personnel, cargo, or equipment. It may be suspended from the buoy› ant portion or it may be built close up against it. It is not to be applied to parts of the keel of a rigid or semirigid airship which have been fitted for the purposes mentioned. 

Oeiling, static.-Altitude in standard atmosphere at which an aerostat is in static equilibrium a.fter removal of all dischargeable weights. 

Oenter of presswre coetflaient.- Ratio of distance of center of pressure from leading edge to chord length. 

Oenter of pressure of cd1•foil section.-Point in chord of airfoil sec› tion, prolonged if necessary, which is at the intersection of the chord and the line of action of the resultant air force. Abbreviation is C. P. 

Ohord (of airfoil section) .-Line of straightedge brought into con› tact with lower surface of the section at two points; in the case of an airfoil having double convex camber, the straight line joining the leading and trailing edges. (These edges may be defined for this purpose as the two points in the section w~ich are farthest apart.) The line joining leading and trailing edges should be used also in those cases in which lower surface is convex except for a short flat portion. The method used for determining the chord should always be explicitly stated for those sections concerning which ambiguity seems likely to arise. 

Ohord length.-Length of projection of airfoil section on its chord. Its symbol is c. 

Oont1•ols .-General term applied to means provided to enable the pilot to control speed, direction of flight, altitude, and power of aircraft. 

D1•ag.-C omponent parallel to relative wind of total air force on aircraft or airfoil. Its symbol is D . 

Dynamic (or impact) pressure.-Product 1;2pV 2 , where p is density and V is relative speed of the air. It is the quantity measured by most air speed instruments. Its symbol is q. 

:Elervator.-Movable auxiliary airfoil, function of which is to impress pitching moment on the aircraft. The elevator is usually hinged to the stabilizer. 

Envelope.-Outer covering of aerostat, usually of fabric. It may or may not be also the gas container. It may be divided by dia› phragms into separate gas compartments or cells, and it may also contain internal air cells or ballonets. 

F light path.-Path of center of gravity of aircraft with reference to the earth. 

H orsepower of engine, ma.:vim-wm.-Maximum horsepower engine can develop. 

