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The propulsive efficiency is always less than one, because conservation of momentum requires that the exhaust have some of the kinetic energy, and the propulsive mechanism (whether propeller, jet exhaust, or ducted fan) is never perfectly efficient. It is greatly dependent on exhaust expulsion velocity and airspeed.
It helps in understanding the efficiency of the propeller at different speeds and is particularly useful in the design and analysis of propeller-driven vehicles.It is the ratio of the freestream fluid speed to the propeller, rotor, or cyclorotor tip speed. When a propeller-driven vehicle is moving at high speed relative to the fluid, or the ...
A propeller imparts momentum to a fluid which causes a force to act on the ship. [1] The ideal efficiency of any propulsor is that of an actuator disc in an ideal fluid. This is called the Froude efficiency and is a natural limit which cannot be exceeded by any device, no matter how good it is.
The efficiency of this conversion (Froude or propulsive efficiency) reflects work done in the 1800s on ship propellers. The relevance for gas turbine-powered aircraft is the use of a secondary jet of air with a propeller or, for jet engine performance, the introduction of the bypass engine.
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. The metric formula is:
Efficiency of conversion of input energy to reactant energy also matters; be that thermal energy in combustion engines or electrical energy in ion engines, the engineering involved in converting such energy to outbound momentum can have high impact on specific impulse. Specific impulse in turn has deep impacts on the achievable delta-v and ...
Aircraft engine performance refers to factors including thrust or shaft power for fuel consumed, weight, cost, outside dimensions and life. It includes meeting regulated environmental limits which apply to emissions of noise and chemical pollutants, and regulated safety aspects which require a design that can safely tolerate environmental hazards such as birds, rain, hail and icing conditions.
In reciprocating and propeller engines, disk loading can be defined as the ratio between propeller-induced velocity and freestream velocity. [citation needed] Lower disk loading will increase efficiency, so it is generally desirable to have larger propellers from an efficiency standpoint.
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