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Aviation History
1920
1920 - 0159.PDF
FEBRUARY 5/ 1920 THE PRINCIPLES OF RIGID AIRSHIP CONSTRUCTION BY A. P. COLE, R.C.N.C., A.M.Inst.N.A. (Continued from page 133.) Structural Strength of the Ship THE" following remarks will apply particularly to the type of rigid airship known as the Zeppelin or Schutte-Lanz, i.e.,pwhen the structure is built up of a number of longi- tudinal girders running throughout the ship, connected at intervals by transverse girders forming a frame round the ship (Fig. 2). This scheme of construction has been found to give^the lightest ship, and to be most readily adaptable will tend to become slack. The wires which will be in tension can be readily ascertained from the sign and slope of the shear- ing force in the shearing force diagram. In addition to these stresses due to being a part of the ship's structure, the longitudinals are subjected to lateral loads due to the gas pressure. Each longitudinal is assumed to take the gas pressure on a panel of width on either side of the longitudinal equal to half the spacing of the longitudinal Fig. 2.—Skeleton structure of Zeppelin to use in very large ships. Considering first the ship as a whole: At any section there will be a bending moment and a shearing force in the vertical plane due to a combination of the static forces (i.e., lift and weight) and aerodynamic' forces (due to air pressure on the outer cover when the ship is running trimmed by the bow or the stern) ; and a bending moment and shearing force in the horizontal plane due to aerodynamic forces when turning. To meet these bending moments and shearing forces we have a built-up structure consisting of successive panels round the ship formed by the longitudinals and transverse girders, each panel being internally braced by wires, called diagonal wires (Fig. 3). It is assumed that the whole of the forces due to bending moment are taken by the longitudinal girders ; the distribu- tion of stress being according to the well-known beam formula P = My/I, y being measured from the neutral axis of the ship's section ; I being measured by the product of the area of the continuous members of the longitudinal girders and the square of the distance of their centroid from the neutral axis of the ship. For the purpose of this calculation, all continuous main longitudinal girders, including those in the corridor, are included. The shearing force is assumed taken entirely by the diagonal wires. For the simple diagojaal wire system, i.e., with the wires being contained entirely in the one panel, it can be easily shown that the tensions in the wires which take the shearing force at a frame are given by the formula LS.F. g A sin A sin where S F ^ , L Shearing force at the frame* " Length of wire in the panel: ' * Length of side of transverse frame. * •."•"/" A = Area of cross section of diagonal wire.J —• 4, = Angle which panel containing the wire makes with the horizontal plane. The summation 2 is taken over the whole section. Obviously at any one section, only the wires running in one direction will be stressed, while those in the opposite direction girders from each other. The gas pressure is obtained from the formula—• Pressure in kilograms per sq. metre = maximum blowing off pressure of automatic valve in mm. water above atmosphere -+- (height of base of longitudinal above automatic valve in metres) x 1 -09. The pressure obtained from this formula is the maximum that can be obtained in practice, and corresponds to the ship rising rapidly near the ground. Further the pressure of the air on the outer cover due to motion is, in general, in the opposite direction to the gas pressure, and conse- quently tends to diminish the lateral load. Hence the stress obtained from the combination of the end load and lateral load as calculated above is in general the highest that can occur in flight. The maximum lateral load will occur on the top longi- tudinals. It is, therefore, extremely desirable that the top longitudinals should be in tension rather than compression. Hence the distribution of loads should be such that the static bending moment at any section of the ship in all conditions should be hogging, i.extending to put the top longitudinals in tension. This principle, incidentally, is also used in some types of semi-rigids, the top longitudinals being replaced by fabric, which is thus always in tension. With a view to decreasing the lateral load on those longi- tudinals which in addition have to take an end load due to the bending moment on the ship, in R 33 and similar ships intermediate longitudinals have been fitted. The end con- nections of these intermediate longitudinals are so arranged that the lateral load is transferred to the main frames, whilst practically no end load comes on the longitudinals. The stresses in the main longitudinals are obtained from the generalised equation of three moments, assuming that each girder is continuous throughout the ship, and that the deflection at each transverse frame is zero. The necessary conditions for obtaining the bending moment are obtained by assuming that in the parallel body the slope of the girder at the transverse frame is zero. (To be continued.) 159 /••--.
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