STANDARD ATMOSPHERE —A hypothetical atmosphere based on averages in which the surface temper- ature is 59°F (15°C), the surface pres- sure is 29.92 in. Hg (1013.2 Mb) at sea level, and the temperature lapse rate is approximately 3.5°F (2°C) per 1,000 feet. ROTATIONAL VELOCITY —The component of relative wind produced by the rotation of the rotor blades. PARASITE DRAG —The part of total drag created by the form or shape of helicopter parts. ROTOR —A complete system of rotating airfoils creating lift for a heli- copter or gyroplane. PAYLOAD —The term used for pas- sengers, baggage, and cargo. PENDULAR ACTION —The lateral or longitudinal oscillation of the fuse- lage due to it being suspended from the rotor system. ROTOR DISC AREA —See disk area. STATIC STOP —A device used to limit the blade flap, or rotor flap, at low r.p.m. or when the rotor is stopped. ROTOR BRAKE —A device used to stop the rotor blades during shutdown. PITCH ANGLE —The angle between the chord line of the rotor blade and the reference plane of the main rotor hub or the rotor plane of rotation. PREROTATION —In a gyroplane, it is the spinning of the rotor to a suffi- cient r.p.m. prior to flight. ROTOR FORCE —The force pro- duced by the rotor in a gyroplane. It is comprised of rotor lift and rotor drag. STEADY-STATE FLIGHT —A con- dition when a rotorcraft is in straight- and-level, unaccelerated flight, and all forces are in balance. SYMMETRICAL AIRFOIL —An airfoil having the same shape on the top and bottom. TAIL ROTOR —A rotor turning in a plane perpendicular to that of the main rotor and parallel to the longitudinal axis of the fuselage. It is used to con- trol the torque of the main rotor and to provide movement about the yaw axis of the helicopter. SEMIRIGID ROTOR —A rotor sys- tem in which the blades are fixed to the hub but are free to flap and feather. SETTLING WITH POWER —See vortex ring state. PRESSURE ALTITUDE —The height above the standard pressure level of 29.92 in. Hg. It is obtained by setting 29.92 in the barometric pressure win- dow and reading the altimeter. SHAFT TURBINE —A turbine engine used to drive an output shaft commonly used in helicopters. TEETERING HINGE —A hinge that permits the rotor blades of a semi- rigid rotor system to flap as a unit. SKID —A flight condition in which the rate of turn is too great for the angle of bank. PROFILE DRAG —Drag incurred from frictional or parasitic resistance of the blades passing through the air. It does not change significantly with the angle of attack of the airfoil section, but it increases moderately as airspeed increases. THRUST —The force developed by the rotor blades acting parallel to the relative wind and opposing the forces of drag and weight. TIP-PATH PLANE —The imaginary circular plane outlined by the rotor blade tips as they make a cycle of rotation. TORQUE —In helicopters with a sin- gle, main rotor system, the tendency of the helicopter to turn in the opposite direction of the main rotor rotation. TRAILING EDGE —The rearmost edge of an airfoil. SKID SHOES —Plates attached to the bottom of skid landing gear pro- tecting the skid. SLIP —A flight condition in which the rate of turn is too slow for the angle of bank. SOLIDITY RATIO —The ratio of the total rotor blade area to total rotor disc area. SPAN —The dimension of a rotor blade or airfoil from root to tip. RESULTANT RELATIVE WIND — Airflow from rotation that is modified by induced flow. RETREATING BLADE —Any blade, located in a semicircular part of the rotor disc, where the blade direction is oppo- site to the direction of flight. RETREATING BLADE STALL — A stall that begins at or near the tip of a blade in a helicopter because of the high angles of attack required to com- pensate for dissymmetry of lift. In a gyroplane the stall occurs at 20 to 40 percent outboard from the hub. TRANSLATING TENDENCY — The tendency of the single-rotor heli- copter to move laterally during hover- ing flight. Also called tail rotor drift. SPLIT NEEDLES —A term used to describe the position of the two nee- dles on the engine/rotor tachometer when the two needles are not superim- posed. RIGID ROTOR —A rotor system permitting blades to feather but not flap or hunt. G-4