afp2

8/9/2019 afp2

http://slidepdf.com/reader/full/afp2 1/7

AIRFOILS

Part II

By Chris Heintz, P. Eng.

[This article is part !" a #$part series, %here aer!na&tical engineer Chris

Heintz 'isc&sses light aircra"t air"!ils.(

In part ) !" this series !n air"!ils %e 'isc&sse' the signi*cance !" relati+e

!ti!n, Reyn!l's n&-ers an' the B!&n'ary Layer lainar !r t&r-&lent/.

An' there %as s!e h!e%!r0 !n -asic air"!il 'esign an' ge!etry s! that

the rea'er sh!&l' n!% -e "ailiar %ith ch!r', lea'ing e'ge. trailing e'ge,

ean line c&r+at&re, thic0ness etc., as sh!%n in Fig&re ).

As this 'isc&ssi!n is liite' t! air"!ils &se' !n light planes, %e %ill as0

!&rsel+es1 2hat are the !st signi*cant "eat&res %e %!&l' li0e t! achie+e

%ith !&r air"!ils3

8/9/2019 afp2


2e %ant t! 4y as 5&ic0ly as p!ssi-le sh!rt ta0e$!6/, th&s nee' high li"t at

l!% spee'.

2e %ant t! ha+e "&ll c!ntr!l !" the aircra"t as s!!n as air-!rne an'

thr!&gh!&t the %h!le spee' range.

F!r a gi+en thr&st engine h!rsep!%er, inta0e, e7ha&st an' pr!peller/ %e

%ant a high cr&ise spee', th&s nee' l!% 'rag at high spee'.

In all c!n*g&rati!ns %e %ant a str!ng air"rae %ith!&t &n'&e %eight $ th&s

%e nee' a thic0 air"!il t! -e a-le t! &se a 'eep spar an' ha+e a large 8t!r5&e

t&-e8 %hich %ill gi+e -en'ing strength an' sti6ness as %ell as t!rsi!nal

strength an' rigi'ity.

2e als! %ant g!!' cli- characteristics %hich eans high li"t an' l!% 'rag in

the cli- attit&'e $ %ith g!!' engine c!!ling an' a light air"rae t! -e a-let! &se "&ll p!%er an' cli- at sl!% spee' %ith a high rate !" cli- steep

cli-/.

As alrea'y !-+i!&s "r! the a-!+e listing. there %ill -e s!e c!pr!ising

in the selecti!n !" the air"!il $ -&t the air"!il is !nly !ne paraeter. Others are

%ing plan"!r rectang&lar, tapere', etc./, %ing tips, %ing t%ist %e %ill

'isc&ss this t!pic, as it sees t! -e !ne that is "re5&ently is&n'erst!!' !r

&n'erestiate'/, %ing aspect rati!, !r span l!a'ing %hich sees t! -e

!+erestiate'/.

O+er the years, it is +ery interesting t! "!ll!% the 'esigns !" aircra"t that are

p!p&lar $ there is a "ashi!n in aircra"t 9&st as there is in cl!thing $ pants, l!ng

s0irts. ini s0irts $ racy l!!0ing aircra"t, !l' "ashi!n classics. ne% l!!0s, etc.

8/9/2019 afp2


The F!rce !n an Air"!il

B&t let &s l!!0 t!'ay at "!ll!%ing -asic air"!il re5&ireents an' later !n seeh!% %e ha+e t! 'esign the %ing s! that the sae re5&ireents can -e

"&rther ipr!+e' &p!n !r at least n!t l!st:/.

2e;ll c!nsi'er1

8/9/2019 afp2


High Li"t

l!% 'rag

Str!ng an' sti6 str&ct&re

2ith classic air"!ils, th!se &se' !+er the last <= t! >= years, %e ha+eacc&st!e' t! a a7i& li"t c!e?cient !" ).# t! ).> %ith a ) t! )>@ thic0

'e/ air"!il an' a 'rag c!e?cient !" .=) in cr&ise c!n*g&rati!n ie. ACA

##), ACA =) an' ACA <=) !r <=)>/. All !" these air"!ils are

relati+ely insensiti+e t! r!&ghness 'irt !r an&"act&ring iper"ecti!n/ !n the

lea'ing e'ge an' e7cept "!r the ##) !r ##)>, their !ent c!e?cient is

relati+ely l!% s! that the %ing is n!t s&-itte' t! +ery large t!rsi!n at high

spee' large S+ $ see "!rces !n an air"!il/.

T! increase the a7i& li"t, tra'iti!nally an apprecia-le increase in theca-er is 5&ite e6ecti+e. Practically, this is '!ne -y 'e4ecting the rear part

the air"!il %ith s!$calle' 4aps !r ailer!ns, ele+at!r !r r&''er/.

It is als! 0n!%n that this s&-stantial c&r+at&re increase is ass!ciate' %ith a

large C e5&i+alent t! .=D S 4ap 'eg./ T! a+!i' 'esigning the %h!le aircra"t

"!r this c!n'iti!n the 'esigner liits the 84ap !&t8 spee' F/ t! a reas!na-le

&se"&l range.

nless relati+ely c!plicate' t! -&il', %hen 4aps see ne7t paragraph/ are

&se', the increase in li"t is !'erate1 t! a CLGA ., at S" e5&i+alent t!

8/9/2019 afp2


)> 'egrees. This is a pr!*le +al&e $ the %h!le %ing %hich &s&ally has !nly

) span %ith 4aps, has a CLGA e5&i+alent ).J/ an' -eca&se the 4aps are

stalle' !n the &pper s&r"ace -eca&se the airstrea has n!t en!&gh

!ent& t! !+erc!e a sharp change !" 'irecti!n $ '&e t! an alrea'y thic0

-!&n'ary layer/, the 'rag increase -ec!es 5&ite s&-stantial %ith S" larger

than )> 'egrees an' there is n! ipr!+eent in the cli- spee' range.

This 'ra%-ac0 !" the 8plain8 4ap is partially !+erc!e %ith the !re

s!phisticate' 8F!%ler8 4ap, %here a gap is create' in s&ch a %ay that, %hen

'e4ecte', air "r! &n'erneath is ca&ght in a 8"&nnel,8 accelerate' an'

8-l!%n8 int! the -!&n'ary layer at the &pper s&r"ace, th&s all!%ing the

air4!% t! "!ll!% the 4ap c!nt!&r t! 'e4ecti!ns &p t! > 'egrees appr!7iate.

s&ally the F!%ler 4aps are n!t !nly 'e4ecte' -&t als! !+e' rear%ar's

8/9/2019 afp2


%ith a hinge p!int sit&ate' -el!% the air"!il !r a s!phisticate' trac0 syste

as !n the Cessna )> an' )D, etc./, %hich increases the %ing area S -y

increasing the ch!r'.

2hen h!e-&il'ers install this 0in' !" 4aps it is +ery ip!rtant that they

stic0 t! the 'esigner;s ge!etry -eca&se the 4ap n!se p!siti!n %ith respect

t! the %ing rear en' is +ery critical t! !-tain a7i& pr!*le li"t c!e?cient

!" &p t! . see ACA Rep. !. # $ )J<J/

Split 4aps, alth!&gh 5&ite e6ecti+e, ha+e "allen !&t !" "ashi!n an' %ill n!t -e

'isc&sse' "&rther [th!&gh I ha+e re$intr!'&ce' the %ith the KEITH CH

===(.

A +ery interesting 4ap is the 8&n0er8 type. It is a separate sall air"!il &n'er

the %ing trailing e'ge an' hinge' in s&ch a %ay as t! al%ays create the

8"&nnel e6ect8 t! reacti+ate the &pper s&r"ace -!&n'ary$layer.

The &n0er 4ap is especially interesting %hen &se' as ailer!ns the ailer!ns

are 4ap secti!ns !n the !&t-!ar' %ing panels, !ne -eing 'e4ecte' '!%n, the

!ther &p, s! that the pil!t has 8r!ll c!ntr!l8 !+er the aircra"t./ As alrea'y

enti!ne' the &s&al -!&n'ary layer is 5&ite thic0 !+er the rear part !" theair"!il an' the ailer!ns nee' a certain ini& 'e4ecti!n t! -e e6ecti+e.

This is &s&ally sall 8ine6ecti+e8 r!ll c!ntr!l 'e4ecti!n "r! its ne&tral

p!siti!n. 2ith the &n0er type ailer!n, this is n!t the case i" "&ll a'+antage !"

the p!ssi-le 8"&nnel e6ect8 is achie+e' -y care"&l 'esign !" the hinge p!int

l!cati!n an' care"&l c!nstr&cti!n.

The 'ra%-ac0 !" this 4ap is that at high spee' the "&nnel is al%ays

c!ns&ing s!e energy s! that the 'rag c!e?cient is slightly higher than "!r

a c!n+enti!nal 4ap.

B&t the ;&n0er; 4ap is a +ery g!!' c!pr!ise %hen e7cellent l!% spee' in

ailer!n c!ntr!lla-ility is 'esire', ass!ciate' %ith high li"tl!% 'rag in cli-

c!n*g&rati!n, an' the t!p spee' en' is n!t s! ip!rtant. [That is %hy I ch!se

the &n0er 4aper!n "!r y STOL CH D=) 'esign(.

8/9/2019 afp2


The ;&n0er; 4ap al%ays, an' the ;F!%ler; 4ap %hen e7ten'e', pr!+i'e a

certain ;-!&n'ary layer; c!ntr!l, -e ca&se they ;tri8 this layer !&t -y -l!%ing

accelerate' air int! it, th&s all!%ing the air4!% t! a'here t! the s!li' air"!il

&p t! s&-stantial 'e4ecti!ns <= t! #> 'egrees/ %ith!&t l!cal stalling !" theair4!%.

In !&r ne7t article in this series, %e %ill l!!0 at lea'ing e'ge high li"t 'e+ices.

This article is presente' as part !" a series, %here aer!na&tical engineer Chris

Heintz 'isc&sses the technical aspects !" his light aircra"t 'esigns in layenters.

This article %as p&-lishe' in EAA Light Plane 2!rl' agazine &ne )JMD/. N

)JMD, Chris Heintz.

afp2

Documents