Head-Resistance—Drift. The resistance of the air to the passage of a body.
Helicopter—An air-screw revolving about a vertical axis, the direction of its thrust being opposed to gravity.
Horizontal Equivalent—The plan view of a body whatever its attitude may be.
Impulse—A force causing a body to gain or lose momentum.
Inclinometer—A curved form of spirit-level used for indicating the attitude of a body relative to the horizontal.
Instability—An inherent tendency of a body, which, if the body is disturbed, causes it to move into a position as far as possible away from its first position.
Instability, Neutral—An inherent tendency of a body to remain in the position given it by the force of a disturbance, with no tendency to move farther or to return to its first position.
Inertia—The inherent resistance to displacement of a body as distinct from resistance the result of an external force.
Joy-Stick—See “Control Lever.”
Keel-Surface—Everything to be seen when viewing an aeroplane from the side of it.