Ceiling: Service.—The height above sea level at which a given aircraft ceases to rise at a rate higher than a small specified one, say 100 feet per minute. This specified rate may be different in the services of different countries.
Absolute.—The maximum height above sea level to which a given aircraft can rise.
Theoretical.—The limiting height to which a given aircraft can rise determined by computations of performance, based upon the drawings and wind tunnel data.
Center of pressure of an aerofoil: The point in the plane of the chords of an aerofoil, prolonged if necessary, through which at any given attitude the line of action of the resultant air force passes. (This definition may be extended to any body.)
Chord of an aerofoil section:
For theoretical purposes.—The zero lift line, i. e., the limiting position, in the section, of the line of action of the resultant air force when the position of the section is such that the lift is zero.
Practical.—The line of a straightedge brought into contact with the lower surface of the section at points near its edges. In the case of an aerofoil having double convex camber, the straight line joining the entering and trailing edges.
Length.—The length of the chord is the length of the projection of the aerofoil section on its chord.
Clinometer: See Inclinometer.
Concentration ring: A hoop to which are attached the ropes suspending the basket of a spherical balloon.