Prony brake.  An instrument used to measure the amount of  horsepower an engine is delivering to its output shaft. The  engine is operated at a specific rpm, and a brake is applied  to its output shaft. The amount of torque applied to the brake  is measured, and this, with the rpm, is converted into brake  horsepower. Propeller.  A device for propelling an aircraft that has blades  on an engine-driven shaft and that, when rotated, produces by  its action on the air, a thrust approximately perpendicular to  its plane of rotation. It includes control components normally  supplied by its manufacturer, but does not include main and  auxiliary rotors or rotating airfoils of engines. Propeller end.  The end of a reciprocating engine to which  the propeller is attached. PropFan engine.  The registered trade name by Hamilton  Standard of an ultra-high-bypass turbine engine. See UHB  engine. Propulsive efficiency.  A measure of the effectiveness with  which an aircraft engine converts the fuel it burns into useful  thrust. It is the ratio of the thrust horsepower produced by a  propeller to the torque horsepower of the shaft turning the  propeller. The nearer the speed of the aircraft is to the speed of  the exhaust jet or propeller wake, the less kinetic energy is lost  in the jet or wake, and the higher the propulsive efficiency. PRT.  Power recovery turbine. A turbine driven by exhaust  gases from several cylinders of a reciprocating engine.  Energy extracted from exhaust gases by the turbine is  coupled, through a fluid clutch, to the engine crankshaft. Pulsating DC.  Direct current whose voltage periodically  changes, but whose electrons flow in the same direction all  of the time. Pulse-jet engine.  A type of air-breathing reaction engine used  during World War II to power jet-propelled missiles. Fuel  is sprayed into the combustion chamber and ignited. As the  heated air expands, it closes the one-way shutter valve in the  front of the engine and exits the engine through the nozzle at  the rear. As soon as the pressure inside the combustion chamber  decreases, air enters through the shutter valve and more fuel is  ignited. The thrust is produced in a series of pulses. Push fit.  A fit between pieces in a mechanical assembly that  is close enough to require the parts to be pushed together. A  push fit is looser than a press fit, but closer than a free fit. Pusher engine.  An engine installed with the propeller  facing the rear of the aircraft. Thrust produced by the  propeller mounted on a pusher engine pushes rather than  pulls the aircraft. Pusher propeller.  A propeller installed on an aircraft  engine so that it faces the rear of the aircraft. Thrust from the  propeller pushes rather than pulls the aircraft. PV diagram.  A diagram showing the relationship between  the volume of a cylinder and the pressure during a cycle of  engine operation. Quill shaft.  A type of shaft used to couple parts of an engine  that are subject to torsional loads. A quill shaft is a long,  hardened steel shaft with splines on each end. One end splines  into the drive shaft and the other end splines into the device  being driven. Torsional vibrations are absorbed by the quill  shaft twisting. Radial bearing load.  The load on a bearing perpendicular  to the shaft on which the bearing is mounted. Centrifugal  loads are radial loads. Radial engine (static radial).  A form of reciprocating  engine in which the cylinders radiate out from a small  central crankcase. The pistons in the cylinders drive a central  crankshaft which in turn drives the propeller. Radial-inflow turbine.  A turbine, similar in appearance to a  centrifugal compressor rotor. Radial-inflow turbines are used  to drive the compressor in reciprocating engine turbochargers  and some of the smaller APU turbine engines. Hot gases flow  into the turbine from its outside rim, then radially inward  through the vanes and out of the turbine at its center. Radiation.  See electromagnetic radiation. Ram air.  Air whose pressure has been increased by the  forward motion of the aircraft. Ram air pressure is the same  as pitot pressure. Ram drag.  The loss of thrust produced by a turbojet or  turbofan engine caused by the increase of velocity of air  entering the engine. Ram drag is the difference between gross  thrust and net thrust. Ram pressure.  Pressure produced when a moving fluid is  stopped. Ram-recovery speed.  The speed of an aircraft at which the  ram effect caused by the forward movement increases the  air pressure at the compressor inlet so that it is the same as  that of the ambient air.  G-22