TM 1-320  4  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.  6  AIR CORPS 