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National Advisory Committee for Aeronautics, Technical Notes - Observations in Flight of the Region of Stalled Flow Over the Blades of an Autogiro Rotor

The flow over the inner halves cf the rotor blades on
a Kellett IG—lB autogiro was investigated in flight by
making camera records of the motion of.silk streamers at~
tached to the upper surfaces Of the blades. These records
were analyzed to determine the boundaries of the region
within which the flow over the blade sections was stalled
for various tip—speed ratios. For the sake of comparison,
corresponding theoretical boundaries were obtained. Both
the size of the stalled area and its rate of growth with
increasing tip-speed_ratio were found.to be larger than
the theory predicted, although experiment agreed with the—
ory with regard to shape and general location of the
stalled area. The stalled region may be an important fac~
tor in both the rotor lift—drag ratio and the blade flap—
ping motion at the-higher tip—speed ratios. The method of
study used in this paper should be.useful in further stud-
ies of the problem, including the reduction of the size of
the region.

The theoretical analysis of the autogiro rotor, devel—
oped by Glauert, Lock, and Wheatley,'includes expressions
from which the angle of attack of a blade element at any
position in the rotor disk can be calculated; These ex-
pressions indicate the existence of three distinct regions
on the rotor disk. In one of these regions, the blade el—
ements are unstalled} in another, they are stalled; and,
in the third, they are subjected to a.reversed flow, with
the air moving from trailing edge to leading edge; The
boundaries of the stalled region, which lies between the
other two regions on the.rotor disk. can be calculated
from the theoretical expressions, provided that the angle
of attack at stall of the blade airfoil section is known.

The size, the shape, and the importance of the
stalled region as obtained'from the theory are not quite ’
accurate because: (1) the blade elements passing through
the region are subjected to high angles of attack for an
extremely short interyal} (2) the angle of attack at
stall can be markedly affected by the conditions of oper—
ation in the rotor, namely, the large and rapid changes
in angle of attack, angle of yaw, and velocity; and (3)
the local values of the inflow velocity are known to de-
part considerably from the mean value used in the theory.