NACELLE: The inclosed shelter for passengers or for an engine. Usually in the case of a single-engine pusher it is the central structure to which the wings and landing gear are attached. NET: A rigging made of ropes and twine on spherical balloons which supports the entire load carried. ORNITHOPTER: A form of aircraft deriving its support and propelling force from flapping wings. OVERHANG: One-half the difference in the span of the upper and lower planes of a biplane. PANCAKE: To “level off” an airplane, just before landing, at too great an altitude, thus stalling it and causing it to descend with the wings at a very large angle of incidence. PANEL: The unit piece of fabric of which the envelope is made. PARACHUTE: An apparatus, made like an umbrella, used to retard the descent of a falling body. PATCH SYSTEM: A system of construction in which patches (or adhesive flaps) are used in place of the suspension band. PERMEABILITY: The measure of the loss of gas by diffusion, through the intact balloon fabric. PITCH OF A PROPELLER: (_a_) _Pitch, effective._—The distance an aircraft advances along its flight path for one revolution of the propeller. (_b_) _Pitch, geometrical._—The distance an element of a propeller would advance in one revolution if it were turning in a solid nut—i. e., if it were moving along a helix of slope equal to the angle between the chord of the element and a plane perpendicular to the propeller axis. The mean geometrical pitch of a propeller, which is a quantity commonly used in specifications, is the mean of the geometrical pitches of the several elements. (_c_) _Pitch, virtual._—The distance a propeller would have to advance in one revolution in order that there might be no thrust. (_d_) _Pitch speed._—The product of the mean geometrical pitch by the number of revolutions of the propeller in unit time—i. e., the speed the aircraft would make if there were no slip. (_e_) _Slip._—The difference between the effective pitch and the mean geometrical pitch. Slip is usually expressed as a percentage of the mean geometrical pitch. PITCH, ANGLE OF: The angle between two planes, defined as follows: One plane includes the lateral axis of the aircraft and the direction of the relative wind; the other plane includes the lateral axis and the longitudinal axis. (In horizontal normal flight this angle of pitch is, then, the angle between the longitudinal axis and the direction of the relative wind.)