Re: Why don't gyrocopters stall?

Greetings:

Gyroplanes, also known by Gyrocopter and Autogiro, have been around since the 1920s. During the '20s and '30s, they enjoyed a brief bit of renown, before being replaced by Igor Sikorsky’s practical design for a helicopter. The military put its money behind Sikorsky’s remarkable aircraft. In so doing, it created the manufacturing base for the civilian Gyroplanes virtually disappeared. In the 1960s and '70s, only two designs in the U.S.A. -- built by Air + Space and McCulloch Aircraft, respectively -- received Government certification. Only the enthusiasm of several generations of sport pilots who drew up plans and built kitplanes has kept the dream of gyroplanes alive.

The military's decision to shun gyroplanes points out the essential difference between the two types of rotorcraft. A helicopter uses engine power to turn its main rotor, like an electric fan. The blades function as the craft's wings, providing lift by moving through the air and forcing it downward. Typically, the aircraft has a tail rotor to prevent it from spinning in a direction opposite that of the main rotors. The pilot exercises directional control by tilting the rotor disc which is also called the wobble plate which changes the pitch of each rotor blade during each revolution. This arrangement allows a helicopter to move in any direction, to compensate for torque effects and to hover in place.

By contrast, the gyroplane's engine drives a prop that propels the craft forward. The overhead rotor is freewheeling, like a windmill. Lift is generated by air moving up through the aft-tilted blades, not down. Thus, the aircraft is in a constant state of torque-free autorotation. Compared to a helicopter, a gyroplane is simple. Like the helicopter, it can take off and land vertically. The crucial difference, at least as far as the military was concerned, is that gyroplanes cannot hover--they must always be in forward motion. However, the gyroplane has more lift at low forward speed than a fixed wing aircraft because the rotary wings are moving at high speed, typically 400 revolutions per minute. A few gyroplaes were built with gear and clutch mechanisms to spin up the rotor before take off to get into the air is less distance.

Most gyroplanes are inherently stable. Pilots have been showing off this characteristic by flying "hands off" since the 1930s. When a helicopter suddenly loses power, the pilot must immediately unload the rotor system and begin autorotation to attempt landing--a delicate, demanding and critical procedure under the best of circumstances, and virtually impossible with a low- or slow-flying aircraft.

By contrast, a gyroplane--which is in a constant state of
autorotation--simply settles to the ground if it loses an engine, regardless of
the altitude and airspeed. What's more, gyroplanes can neither stall nor
spin. If forward airspeed slows too much, below 15 mph in a typical
design, the aircraft descends gently.

The physics of rotary winged aircraft are quite complicated and difficult to
compare with the aerodynamics of conventional fixed wing aircraft for all
parts of the rotary wing are moving at different velocities in the air mass.
Also, the rotors of the helicopter and the gyroplane are quite different. In
some helicopters the blade tips approach the speed of sound, this is not
true for gyroplanes. Also, the rotor blade pitch in a helicopter is continually
being changed by the wobble plate as the rotor makes each revolution. As
your question proposed, in some cases, as in the recent fatal accident with
the V-22 tilt rotor aircraft, a helicopter rotor blade can stall during rapid
decent with the craft falling sideways as your question suggested. Although
some recent gyroplane designs are using variable pitch rotors, flattening the
pitch is used to reduce drag and to increase their top speed.

In engineering school I was taught that the faster airflow over the top of a
wing (air foil) reduces the pressure above the wing , lifting the aircraft.
This is from Bernoulli’s principle. Later, when I became a
military pilot they told us to forget about the lift on top of the wing,
consider the wing bottom to
be pushing down on the air mass and the upwash and down wash causes the flow lines
above the wing to be compressed and the wing to lift. It turns out that both concepts are
basically the same; however its easier to understand how to fly fixed and rotary wing
aircraft using the second concept, while designing an aircraft requires knowledge
of the first concept.

NASA’s Aeronautics Learning Laboratory for Science Technology and Research (ALLSTAR) web
site covers both these concepts, the engineer’s and the pilot’s, and adds much more detail.
This site is enhanced with animated diagrams using software that you can quickly
download on the web site. http://www.allstar.fiu.edu/

The pilot’s view of gyroplane operation is probably much more
understandable than the engineers. In normal level flight the plane of the
rotors rotation for a gyroplane is always tilted up in the front of the aircraft
and down in the rear (the opposite of helicopters) so that forward motion
causes greater air pressure on all of the underside area of all of the rotors
(and vacuum above), no matter where the rotors are in the rotation cycle.
The air then flows up and through the rotors which also is opposite of the
helicopter air flow. A typical rotation rate for a small, low speed, gyroplane
is about 400 revolutions per minute
http://taggart.glg.msu. edu/gyro/gbfeatr.htm/
No matter at what speed a gyroplane is moving, either forward motion or
gravity continues to force air pressure against the bottom of the rotors
making them very stable. The only cause for concern would be to get into a
flight attitude where the plane of rotation becomes more vertical than
horizontal. A pilot must force this situation to happen and still might be
able to recover if the gyroplane has enough altitude.

Best regards, Your Mad Scientist
Adrian Popa