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"David CL Francis" wrote in message
... The extreme of this is the enclosed fan where the enclosure markedly reduces tip losses. The fan runs nicely along like this with a high blade area and little daylight visible through the disc. Whatever happened to the concept of piston engines running a ducted fan? That aerocar thing has them, but what about on other more normal planes? How efficient is a ducted fan compared to a prop? I seem to remember hearing in model aircraft settings, a prop is more efficient. Paul |
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"Paul Sengupta" wrote in message . ..
"David CL Francis" wrote in message ... The extreme of this is the enclosed fan where the enclosure markedly reduces tip losses. The fan runs nicely along like this with a high blade area and little daylight visible through the disc. Whatever happened to the concept of piston engines running a ducted fan? That aerocar thing has them, but what about on other more normal planes? How efficient is a ducted fan compared to a prop? I seem to remember hearing in model aircraft settings, a prop is more efficient. Paul Unducted props tend to be more efficient simply because they are of larger diameter, and it's much more efficient to accelerate a large column of air to a lower speed that to accelerate a small column of air to a high speed. The higher RPMs necessary for small props cause much more drag on the prop and horsepower is lost to turbulence, noise, heat and so on. A 150 HP lightplane driving a six-foot propeller at 2700 RPM would never generate more than about 500 pounds of thrust, no matter what the blade pitch might be. A small helicoper with 150 HP driving a 27-foot rotor at about 300 RPM will generate far more thrust, enough to lift the entire helicoper, which might weigh 1500 lbs. Dan |
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