilsby George Colling?, EAA 67 Illustrations by the author

THEY

T he differing explanations as towhy and how an airfoil exerts

l if t have always been of interestto me. There are, of course, manytechnical books which treat thesubject accurately. However, theyare for the most part written inanother language - the language ofmathematics. These books are forthose trained and accustomed toaccept mathematical formulae asthe best method of describing suchthings.

At times, translations into so-called "layman's" language takesplace - and here is where the funstarts. Or rather, where the in-terpretations seem to vary. Somemay say something like this, "Theupper surface is curved like the in-side of a venturi tube. The air issqueezed as it goes through thenarrow neck. At this constriction,according to Bernoulli's theorem,a reduction in pressure occurs. Thepressure over the upper surface ofthe airfoil is then less than the

pressure under the lower surface,so the wing rises".

I believe that this type of ex-planation is incomplete and there-fore misleading. It is only a por-tion of what is really happening.The work of the upper surface ofan airfoil cannot be separated fromthe work of the lower surface.They both interact on each other.

Another prevalent description isone that states that the air goesfaster over the top surface becauseit has to meet the air from thebottom surface at the trailing edgeat the same time! Other miscon-ceptions have been popular, suchas the statement that a biplanenever lifts as much as a monoplane.

Nieuport 28 replica for the crowd,while Tallman's Sopwith Camelwas pushed out of the hangar andthe Le Rhone rotary engine wasrun to demonstrate the uncommonfeature of revolving cylinders —something not seen on today's air-planes. Tallman's fleet of movie

aircraft are now based at West Ri-verside Airport.

Since the rain curtailed activitiesof this Fly-In, the Southern Cal-ifornia Chapters are looking for-ward to the next Fly-In, tentative-ly scheduled for June or July inPaso Robles.

PLANES AT THE SOUTHERN CALIFORNIA FLY-INRegistration Owners

N93Y Harvey MaceN70P Lou Stolp

N90P

N17K

N95P

Frank Smith

Lee Wainscott

Harold Terrill

Builder & YearArt Chester (1937)Lou Stolp (1957)

Frank Smith (1956)

Dick Johnston

Harold Terrill (1957)

Salvoy-Stark (1945)

Airplane1. Chester "Goon"2. Stolp-Adams SA-100

"Starduster"3. Smith DSA-1

"Miniplane"4. Jeaco 2

"Monster"5. Terrill HTL-100

"Poopsie Doll"6. Solvay-Stork 10 Mod. N41770 Al Trefethen

"Skyhopper"7. Stits Playboy Mod. N75P Joan Trefethen J. Trefethen (1957)8. Stits Flut-R-Bua SA-6B N6065C Charles Bray Matt Peck (1956)9. Stits Playboy SA-3B N91P Robert Gillespie Gillespie & Fall (1957) Fullerton

Roland Fall10. Corben Baby Ace N49T Dennis Newton Dennis Newton (1957) Grand Central.

Glendole11. Glen-Lee 2 N5155V Lee Kennedy Kennedy & Glendenning Cable-Cloremont

G. M. Glendenninq (1957)12. Stits Skycoupe SA-7B N5594V Roy Stits Roy Stits (1957) West Riverside13. Stits Playboy SA-3A N47K Don Finn Frank Smith San Fernando

Valley14. Porterfield LP-65 Mod. Mr. Barclay Grand Central,

GlendaleSPORT AVIATION

Home AirportSacramentoCompton

Fullerton

Fullerton

Torronce

Torronce

TorranceForrance

area for area. However, at someangles of attack the total lift fora biplane can be greater becauseof the slot effect delaying separa-tion over the lower wing (see Fig.1) which in turn scavenges the up-per wing.

To further indicate the scope ofthese "variations on a theme", the

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following three examples are pre-sented. The first is taken from thebook "The ABC of Aviation", 1919edition, by Captain Victor W.Page. A true facsimile of an il-lustration shown on page 21 of thatbook is shown in Fig. 2, with theoriginal annotations. On page 75this explanation is given: "Thegreatest lifting effect is due to thenegative pressure or suction lifton the upper surface".

The second example is from anarticle in an aviation magazinepublished during the 1940's. Itwas entitled "Smoke Tunnel" andwas written by Henry Struck andC. Townsend Ludington. It des-cribes various experiments madein a two-dimensional flow smoketunnel. Here's what it says: "TopSurface Lift". Most of us know thatthe principal thing which keepsan airplane flying is the fact thatair moves faster over the top ofa wing than it does over the bot-tom. This is particularly true ofa section highly cambered on theupper surface. In 1738 Bernoullidiscovered the principle whichbears his name - namely, that pres-sure of air or water on a surface islessened the faster it moves overthat surface. Therefore airplanewings are designed so that the airflows faster over the top. Thismeans that the pressure on the up-per surface is less than the pressureon the lower one and the result is

in the influence each air particlehas on another. They appear toform a circular path. This doesn'tmean that each particle flows com-pletely around the airfoil, rathereach one moves a relatively shortdistance, describing quite a differ-ent sort of pattern depending onits location to the moving body(see Fig. 8b). But at any onemoment they all do have a kinship

to each other in the form of a cir-culatory pattern or vortex flow(see Fig. 9). The more the airis moved, the greater the circula-tion is said to be. Expressed an-other way, there would be no l if t ifthere was no circulation! This wasproven by the German mathema-tician Kutta as long ago as 1902.

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The vortex flow at the wing tipsmay be imagined as a continuationof this circulation, turned through90 deg. This entire formation isdescribed by Prandtl as a "Horse-shoe Vortex System" (see Fig. 10).

Up to now an airfoil moving intostationary air has been described.As it passes it imparts a velocity(with a direction, of course) to theair, shown simply as a vector inFig. 11. In transposing this oc-curence to a relative sense, imaginethat now the airfoil is stationaryand the air is moving past it(see Fig. 12). It goes without say-ing that the airfoil has exactly thesame effect on the air. Whereas inthe first description the air wasstationary and was then acceleratedto a finite speed, it is now acceler-ated from a given airstream veloci-ty to a speed in excess of this rela-tive air flow speed (see Fig. 13).Adding the two velocities or vec-tors (a + b), while not straightaddition, does give a greater valueto the resultant vector (c). In arelative sense the airfoil deflects

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or bends the airstream downwardand at the same time acceleratesit. It couldn't bend it unless it ac-celerated it, and conversely itcouldn't accelerate it without bend-

ing it. The air leaving the trail-ing edge of the wing of an airplanein flight is moving faster thanwhen it first was influenced by thewing. But it is only the reactionto the bending of the airflow whichsupports the weight of the airplane,shown crudely in Figs. 14 and 15.

On the other hand the propeller,working the same way as the wing,also bends and accelerates its air-flow. Unlike the wing, it utilizesonly the acceleration of air to pro-vide a forward reaction called"thrust". The bending of the air-flow (corkscrewing slipstream) isin fact a nuisance, but as the twoare inseparable, it has to be (seeFig. 16). A further analogy maybe given. The increased down-

wash from a wing at low airspeedsand high angles of attack is similarto the greater rotational value ofa propeller slipstream at low air-speeds and high rpm.

When a fish moves its tail finfrom side to side (see Fig. 17) italternately bends the flow first toone side and then the other to ac-celerate the water rearward, the

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reaction to this being in a forwarddirection. When our swimmingcoaches had us kick our feet (seeFig. 18) they asked that the wholeleg be stiffly moved. This wouldaccelerate more water rearwardand give us a greater forward

SPORT AVIATION 11

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speed, and incidentally is why"swim-fins" are so popular.

If the angle of the airfoil is great(see Fig. 19) the air is compressedto a very high degree under thelower surface. This tends to pusha lot more air up around the frontof the airfoil and over the top sur-face. Due to the greater pressurebehind it, and the very low pres-sure over the top surface caused bythe larger "hole" the airfoil wouldleave, the air is moving extremelyfast over the upper front portionof the body. So fast in fact, thatit cannot easily be diverted orpushed into the low pressure areaby the surrounding atmosphericpressure. As it slows down a little,say at point "A", it is then pushedmore easily into the "hole".

The actual particle velocity ofthe air in the "hole" has sloweddown regaining its static pressurewhich also aggravates the situationby discouraging the fast movingair near the leading edge to comeinto this region. This entire action

may take place gradually or sud-denly, depending upon many fac-tors, including the type of airfoil ,its surface condition, the airspeed,the air pressure, the physical sizeof the airfoil , the rate of change ofangle of attack, etc. This burbling,stalling or separation spoils theeven downward flow pattern overthe greater part of the upper sur-face. Some of the air just millsaround, dissipating its velocity,while some of it is dragged alongand continues to rotate in a space

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between the fast moving air andthe airfoil surface. This thrash-ing about of the air uses more ener-gy than is required to simply movethe airfoil through the air at thelower angles of attack as describedearlier. This has to be matchedby an increase in propulsive ener-gy, else the airfoil will quicklyslow down, reducing the lift thatis remaining to an even lowervalue (see Fig. 20). In many in-

stances, this rapid deceleration ofthe airplane, caused by the highdrag build-up, is the one factor thatdrops the machine, because con-siderable "lift" usually remainsafter separation has begun. A casein point is given in Aviation Weeklor January 13. 1958, where indescribing the new Northrop T-38it is stated: "the T-38 has excel-lent lift characteristics with liftcoefficient increasing with angle ofattack well beyond the stall."

Next month we'll continue thisdiscussion about airfoils and howthey work. The following books,in addition to those publicationsdirectly referenced in the text, con-tain material which substantiatesthis article:

1. Mechanics of Flight by A. C. Kermode— Pitman. 1942

2. Aeroplane Construction and Operation,by J. B. Rothbun—Stanton & Van Vliet, 1918

3. Elements of Technical Aeronautics,Not'l. Aeronautics Council, 1942

4. Experimental Aerodynamics, by H. C.Pavian — Pitman, 1940

5. Aerodynamics for Model Airplanes, byD. K. Foote — Barnes, 1952

6. Simple Aerodynamics and the Airplane,by C. C. Carter — Ronald Press, 1940

7. Wing Tunnel Testing, by A. Pope —Wiley & Sons, 1954

8. Principals of Flight, by E. A. Stalker—Ronald Press, 1931

9. Technical Aerodynamics, by K. D. Woo—McGraw-Hill, 1935

10. Principals of Aerodynamics, by D. O.Dommasch^Pitman, 1953

11. Elements of Practical Aerodynamics, byB. Jones—Wiley & Sons, 1942

12. Principals of Aerodynamics, by J. H.Dwmnell-McGraw-Hill, 1949

13. Dynamics of Airplanes & AirplaneStructures by Younger & Woods—Wiley &Sons. 1931

14. N .A .C.A . Reports.

More on the Pretty Prairie Special IIIT oo late to include with the

story on the "Pretty PrairieSpecial III" which appeared in theFebruary issue of SPORT AVIA-TION, we received a letter fromthe builder, Colonel Marion D. Un-ruh of Austin, Tex., who was re-cently transferred to Alaska. Hewrites:

"Our departure date for Alaskahas been moved up and this has in-terrupted a good many of my plans,including a plan to fly the airplaneup to Kansas.

"I have finally finished 'experi-menting' with the plane with thefollowing results: Top speed hasbeen upped to 148 mph; cruisingspeed at 1950 rpm (manufacturer'srecommended rpm for the engine)128 mph; cruising speed at 1875rpm is 121 mph; climb first min-ute after take-off for one minuteat 134 hp is 1100 ft.

"I think I told you I was build-ing a new oil tank to be placedbehind the pilot's seat up in thehead rest area. This has been com-pleted as well as installing a longertail spring that raises the tail ofthe aircraft, with an additional in-crease of two pounds of weight.I was able to remove 5'2 Ibs. fromthe McCauley prop and have itrepitched and rebuilt. All this re-sulted, including the oil tank modi-fication, in a weight reduction of62 "2 Ibs.

"Some of the members might beinterested in the oil tank install-ation. It is now 8 ft. 7 in. fromthe engine oil inlet and is workingbeautifully. However, the Menas-co engine has a positive displace-ment type oil pump as well as ascavenging pump to handle pullingthe oil that distance. Also thereis always a head of oil on the oilpump proper.

"Present plans now are to leavethe airplane in Georgetown, Tex..in the care of A. O. Williams, theGeorgetown operator. I feel surehe will take good care of it. I amhoping that I may be able to comehome on leave next summer, andif it is at all possible, I will seeyou at the 1958 Fly-In." A

Cartoon by John Bowlly

12 MARCH 195»