Slip.-Diiference between mean geometrical pitch and effective pitch. Slip maY’ be expressed as a percentage of the mean geometrical pitch’ or as a linear dimension. • 

SliiJ junction.- Ratio of speed of advance through undisturbed a:ir to the product of propeller diameter by number of revolutions in unit time,_ that is, Jv• Slip function is the primary factor con› trolling propeller performance. It is 1r times ratio of forward speed to tip speed of propeller. 

Slipstream.- Stream of air driven astern by propeller. (The indraft ~- is sometimes included also.) • 

Speed, grouou.l .-Hor~zontal c•mponent of velocity of aircraft rela› tive to the earth. 

Stability.-That property of a body which causes it, when disturbed • from a condition of equilibrium or steady motion, to develop .forces or moments which tend to restore the body to its original condi› tion . 

.Automatic.-Stability dependent upon movable control surfaces automatically operated by mechanical means. 

Directional.-Stability with reference to rotations about the nor› mal axis, that is, an airship possesses directional stability in its simplest form if a restoring moment comes into action when it is given a small angle of yaw. Owing to symmetry, directional stability is closely associated with lateral stability. 

Inherent.-Stability of an aircraft due solely to disposition and arrangement of its fixed parts, that is, that property which causes it when disturbed to return to its normal attitude of flight without use of controls or interposition of any mechani› cal devices. 

Lateral.-Stability with reference to disturbances involving rolling, yawing, or side slipping, that is, disturbances in which position of the plane o£ symmetry of the aircraft is affected. 

Longitudinal.- Stability with reference to disturbances in the plane of symmetry, that is, disturbances involving pitching .and variation of longitudinal and normal velocities. 

Static.-Stability of such a cha-racter that, if the airship is dis› placed slightly £rom its normal attitude by rotation about an axis through its center of gravity (as may be done in wind tunnel experiments), moments come into play which tend to return the airship toward its original attitude. 

Streamline.-Path of a small portion of a fluid relative to a solid body with respect to which the fluid is moving. The term is coin- 

