From the Wellsville Area Small Plane Society, Wellsville, New York
Autogyro Aerodynamics
by Ken Gough
After reading many explanations of
how autogyros fly, I have come to the conclusion
that the discussion can get too technical too
quickly. Let’s start with a simple analogy.
Some kinds of maple seeds have a wing with an
airfoil. It spins as it falls, and the upward force
on the blade slows the fall. The force that keeps an
autogyro airborne is the same that acts on the maple
seed. Now if you attached a thread to the seed and
pulled in horizontally as it fell and spun, it would
fly! Well, maybe it would need a tail and a more
efficient blade, but you get the point. The arc of
the rotor would angle back a bit, and the resulting
upward force would be greater than the gravitational
force.
Hopefully, from this explanation, you can see the
differences between an autogyro and a helicopter.
Most importantly, an autogyro rotor is not powered.
It is simply freewheeling in the wind. The plane of
the blades is titled back opposite the direction of
travel, not forward like a helicopter. And the
blades have a negative angle of attack. This makes
sense because if they had a positive angle of attack
they would spin backward when the wind hits from
underneath.
Most autogyro blades have a special hinge to keep
them at the proper angle of attack.
The hinge line is at an angle to the blade, so when
the blade tilts up, the angle of attack decreases
(leading edge is lower.) And if the blade tilts
down, the angle would increase, but a stopper blocks
the downward bend. You don’t want the blades
dropping and hitting the airplane. Hinging the
blades also helps to decrease unwanted roll forces
caused by the differences between advancing and
retreating aerodynamics. Q |
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