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NOMENCLATURE FOR AERONAUTICS 17 surface of the water. It contains accommodations for the crew and passengers, usually combining the functions of both float and fuselage. hump speed- See SPEED, HUMP. hydrofoil (or hydrovane)- Any surface designed to obtain reaction from the water through which it moves. hydroplane- See PLANE. identification light- See LIGHT, IDENTIFICATION. Immelman turn, normal- A maneuver made by completing the first half of a normal loop; from the inverted position at the top of the loop, half-rolling the airplane to the level position, thus obtaining a 180° change in direction simultaneously with a gain in altitude. (See fig. 8.) impact pressure- The pressure acting at the forward stagnation point of a body, such as a pitot tube, placed in an air current. Impact pressure may by measured from an arbitrary datum pressure. incidence, angle of- See ANGLE OF WING SETTING. inclinometer- An instrument that measures the attitude of an aircraft with respect to the horizontal. induced drag- See DRAG, INDUCED. induction compass- See COMPASS, EARTH-INDUCTOR. induction system, rotary- A carburetor induction system used on radial engines, in which a rotary fan assists in distributing the fuel charge to the cylinders. inflation net- See NET, INFLATION. inflation sleeve- See SLEEVE, INFLATION. inflow- The flow of air into a propeller. instability, spiral- A type of instability, inherent in certain airplanes, which becomes evident when the airplane assumes too great a bank and sideslips; the bank continues to increase and the radius of the turn to decrease. instrument flying- The art of controlling an aircraft solely by the use of instruments; sometimes called "blind flying." intake header- A short duct extending from outside the engine cowling to the supercharger intake. interceptor- A lateral-control device consisting of a small plate placed just back of a wing slot to spoil the effect of the slot at high angles of attack. (See fig 1.) (Cf. spoiler.) interference- The aerodynamic influence of two or more bodies on one another. inverted flight (stress analysis)- A loading condition for the wings simulating the conditions of flying upside down and of commencing a dive (cf. dive(stress analysis).) jackstay- A longitudinal rigging provided to maintain the correct distance between various parts in the fittings of an aerostat. keel, airship- The assembly of members at the bottom of the hull of a semirigid or rigid airship, which provides special strength to resist hogging and sagging and also serves to distribute the effect of concentrated loads along the hull. (See fig. 4.) 703-34-3 kite balloon- See BALLOON, KITE. laminar flow- See FLOW, LAMINAR. landing- The act of terminating flight in which the aircraft is made to descend, lose flying speed, establish contact with the ground, and finally come to rest. glide landing- A landing in which a steady glide is maintained to the landing surface without the usual leveling-off before contact. level landing (stress analysis)- A loading condition for the fuselage and landing gear, representing a two-point landing with the fuselage horizontal. normal (or three-point) landing- A landing in which a path tangential to the landing surface and the loss in flying speed are attained at approximately the instant of contact. pancake landing- A landing in which the leveling-off process is carried out several feet about the ground, as a result of which the airplane settles rapidly on a steep flight path in a normal attitude. three-point landing (stress analysis)- A loading condition for the fuselage and landing gear, representing landing with the wheels and tail skid touching the ground simultaneously (cf. level landing). landing angle- See ANGLE, LANDING. landing area, effective- That portion of the landing area, with approaches clear within the allowable safe climbing and gliding angle, available for the take-off and landing of aircraft. landing-direction light- See LIGHT, LANDING-DIRECTION. landing field- Any area of land designed for the take-off and landing of aircraft. It may or may not be part of an airport. landing gear- The understructure which supports the weight of an aircraft when in contact with the land or water and which usually contains a mechanism for reducing the shock of landing. Also called "undercarriage." (See fig. 5.) retractable landing gear- A type of landing gear which may be withdrawn into the body or wings of an airplane while it is in flight, in order to reduce the parasite drag. landing light- See LIGHT, LANDING. landing strip- A narrow and comparatively long area forming part of a landplane airport or of an intermediate or auxiliary field, which is suitable for the landing and take-off of airplanes under ordinary weather conditions. landplane- An airplane designed to rise from and alight on the land. lateral axis- See AXES OF AN AIRPLANE. leading edge- The foremost edge of an airfoil or propeller blade. length, chord- See CHORD LENGTH.