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. 

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. 

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. 

d. Referring again to the turn described in b above, it appears, curiously enough, that the first effect on putting the rudder over to 

