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post #7 of Old 11-11-2004
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2 blade or 3 blade propeller

It is not quite that simple. You can get greater blade area in a number of ways. On a two blade prop you can increase the width of the blades without increasing diameter (often called a ''power prop''). This adds some power but is relatively inefficient since it does not increase the leading edge of the prop but does increase the tip drag. In order to get the full benefit of increased blade area out of a two blade prop, the two blade propellor needs to have a larger diameter. Because of the longer lever arm of the larger diameter prop it requires greater torque to turn a two blade prop of equal area to a three blade at the same RPM. Since the determinant of horsepower in a rotating shaft is rpm x torque (times a constant I believe, but that I can''t recall) the greater torque will require more horsepower for a given thrust. That is the theory. The reality can be a little different because the greater reciprocating mass and lower efficiency of a three blade can offset this increase in HP a little.

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