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The calculation is somewhat different for reciprocating and turboprop engines which rely on a propeller for propulsion since their output is typically expressed in terms of power rather than thrust. The equation for heat added per unit time, Q , can be adopted as follows:
In many cases, propulsion systems with very high specific impulse—some ion thrusters reach 25x-35x better I sp than chemical engines—produce correspondingly low thrust. [4] When calculating specific impulse, only propellant carried with the vehicle before use is counted, in the standard interpretation.
The thermodynamic and propulsive efficiencies are independent. For the turbojet though, any improvement which raised the cycle pressure ratio or turbine inlet temperature also raised the jet pipe temperature and pressure giving a higher jet velocity relative to aircraft velocity. As the thermal efficiency went up the propulsive efficiency went ...
Therefore, turbines are more efficient for aircraft propulsion than might be indicated by a simplistic look at the table below. SFC varies with throttle setting, altitude, climate. For jet engines, air flight speed is an important factor too. Air flight speed counteracts the jet's exhaust speed.
For jet engines the useful power is equal to the flight speed of the aircraft multiplied by the force, known as net thrust, required to make it go at that speed. It is used when calculating propulsive efficiency.
where is propulsive efficiency (typically 0.65 for wooden propellers, 0.75 metal fixed pitch and up to 0.85 for constant-speed propellers), hp is the engine's shaft horsepower, and is true airspeed in feet per second, weight is in lbs.
Characteristic velocity or , or C-star is a measure of the combustion performance of a rocket engine independent of nozzle performance, and is used to compare different propellants and propulsion systems. c* should not be confused with c, which is the effective exhaust velocity related to the specific impulse by: =. Specific impulse and ...
Nevertheless, high-bypass engines have a high propulsive efficiency because even slightly increasing the velocity of a very large volume and consequently mass of air produces a very large change in momentum and thrust: thrust is the engine's mass flow (the amount of air flowing through the engine) multiplied by the difference between the inlet ...