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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.
The ratio between a propeller's efficiency attached to a ship and in open water (′) is termed relative rotative efficiency. The overall propulsive efficiency (an extension of effective power ()) is developed from the propulsive coefficient (), which is derived from the installed shaft power modified by the effective power for the hull with ...
A 6-bladed Hamilton Standard 568F propeller on an ATR 72 short-haul airliner. Lowry [27] quotes a propeller efficiency of about 73.5% at cruise for a Cessna 172.This is derived from his "Bootstrap approach" for analyzing the performance of light general aviation aircraft using fixed pitch or constant speed propellers.
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 ...
Prop efficiency is usually at least 2–5% less and in some cases more than 15% less than an equivalent tractor installation. [23] Full-scale wind tunnel investigation of the canard Rutan VariEze showed a propeller efficiency of 0.75 compared to 0.85 for a tractor configuration, a loss of 12%. [ 24 ]
The thrust efficiency is the actual thrust as percentage of this. If, e.g., solar power is used, this restricts a {\displaystyle a} ; in the case of a large v e {\displaystyle v_{\text{e}}} the possible acceleration is inversely proportional to it, hence the time to reach a required delta-v is proportional to v e {\displaystyle v_{\text{e ...
The efficiency of a contra-rotating propeller is somewhat offset by its mechanical complexity and the added weight of this gearing that makes the aircraft heavier, thus some performance is sacrificed to carry it. Nonetheless, coaxial contra-rotating propellers and rotors have been used in several military aircraft, such as the Tupolev Tu-95 "Bear".
An advanced type of propeller used on the American Los Angeles-class submarine as well as the German Type 212 submarine is called a skewback propeller. As in the scimitar blades used on some aircraft, the blade tips of a skewback propeller are swept back against the direction of rotation. In addition, the blades are tilted rearward along the ...