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NOMENCLATURE FOR AERONAUTICS 21 overshoot-To fly beyond a designated mark or area, such as a landing field, while attempting to land on the mark or within the area. pancake landing-See LANDING, PANCAKE. panel (aerostat)-(1) The unit piece of fabric of which the envelope or outer cover of an aerostat is made. (2) In rigid airships, the area bounded by two consecutive longitudinals and two consecutive transverses. panel (airplane)-A portion of an airplane wing constructed separately from the rest of the wing to which it is attached. panel, rip-A strip of fabric, inserted or fitted in the upper part of the envelope of a balloon or semirigid or nonrigid airship, which is torn or ripped open when immediate deflation is desired. (See fig. 3.) parachute-An umbrella-like device used to retard the descent of a falling body by offering resistance to its motion through the air. pilot parachute-A small auxiliary parachute attached to the apex of the main parachute, designed to pull the latter out of its pack when the rip cord is pulled. parachute flare-See FLARE, PARACHUTE. parachute harness-A combination of straps, buckles, and fastenings used to attach a parachute to the wearer. parachute pack- A parachute and its container. parachute rigger-A person who packs, repairs, and inspects parachutes. parasite drag-See DRAG, PARASITE. parasol monoplane-See MONOPLANE, PARASOL. patch-A strengthened or reinforced flap of fabric of special shape and construction cemented to the envelope or gas cell of an aerostat. It usually forms an anchorage by which some portion of the structure may be attached to the envelope, or by which the positioning lines controlling the gas cell may be attached to the cell. channel patch-A channel-shaped fabric fitting secured to the envelope of an aerostat to allow a rod or spar to be laced to the envelope. finger patch-A special form of patch having "fingers" extending from the central portion to distribute the load more widely to the fabric of an envelope or gas cell. suspension patch-A patch, secured to the envelope or to a gas cell of an aerostat, to which a suspension line may be attached. path, flight-See FLIGHT PATH. pay load-See LOAD, PAY. pendant, sighting-A vertical wire on the center line and forward of the control car of an airship, used as a guide in steering and to assist in determining the direction of the wind. permeability-The measure of the rate of diffusion of a gas per unit area of any material used in the construction of a gas container. pilot-One who operates the controls of an aircraft in flight. pilot, automatic-See AUTOMATIC PILOT. pilot balloon-See BALLOON, PILOT. pilot parachute-See PARACHUTE, PILOT. pilot plane-An auxiliary airfoil pivoted near the leading edge of a main airfoil and free to take up a position in line with the wind. pitch-An angular displacement about an axis parallel to the lateral axis of an aircraft. pitch, angle of-See ANGLE OF PITCH. pitching-Angular motion about the lateral axis(fig. 12). pitch (or pitching) indicator-An instrument for indicating the existence and approximate magnitude of the angular velocity about the lateral axis of an aircraft. pitch of a propeller: effective pitch-The distance an aircraft advances along its flight path for one revolution of the propeller. geometrical pitch-The distance an element of a propeller would advance in one revolution if it were moving along a helix having an angle equal to its blade angle. zero-thrust pitch-The distance a propeller would have to advance in one revolution to give no thrust. Also called "experimental mean pitch." pitch ratio (propeller)-The ratio of the pitch to the diameter. pitot-static tube-A parallel or coaxial combination of a pitot and a static tube. The difference between the impact pressure and the static pressure is a function of the velocity of flow past the tube. pitot tube-A cylindrical tube with an open end pointed upstream, used in measuring impact of pressure. pito-venturi tube-A combination of a pitot and a venturi tube. plane (or hydroplane)-To move through the water at such a speed that the support derived is due to hydrodynamic and aerodynamic rather than to hydrostatic forces. plan form, developed-The plan of an airfoil as drawn with the chord lines at each section rotated about the airfoil axis into a plane parallel to the plane of projection and with the airfoil axis rotated or developed and projected into the plane of projection. plan form, projected-The contour as viewed from above. platform, observation-A small deck fitted on the top of an airship for a lookout and defense or for making observations used in navigating the airship. plowing-Taxying a seaplane at low speed before rising on the step. pontoon-Obsolete as applied to aircraft. (see FLOAT.) porpoising-An undulatory movement of a seaplane consisting of a combination of a vertical oscillation about its transverse axis, which occurs at certain stages of planing.