Chapter XXII: Introduction

The following nomenclature was adopted by the National Advisory Committee for Aeronautics at its annual meeting October 10, 1918.

The purpose of its adoption and publication is to help secure uniformity in the official documents of the government as well as in the technical journals.

AERONAUTICAL NOMENCLATURE

AEROFOIL: A winglike structure, flat or curved, designed to obtain reaction upon its surfaces from the air through which it moves.

AEROFOIL SECTION: A section of an aerofoil made by a plane parallel to the plane of symmetry of the aerofoil.

AEROPLANE: See Airplane.

AILERON: A movable auxiliary surface, usually part of the trailing edge of a wing, the function of which is to control the lateral attitude of an airplane by rotating it about its longitudinal axis.

AIRCRAFT: Any form of craft designed for the navigation of the air—airplanes, airships, balloons, helicopters, kites, kite balloons, ornithopters, gliders, etc.

AIRPLANE: A form of aircraft heavier than air which has wing surfaces for support in the air, with stabilizing surfaces, rudders for steering, and power plant for propulsion through the air. This term is commonly used in a more restricted sense to refer to airplanes fitted with landing gear suited to operation from the land. If the landing gear is suited to operation from the water, the term “seaplane” is used. (See definition.)

_Pusher._—A type of airplane with the propeller in the rear of the engine.

_Tractor._—A type of airplane with the propeller in front of the engine.

AIRSHIP: A form of balloon, the outer envelope of which is of elongated form, provided with a propelling system, car, rudders, and stabilizing surfaces.

_Nonrigid._—An airship whose form is maintained by the pressure of the contained gas assisted by the car-suspension system.

_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.