Donely, Philip

Shufflebarger, C C

English

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NO \-W 
TECHNI CAL NOT ES' 
NATIONAL ADVISORY COMMITTEE FOR AER ONAUTICS 
No. 731 
fiSTS IN THE. GUST TUNl~EL IF A MODEL OF THE. 
XBH-l AI RPLANE 
By Philip Donely and C. C. Shufflebarger 
Lani;l.ey ~f.em.o r ia:. A.e'N)nauti cal La bo rata ry 
Wa.sh.in~ t on 
October 19;;9 
IlE C P 
T. 1M returned tQ 
the files of the Nltf'na1 
Ad-,itory Coorrwttae 
for ~ronautics 
gron, D. C. 
https://ntrs.nasa.gov/search.jsp?R=19930081817 2020-06-17T22:50:27+00:00Z

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS 
TECHNICAL NOTE NO. 731 
TESTS IN THE GUST TUNNEL OF A MODEL OF THE 
XB~v1-1 AI RPLANE 
By Philip Donely and C. C. Shuffleoar~er 
SmLiARY 
A dynamically scaled model of a Navy X5M-l oiplane 
was tested in the ~ust tunnel to provide data for use in 
determining the structure of atmospheric gusts from meas-
urements of the motions of the full-scale airplane in 
rou"Sh air . 
The a~reement of the test results with values computed 
oy means of the theory of unsteady lift of monoplanes was 
~ood for gust g radient distances up to 12 chord lengths 
when the computations were oased on the assumption that the 
wings of a oiplane act independently in unsteady flow and 
when the mean slo pe of the lift curves of the two wings and 
the mean chord of the c el lule were used in conjunction with 
data on the unsteady lift of finite wings . 
INTRODUC TI ON 
In connection with a ~eneral investi~ation of the 
fli~ht loads imposed oy atmospheric gusts on airplane 
structures, flight tests have oeen made at Langley Field 
for the purpose of ootainin detailed infor ma tion regard-
ing th e charact er istics of gusts in thermal conditions at 
va rious a ltitudes . A Na vy XB -1 oiplane was utilized for 
this pu r p ose, the characteristics of the ~usts to De de -
termined from measurements of the motions of the airplane . 
As availaole theoretical relationships oetween gust 
shape and ai rplane motion could not with confidence De ap -
plied to the ca se of a oiplane to det ermine the gust char -
acteristics , th ese relationships for the XBM-l airplane 
had to be experimentally determined by testing a dynamic -
ally scaled model of the airp lane i n the gust tunnel with 
seve ral ~ust s'hape s . 
2 N.A.C.A. Technical Note No. 731 
In this paper, the experimental results of the ~ust­
tunnel tests of the XBM-l model are presented and compare d 
with computed results based on the theory of the unsteady 
lift of mono~lanes. 
APPARATUS 
The ~ust tunnel and its related equipment are described 
in reference 1. 
The model is shown in fi~ures 1 and 2. Its pertinent 
characteristics, as well as those of the full-size airplane, 
are ~iven in table I. Althou~h the principal aerodynamic 
features of the airplane were faithfully represented in the 
model, no attempt was made to reproduce small ex crescences 
such as engine cylinde rs, windshields, etc . , in view of 
their small importanco in these tests. The fuselage diam-
e t e r was also slightly enlarged to accommodate the acceler-
ometer. 
The wings were made as ri~id as practicable in order 
to eliminate effects due to their deflection irr the steep-
est gust gradient. The natural ~eriod of the uings is given 
in table I and the deflection curve for a load factor of 
1.0 is given in fi ~ure 3. 
The thr ee gust velocity distributions or profiles in 
which the tests were made were approximately linear and 
are shown in figure 4 as plots of the ratio of local gust 
velocity, U, to the average maximum gust velocity 
Umaxav' a~ainst the distance, in chord lengths, in the 
direction of flight. 
TESTS 
The tests consisted of fli~hts over ~h e gust tunnel 
at fixed values of the forward velocity and of the average 
maximum gust velocity. A minimum of five fli g hts was made 
for each of the three ~ust gradients. Measurements were 
made of flight velocity, gust velocity, normal acceleration, 
vertical displacement, and pitch. 
N . A . C . A . Technical No te No. 731 3 
PB.ECISION 
The me asured quantities a re estimated to be accurate 
within the following limits for any singl e t es t or run. 
Acc ele rati o n - - - - - - - -±O.lg 
Forward v e locity - - - - - ±1 . 0 foot pe r se cond 
Gust velocity - - - - - - -±0 . l foot per second 
Pitch - - - - __ _ ± 0 . 2° 
Vertic a l d i sp l acement - - - ± O. Ol fo ot 
App roxi ma te co mputations of the effect of wing flexi-
bility indicat ed an error of 2 percent in the acceleration 
in a sharp - edge gust and smaller errors in the· gradient 
gu~ts . The error due to wing flexibility is thus within 
the limits of a ccura cy of the re st of the data . 
RESULTS 
Records of two flights fo r ea ch gradient were e valu-
ated to · give histories of events during passage through 
the ~u~t . These re sults are shown i n the uncorr e cted form 
in figu re s 5 , 6, and 7 . The oscillations superimposed on 
the acceleration curve for the sha rp - edge ~us t ( f i g . 5 ) 
we re due to the flexing of the wings as a result of the 
gust . 
I n order to el imina te t he effects of slight departure s 
f rom the no mi na l values of ai r speed and s ust velocity, the 
maximum acceleration i n cre ment s we r e corr e ct e d to a forward 
v e locity of 60 feet per second ( 40 . 9 miles pe r hour) and a 
s ust velocity of 6 . 0 foet pe r second . These r esults are 
shown in figure 8 plotted ag~inst the gradient distance H . 
Two met hods of computi ng th e a cc e l e ration incr ements 
a~e available : name ly, · the me thod presented in reference 2; 
an d a me thod which is based on the unst ea dy lift functions 
of refer e nce 3, and ~hich results in formulas correspondi ng 
to eQuations ( 3) an d ( 4) of refer e nce 2 . Computations were 
made by both meth ods on th ~ ~ssump tion that t he biplane 
cellule could b e repr esent e d by an equivalent monoplane 
____________ ____ ~ __ ~ ____ ~ ________________ J 
\ 
L-
4 N . A.C.A. Techni c n l Note No . 731 
hnvin~ the same slo~ e of th8 lift curve and a chord equa l 
to t he mean cho rd of the cellule . F o r these computations , 
a me~sur ed va l le o f t he sl ope o f th e lift curvo was used . 
Th e theoretical curves a ro sho?n in f i ~u r e 8 t o~e th ar with 
t he experim e~t a l curve. Ins pe ction o f fi~ure 8 shous that 
th e theoret ica l cu rve s are louer than th e expe ri mental for 
~ radient distances from 0 to 16 chord len~ths , the values 
c omputed by t he metho d of ref e rence 2 and the method based 
on r efe r en ce 3 boin~ 80 pe rcent and 92 pe rcent of the ex-
per i mental values, respect ively . 
I n an attempt to arrive at bet ter a~ r e em e nt, the win~s 
of a b i p l ane were assumed to a ct independently ; i . e. , there 
i s n o interfer ence between the win~s in unsteady flow . Ac -
cordingly, th e mean of t~ e sloyes of the lift curves of the 
ind i v i dua l wings an d the mean chord were used to de f ine the 
bi p l ane c e llule , and calculations base d on the theory of 
ref e rence 3 wore made . Th e conputed curve for thi s case is 
shoun in figure 8 and is in al most pe rfect a~reement with 
the experimental curve for gradient d istances up to 12 
chord leng ths. 
As would be expected , the t he oretica l curve for the 
case in ~hich th ere ~ere a ssumed to be no interf e r en ce 
effects do es n ot a~~ly for th e lon~ o r gradien t distances 
becaus e of the fac t that the interf e rence effects were ap -
p roaching their steady - flow values and because of the pit ch 
of t he model . Calculations indicated that a pproximately 30 
pe rc e nt of the discr epan cy for the gust g radient distance 
of 20 chord lengths was due to the effect of pitch . 
CONCLUDI NG RE MARKS 
For a model of the XE M- l ai r plane , the theoretical 
acce leration increments, computed by the use of data on 
the unsteady lift of f init o wings under the assumption 
tha t t ho win~s of a biplane act independently in unsteady 
flow, were in ~o od a~reoncnt wi th expe ri men t for ~ radient 
distances u p to 1 2 c h ord le n~ ths. Beyon d 1 2 chord lengths, 
the rap i d decre a se of t ho 8xpor im entaf values of the accel-
e ration increment in tho lon~ e r ~ radient distan ces is prob-
ably acc ount ed f o r by the p itch of tho mode l and by the 
fac t tha t the interf e r ence effects were approachin~ th e ir 
steady- f l ow values . 
Lang ley Memorial Ae r onautical Lab o ratory, 
Natinnal Advi sory CO IDn i tte e fo r Aeronaut ics, 
Lan~ le y Field , Va ., September 19, 19 3 9 . 
- --- -------' 
N . A . O. A . Tec > .. ll ical lote No . 731 5 
REFERErT OES 
1 . Done l y. Philip : An Exp 8 ri me ntal I nvest i ~at i on of the 
Nornal Acce l e r a tio n of ~n Airp lane Model in a Gust . 
=r . N . ·'~o . 7 0 6, N .A. O.A., 1939 . 
2 . Rhode , Richa r d V. : Gunt Lo ad s o n Airp lane s. S . A.E . 
Trans .. vol. ~2 , 1 9::>:7 , PI). 8 1- 88 . 
0 . J ones , Robe rt T .: Th e Unsteady Lift of a F inite Win~ . 
T . N. No . 682, N .A. O. A .• 1 939 . 
TA3LE I 
Oharact e ri st ic s of t he XBM- l Mode l 
~i()del 
We i -'Sh t, 1 b . - - - - - - - - - - - 2 . 0 29 
Win~ area , sq . ft . - - - - - - - 2 . 195 
Win.g l oedi n.g , lb. per sq . ft . . 924 
Span , f t . - - 3 . 0 
~ean Rc r odynamic c ho r d , f t. . 4 14 
Oentor of ~ rav1ty, p e rc e n t m. a . c . 27 .1 
Na tur a l wing per i od , sec . - - - - . 019 
i 'oment of i ~e rti a Tnk al b . - f t . a -, ... y , . 368 
Slope o f lift c u rve , pe r radian - - 4 .1 6 
Gust velocity, f . p . s . - - - - - - - 6 . 0 
Fo r ward veloc it y, m. p . h . - - - - - 40 . 9 
Oo rrespond in.g 
va lu e s for 
X3 M-l a irp l ane 
5 , 200 
4 1 2 
1 2 . 6 
4 1 
5 . 66 
27 .1 
1 80 , 000 
4 . 16 
22 . 2 
15 2 
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Tests in the Gust Tunnel of a Model of the XBM-1 Airplane

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