NACA Committee Report

i4r,

fu,o,1}-i.: COruHDENTIAL

REPORT TO THE NACA COMMIT TEE ON AERODYNAMICS

By Alexande r Kartveti Republie Aviati'on Corporation Farmingdale, New yort .

I'o r Me e ting of the NACA Comnlittee oni Aerod.ynamics

I Aviation corporation, sorne design problerns have evolved which help believe will be of interest to rnembers of the comrnittee, and a

to the NACA in forrning their future research program"

BUFT'ETING

Recentftighttestsofsupersonicairplanesinthetransonic a superregion have reernpha sized. the problern of buffeting, since in cruising sonic airplane is often called upon to fly at transonic speed reduces ot' rnaneuvering, this problern becorrres irnportant. Buffeting cuts down the usable value of rnaxirnurn lift coefficient and consequently

is very desirthe rnaneuverability of the airplane. For this reason' it j).

able to decrease the arnount of buffeting to a rn'r'nun'urr1' The problern of buffeting is not new but has,been experienced on previous types of airplanes throughout the whole spee'd

range' In sub-

occurred sonic airplanes, using straight and rh.ther thick wings, buffeting lift coefat high angles of attack practically coincident with the rnaxirnurn

ficient and was considered beneficial as a stall warning- In transonic cjccurred airplanes, with sweptback and. moderately thin wings, buffeting

below

and cut down the usable value of

tift coefficient asafunction

""*"*. swept' extrernely of Mach No. In supersonic airplanes equipped with highly lift with very sharp leading edges, the arnount of loss of usable thin wings

due to buffeting

is rnuch greater'

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2

figure No. I shows, on the sarre diagram, the buffeting boundaries as deterrnined by flight tests of typical transonic and supersonic airplanes, together with the respective curves of CLMa,,,. predir:ted frorn available

data. It is

seen that.the loss of

C"M"*. over the

range of Mach Nurnbers for a supersonic airplane is rnuch greater than

for a transonic airplane. It is also

seen that the use of leading edge

flaps improves the buffeting boundary of a supersonic airplane. The curve on Figure No. I with leading edge flap down was obtained using 30%

deflection. It is felt, however, that there is an.optirrrum position :

of the leading edge flap which will give considerably better buffet

boundary. This optirnurn positi.on has not yet been deterrnined, It is of interest to note the different shape of the buffet boundaries for supersonic and transonic

airplanes. The former has lesser slope and does not pre-

sent a pronounced dip near Mach No.

l.

These boundaries are drawn

for so-called rnoderate buffet,

or buffet corresponding to a vertical acceleration of 0.2g. The incipient,

or first noticeable buffet boundary, however, lies some distance below the rnoderate buffet boundary and would therefore cause

still greater loss

of the available lift coefficient. It is up to tire judgernent of the pilot which of these boundaries to

use. The pilot is unabLe to distingriish between

' harrnless and dangerous vibrations and therefore unconsciously classifies

'

unaccounted

for vibrations as the beginning of trouble. Als6, the pilot very

often believes that a slight increase of angle of attack beyond the buffet

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coNruFryTieL boundary

will

cause stalling of the

wing.

The pilot witrl not exceed a

certain lirnit of buffeting, which he accepts as tolerabtre or safe.

The origin of buffeting is usually traced to shock induced separation governed by thickness, carnber, planforrn and sharpness of leading edge. Near Mach No. 1, the picture

is further cornplicated

by

so ca1led rrArea Progression Shocksrt resulting frorn flow lnterference between the wing and the fuselage. As the problern stands now, the pre-

diction of buffet boundary is difficult and uncertain. 'Wind tunnel model tests yield no useful inforrnation. The only data available are a series of flight tests conducted a few years ago by the NACA on a transonic

fighter with a rather thick wingr and sorne rqcket firing tests in which incrernental gts, due to buffeting, were measured. Experience thov/s that buf{et boundaries computed frorn these data are conservative when compared with actual flight test results of the modern suPersonic airplane,

It

cannot be over-ernphasized that better analysis of the buffetlng

problern would be of irnmense technical value. A rnore detailed experirnental knowledge of the problern is ndeded. 'What is the best plan{orrn -).

to gain the best cornpr,olnise for delaying buffeting?. Is boundary layer removal or rrreans of energizing the boundary layer practical? In.other

rules to attain the words, what are the design rules

optrirnurn

practical buffeting

Iimit?

,

It is possible that relative inforrnation

wind tunnel

rnay be.obtained by

tests. such tests, however, should be coo,rdinated with

exteneive flight testing on a prototype airplane, or airplanes embodying

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typical comblnations of the pertinent variableg.

It is therefore

suggested that the NACA reconeider the problem

of buffeting as related to the design of supersonic

DESiGN

airiraft.

Or AT.TER.BODY

].-

The design of the after-body enclosin! the Jet ie another

irob-

lem of great interest to designers of Bupersonic airplaneg.

Recent flight tests have shown that an improper shape of after'body may considerably increage the drag and cause turbulent flow with

consequent undeeirable

vibration. The problem becotnes more

lnvolved.

.

because of the fact that the ahape of the after-6ody should. be a reasonable

compromise between cruising and

full afterburner

opening of the engine

nozzle.

While a reagonably accurate estimate can be made of the wave drag and friction drag of a normally streamlined body, the effect of the

lnteraction of the jet with the after-body is not readily subject to accurate

calculation. Refairlng the after-bod,y of a aupersonic airplane to a lowe r boat-tail angle had an important effect in reducing uuffeting of the aircraft aod Figure

No. 2 shows the original and modified, shape of the fueelage after-bod,y. liflhile the meaeurement of the d.rag saving by that modification wae not conducted in flight teste,. all.IeveL flight buffeting was ellminated

includlng fli.ghte at high

, and

rrqrr.

An excellent paper by Nelaon and, Henry, rrJet Effecta on Baee

After-Body Dragtt, ghows how the jet ln lts effect on the afflr-body

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5

andthebaseareafilaycauselarge.dragpenalties.Thiswork,however' sonic trozzles only. is restricted to the transonic range and to

Itisevidentfrornthispaperthatreductioninthebaseareaand finally elirninate the drag' rn pracwill angre boat-tail srnall a of use the tice,however,itisnotdesirabletomaketheboat-tailangletoosrnall. fuselage and necessiitate the backThis would increase the rength of the

wardmovernentoftheengineinstallationwithaconsequentweightpenalty

andrearwardrnovernentofthecenterofgravityoftheairplane.Anoptilarger l0o is given in this paPer' but for murrr boat-tail angle of B to of say 15o, the drag rnay va:y between. rnagnitude practical rnore of angles wide lirnits.

Itwouldbegreatlydesirabletoextendthlsresearchtocover in rnore detail' Also' thre uSe Of rnore practical cases and to give these divergentnozzlesshouldbeincluded,asthismayaffecttheflowpattern +a extlnd th test to the -^f-.1 to -*tanrl would be very useful on the after-body and base' It

,

suPersonic}vlachNurnberstoobtainknowledgeofthebestpracticalcom.

prorniseforthedesignforbothtopspeedandcruising.Thegelectionof pertinentdiarneterratiosclosetopractical..valuesofmodernje!engines would be verY valuable' ' --i''l^ +L^ the aassociated pertin< :nt area Given a certain jet engine with

ratioswithandwithoutafterburner,whatistheoptirnurndesignofthe after.bodyandtheperrnissiblebaseareaallowingforengineeringco[I. cruising? promises to cover both top speed and

Measurernentsonpressuredistributionasdroyninthepaperby

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Nelson and Henry should be continued. to atternpt to explain observed data and to

try

to provide at least a semi-ernpirical .rnethod of predicting

the perforrnance.

ESCAPE CAPSULE

An irnportant itern of new advanced aircra{t is a safe, fool-

proof rnethod of escape for the pilot under all flight cond.itions.

Medical research on the present ejection seat has proven it

to be unsuitable for use at supersonic Mach Nurnbers because of wind blast and turnbling. As a result, present efforts are concentrated on

'

the clcveloprnent of ejection capsules.

For use on our superaonic airplane, the XF-103, Republic has rnade extensive design studies, tests and anal.yses of the escape

capsule. Test results so far have varied with conditions, sorne quite good, others unsatisfactory. Ab of today, the pilot cannot be guaranteed safe escape in

all

cases

A nurnber of full scale capsule drops were. rnade frorn the bornbbay of a B-47 at Mach No, . B, frorn 20, 000

feet. To satisfy rnedical

requirernents for a very low tumbling rate, retractable fins were

ad.dq:d

to the Republic capsule. These fins proved effective in stabilizing the capsule longitudinally and directionally. However, the capsule ehowed

rolling tendencies increasing to a point where it became unsteady developed a spinlike rnotion.

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