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. CONTROLS: A general term applying to the means provided for operating the devices used to control speed, direction of flight, and attitude of an aircraft. CONTROL COLUMN: The vertical lever by means of which certain of the principal controls are operated, usually those for pitching and rolling. CROSS-WIND FORCE: The component perpendicular to the lift and to the drag of the total force on an aircraft due to the air through which it moves. CROW’S-FOOT: A system of diverging short ropes for distributing the pull of a single rope. DECALAGE: The angle between the chords of the principal and the tail planes of a monoplane. The same term may be applied to the corresponding angle between the direction of the chord or chords of a biplane and the direction of a tail plane. (This angle is also sometimes known as the longitudinal V of the two planes.) DIHEDRAL IN AN AIRPLANE: The angle included at the intersection of the imaginary surfaces containing the chords of the right and left planes (continued to the plane of symmetry if necessary). This angle is measured in a plane perpendicular to that intersection. The measure of the dihedral is taken as 90° minus one-half of this angle as defined. The dihedral of the upper planes may and frequently does differ from that of the lower planes in a biplane. DIRIGIBLE: See Airship. DIVING RUDDER: See Elevator. DOPE: A general term applied to the material used in treating the cloth surface of airplane members and balloons to increase strength, produce tautness, and act as a filler to maintain air-tightness; it usually has a cellulose base. DRAG: The component parallel to the relative wind of the total force on an aerofoil or aircraft due to the air through which it moves. In the case of an airplane, that part of the drag due to the wings is called “wing resistance”; that due to the rest of the airplane is called “parasite resistance.” DRIFT: See Drag. Also used as synonymous with “leeway,” q. v.