Document
MR July 1944
NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS
ORIGINALLY ISSUED July 1944 as Memorandum Report WIND-TUNNEL INVESTIGATION OF THE EFFECT OF JET-M OTO R OPERATION ON STABILITY By Wallace F. Davis and Sherwood H. Brown Ames Aeronautical Laboratory Mo ffett Field, California WASHINGTON NACA WARTIME REPORTS are reprints of papers originally issued to provide rapid distribution of advance research results to an authorized group requiring them for the war effort. They were pre- viously held under a security status but are now unclassified. Some of these reports were not tech- n!ca1l~7 pcUted. All have been reproduced without change in order to expedite general distribution.
I ~ I A-3l NATIONAL ADVISORY COMl"1:I'I'TEE FOR AERONAU'I'i:CS ~vn~D - TUrmEL INVESTIGATION OF TEE ~B'FECT OF JET - BOTOR O?ERhTION ON ~ TkaILITY By Wallace F. Davis an~ Sherwood H. Brown SUMl-1ARY The effec ts of jet - notor operation on the stability and control characteristics of two fi sh ter -t ype airplanes as determined by wind -t unnel test s of 1/5 - scale mode ls are pr e- sented . It is s ho wn that the ~ction of the jets is to cause a small los s in stick - fixed stELbili t y which is predic table from known theories.
IN:'RODUCTION A jet - propulsion engine draws L"1.ir into a com pressor and delivers it to E.. chambe r where the Ro.dition ana. combus t ion of fuel results i n a high -t emperature gaseo us m ixture under high pressure . This mixture i s then part ially Expanded t ~ rough a gas ·t ur b ine and is f in c~lly e j ec ted as a high - velocity high - temper&ture jet . In gene r a l, the diffusion of T such a jet is trJ.e result of the absorption of its 8nergy b J the su rroun ding flui d through tur bulent mixi ng. Thc diamete r of the ~et increases SlOily at a ma ximu~ diffusion &ng le of
about 7 {references 1 and 2) 8.no. tL1C increase in cross -
sectional area is 9ccompan ied by n decrease in velocity. If the momentum of the spread in g jet is to remain constant) the decree . se in velocity n:ust be Q.ccomp[',nied by an increase in mass flow. This means ) of course, that part of the fluid in the surrounding streem is drawn into the jet and the stream - lines of the s treF. .:r1 ere def l ected to"'!8.rd the ax is of the high - velocity jet . Theoret ic al treatm en ts of this flow problem are g iven in reference 3.
Prior to the development of these theoretical studiOS , tests of rr:od0ls of two fighter -t ype 8. ir plA.nes ) one with a single jet motor discharging its hot g~scs from the tail a nd the other e. t wi n-en gine e irpl p.ne ','.'ith jets exhausti ng ne ar the
wing roots, were made in the Ames 7- by lO-foot wind tunnels
to determ in e the effects of jGt oporE t tion on stEtbility r. nd control . This report presents the results of these tests with a cold jet and comD~res them with the change in stability calculRted from theory .
JET - PROPULSIO:'"! ST-illL.ATIOJ.\T If pctual jet-propulsion engine-operating conditions were to be used in w ind-tunn el testing, a jet of the same outlet veloci ty p.nd tenperature as generated by the full-scE'..le engine would be necesssry . The difficulties met in producing these cO:lditions in a wind-tunnel moclel hnvc m2c1e actual simulp.tion impractical at the present time, so the following approxir:late method has been developed .
If it i s Rssumed that jet operQtlon cloes not affect the external drag of the engine housing B~d that the static pressure at the jet outlet is eo_ual to atmospheric pressure , the thrust of a jet - propulsion uni t Iill be equE'.l to the rate of change in the momentum of the fluid emerging fro~ the jet or where T thrust force, pounds HA 1.!ass floir! of air , slugs per second V velocity of fluid at jet outlet, feet per second J V velocity of flight, feet per second MF ~ass flow of fuel, slugs ocr second Then, defining the jet thrust coefficient ~cJI as the thrust divided bv the product of the d- v nf'JT.ic pressure of the free strea~ q" (lb/s~ ft) &nd the wing are~ S (sq ft), and since _ ..1 1r:he rc PJ density of f l uid at jet outlet , s l ugs per cubic foot ~J area of je t ou tl et , sque..re fee t I MA + ~JfF \ lr. + l'Jlp ) A PJ rtJ
= 2
pS J With the jet - propulsion engines used at the p resent tiDe l IF will not be :nore thF .n 2 percent of EA even under extrec e conditions . Therefore , if HF is l":egleeted , and the thrust will be sirr.ulated if the product of the ratios of the above eouation is the desired conEtant . If p cold jet is l.:.sed in -JlJind-tunne 1 t e stint"} 'c:l.e density rat i o will not be the same as found under 2ctual c o~d itions; however, a g iven
TCJ ,I can be attained by ad j u ot in g the velue of ~J ISo This
adjustment was made in the case of the mod els by reducing the jet - outlet area with a stre a m lin cQ p l ug .
The p rincip a l sources of error in using this ~ethod of jet simul a tion are the incorrect jet temperature and viscosi ty and th e chang-o in the manner of ,iet diffusion by the stream - lined p lug. Both of th e se errors l' ill e ffect th e infl'Jw about the jet , end thus po ssibly the stacility and control chE'.racter - istics of a wind-tunnel ~odel . 30~ ever) it is believed that these errorS \.'.Iill be s ma ll and of s econdary iE lp ortc nce .
APPAHATU SAND !1ETHOD The arran g ement of the 1/5 - scale models of the fighter 6. irpJ.enes in the (- by lO-foot wil1l1. tun l 1el is shown in figures 1 and 2 and the model dimensions are ~ iven in figures 3 and 4 .
The jet was simulated by compressed E'.ir supplied to the model by the piping arrangement sho~n in figures 1 and 2 . Thi6 air passed through 8. mercury seal that removecl any restraint from the balance system and entered the floating frame alon g the center line of rotation in yaw perpendicular to the dra g and cros s- force-wind axes and along the axis of the links of the front-lift sc a les . The model Ditched about a fixed vertical p ipe, and the rotation of the jet outlet c ~ bout the pi v ot waS taken by flexible tub i ng. In order to reduce interference and tare forces, the length of the pipe t h at projected i nto the wind-tunnel air stream was surrounded by a fairing that was free of the wind'-tunnel balance syste r,l . The jet - outlet velocity was calculated fram measure me nts obtained from a thermocouple and calibrated orifices L1 the pipe leading to the jet outlet .
The forces due to jet operation were measured through the av a ilable jet - vel o city range (0 to 10 0 0 ft/sec) in a tunnel - off calibration . Since only the effects of the change in flow about the model were deSired, t he p itching mo~ent and the force components resulting from the thrust of the jet have been subtracted from the test re s ults . All the data have been corrected for wind-tunnel-wall effects and tares .
The method of conducting th e tests consisted of setting the outlet ve locity of the jet at the h i g hest attainable value and varyinf the outlet - velocity ratio VJ/V bY ' 0hanging the dynamic pressure in the wind tunnel . This procedure caused a change in Reyn o lds number (from 700,000 to 1)700, O OO } based upon the mean aerodynamic chord of the m odels) that aff e cted the aerodynamic characterist i cs of the models to some extent , especially when the flaps were deflectecL In order to elimi - nate this variable } jet - off dateS. 1fIie r e obt a .ined at the s a me dynamic pressures as were the jet-on data. Comparison of the results showed an increment i n pitching moment and control hinge moment due to jet operation . In order to illustrate the effect of outlet - velocity ratio, it has been assumed that Reynolds number has a negligible effect on these inc . rements , and they have been added to the basic mo n ent and h i ng a- moment curves obtained at the hi g her Reynolds number.
The natur~ of the flow in the re g ion of a hi g h-velicity jet we .s investi g cS . ted by visual and (,ua litative obs(;rvations .
The apparatus for the form e r method consisted of a 5 -inch by
5-inch plexi g lass flow ch a nnel, a t 1 o-dimensional jet } and a smoke generator. The jet was supplied by compressed air from a source of vari2 . ble pressure anc1.. issued from a 1/16-inch slot in a plenum chamber that extended across t.he flow channel.
S moke was produced by blowing an oil mixture through a heated
coil and was introduced into the low-velocity air stream (6
to 12 ft/sec) above the jet outlet. High-speed photo g r~phs were taken of the smoke streamers through e.n outlet-velocity ratio range of 0 to S.
The magnitude and the direction of the chan ge in flow about the jet outlet of the sin g le-en g ine e.irple.ne modei were measured with a directional pitot -t ube. The accuracy of this instrument in measurin g angles of pitch and yaw is within about ±O.25°, and the dyn[ 1. mic pressure measurements ' are within about ±l percent .
The chan ge in pressure distribution on the horizontal tail surface of the single-engine fi gh ter model was ~ easured by static p ressure orific e s at 25 pe rcent and 50 percent of the elevator se m ispan on 2n airfoil located 6 inches (model scale) above the center line of the jet outlet.
RESU:"TS AND DI S CUSSION Flow Pe.ttern Since the hi g h-velocity jets of both 8.irplanes could influence the flo"\.l. ' in the vicinity of the tail and thereby change the stability, tests we r e made in a small flow channel to study the flow around the jet 1IJi th smoke streamers . Figure 5 shows photographs of t h ese smoke streamers . These studies indic a te that the streamlines curve toward the jet as the outlet-velocity ratio is incre £1. secl nnd that the greatest change in stream direction is near the jet outlet . l1easurements , with a directional pitot tube, of the magnitude of the flov.l-angle .. . ch?:.nges about the jet of the sin g le-engine airple .ne model are not of sufficient ~ccuracy to determine absolute values, since the ' variation i s of the same. ma g nitude as the accuracy of the
instrument. However, the data of fi g ure 6 do show that the
change in stre aD d irection ip the region of the horizontal surface is sma ll (less than 1°). No measurable chan ge in the vel06ity of th e str e am at the tail was noticed.
I .
Change in Tail Load The flow inclination caused b y the jet produced an increase in the download on the horizontal tail of the single-engine airplEne model bec a use of an increase in downw E. sh ang l e e.nd a . change in the effective ca m ber of the 0i rfoil section. Mea surements of th e press ur e -coeffiQient
lP = ~p/q) distribution show that t he jet causes a decr ea s e
in the p ressure on the lower surfac e t hat increases with jet t h rust co e ffici e nt (TcJ ' ) and d ec r eases wi th distance fro m t he
center l i ne of the jet . ( See fi g . 7. ) The . ch a nge in pr esQur e
co e ffici en t is small ) being equal to about 0.15 at 25 pe rcent of the semispan of the horizont a l t c il for an outlet - vc locity ratio of 12 .
A decre2se in p res su re on the lower sur f a c e of th e h ori - zonta l t a il c auses a mo re pos itiv e ~itch ing-moment co e fficien t.
Eeasurerr,ent s of the p itchin g mome nt 'i.'j ~.th the same hor izontal tai l as was us ed for t he pressuro-distrihutjon "tes ts showed a p ositive shi f t of the p i tc hing - moT.ei1'c-cof·:ff ici en t curv e of about 0 . 03 at an outl et - velocity ratio of 12 . S ince t he pitching moment of t he m od el without c, t2il wc,s not affected ~~ by jet operation) this ch a n ge in C~ agreed wi th that pr edicted by t he p res su re measurcnents .
Ch 2nge in Longitudin al Stc~ility For the Single - eng ine model ) m ili t2ry - rated - po we r op e ra - t io n of the j e t - propuls ion eng ine of the airp 1 8.n e is char 8, ct e rized by the vari a, tion of thrust coefficient T I CJ and out l et - velocity ratio VJ/V with s ') eed t ha t is shown
i n fi g ure e. The -1 a riations of :p itc h in g-moment a n d, elevator
hin l?~ e - moment co e ff icient s re s ul t in g froD 0po' ration of the jet ere shown in figure 9, 10 ) and 11 . From these data, the effect ~f jet op erat ion upon an gOOO - po und single-engine airplane fly i ng at sea l e ve l h as becn computed a nd is sum ma riz cd in th e follo in g table : iJi easL:.red che.ng-e- ' i i~ ~stimatea change in Condition stick-fi xed net.;.tra l- neutral - po int loca - ; ____________________ 4- ____ ~ p~ o~i ~ n ~ t ~~ l ~ o~c~a~t~i~o~n ___ tion (fro m r e f . ) .
,.. '----'
I , Flaps retracted r1 . h . C . 2 perc e nt I· LA ,C.
1 pe rcent 1.f . A. C.
Flaps 55
---------------------------- -- ---- --- --------~--------------------~
For the twin - e ng ine a irpl e ne, th e v ar i at ion of outl e t-v e locity ratio , thrust co e ffiCi e nt, and lift coefficient with airspeed
~re g iven in fi g ure 12 . Figures 13) 14 ) a nd 15 show the
eff e cts of jet op erat ion on the stabi lity ~nd control a nd
, I
elevator hinge m o ments . The measured and estimated changes in stick - fixed stabili t y for the ti;·Jin- engine. airplane are g iven in the following table~ l1easured chaE ge in - 2stima ted chan ge in Oondition stick-fixed neutral neutral Doint location _ __ _ ,_ _____ ---"P ;,... O ;,... l_·l!t locati .on (from reference 3).
FIE.ps retrE . cted 3 perce'nt l·i . A. O. 4. 2 percent M. A.O.
Flaps deflected 4 percent H. A. C.
- -- . ---~ -- , From these data it is concluded that the effects of jet oper - ation on the stick - fixed stability are small and may be preciicted w ith reasonable accuracy frC?lTI l~~own the aries.
The effects of jet operation on the elevator hinge moments of the single-engine airplane r es ult in an increase in stick - free stability because of the increase in Ch ' u
Oalculated stick - free neutral - p aint shifts of 30 percent
for this condition are not of Si g nificance , however , since the force va ri a tion with speed is very slight l y increased . For the twin - engine airplane the effect of jet op eration on the hinge-moment charact e ri a tics is ne glig ible, insofar as changes in stick - free stability are concerned .
Dir e ctional S tability Tests of the directional stability ith jets operating show negligible changes in stability a nd trim .
OONCLUDlhG TIJ:.iAnKS It is conclu ded th a t the effects of jet operation on the stability char e~ ~er istics of a irplanos si m ilar to those for which model teS G8 we re made will be s ffia lJ pr ovided the jet does not impii1 [8 on the tai l. Tho st a bil j,1.: y changes ca n be predic t ed with reaso nabl e accur&cy from kn0wn theories . It is p08sible th at for certain locations of the t a il plane relative to th e jet , stability chan ges appreciably greater I th a n those foun d for the models of this report may be I • I experienced . Study of the the o reticnl treatises wi ll sh ow regions of l arge f lo w ang l es wh i ch should be avoided in fixing the positi on of the tai l.
Ames Aeronautical Laboratory , }ational Advisory Committee for Aero~autics , r: o ffett Field , Calif ., Nov . 6, 1945.
REFE§~JCt:: S 1. Corrsin , Stanley : Investigatio!1 of Flow in an Axially SymQetrical Heated Jet of Air . NACA ACR No . 3 L 23 , 1943 .
2 . Abramovich , G. N. : The 1heory of ~ FreG Jet of a Compressible Gas. NACA ~r No . l05g , 19 4.
3. Ribner , a e r bert S. : Field of Flo~ About a Jet and Effect
of Jets on Stabilitv of Jet - Propelled Aircraft .
Ym L5EOI , 1945 . '
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