Houbolt, John C

English

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CB Jan. 1943 
NATIONAL AOy'.ISOR:-Y C 
:J 
OR AERONAUTICS 
ORIGINALLY ISSUED 
January 1943 as 
Confidential Bulletin 
()II 
A PR1'LIMINARY STUDY OF THE EFFECT OF COMPRESSIVE LOAD 
OK THE FAlRNRSS OF A LOW-DFAG 'YrrnG SPECIMEN 
WITH Z-SECTlm STTI'F]NERS 
By Eastman K. Jacobs, Eugene E . Lundquist, Milton Davidson, 
and John C. Houbol t 
Langley Memorial Aeronautical Leboratory 
Langley Field , Va. 
WASHINGTON 
NACA WARTIME REPORTS are reprints of papers origirially issued to provide rapid distribution of 
advance r esearch r esults to an author ized group reqUiring them for the war effort. They were pre-
viously held under a security sta tus but are now unclas s ified. Some of these reports were not tech-
nically edited. All have been r eproduced without change in order to expedite general distribution. 
L - 527 
J 
https://ntrs.nasa.gov/search.jsp?R=19930092746 2020-06-17T00:58:30+00:00Z
NATIONAL ADVISORY COMMITTEE FOR AERONAUT ICS 
CONF I DENTIAL BULLETIN 
A PRELIMINARY STUDY OF THE EFFEC T OF COMPRE SSI VE LOAD 
ON THE FAIRNESS OF A LOW- DRAG WIEG SPECIMEN 
WITH Z- SECT I ON STIFF ENER S 
By Eastman N. Jacobs , Eu g ene E . Lundqu ist, Mil t on Davidson, 
and John C. Houbolt 
SUM~~ARY 
A low-dr ag airfoil s~ ecimen ~as loaded in compress ion, 
and sur veys o f the fair n ess of the su r fa c e were made at 
differe n t stresses as ell as a t zer o l oad after succe s-
sively higher stresses had been applied and removed. Th e 
tests rep orted and t he discussio ~ of the significance of 
the re su lts obtained sugg est a procedure for determining 
the probable outeo e of a part icular type of co nstruc tion 
for a low- dra g wing. 
I N RODUCTIO.J 
If full advantage is to be taken of the aerodynamic 
propertie s of a low- dra g wing, t he wing s hould retain i ts 
aer od y nam i cally s mooth and fair surface after the airpla ne 
has be e n subjected to its maximum applied flight load . In 
orde r to study the effect of compressive stresses on the 
fairness of t h e wing surface, a ser ies of co mp ression t ests 
was made on a low- drag-airfoil specimen of NACA section 
67 , 1- 113 . The wing s tructure consisted of a skin vith 
spanwise Z-section stiffeners suuported by two end ribs and 
two i n termediate ri b s. The s t if feners were attached to the 
skin with rivets dr i ven by riveti ng method E of reference 
1 . Nominal di me nsions of the ski n and stiffeners are g iven 
in figure 1 . As sho wn in fi gure 2, the tests we re made in 
the 1 , 200 . 000-pound- ca~acity testi ng ma chine in the NACA 
structures research lab orator y . Co mp r e ssive loads c orre-
sponding to average stresse s of 0, 5000 , 0 , 15,COO, 0 , 
21,500 , 0 , 25,000 , and ° pounds per squar e inch were suc-
cess i vely applied , and surveys of the fai rnes s of the sur-
f aces were made at a~l except the t wo highest loads. 
2 
MEASURE~ENT OF AERODYNAMIC FAIRNESS 
The aerodynamic fair ness of the wing was judged by 
the method outlined under "Practical surface conditions 
and cOi1struction accuracy," a section of the Discuss·ion 
in reference 2. The method e mploye d is summari zed in the 
followin g statement from reference 2: "In general, it has 
been found that the fairness of the important f orw a rd part 
of the wing ia satisfactory if a straight edge may b e 
rolled smoothl y over the surfa ce, thus i nd icating freedom 
from flats or concavities in the normally convex surfaces." 
A 6-inch-square grid was marked on the up~ er and lower 
surfaces of t h e airfoil. The Gurveys of fai rness were made 
at the spanwise center of each b loc k in the g rid and ex-
tended from 20 to abo t 70 perce n t of the chord from the 
leading edge . The deviati o ns f r om f a irness, as detected 
by the strai ght-edge rolli ng te s t a nd recorded in fi gu re 3, 
were classified as follo ws: 
(a) The lig ht li ne s indica te flat areas just percep-
tible to the observer b y means of the str a ight~ 
edg e test. 
(b) The heavy li n es i ndic a t e fl a t spots t hat are 
clearly evident but are no t, i n s eneral, bad 
enough to show any li sh t und er the straigh t 
edge. 
(c ) The medium lines indica te flat s p ots .tha t are 
inter mediate bet ween the t wo foregoin g 
classif icat ions. 
RESULTS OF FAIRNESS SURVEYS 
The first survey was made at zero load and, from the 
results obtained (fi g . 3) , the wing could not be clearly 
shown to be aerodyna micall y fair before loading . The sur-
veys made at average co mpressive stresses of 5000 and 
15,000 pounds per square inC l s h owed a tende n c y toward an 
i n creasing number of flat areas with load. When the stress 
was reduced to zero from 15 , OQO pounds per square inch, 
ho wever , no clearly evident c hanbe in the extent of flat 
areas from t h e original zero- load condition wa s ~bserved. 
3 
At a stress of 21,500 p ound s per s quare i nch , a vis-
ible u ave pa tter n h a d deve ioped . (see fig . 2 . ) When this 
stress was r em ov e d, the s u rface su r vey s how e d that some 
a dd i t iona l dep r e ssio n s or fla t a r eas had resulted . (see 
fi g . 3 .) On t he ba sis of e xp er i e n c e ga i ned in wind- tunn e l 
test ing of lo w-dr ag a irfoils ove r a per i od of severql 
ye a rs, it is b eli eved tha t t hese additional flat areas 
TIould not d efini t e l y be e xp ec ted to increa se the drag of an 
ajrfoil. The s u r vey made a f ter applicatio~ and remoyal of 
an a v erag e c om~ r e ssi v e ~ tress of 25,000 pounds per squa re 
i n c h re v e a l ed su c h a n i n c rease in the number and severity 
of t h e p er ma ne n t d ep r es sio n s and flat areas that the wing 
speci me n cou ld definite l y b e r e~arded as not aerodynam-
icall y fa i::.' . 
SI GN IF ICANCE OF AEROD'NA~ IC FArR .. ESS IT DIFFERENT STRESSE S 
Fr o m data tak e n i n a n i nve s t.igati.on of the compress i ve 
s t re ngt h o f f l a t panels wit h Z- section sti~fene~s, it wa s 
fo und ~ha t the ave r a 5 e u l t i mate stress for a skin-stiffene r 
cc mb i na tion o f t h e s ame p r oportio n s as that used in the 
a irfoi l specimen wa s 33 , 000 pounds per square inch. (Se e 
re f er en c e 3 , f i g . 5 .) As p re vio u sl y stated, no definite l y 
s e rious depress i on s and f l at a r eas remained in the wing 
s u rf a c e a ft er remova l of a stress of 21 , 50 0 pounds per 
s quare i n c h . Thi s s t r e ss i s 6 5 percent , or approximate l y 
t wo-t h irds, of t h e aver ag e ultimate stress developed by 
t h e s t iffene d p a n els. As th e maxim~m l o ad that is norma ll y 
e xp ec t ed to b e c ar r i e d by a n a irplane is t~vo-thirds of th e 
desi g n f a il ing load , i t is c on c luded that a skin-stiffene r 
structu re o f the relative p r opo r ~ions shown in figure 1 
may be re ga r de d a s s ati s fa c tory for retaining an aerody -
n am ica l l y f a ir s u r fa c e af t e r a c o mpression load correspond-
ing to th e max imum fli gh t load has been applied and removed . 
At zero str es s o r lo a d , the skin of an Air~lane may 
have dep r e ssi on s or wave s that ar13 due to imperfect con-
s t r u ctio n or a r e the r esult of a Dr e viously applied flight 
l oad t ha t pr o du c ed pe r ma nent se t in the skin. These wave s 
i n th e sk i n , regardle ss of t ~e ir origin , are magnifi e d at 
str es s e s b e l ow th e bu c kling s t r ass according to the foll o w-
ing appr ox i ma t e e qua ti o n 
1 ( 1 ) 
1 -
- _. -----------------------
4 
where 00 and of 
stress and stress 
buc k ling s tress . 
are t h e depths of the wave at 
f, resp ectively, and fcr is 
The quantity 
1 
1 - f 
fcr 
zero 
the 
is a magnification f a ctor because it show s how the i n itial 
deflection 00 is ma g n i fied b y the stress f. 
In high-speed level fli ght, the magnifi cation factor 
shou ld be s mall in ord e r t h Rt sl ight wave s ~ nd depressions 
which way have been p r esent at z e ro stress are not magn i-
fied t o such a de g r ee that th e " cause a significant in-
crease in d r ag . If f is t h e st r ess in level flight , 
the n .' fer rnu s t bel a r g e b y c 0 i,rp a r i son wit:n fin 0 r d e r 
to k ee.p the magnification fector s mall . Th ab so lute mag-
nitude of fcr s houl d als o be l a r g e e nough t hat the stress 
corr espondi n g to the max i mum ap p lied flight lo ad does not 
exceed th e ~u ck l ing st r e ss to s uch an e xt e nt that s eV 0 r c 
permanent buck l e s a r c form e d in t ~ e s k in . 
If an airplane w i ng were c o n structe d with the s am e 
s k in-stiffe n er p rop ortio n s as t h e airfo il tested in thi s 
i nvestigation an d if the design load factor were taken as 
6 . 6 or higher , the wing stru ctu re would have a value of 
fcr in accorda n ce wi th t he f or e g oing requirements. With 
the desi g n load factor t aken as 6 . 6 , the co mpressi v e stress 
i n level fli ght wou ld be 33000 ---
6.6 
or 5000 pounds per square 
inch . If the buc kl in g stress is tak en a s 20,000 pou~d s 
per square i n ch, an ap~rox imately correct " value, the mag-
nification factor for l e vel flight is then o nly four-thirds. 
The resu lts of th e survey in fi gu r e 3 at a str e ss of 5000 
pounds p e r squa r e inch i ndi cate tha t i n t h e lev el- fl i ght 
condition s u ch a wing would show on l y a sli g ht extens ion 
of t h e i n itial flat areas a n d depressi o n s. As has been 
mentioned previously, furt h ermore, the s u rveys s ho~ed that 
no defi n itel~ serious pe r manen t buckles de veloped in the 
test specime until a stress of more t han t wo-thirds of 
t h at corresponding to maximum strength wa s appl ied . 
It i s possible that a h i gher level-flight stress and 
a corresp o ndi l1g 1y lo wer desi gn load factor could be u sed I j 
l 
t -
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IJ'I 
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-~~--------- ',-------~-~'. - .. - - -
5 
with the ski n-stiffener combi n ation of fi gu re 1 wit h ou t 
serious loss of fairness of the wing ski n in l evel flight . 
The r esults of the fairness survey s in figure 3 do n ot 
per wit a p r edict io n of h ow much the level-flight str es s 
mi g ht exceed 5000 pounds per square inch. The fairness 
surveys do indicate, however, that the level-flight stress 
s hould probably be less than 15,000 pounds per s quare inch. 
CONC LUSION 
The tests report ed he rei n and t h e discussion of the 
significance of the res u lts obtained suge est 8 procedure 
for determini ng in advance the p rob ab le outcome of a par-
ticu lar ty~e of co struction for a low-dra g wing. 
Lang ley Memoria l Aeronauti cal Laboratory, 
Natio nal Advisory Co mm it tee for Aeronautics, 
Langley F i e ld, Va. 
REFERE NC:C S 
1. Lundquist, Eugene E ., and Gottlieb, Rob e rt: A Study 
oft h e T i g h t n e s san d Flu s h n e s s 0 f 1" a chi 11 e - C 0 un t e r sunk 
Rivets for Aircraft . NACA R.B., J une 1 942 . 
2 . Jacobs, Eastman N., Abbott, Ira H., and Davidso n , ~ilton : 
Pr eliminary Lou- Drag-Ai rfoil and Flap Date fron Tes ts 
at Large Reynold s Numbers and Low Turbulence . NA CA 
A.C . R ., Ma rch 1942. 
3 . Rossman, Carl A., and Schu e tte, Evan H.: Data on Com-
pressive Str e n g th of Fl at Panel s wit h Z-S ection 
Stiff ener s. NACA R . E ., Sep t. 1942. 
J 
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Fig. 1 
~--~.--~~------
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Os ~ bf I~ 
ts =: 0 . 064 ~-t - 0.63 s 
b s L 12 
= 35 0;= 
ts 
bw 
= 25 bf _ 0.4 
t Vl bw -
Figure 1.- Relative proportions of sk in- stiffener combination used in 
test specimen L, rib spacing . 
NACA 
'-.) 
Figure 2.- Low-drag-airfoil specimen buckled in 
testing machine. Average compressive 
str ess, 21,500 pounds per square inch. 
Fig. 2 
NACA 
stress 
hislory 
(/b/~q/n 
F1g.3 
Lower surface 
. 
Upper surface 
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figure J. - Loco/Ion 0/ /lo! .!pols on low-drag 
airfoil specimen under end compression, 
Weighl end I~nglh 0/ lines indk;o/e 
severily and ex/enl of dgpr{?sslons. 


A Preliminary Study of the Effect of Compressive Load on the Fairness of a Low-Drag Wing Specimen with Z-Section Stiffeners

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