TM 1....:.:320  29-Sl  AIR CORPS  those necessary on a small one traveling at the same speed. An un› stable airship requires closer attention from the pilot than does one  which is stable, but it is not necessarily either difficult or dangerous  •to operate and has the advantage of being more easily maneuverable  than the more stable types.  30.  Directional stability.-a. Directional stability is maintained  in part by use of vertical fixed fins and rudder.  When the rudder  is set in neutral it acts as additional  fin  surface, but the total  fin  sur› face is never large. enough to provide complete directional stability  Since there is no statical restoring moment to overcome a horizontal  deviation from the flight path, maintenance of directional stability  devolves upon the pilot who must correct any deviations as soon as  they appear.  Otherwise a deviation once started will tend  to  in› . crease until the airship is traveling in a circle of so small a radius  that the  d~mping  moment balances the turning moment due to pres› sure on the nose.  This is quite different from the condition of longitu› dinal stability where the elevator can  be  left locked in any particular  position and the airship will return to its original attitude if atmos› pheric disturbances have momentarily changed that attitude.  b.  As soon as there is any deviation from the straight line of flight  the a.ir strikes on the side of the envelope and sets up a moment tend› ing to turn the airship farther from its original course.  This moment  corresponds exactly to the upsetting moment,  Me,  which opposes lon› gitudinal stability. There is then an unbalanced moment which tends  to give the airship an angular acceleration and so to turn her more  and more rapidly.  At the same time the lateral force on the envelope,  which corresponds to the dynamic lift, is increasing and .furnishes the  necessary centripetal force to keep the airship traveling in a circular  path.  It  is quite true that a force resisting this circling is exerted by  the vertical surfaces, but, as mentione.d above, the vertical fin surfaces  are never large enough to provide full stability, and the rudder must  be used to assist them.  Use of the rudder will be more fully discussed  in sec6on V.  31. Lateral stability.-a. Stability in roll, which is a very diffi› cult problem in airplanes, is taken care  of  almost automatically in air› ships; since the same statical restoring moment acts with regard to roll  as with regard to pitch and there is no dynamic upsetting moment to  oppose it. The only rolling motions are those due to side gusts against  the car and bag and those due to centrifugal force when turning. The•  moments of these forces are overcome immediately by the large re› storing moment due to the low position of the center of gravity. Roll- 46 