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ship is statically stable if it tends to return toward initial condition of steady motion whenever slightly disturbed from that motion. This requirement is not dependent upon the plane in which deviation from steady motion occurs, and, as will be shown later, an airship is statically unstable in yaw. 

(2) Dynamic stability is the stability effected by action of the air ~tream upon controlled surfaces. Were it not for these surfaces air› ships would become unmanageable at very slow speeds. 

c. Stability may be classified further. An airship in steady flight has three types of stability, pitch or longitudinal, yaw or directional, and roll about the longitudinal axis. While these stabilities are all correlated in the case of an airplane, this is not the case with an air› ship, the three types of stability being independent of each other. 

Airs/lip l'rtll'~h/1~ hDri'zonl"t~l!y in Sl"t~flc ~9uiliiJri11m. Lon~ifudintJ! t~xis coinicidenr with direction o/ mot-ion 

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d. The following discussion will be based upon the assumptions for each situation that- 

(1) Ascensional force remains constant. 

(2) Total weight remains constant. 

(3) Speed remains the same. 

(4) Form of airship remains unchanged. 

(5) Center of gravity and center of buoyancy remain fixed. 

(6) Controls remain in neutral. 

27 .. Forces and moments acting on airship.-a. Suppose an airship flies along a horizontal right-line trajectory .while its longi› tudinal axis makes an angle of oo with the flight path, then the airship will be acted on by the following forces and moments (see fig. 20). 

(1) Forces: 

(a) L$_{0}$=Lift of inflating gas acting through center of buoyancy, G. 

