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• January 17, 2017 Create Date
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National Advisory Committee for Aeronautics, Technical Notes - Experimental Investigation of Transition of a Model Helicopter Rotor from Hovering to Vertical Autorotation

An experimental investigation has been carried out to study the
variation of average induced flow around a model helicopter during
transition from a hovering condition to steady autorotative vertical
descent. Test data were Obtained from simulated power failures under
many different conditions. Results are summarized for variations in disk
loading, blade angles, and rate of pitch change. Calculations were made
of "effective induced velocity" for the various conditions. Sample
comparisons of calculated and experimental performance were made.

The results of these tests show that the manner in which effective
induced velocity varies during the transition to autorotation often differs
greatly from the exponential variation assumed in the theoretical analysis
(NACA TN 1907). It is also shown that conditions peculiar to vertical
descent in the transition range cause variations in performance of the

This report covers one phase of an experimental program of model
testing to study accelerated vertical flight of helicopters. This phase
considers only the power-off vertical descent of a helicopter model during
the first few seconds after power failure at a hovering condition.

In considering this transition from hovering flight to steady
autorotation, one of the factors which must be known is the manner in
which the induced flow of the rotor varies with time after power failure.
Methods are well-known for calculation of initial and final values of
average induced velocity (reference 1), but there has previously been
neither theory nor empirical data to predict the manner in which the
transition occurs between the two.