Ask Elton
Your Flight Training & Ground School Specialist

Glossary - Propulsion

Accessory Drive Gearbox An auxiliary drive shaft and unit to power generators, fuel pumps and hydraulic services.
Afterburner/ing The addition of fuel to the hot exhaust to take advantage of the unused oxygen to increase thrust.
Ambient The conditions that exist in the local environment – temperature, pressure and density.
Annular The circular chamber that forms a concentric cylinder around the axis of the engine. See Cannular.
Athodyd Aerothermodynamic duct – another name for a ramjet.
Auto-Feather Automatic feathering of a propeller when predetermined parameters are sensed.
Axial In the direction of the engine axis (straightthrough).
Beta Range Propeller blade angles that produces zero or reverse thrust.
Blade Angle The angle of the propeller blade relative to the plane of rotation (measured at a point 75% of the blade length from the hub).
Blade Creep The increased length of a turbine blade due to heat stress. This often determines the temperature limits of the engine.
Bleed Air Air that is extracted from the compressor to provide pressurisation or auxiliary drive.
Bore The diameter of a cylinder of a piston engine.
Cannular The annular arrangement of individual combustion chambers around the circumference of the engine.
Centrifugal Flowing outwards from the centre e.g. centrifugal compressor which propels air outwards and centrifugal force due to acceleration during turns.
Compressor Stall/Surge Interruption of airflow to the compressor resulting in a loss of thrust and possible reverse flow from the turbine to the rear of the compressor.
Continuous Ignition A manual selection where the igniters are firing continuously to counter possible flame out due to bird or water ingestion.
Critical Altitude The maximum altitude at which the engine can maintain sea level maximum power is the maximum power critical altitude for the engine. Similarly there is a maximum continuous power critical altitude. Note that power relates to critical altitude and MAP relates to full throttle height.
Dry Thrust Thrust without afterburning.
Engine Core The gas generator or power producing core of the engine which then drives the fan or propeller shaft.
Engine Pressure Ratio EPR The ratio of the gas turbine engine total output pressure to compressor inlet pressure (P7/P1).
Fir Tree A means of attaching turbine blades to the turbine wheel.
Flame-Out Extinction of the combustion flame.
Flash Point The lowest temperature, corrected to a barometric pressure of 1013.2 kPa, at which application of a test flame causes the vapour of the test portion to ignite under the specific conditions of test.
Four-Stroke Cycle (Otto Cycle) The cycle of the piston engine – intake, compression, power and exhaust.
Fuel Control Unit The mechanism that controls the rate of increase of fuel flow to the engine to achieve optimum acceleration (spool-up).
Full Throttle Height Full throttle height is the maximum altitude at which a supercharged or turbocharged engine is able to maintain a specified MAP. Note that power relates to critical altitude and MAP relates to full throttle height.
Gross Fuel Consumption GFC The total fuel used per unit of time. At any moment GFC equates to fuel flow. See Specific Fuel Consumption.
Helix Angle The angle formed between the resultant of the blade motion and the plane of rotation.
Hung Start A start where engine acceleration deviates from the normal schedule such that fuel may be excessive and the resulting heat may rise beyond limits. Generally the rpm hangs below self-sustaining speed.
Idle rpm The rpm which the engine maintains at minimum power – a small margin above minimum self sustaining rpm.
Inlet Guide Vanes IGV Guide vanes that direct airflow to the first stage of the compressor.
Intake Buzz Vibration and airflow disturbance due to the oscillation of the shock wave into and out of the intake.
Intake Ramps Devices used to control shock waves in the engine intake.
Inter Turbine Temperature ITT The temperature between the turbine stages (T5).
Jet Pipe Temperature JPT The absolute temperature in the jet pipe before the afterburner.
Mach Number The ratio of TAS to the local speed of sound – expressed as a decimal e.g. M = 0.8 or 0.8 IMN
Mach One The local speed of sound (which varies with ambient temperature).
Manifold Air Pressure The total pressure in the inlet manifold of a piston engine.
Modified Otto Cycle The modified four-stroke cycle which changes valve timing and ignition timing to increase power and efficiency.
Nozzle Guide Vanes NGV The fixed or variable guide vanes which direct airflow at the optimum angle to the first stage of the compressor.
Octane Rating Grades of AVGAS with ratings up to 100 are identified by their nominal minimum lean-mixture anti-knock rating (resistance to detonation) relative to a blend of pure iso-octane which is designated with a value of 100 and n-heptane which is designated as zero. Grade 80 fuel has the detonation resistance equivalent to a blend of 80% iso-octane and 20% n-heptane. Grade 91 fuel (car petrol) has a detonation resistance equivalent to 91% iso-octane and 9% n-heptane.
Over-Boost Exceedance of allowable manifold air pressure.
Over-Temp Exceedance of temperature limits.
Over-Torque Exceedance of torque limits.
Performance Rating (Fuel) Grades of AVGAS with ratings of 100 or more are identified by their nominal minimum lean-mixture anti-knock rating (resistance to detonation) relative to a blend of pure iso-octane which is designated with a value of 100, and the quantity of an antiknock additive tetra-ethyl lead require to achieve the level of detonation resistance. Grade 100 fuel has the detonation resistance equivalent to 100% iso-octane. This quality is achieved by blending a fuel with a quantity of tetre-ethyl lead. Grade 100 fuel and Grade 100 LL (low lead) have same detonation resistance but Grade 100 LL has half the lead content of Grade 100. Grade 100 LL is manufactured from a higher grade base fuel needing less additive to achieve the same detonation.
Pitch The linear dimension of the blade angle – measured at a point 75% of the blade length from the hub.
Radial Radiating from the centre e.g. radial engine cylinders.
Ramjet A fuel burning engine which has no moving parts.
Rated Thrust/Rated Power Any specified thrust/power condition for which the engine is rated (guaranteed) e.g. rated take-off thrust, rated maximum continuous power.
Red Line Limiting speed, power, torque, rpm or temperature.
Reheat Active (spark) re-ignition of combustion in a gas turbine engine.
Relight Active (spark) re-ignition of combustion in a gas turbine engine.
Reverse Pitch Negative propeller blade angle generating thrust opposite to the direction of aircraft monitor.
Reverse Thrust Jet or fan efflux partly or wholly opposite to the direction of aircraft motion.
Rotor (Helicopter) Rotating aerodynamic surfaces that convert engine torque to lift and thrust.
Rotor (Piston Engine) The distributor has a rotor which directs electrical current to each spark plug at the optimum time.
Rotor (Turbine Engine) The rotating assembly consisting of a central wheel and blades – in both the compressor and turbine.
Scavenge Pump A pump that acts to return oil to the oil reservoir/tank.
Shock Cones Shaped devices which form part of an air intake and whose purpose is to move the shock wave out of the intake.
Shock Wave Pressure wave that forms when a volume of supersonic flow decelerates.
Slam Acceleration Rapid application of full power. The ACU accelerates the engine at its maximum rate.
Specific Fuel Consumption SFC The ratio of gross fuel consumption to power or thrust.
Specific Gravity SG The mass of fuel per unit volume in comparison to that of water at a given temperature; also relative density. The SG of AVTUR is deemed to be 0.8 at +20° Celsius – therefore one litre of AVTUR at 20° C, will have a mass of 0.8 kg.
Spool-Up The delay in engine acceleration due to the FCU.
Static Thrust The maximum force generated by the jet efflux, fan and propeller when the aircraft is stationary.
Stators Fixed aerofoils between the rotating assemblies in an axial compressor to optimise the angle of attack of succeeding rotors.
Stroke The distance the piston travels between BDC and TDC.
Supercharger An engine-driven compressor which provides a higher MAP to a piston engine.
Thrust Augmentation Boosted thrust due to water injection or afterburning.
Top Dead Centre The point at which the piston reaches the top of its travel on the compression stroke.
Turbine Inlet Temperature TIT The absolute temperature at the face of the turbine’s first stage (T4).
Turbocharger An exhaust-driven compressor which provides a higher MAP to a piston engine.
Turbofan A turbine engine where not all of the compressor/fan output passes through the engine core.
Turbojet A turbine engine where all of the compressor output passes through the engine core.
Turboprop A turbine engine whose primary output is torque used to turn a propeller.
Turboshaft A turbine engine which drives an output shaft (which in a helicopter drive a transmission/rotor assembly).
Variable Nozzle Variable exhaust nozzle which changes crosssectional area in response to afterburner selection.
Vectored Thrust Deflection of the jet exhaust nozzle for vertical lift or increased manoeuvre capability.
Volumetric Efficiency The ratio of the actual mass of fuel and air drawn into the cylinder of a piston engine to the maximum possible mass of fuel and air at standard temperature and pressure that could be drawn into the cylinder.
Wastegate The manual or automatic valve which varies the area of the exhaust and therefore changes turbocharger output. If the wastegate is closed the most exhaust charge is directed to drive the turbocharger.
Windmilling A state where the propeller is driven by the airflow.