Angle of Incidence, Optimum—The angle of incidence at which the lift-drift ratio is the highest.
Angle, Gliding—The angle between the horizontal and the path along which an aeroplane, at normal flying speed, but not under engine power, descends in still air.
Angle, Dihedral—The angle between two planes.
Angle, Lateral Dihedral—The lifting surface of an aeroplane is said to be at a lateral dihedral angle when it is inclined upward towards its wing-tips.
Angle, Longitudinal Dihedral—The main surface and tail surface are said to be at a longitudinal dihedral angle when the projections of their neutral lift lines meet and produce an angle above them.
Angle, Rigger's Longitudinal Dihedral—Ditto, but substituting "chords" for "neutral lift lines."
Angle, Pitch—The angle at any given point of a propeller, at which the blade is inclined to the direction of motion when the propeller is revolving but the aeroplane stationary.
Altimeter—An instrument used for measuring height.
Air-Speed Indicator—An instrument used for measuring air pressures or velocities. It consequently indicates whether the surface is securing the requisite reaction for flight. Usually calibrated in miles per hour, in which case it indicates the correct number of miles per hour at only one altitude. This is owing to the density of the air decreasing with increase of altitude and necessitating a greater speed through space to secure the same air pressure as would be secured by less speed at a lower altitude. It would be more correct to calibrate it in units of air pressure. [1]