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ing may be very uncomfortable because of the short and snappy period, but there is never any danger of its reaching an excessive value. 

b. The static stability of an airship wit~ regard to both roll and pitch may be increased by lowering the car, but this gives equilibrium only at the sacrifice of ease of control and efficiency, since lowering the thrust line increases the thrust moment and lowering the car increases length of suspensions and hence parasite resistance. 

32. Summary.-a. Airship stability may be summarized as follows : 

(1) Airships are very stable about their lateral axis. In this nr gard the designer has no trouble whatsoever. 

(2) Airships must be designed carefully to give longitudinal sta› bility. This problem is however of more interest to the designer than to the pilot. 

(3) Airships are statically unstable in yaw, necessitating the closest attention on the part of the direction pilot to counteract circling by means of the rudder. 

b. No concrete problems have been given in this section as the appli› cation of fundamentals covered therein will be shown in section V. 

## SECTION v CONTROL 

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33. General types.-a. Control of airships may be subdivided into two classes, directional and altitude. On nearly all airplanas these two types of control are so interrelated as to necessitate their both being performed by one pilot. In airships this is not the case, and on all but the smallest airships two pilots are utilized, one for direction, one for altitude. . 

b. For efficient performance the two pilots should be familiar with each other’s style of flying and constantly alert to render each other assistance. For instance, to obtain the proper additional superheat to effect a landing (see TM 1- 325), the altitude pilot may desire a longer approach than usual. The direction pilot should so arrange the course as to meet needs of the situation. Instances of the value of coordination are too numerous to mention, but fortunately capable pilots have little difficulty in achieving desired results. 

