REDUCING VORTEX DRAG ON PROPELLERS

The least vortex drag for a wing is found with a perfectly elliptical chord distribution. The equivalent shape for a propeller can be found by calculation but it is not an ellipse. This is because the tips of the blades move faster through the air than the roots, so that, in a given time, more air mass is affected by them. The effect of this and other factors is to require, for least vortex drag, a narrower blade profile near the tips than a purely elliptical form, and, near the blade roots, a somewhat broader than elliptical shape. (This refers to the developed planform, when the chord at each place along the blade is plotted in two dimensions as if the blade were straightened out)

The importance of this discovery, largely due to Eugene Larrabee, may be gauged from the fact that the manpowered aircraft, Gossamer Albatross, which crossed the English Channel, was incapable of staying in the air for more than a few minutes, until Larrabee’s design of propeller was adopted. Unfortunately, model aircraft propellers generally are constrained in design by factors other than aerodynamics, especially the need for strength and stiffness under very high loads, and ease of manufacture.

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