Rigid.—An airship whose form is maintained by a rigid structure contained within the envelope.
Semirigid.—An airship whose form is maintained by means of a rigid keel and by gas pressure.
Air-speed meter: An instrument designed to measure the speed of an aircraft with reference to the air.
Altimeter: An aneroid mounted on an aircraft to indicate continuously its height above the surface of the earth. Its dial is marked in feet, yards, or meters.
Anemometer: Any instrument for measuring the velocity of the wind.
Angle:
Of attack (or of incidence) of an aerofoil.—The acute angle between the direction of the relative wind and the chord of an aerofoil; i. e., the angle between the chord of an aerofoil and its motion relative to the air. (This definition may be extended to any body having an axis.)
Critical.—The angle of attack at which the lift-curve has its first maximum; sometimes referred to as the “burble point.”
Gliding.—The angle the flight path makes with the horizontal when descending in still air under the influence of gravity alone; i. e., without power from the engine.
Angle of incidence (in directions for rigging): In the process of rigging an airplane some arbitrary definite line in the airplane is kept horizontal; the angle of incidence of a wing, or of any aerofoil, is the angle between its chord and this horizontal line, which usually is the line of the upper longitudinals of the fuselage or nacelle.