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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.)

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.