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Aviation History
1943
1943 - 1965.PDF
AUGUST 5TH, I943 FLIC HT PLYWOOD AND PLASTICS-MI stiffeners and longerons without ill-effects. Shear forces m the fuselage side panels are greatest between the wing spars, therefore thicker plywood must be used in this region than suffices elsewhere. As this ply can take compression it should be made to do so by adding stiffeners where test shows them to be advantageous. In this instance the simple analysis given above, in which tension forces only are assumed to be acting in the ply, is no longer valid. A fuselage of this type supplied by Aero-Research Ltd., was bought bj' the Air Ministry for investigation, and measurements of the deflections of the fuselage under bending and torsional loads were obtained. Notes from these investigations will be now dealt with, the overall dimensions of the rear portion submitted to test being illustrated in Fig. 16. If such a fuselage is regarded purely as a plywood tube, the torsional deflection in radians can be calculated from Bredt's or Batho's formula : o _ n I ££ where T = torqueI t dp = element of perimeter I = length of tube A = area enclosed by tube G = rigidity modulus of» material. In the graph Fig. 17 is shown the torsional twist in degrees under a torque of 10,000 lb.-in., applied at the stern post, assuming the value of G for plywood to be 1.53 X io5. The experimentally determined points are shown, and it will be seen that except at the extreme end the agreement is good. It is stated that in a fuselage of this type the skin buckles under load, and is subjected only to diagonal tension stress ; the skin has no rigidity against shearing stresses. In these circumstances the conception of a rigidity constant becomes meaningless, and G must be replaced by some other modu- lus. Consequently the use of Bredt's formula in the above form is open to objection. The deflection of a shear resistant panel is given by ^' _ PI where P = applied load 4f,~ Oth I = length of panel h = depth of panel t = thickness of panel The deflection of a panel under diagonal tension is ->. _ 4Zf where E = Young's modulus along ~ Kth the tension lines. Therefore in the derivation of Bredt's formula it is sub- stituted for is giving the modified equation 6 = A1! T From this it will be seen that when the tube is so thin that it is in a state of diagonal tension G must be replaced by 2-8 LU 3 2-0UJ 0 1-6 m £0-8 0-4- —• EXPERIMENTALLYDETERMINED POINTS X —— ^*- *^ \ / / 10 Fig. 17. 13020 30 40 50 60 70 80 90 100 110 120 DISTANCE FROM FIXED END IN INCHES Torsional twist in degrees under torque loadapplied at sternpost. Vb 1-4° i rf V),.rf ION 3 JLl-In.c" Db P 0-2* OBSERVEDDEFLECTIONS \ a w / / 4 I*A *is a Fig. 16. Overall dimensions of the rear fuselage submitted to test. 0 10 20 30 40 50 60 70 80 90 100 110 120 130 DISTANCE FROM FIXED END IN INCHES Fig. 18. Deflections of the fuselage under test. E/4. The value of Young's modulus for birch plywood with Bakelite adhesive at 45 deg. to the grain is about .5 X io6 lb./ sq. in. The value of E/4 should, therefore, be 1.25 x io5 which, in view of the difficulties in obtaining consistent results in measuring the deflections, may be said to be in good agreement with the value 1.53 X io5 used previously. Curves are also shown in Fig. 18 for the computed deflec- tions of the fuselage under an up-load of 1,000 lb. at the stern post, using three different values of E and assuming that the deflection is due to pure bending and is resisted by four longerons only, without assistance from the skin. The three values of E correspond to the mean and two extreme values for the spruce used. The observed deflections are also shown and are rather below the computed curves, due to the fact that a small portion of the plywood oh the com- pressive side and a larger portion on the tension side does . -.. . assist the longerons to resist bending. Moulded Plywood Construction Having briefly dealt with these aspects for the most simple type of fuselage incorporating plywood con- struction, the recent development of moulded plywood construction will be considered. One of the first things an engineer must know about any material is the modulus of elasticity. In this respect wood is a little more difficult than other materials since each species has a different value parallel and perpen- dicular to the direction of the grain. To calculate the modulus of elasticity for any construction of plies and species, Gassner, in the Journal of the Aeronautical Sciences, March, 1942, offers a means of working backwards from test data on three-ply panels to
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