Paul J. Carpenter
naca-tn-1698
Oct 1948
Two representative helicopter rotors with widely different characteristics were investigated to determine the effect of wind velocity on the power required for sustentation. The rotors were investigated with the Langley helicopter apparatus and were powered by a 1350-horsepower engine.
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http://naca.central.cranfield.ac.uk/reports/1948/naca-tn-1698.pdf