To : Helidrvr
In response to your request to compare the helicopter rotor system to that of an autogyro I provide the following. I know that I put much of this on the Just Helicopter forum and I may well have put it on the Rotorheads forum as well.
In order to understand what I am about to say the reader has to make a minor adjustment in how he/she views a spinning rotor system. The discussion below applies to any type of helicopter rotor system no matter if it is rigid, semi rigid or, fully articulated. Hopefully I don’t come off as being pedantic.
I would suggest that you stop thinking of individual blades flying in a circle. Instead, think of the blades as a solid disc just like the rotor on a gyroscope. And, like a gyroscope the rotor system has rigidity in space and a phase angle of 90-degrees. If you apply a force to a gyro that is on gimbals, the gyro because of precession will react 90 degrees later in the direction of rotation. The same is true on a helicopter. Cyclic input will change the pitch relationship across the disc and this will result in an imbalance of forces. If the pilot pushes the stick forward, the greater force is over the left side of the disc. This upward force (due to precession) will cause the disc to raise over the tail and drop over the nose. Aerodynamics plays a minimal part in this action. The change in disc position was caused
by the change in lift forces but the actual movement was caused by the gyroscopic forces and characteristics of the spinning disc.
On two blade rotor system the pitch horn leads the blade by 90-degrees. On this type of rotor system the swashplate tips down over the nose and the pitch horn introduces the maximum pitch input while the blades are over the lateral axis. With this 90-degree disposition of the pitch horn the phase angle will cause the blade to dip down over the nose and the helicopter flies forward.
On multi blade systems the pitch horn leads the blades by 45-degrees and the swashplate tips 45-degrees ahead of the direction of flight. 45 + 45 = 90 degree phase angle and the same thing happens. The helicopter flies forward.
Individual blades do not fly to a position, The disc is moved by gyroscopic forces. The pitch change of the individual blades simply change the balance of forces across the disc and physics does the rest.
Now, we get to autogyros. There are several types but I will address the two blade and three blade variety. On the two blade type the blade assembly (There is no individual pitch change) is tilted. This in effect decreases the pitch on one blade and increases the pitch on the opposing blade. This, like on the helicopter, creates a differential of lift across the blade assembly and this differential of forces triggers, guess what, gyroscopic precession and the disc tilts but it tilts a limited amount so that it is almost parallel with the direction of flight.
On a multi blade autogyro the rotor head is fully articulated and pitch input is exactly the same as on a helicopter with the same restrictions as on a two blade system. Limited forward tilt.
The closest thing to an autogyro that was a helicopter was the Cheyenne. In flight the helicopter was powered by a propeller in the rear and most of the lift was provided by wings. The spinning rotor provided some of the lift but it was mainly used to control the direction and attitude of the helicopter.
What it boils down to is that both the helicopter and autogyro operate under the same physical laws relative to gyroscopic precession.
[This message has been edited by Lu Zuckerman (edited 05 December 2000).]