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F:te D 52.333 / 63 McCOOK FIELD REPORT SERIAL No. 1376
AIR SERVICE INFORMATION CIRCULAR
<AVIATION)
PUBLISHED BY THE CHIEF OF AIR SERVI<;:E, WASHINGTON, D. rc.
J
Vol. II January 15, 1921 No. 172
STRUCTURAL DESIGN OF CABANE STRUTS FOR THE
PW-1 WITH R. A~ F. 1 S TAPERED WINGS
(AIRPLANE SECTION, S. & A. BRANCH) 1
/
Prepared by Engineering Division, Air Service
McCook Field, September 21, 1920 ·
-Ralph Brown iJ! ; . H i: 1 u i1
LIBRARY
WASHINGTON
GOVERNMENT PRINTING QFFICE
1921
MAR 28 2013
Non-Depoitory
Auburn University
)
STRUCTURAL DESIGN OF CABANE STRUTS FOR THE
PW-1 WITH R. A F. 15 TAPERED WINGS. ,
ASSUMPTIONS.
1. The stresses in the cabane were investigated for three
conditions of loading-high incidence, low incidence, and
reversed flight.
2 The value of R0 for the PW-1 for the two normal flight
conditions was obtained by multiplying the corresponding
values in McCook Field Report No. 660 for the VCP-1 by
the .ratio of the weights of 1the two airplanes, 2,600 to
2,300. .
3. For reversed flight the same values were modified to
take account of the change in location of the center of
pressure and the change in wing loading.
4. The following table shows the location of the center
of pressure, etc., assumed for the three conditions investigated:
Location Load
of c. p. factor.
-------=----1------------
High incidence . . .. .......... .
Low incidmc.e ....... . . . .... .
Reversed flight ..... .. ...... . · I
12
- 1~ ······1:0·
Per cent.
28
50
25
8.5
5.5
3.5
5. The vertical load on the cabane was taken as the
values of R2 obtained as above.
6. The drag load in the normal flight condition was
taken as the corresponding value for the PW-2 with
U. S.· A. 27 wings, obtained independently.
7. In reversed flight the upper wing was assumed to be
0.9 as efficient as the lower.
. 8. The drag load in reversed flight was assumed to be
equal to one-fourth the lift on half of the upper wing.
9. The drag loads were assumed to act at the front spar
10. The stress in the compression rib due to loads at the
cabane strut point in the rear spar was first computE,d
This stress was added to the drag load when figuring the
stresses at the cabane strut point in the front spai.·:
DISCllJSSION.
1. The wings for which this cabane is designed are the
same as those used in the VCP- 1. Therefore it was
practicable to use the computation in McCook Field
Report No. 660 in this design, correcting for the change
in the total weight of the airplane.
2. It was not accurate to assume that the drag loads in
normal flight would be the same for the R. A. F. 15 wings
as for the U. S. A. 27 wings, but the error is not great,
and none of the cabane struts was limited by these loadings.
The chief object in investigating these loadings
was to show that they were not limiting conditions, and
the assumption was close enough for that purpose.
3. The reversed flight conditions were those required
by Air Service specifications,
~§664-2t
4. The computations showed that if a drag load were
applied at the cabane strut point in the upper rear spar,
practically the entire load would be carried through the
compression rib to the front spar.
COMPUTATIONS.
The first step was to determine the components of the
lengths of the cabane st.ruts along the axes of reference,
and the ratios of these components to the lengths of the
struts. These values are given in Table I, where the
columns h eaded "V," "D," and "H" give the vertical,
drag, and horizontal components, respectively, in inches.
Table I shows the directions of these components. The
column headed "L" gives the lengths of the members
obtained by the formula L2= V2+D2+H2• The column
beaded "Per cent" gives the ratios of the corresponding
components to the total lengths.
TABLE !.-Dimensions of cabane struts.
V. Per D. Per H. Per cent. cont. cent. L.
------------
Front .......... ... . 24.63 80.4 15.89 51. 8 9.0 29 .3 30.66
Center ......... . ... 18.69 52.0 28.23 78. 6 12. 0 33.4 35.92
Rear ........... . ... 17. 74 77.6 8.03 35. l 12.0 52.5 22.88
Comp. rib ......... .95 2.6 36.27 100.0 0 0 36.28
The stresses in the struts were first found due to unit
1oads at the cabane strut points of the spai.·s.
Assume an upward load of unity at the front spar.
~ V=0 0.804SF+0.520 sc+O= -1.00
~D=0 0.518 SF-0.786 so+O=O
~H= 0 0.293 sF+0.334 S0+1.o s.=O
Whence SF=0.873 tension;, stress in front cabane strut.
S c=0.575 tension; stress in center cabane strut.
S, =0.448 compression; stress in front spar.
In the same manner the stresses in the cabane struts
due to unit drag loads at the front and rear spai.·s and unit
vertical load at the rear spar were obtained. The results.
are tabulated in Table II, in which the plus sign indicates
tension and the minus sign compression.
TABLE II.
Unit load. Front. Center. Rear. Comp. Front
rib. spar.
-------,--------- - -----
Rear
spar.
Up at front spar . .. + o. 873 1 + o. 575 o O -o. 448 o
Upat rear spar .... - 0.258 + 0. 398 + 1. 272 -0.447 -0.0578 -0.669
Back at front spar +o. 577 -o. 891 O O +o. 129 O
Backatrearspar .. +0.577 -0.891 +0.0334 +0.989 +0.129 -0. 0175
In normal flight the wing loading on the VCP-1 is 9.36
pounds per squai.·e foot on the upper wing and 7.66 pounds
per square foot on the lower.
(2)
Area of upper wing=l46.8 square feet.
Area of lower wing=l21.3 square feet.
In reversed flight:
2,600=0.9Xl46.8 x+l21.3 x.
X=l0.26 pounds per square foot load on lower wing
of PW-1 in reversed flight.
0.9 x=9.23 pounds per square foot load on upper
wing of PW-1 in reversed flight.
Proportion of the load carried by each spar in high
incidence and reversed flight conditions.
Front . .. . ... .. .. .. ............... .
Rear .. . . ........... . .. ... ........ .
High inci- Reversed
clence. flight.
70
30
75
25
Comparing the PW-1 in reversed flight and the VCP-1
at b.i.gh incidence, the front upper spar of the PW-1 will
75
carry 70 as much of the total load as the front spar of the
VCP- 1, and the total loads in two cases will bear the ratio
f 9.23
o 9.36.
Therefore, R2 on the front upper spar of the PW-1 in
reversed flight will b&-
75 9.23
R2=220X70x9.36=233 pounds for the front upper spar.
and
25 9.23
R2= 95X30x9.36=78 pounds for the rear upper spar.
Drift load in reversed flight, 0.25 X 9.23 X 146.8 X 0.5=
170 pounds.
Values of R2 and drag load for normal flight ·conditions:
VCP- 1. PW-1.
H. I. L. I. H. I. L. I.
--------------- ------------
Front upper spar ...... . .. . ... . ..... . 220
Rear upper spar........ .... .. . ....... 95
Drag load ...... ...... . .. ... .. . ... . . . . ... .. .. .
95 248
220 107
37.1
107
248
136. 3
3
TABLE III.-Loads on cabane.
High mcidence.
Low incidence.
Reversed
flight.
Load factor ..... , . . . . . 1. O 8. 5 1. 0 5. 5 1. 0 3. 5
--------- 1----- - ------
Front upper R,.. . . . . . . . . . 248
Rear upper R, .. ... ....... 107
Front upper drag load. . . . 37. 1
2, 110 107
910 248
315 136. 3
590 -233
1,365 - 78
750 170
TABLE IV .-Stresses in members.
H. I. L. I. R. F.
Load .. . F. C. R. F. C. R. F. C.
-825
-274
595
R.
----------- - ----- -
Front upper 1, 840 1,210 0 51.1 339 0 -720 - 475 0
Rear upper. -234 362 1, 158 -352 543 1,738 71 - 109 -350
Drift ....... 182 - 281 0 433 -669 0 343 - 530 0 --- - --- - --- ---
Net . . 1, 788 1,291 1, 158 596 213 1, 738 -306 - 1, 114 1-350
TABLE V.- Maximum stresses.
Max. Max. Circu- Member. Length. I SUN· A s,i- C. 'l' . I reqd. Areqd. tuJabre . plie . plie .
------------- --
Front ... .. 30. 66 306 1,788 o. 00101 o. 0326 A" 22 o. 00116 o. 0416
Center .... 35.92 1, 114 J:,291 . 0050 . 0235 !1<20: . 00504 . 07 7
Rear . ... . . 22. 88 350 1,738 . 0006 . 0310 f',22, . 00116 . 0416
Although the tubes listed in Table V would carry the
stresses computed, for practical reasons they are too small
and it is advisable to use larger sizes. The rigidity of the
cabane structure would be greatly increased at a small
expense in weight by using i 11, 20g tubes for the front
and rear struts and 111, 20g or i'', 18g for the center.
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