Shape and Framing. In the early days of dirigible design the data upon which the shape and proportions of the envelope were based were purely empirical. Schwartz, Germany’s pioneer in this field, adopted the projectile as representing the form offering the least air resistance and accordingly designed his envelope with a sharply pointed bow and a rounded-off stern, giving it a length four times its diameter. Zeppelin did not agree with these conclusions and adopted a pencil form, rounded at the nose and tapering to a sharp point at the stern, making the length nine to ten times the diameter. Subsequent research work in the aerodynamic laboratory has demonstrated that the most efficient form for air penetration is one having a length six times its maximum diameter with the latter situated at a point four-tenths of the total length from the bow. It has likewise been proved that an ellipse is more efficient than either the projectile or pencil form and that tapering to a sharp point at the stern offers no particular advantage. As a result, the most approved form resembles the shape of a perfecto cigar, the nose being somewhat blunter than the after end. This form is likewise that of the swiftest-swimming fishes and has been shown to have the least head resistance as well as the minimum skin friction; it results in a section to which the term stream-line has been applied, and it is now employed on all exposed non-supporting surfaces on aeroplanes, such as the struts and even the bracing cables. Laboratory research has demonstrated that it is worth while to reduce the head resistance of even such apparently negligible surfaces as those presented by these wires and cables and, therefore, they are stream-lined by attaching recessed triangular strips of wood to their forward sides.
Framing Details. Despite this, the builders of the Zeppelins have adhered to the original pencil shape with but slight modifications at the bow and stern, probably because that shape is much easier to build and assemble from standard girders. The form of girder employed is shown in Fig. 17, while the complete assembly of the frame is illustrated in Fig. 18. The girders form the longerons, or longitudinal beams, running the entire length of the rigid frame and supported at equidistant points by ring members built of similar girder sections. The fourth ring from the nose and each alternate ring after that are further braced by being trussed to the longitudinal beams around their entire circumferences, as shown in Fig. 18. The larger V-shaped truss at the bottom forms the gangway, which is now placed inside the envelope instead of being suspended beneath it, as formerly. This is done to eliminate the head resistance set up by the additional surface thus exposed. In the first instance in which this gangway was incorporated in the envelope, no provision was made for ventilation, and the ship was wrecked by a gas explosion. Regardless of how tight the fabric is made, gas is always oozing out through it to a greater or less extent. This fact is now met by providing ventilating shafts leading from the gangway to the upper surface of the envelope. Additional shafts through the envelope lead to gun platforms, forward, amidships, and aft, and are reached by aluminum ladders.
Fig. 17. Trellis Type of Aluminum Girder used in Longitudinals of Zeppelin Frame
Fig. 18. Aluminum Frame Construction of Zeppelin Hull
Framing of Schutte-Lanz Type. It has become customary to refer to all large German airships as Zeppelins, but many of those used during the past three years have been of the Schutte-Lanz build, which is also a rigid frame type of dirigible but has been designed with a view of overcoming some of the disadvantages of the aluminum frame construction encountered in the use of the Zeppelin. The length and diameter of the latter airships are such that, no matter how rigidly the framing is assembled, there is more or less sag. When the sag exceeds a certain amount, the frame is apt to buckle at the point where it occurs, involving expensive repairs or wrecking the airship altogether. To overcome this difficulty, the Schutte-Lanz type employs a rigid frame of flexible material, namely, laminated wood in strip form, held together at joints and crossings by aluminum fittings and braced inside by cables. As shown by Fig. 19, no rigid longitudinal beams are employed, the only girders used being rings, to which a network built of the wood strips is attached. Starting at the nose, each continuous strip follows an open spiral path such as would be traced in the air by a screw of very large pitch, in fact, approximating the rifling of a gun barrel. It will also be noted from the illustration that the form of the Schutte-Lanz airship is the cigar-shape, which laboratory research has shown to be the most efficient.
Fig. 19. Schutte-Lanz Type of Frame Construction of Laminated Wood with Aluminum Fittings
The use of wood in conjunction with the spiral construction of the supporting members of the framing affords the maximum degree of flexibility, since the displacement of any of these members under stresses of either tension or compression would have to be very great to cause damage to the frame as a whole. The frame not being rigid, strictly speaking, either as units or as a complete assembly, stress at any particular point would simply cause all the members near that point to give in the direction of the strain, and the rest of the frame would accommodate itself to their change of position by either elongating or shortening slightly. In addition to these advantages, the Schutte-Lanz type of construction is said to be lighter than the Zeppelin for an airship of the same load-carrying capacity.