_Ground Angle_--The angle of incidence of an aeroplane surface when the machine is standing on the ground. _Guy_--A wire attached to a more or less distant part of the structure of any aircraft to prevent spreading. Also used to denote controlling wires which transmit the movements of the levers. _Gyroscopic Action_--The resistance which a rotating wheel, or wheel-like construction, exhibits when a disturbing force tends to change its plane of rotation. H _Hangar_--A structure for the housing of aeroplanes. _Head Resistance_--The resistance encountered by a surface moving through the air. _Heavier-than-air_--A term applied to flying machines whose weight is not counterbalanced or buoyed up by the lifting power of some gas lighter than air; and which weigh more than the volume of air displaced. _Helicopater_--A type of flying machine in which propellers revolving horizontally lift and sustain its weight in the air. _Horizontal Rudder_--The rudder surface which is used to steer an aircraft upward or downward: so-called because it lies normally in a position parallel to the horizon; that is, level. _Horse-power_--An amount of work equivalent to the lifting of 33,000 footpounds in one minute. See Footpound. _Hyperbola_--The outline formed by the cutting of a cone by a plane passing one side of its axis at such an angle that it would also intersect another cone placed apex to apex on the same axis. K _Keel_--A framework extending lengthwise under an aircraft to stiffen the construction: usually employed on airships with elongated gas-bags. L _Lateral_--From side to side; that is, crossing the length fore-and-aft, and generally at right angles to it. _Land-speed_--The speed of aircraft as related to objects on the ground. See Air-speed. _Landing Area_--A piece of land specially prepared for the alighting of aeroplanes without risk of injury. _Leeway_--Movement of a machine aside from the intended course, due to the lateral drift of the whole body of air; measured usually at right angles to the course. _Lift_--The raising, or sustaining effect of an aeroplane surface. It is expressed in the weight thus overcome. _Lighter-than-air_--A term used to designate aircraft which, owing to the buoyancy of the gas attached, weigh less than the volume of air which they displace. _Longitudinal_--In a lengthwise, or fore-and-aft direction. M _Main Plane_--The principal supporting surface of an aeroplane. In the biplane, or the multiplane type, it denotes the lowest surface, unless some other is decidedly larger. _Main Landing Wheels_--Those wheels on the alighting gear which take the shock in landing.