d.  Referring again to the turn described in  b  above, it appears,  curiously enough, that the first effect on putting the rudder over  to  48.  o.  The turning radius is governed by the damping moment on the  envelope and is greater for an airship of large fineness ratio than for  one where this ratio is small.  It  should be one of the first concerns of  the pilot whenever he assumes control of a new type of airship to  familiarize himself with its turning radius.  Otherwise he might  very conceivably endeavor to execute a turning maneuver where the  space  limitatio~  was insufficient.  b.  It  is essential that the pilot have a clear conception of the reac› tion to rudder control of the airship in a turn. When it is desired to  turn to the right, for example, the rudder is put over to the right,.  The instantaneous effect of this rotation is to produce a force to the  left acting on the right side of the rudder. This force to the left has  a dual effect. In the first place, it gives the moment about the center  of gravity tending to turn the nose to the right. I n the second place,  it moves the entire airship to the left. As the airship moves to the  left and as its nose turns to the right, both motions combine to cause  the air to strike on the left of the envelope and so to turn the nose  still farther to the right.  After this has proceeded for an interval,  the pressure on the left-hand side of the nos? becomes equal to that on  the right-hand side of the rudder and the total resultant pressure is  therefore zero, but since one force is applied to the front and the other  to the rear, there  is  a resultant turning moment tending to continue  the twisting to the right.  As the motion proceeds still farther, the  force on the left-hand side of the envelope becomes greater than the  force on the right-hand side of the rudder and there is a centripetal  force to the right so that the airship starts to move to the right.  If  the rudder is  left in hard  or even  if  it is turned to neutral, this turn› ing to the right will continue, and in order to check the circling it is  necessary to put the rudder over to the left of the envelope.  34. Directional.-a. As stated  in  paragraph 33, the direction pilot  is charged with control of the course of the airship in a horizontal  plane.  On cross-country .flights his problem resolves itself into that  of holding the course required by the mission of the airship.  Once  the course is set, the airship will hold its own course unless acted on  by some exterior forces such as gusts.  These must be overcome by  prompt application of the rudder in the opposing direction. When  flying  in  very gusty air it is impossible tD prevent yawing, but a good  pilot can keep the magnitude of the oscillations from exceeding a few  degrees.  Then since the gusts strike about equally from both sides  the mean course of the airship will be the one desired.  TM 1-320  34  AIR CORPS 