FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1943
1943 - 2002.PDF
174 FLIGHT AUGUST 12TH, 1943 Plywood and Plastics—IV The Moulded Plywood Fuselage, Effect of Qrain Angle and Stress Analysis : Modern Developments By W. NICHOLS, A.R.Ae.S. IN concluding the previous article reference was madeto tests on plywood with various grain angles to thedirection of load, demonstrating that the effect is pro- nounced, and in Fig. 20 variation of E with grain angle has been reproduced. The background of this curve is a series of tests at grain angle 45 deg. plus the known values of E for grain angles at o and go deg. to the load. The curve is based on the formula as given which, in addition to meeting the above requirements, is recurrent. With the moulded plywood technique a balanced con- struction becomes necessary, clue to the different character- istics across and along the grain directions. By balanced construction is meant that the veneers are symmetrically located in respect to thickness, grain direction and species about the centre line of the panel. II this procedure is not followed, warping of the panels will be evident immediately upon removal from the autoclave. The operation of arranging the veneers prior to moulding is commonly referred to as the " lay-up." and by the use of thin Veneers and careful lay-up a panel that is practically isotropic may be developed. With forces parallel to the grain the lay-up causes a decrease in allowable tensile and compressive values from the plain wood but increases greatly the shear allowables. In practice a predominating stress usually occurs at a certain section, and the veneers can be arranged in the most desirable manner to accommodate this stress. In the design of the main-plane for example, it can be arranged to have high predominating grain direction on top and bottom skins where tension and compression stresses due to bend- ing are the major considerations. Approaching the neutral axis, longitudinal or spanwise grained veneers can be tapered nut until a, lay up exists of ap- proximately as many spanwise as chordwise grained veneers. At this point tension and compression stresses are at a minimum and shear stress is the design criterion. Determination of Effective Widths Returning to the discussion of basic plywood design factors by Marhoefer, stress calculations for a typical fuselage section are briefly dealt with. As with metal monocoque practice, the deter- mination of the proper width of plate to include with the stiffeners ("effective" widths) in calculating section properties arises. A typical section through a fuselage shell is shown in Fig. 21 with various effective " widths indicated on the compression side of the neutral axis, which is the only one on which they apply. 4 " Effective " width will not only vary with the stringer stress, but will also require evaluation of the effect of the curvature of the panel (r/t). Error introduced by im- proper " effective " width is mini- mised where the grain angle of the shell is approximately at 45 deg., since Ew/E is then quite low chosen value of shell width relatively INEFFECTIVE PLATE w< and renders any insignificant. It is claimed for 45 deg. veneers that values of " w " = 12/t each side of the stiffener plus the skin over the stiffener itself have given reasonably good results in practice. The moulded monocoque shell without structural frames, etc., has inherently a rigidity not found in sheet metal construc- tion, and such rigidity has a decided supporting effect upon the stringers. This is borne out by the tendency of the compressive stringers under tests to carry loads beyond those that would normally be anticipated. Likelihood of the skin buckling underload is greater, due to the higher shear stress encountered in the vicinity of the neutral axis, and the problem as to the correct amount of " effective " width is reduced. : Tensile Strength v. Grain Angle Behaviour of tension material, it is stated, in both wing and fuselage has been "more diificult to understand and ex- plain than the compression material. Whereas the latter has almost invariably exceeded the expected stresses, it has not been so with the tension material. The possibility that the effect of grain angle may be the source of trouble has led to investigations of tensile strength variation with grain angle. Results of such investigations are presented in the curve, Tensile Strength v drain Angle, shown in Fig. 22. Actual values have been reduced to a function of the standard tensile strength parallel to the grain to facilitate use with various woods. Tests for this curve were obtained from mahogany specimens with grain angles at 30, 45 and 60 deg. It will be noted that this curve matches closely the formula for effective modulus' of elasticity. Considering some of the stresses on the section illustrated and assuming a bending momen" of 250 000 lb.-in., the tension stres on the skin item ]p will be—• My 16.78 -j- = 250,000 X COMPRESSION IN TOP OF FUSELAGE = 12x0125 =15" 218" 125" FUSELAGE GRAIN ±4-5°TO THRUST LINE 1 = 752-1 IN* NEUTRAL AXIS SCALE IN INCHES 10 12 Fig. 21 Stress diagram : Section through fuselage sheet. F1 = T I " ' " 1,504 = 2,780 Ib./sq. in.. This value must be corrected for the value of K., from curve, and the actual stress will become— F1 = 2780 X .23 =64olb./sq. in,. Examining next the stress in stringer D— 250,000 X 13.18 ~~ I-5°4 = 2190 lb./sq. in. For this stress the stringer ap- pears oversi^d, but it is stated that tension stresses achieved under static test do not agree. Failures in early structures in all cases except one were tension failures, which produced the basic information presented herewith in the curves shown in Figs. 20 and 22 respectively. Recalculations of sections at points of structural failure indicated that even these reductions were not sufficient, and at present an arbitrary maximum value of 5,000 Ib./sq. in. tension stress is used in combination with the calculation of section proper- ties. The author admits that the
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
