Se1nirigid.- Airship whose form is maintained by means of a rigid or jointed keel in conjunction with internal pressure in the gas containers and ballonets (fig. 2). 

The term "airship" is sometimes incorrectly applied to heavier than air aircraft either in full or as "ship." This is a slang use of the word and should be avoided. 

Air speed’.-Speed of an aircraft relative to the air. Its symbol is V. 

Angle, critical.-Angle of attack at which the flow about an airfoil changes abruptly with corresponding abrupt changes in lift and drag. 

Angle, elevator.-Angular displacement of elevator from neutral position. It is positive when trailing edge of the elevator is below neutral position. 

Angle of attack.-Acute angle bet-.veen the. chord of an airfoil and its direction of motion relative to the air. (This defin ition may be extended to other bodies than airfoils.) Its symbol is a. 

Angle of pitch.-Acute angle between two planes defined as follows: One plane includes lateral axis of the aircraft and direction of the relative wind; the other plane includes lateral axis and longi› tudinal axis. (In normal fli ght the angle of pitch is the angle between longitudinal axis and direction of relative wind.) This angle is denoted by 0 and is positive when nose of the aircraft has • nsen. 

Angle of roll, or angle of bank.- Acute angle through which aircraft must be rotated about its longitudinal axis in order to bring its lateral axis into a horizontal plane. This angle is denoted by <I> and is positive when the left wing is higher than the right. 

Angle of yaw.-Acute angle between direction of relative wind and plane of symmetry of an aircraft. This angle is denoted by ’II and is positive when the aircraft has turned to the right. 

Angle, propeller blad’e.-Actual angle between chord of propeller section and plane perpendicular to axis of rotation of propeller. Usually caUed "blade angle." 

Angle, rudder.-Acute angle between rudder and plane of symmetry of the aircraft. It is positive when trailing edge has moved to the left with reference to normal position of pilot. 

Angle, zero lift;.-Angle of attack of an airfoil when its lift is zero. 

Aspect ratio of propeller blade.-Half the ratio of propeller diameter to maximum blade width. 

Awes of aircraft.-Three fixed lines of reference, usually centroidal and mutually perpendicular. The longitudinal axis in the plane 

