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22! Variation of pressure distribution on airship hull.--a. In section II resistance of an airship was shown to be parfly caused by increased n<?Se pressure. Throughout the discussion the airship was considered to be flying on an even keel and in a straight line .. All forces were parallel to the direction of flight. Before entering• the subject of stability proper it will be necessary to show variation in pressure distribution on the hull when the airship is not flying as considered in section II, or, in other words, when transverse aero- . dynamic forces are present on the hull. 

b. Figure 16 shows a typical pressure distribution on an airship hull when the airship is in horizontal flight in a straight line and on an even keel. This pressure distribution will be true whenever the line join~ ing the tip of the nose with the tip of the tail (longitudinal axis) is 

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pa.rallel with the direction of motion. Because an airship can be con~ . sidered as a symmetrical solid of revolution, the pressure distribnt.ion has the following charactertistics : • 

( 1) Distribution depicted is uniform for any plane passed through ~ the longitudinal axis. 

(2) Varying reduced pressure exists from a section just in rear of the nose to a section just forward of the tail. 

{3) Both nose and tail have positive pressure, but that on the tail is too small to be of much assistance to forward motion. 

c. Figure 17 shows the distributions in pressure for an 18° angle of attack to the relative air. Other angles of attack have similar dis› tributions. The distribution shown holds equally true whether the deviation of the axis from the direction of motion is in a horizontal or a vertical plane. When, for instance, the inclination is in the verti› cal plane, the following chara_cteristics are observed: 

(1) Positive pressure on the nose lies almost entirely in a zone be› neath the axis. 

(2) Plane of transition, BO, figure 17, is oblique with regard to the . . aX;J.S. 

