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be equal to produce motion in a constant turn. From this is derived the relation- 

<img src='content_image/534953.jpg'>

where ’I!= angle of yaw. 

a=distance from center of volume to center of pressure of tail surfaces. 

R = radius of turning circle. 

This relation gives results widely at variance :from the results o:f actual. tests on full-sized airships, presumably due to the assumption that the• resultant of the hull forces is zero. Fortunately, the total bending moment due to a steady angle of turn is only about one-fifth as great as that due to an equal fixed angle of pitch where unbalanced weight and centrifugal force are of equal magnitude. 

41. Forces caused by gusts.-a. Very little is known concerning maximum value of forces caused by gusts. The following statement very excellently sums up the situation : 

1 "The existence of veritable fountains of upward rushing air whose sides at times and places are sharply separated from the surrounding atmosphere must be taken into account in the design of airships. The most violent of such currents, the tornado, combines vertical velocity with rotation, but fortunately can be seen from a great distance, and can and must be avoided. The thunderstorm with large fully devel› oped cumulus tops is also conspicuous and avoidable. It would appear to be :folly to enter such a cloud and subject the ship to the unknown dangers of wind, rain, hail, and lightning. Barring •such spectacular hazards, there remain convection currents which the ship may run into at full speed. There is ample evidence that upward velocities as high as 10 feet per second may be encountered. This vertical air velocity u, combined with the relative horizontal speed v of the airship, will give 

-1 the effect of a change of pitch of tan vu., 

b. It remains simply for the pilot, as stated in paragraph 38e, to reduce the speed in bumpy atmosphere, especially if at the same time the airship is developing large dynamic lift, positive or negative, as then the stresses are already large. 

42. Empirical formulas for maximum aerodynamic bendin g moment on hull and for forces on tail surfaces.-a. The follow› ing formula has been developed :for the maximum aerodynamic bend- 

