Spratt May1980 c

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AND SINGLE

SURFACE WINGS

By George G. Spratt (EAA 17426)Spratt and Company, Inc.

P. 0. Box 351

Media, PA 19063

Photo 1 — Chanute double deck glider flying in the Lake Michigan sand dunes. Summer of 1896.

J.HE DECADE SURROUNDING 1900 was the mostactive in fl ight development and experimentation of anyother s im i l a r time per iod . . . and muc h of this develop-ment was done with what we would now call hang glid-

ers.

Visiting a present day ultralight meet one is quicklyimpressed by the similarity: a i r f r a m e weights are all

but identical , much of the control is by the pilot shif t inghis weight, airfoils are usually single surface, air speed

is the same and foot l a u n c h in g is common.This early hang glider period was terminated by the

development of the gasoline engine and cheap fuel . Then

the thirst for power and speed, accelerated by the on-40 MAY 1980

coming world war, obliterated nearly all that had been

done d u r in g this period: the open fuselage was replaced

by an enclosed fuselage, wing warp ing by the aileron,

the single surface by thickened airfoi ls , weight sh i f t ingby dynamic controls and wheels replaced foot or track

launch ing . About the only vestige still remaining is the

vertical rudder used by the Wrights to overcome adverse

yaw, but not even this is used universa l ly . Almost none,

if any, of the present hang glider enthusiasts were pre-sent during that time so it is understandable that much

of the work is being redone and, unfortunately, many ofthe mishaps repeated.

It would be truly worthwhile if someone would write

a full history of this pioneer stage in aircraft develop-

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Photo 2 — Weight double deck glider. Kitty Hawk 1901.

ment before to o much of it is lost, or distorted by non-

a i rc ra f t historians.

It is my hope that th e fo l lowing br ief review of butone of the cont rol and s tab i l i ty prob lems encountered

almost 80 years ago may be of help to someone now ex-

periment ing with single surface curved airfoils . Photo 1is a Chanute glider f ly ing in the dunes on the shore of

Lake Mic h iga n . This was the f irst glider b u i l t in this

country to be f lown consistently, w i t h o u t m i s h a p , b ymany people, some of them novices.

It was f lown in winds from 10 to 31 miles per h o u r ,at air speeds of 20 to 40 miles per h o u r . This g l i d e rweighed only 23 pounds but had a w ing span of 16 feet,chord of 4 feet 3 inches and area of 135 square feet. Th ea i r fo i ls were single surface, c i r cu l a r arc with a camber

of 1 in 10.

Of it Chanute said: "It was found steady, easy to

handle before starting, and under good control when

under way — a motion of the operator 's body of notover 2 inches proving as effective as 5 or more in -ches in the Lilienthal machine."

N ot easily v i s ib l e in the photo is a tail h av i n g h o r i -

zonta l and vertical airfoi ls . These were connected to thewings by an automat ic device, designed by Mr. Herringfor th e purpose of securing stability. Mr. A. M. Her r ingwas then in the employ of Mr. Chanute, having b u i l tthis and several other a i rc ra f t for him.

Dur i ng th is t ime he developed and applied for a pa-tent on this method of spring loading the ta i l so as to

tend to keep the wings at a constant attack. There is

some question about the effectiveness of this device as

Chanute said later, "A few h idde n defects were gradu-

al ly evolved, such as lack of adjustment in the automa-

ti c device." There have also been questions about just

how much of Herring's t h i n k i n g , as well as mechanical

s k i l l , went in to other details of this a i rc ra f t . Later, when

the Wrigh t s decided to b u i l d a glider it was logical tostart with this design. Even as homebui lders today, they

wanted to make some changes.

Foot launching was out as they were thinking even-

tual ly of addi ng a 200 pound motor plus the m u c h

heavier structure required. This meant putting skids

below and the p i l o t above the lower wing. With this

greater weigh t , th e p i lo t m o v i n g h is body would have

l i t t le effect, so dynamic cont rol would be required. Forbank and turn, twisting the w ings had been used by

some previous experimenters so was adopted. For pitchcontrol a small hor i z on t a l mova b le vane ahead of the

wings s h o u l d g ive greater moment than the 2 inch p i lo t

move me nt Chanute c la imed as a mple .B y the fa l l of 1900 the Wrights had their glider

ready to take to K i t ty Hawk for testing. The w ing area

was 165 square feet, the w e igh t 52 pounds and the wingcurvature an arc of 1-22. Wi l b u r took i t to Kit ty Hawkto assemble and test — a lot of work for one man, so

Orvi l le joined him three weeks later. Some tests as akite showed the l i f t far less than expected so it was

t aken 4 miles south to the sand dunes to attempt toglide down th e windward side as C h a n u t e had done nearLake Michigan.

They tried the elevator in the front and in the back,

but s t i l l c o u l d not prevent a d i v e or stop one once

started. Results were in no way comparable to those

c la imed by Chanute and they told him so. Chanute hadnever encountered this condi t ion , even with no controlvane, so was at a loss to k n o w what was h a p p e n i n g .

Chanute asked my f a the r , Dr. George A. Spratt, if he

knew why the action was so different and what could be

done about it. A l though it was to be several years beforemy fa ther would discover th e reversal of center of pres-

sure t ravel in the c i rcula r arc airfoil , he k n e w from his

ow n exper ience as well as that of others that there was

something erratic about this a i r fo i l . Sometimes it would

fly stably and at other times become a d iv ing demon. On

the other hand he knew that a fiat plate, w h i l e very

poor in load carrying ab i l i t y and therefore unsuitable foran a i r f o i l , was stable under al l c ond i t i ons and doci le

t h r o u g h o u t the entire f l ight range. He suggested thatthe curvature be made only at the fo rward portion of the

a i r fo i l then f a i r in to a flatter af ter po rt ion . . . thuscompromising some of the l i f t ing ability of the curve for

the s tab i l i ty of the fiat . The Wrights decided to c on t inue

SPORT AV IATIO N 41

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„••»;•

Photo 3 — Chanute triple deck glider flying at Kitty Hawk in 1902.

t he i r tests in 1901 so used this curvature in a muchlarger machine , so glides could be made in lower windvelocities . The w ing area was an unheard of 308 square

feet. (290 considering cu t cuts.) The span was 22 feetwith a parabolic curve of 1-12.

After the Wrights took this ma c hi ne to Kitty Hawk

my father went down to visi t them an d se e how it wasw o r k i n g . H e w as met at the pier wi th the empha t i cstatement: "I t won't work." Back at camp they found,after some e xpe r ime n t ing , the fl imsy ribs were flexingunder air load into an arc, destroying th e desired effect.Whe n the ribs were braced, as can be seen in the photo-graph of this m a c h i n e (Photo 2), it could be controlled

easily by the small fo rward elevator. Only now that the

aircraf t could be controlled and f lown was it found that

the roll control did not work at all as expected — but

that is another story.

Chanute found this airfoil so interesting that he had

a t r ip lane bui l t and taken to Kitty Hawk for the

Wrights to fly and compare. In Photo 3 it can be seen

flying from one of the dunes. The extreme forward posi-tion of the camber is apparent. My father, thinking ofthis as only a temporary fix and in no way a true solu-tion to pitch stability, cont inued to work on the problem

. . . w or k that led later to his discovery of the reversal of

center of pressure travel on the circular arc.

42 MAY 1980

A flat surface at 90 ° angle of attack has the center ofpressure at 50% chord. Then as the angle is decreased it

moves steadily forward, over th e entire f l ight range, to0° angle of attack. This makes th e airfoil stable in pitchthroughout th e flight range because should th e airfoilt i l t down, as in a dive, th e center of pressure moves for-

ward tending to return the airfoil to the original angle.Likewise an up tu rn moves th e center of pressure afttending to bring it back down.

In th e circular arc at 90" attack the center of pres-

sure is at 50% chord, exactly as with th e flat surface.With decreasing angle the center of pressure continues

to move forward in a manner identical to the flat sur-

face until an angle of about 15° is reached. So far the

circular arc is just as stable as the flat plate and as longas the a i rc ra f t continues to fly in this region is com-

pletely stable. But with the circular arc this changes ab-

ruptly at this angle. Now the center of pressure turns

an d starts to move af t with decreasing an gle of attack atan increasing rate. In some cases it moves to or beyond

the trailing edge. Thus if an aircraft flying in the stablerange accidentally decreases the angle, due to a gust or

increase in speed, to less than 15 ° the center of pressuremoves rapidly aft causing more decrease in angle and

increase in speed. In other words, an uncontrolled dive.

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i- n i l ; l i l > I H ' \ \ i - | . UK T i l l , in : \ \ | i \ y l \ T l u \

K x O l O DOS 0 0 8 001 006 005 00* 003 0 02 00 1 100

—— — Hurfe*. I'Unt-x

————— Kuiter. Curvrd. ('Mtitwr ^

————— Surfor .. - „',

Plate I — Comparison made by Eiffel of C.P. travel on sev-eral single surface circular arc airfoils to a flat surface.

-7.8'

CUKVATUAK.

Plate II — Vector diagram of single surface circular arc air-foil having spars at leading and trailing edge.

NEW YORK UNIVERSITYWIND TUNNEL TES T NO CO4.

NINC FOOT WINC TUNNEL

AIR SPC&D 3O M p HSTANDARD ATMOSPHERE

Plate III — Airfoil characteristics of single sur face circular

ar c having spars at leading and trailing edge.

Later, M. G. Eiffel made accurate measurements ofthis characteristic w i t h h is wind t unne l i n Paris. T h eresults of this work are shown in Plate 1 compar ing sev-

eral different cambers to a flat plate. As my fa ther con-

tinued to work with the c i rcula r arc he realized that the

center of pressure location on the surface is only part of

the picture. You must t h ink o f the resultant of all forcesacting on an air foil as a vector. The center of pressure is

only the po in t at which this vector passes through the

surface. More important is the di rec t ion or slope of this

vector. Obviously any aerodynamic force acting on the

segment of a circle, neglecting f r ic t ion , must be nor ma lto this sur fa c e and thus pass t h r o u g h the center of

radius .Sketch 1 is a typica l single sur face c i r cu l a r arc air-

foil with th e vectors corresponding to several angles ofattack between 0° and 20°. Clearly, if the center of grav-

ity of the a i rc ra f t was at the center of radius there couldbe no p i t c h i n g moments and the aircraft w o u l d have

neutral s t a b i l i t y . No correction would be required when

flying through turbulence, even with the air striking theairfoil at va r ious angles.

I t now s h o u l d be clear why the Chanute glider

worked so well and was so stable w hi l e the Wright copy

was uncontro l lab le . Looking at Photo 1, the pi lo t is wellbelow the wings, his center of gravity nearly coincideswith the center of c u r v a t u r e . The approximate posit ionmarked "C. G. — Chanute" in Sketch 1. Compare this tothe Wr igh t glider in Photo 2 where the pi lo t is actually

above the lower w i n g , in the area marked "C. G. —

Wright" in the sketch. Any moments encountered in the

f l ight range of Chanute's machine could easily be cor-

rected by the 2 inch body movement Chanute c la imedw h i l e those in the W r i g h t machine are more than four

times as high.N o surface is entirely f r ict ionless and some structure

in the form of spars is required. How badly would this

distort the theoretical vector diagrams we have been us-ing?

SPORT AVIATION 43

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To find out I b u i l t an airfoil of 40 inch span and 10

inch chord with a spar at the leading and t ra i l ing edge.Compression members were between these spars to re-

sist tension of the single surface. This airfoi l was tested

in NYU wind tunnel with the results shown in Plates II

an d III. Vector diagram Plate II was a pleasant surprise.

Structural drag did not seriously distort the focal point

but tended to bring it closer to the wing. This is desir-able because it is usual ly this distance that l imits th epossible aircraft size.

For the ultralight bui lder who is not concerned with

speed but enjoys f lying with a m i n i m u m of sophistica-

tion and a m a x i m u m of ease and safety, the single sur-face wing s h o u l d be c o n s i d e r e d . . . p r o v i d e d hethoroughly understands the unusual stability charac-teristics of this wing.

Plate II I shows that even i n c l u d i n g structure this

wing compares favorably in l ift to any modern day air-foil . (Data was taken at 35 miles per h o u r air speed.)

True, the drag is higher than with many thickened

airfoi ls but to offset this one deficiency there are many

advantages. Using a spar at the leading and t r a i l i n gedges, with a compression member between makes the

lightest and simplest const ruct ion possible . T he r ibsneed only be r ig id enough to maintain camber. With alittle mechan ica l i n g e n u i t y the wing could be made

foldable.Plate IV , also prepared by Eiffe l , shows in polar form

th e l ift and drag of several different cambers and a f lat

surface. Coefficients give n are metr ic . T o convert them

to our more f a m i l i a r units mul t ip ly Ky by 15.95 to get

G I and K x by 15.95 to get Cj.

For instance, the surface h a v i n g a camber of 1 in 7

develops m a x i m u m lift at about 20° angle of attack. Atthis angle Ky is .094.Mult ip ly ing this by 15.95 gives

1.499 C j which is high indeed for an airfoi l operating

at low Reynolds n u m b e r .

Sketch 1 — Typical vector diagram of single surface circular

ar c airfoil showing approximate C.G. locations of Chanutean d Wright gliders.

Kx 0 !0 0 OS 001 006 00 6 00* 003 0 02 00 1 "0 0

iirliwr, I'liim- ^

urface. Curved. CiUlllRT .,

Plate IV — Comparison made by Eiffel of lift an d drag ofseveral single surface circular arc airfoils to a flat surface.

Latest version of the Spratt land plane.

C.Ga. - CHANUTI

CENTER CF RADIUS

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Spratt May1980 c