TM 1-320  4  AIR CORPS  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.  Awes of aircraft.-Three  fixed lines of reference, usually centroidal  and mutually perpendicular. The longitudinal axis in the plane  Aspect ratio of propeller blade.-Half  the ratio of propeller diameter  to maximum blade width.  Angle, zero  lift;.-Angle of attack of an airfoil when its lift is zero.  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, propeller blad’e.-Actual  angle between chord of propeller  section and plane perpendicular to axis of rotation of propeller.  Usually caUed "blade angle."  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 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 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 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, elevator.-Angular  displacement of elevator from neutral  position.  It is positive when trailing edge of the elevator is below  neutral position.  Angle, critical.-Angle  of attack at which the flow about an airfoil  changes abruptly with corresponding abrupt changes in lift and  drag.  Air speed’.-Speed  of an aircraft relative to the air.  Its symbol  is  V.  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). 