I have always had an interest in the Physics of Gyroplane control since I first flew one in 1963 (Pacific Ultralights May 98 pg.25). more recently I became a certified trike pilot but have now finally returned to gyros.

Over the Years there has been an ongoing debate as to whether gyroplanes are aerodynamically, controlled or weight shift controlled (Gyro News Winter '91 pg. 17 and pg. 23). and there have been a number of articles describing the gyro as a weight shift aircraft (Gyro News Winter '89 pg. 26. Summer '90 pg. 24).

The truth is that the control is entirely aerodynamic. with gravity acting on the body of the gyro to provide the reference direction. The latter gives the illusion of weight shift which in fact it is not. The purpose of this article is to show how aerodynamics and physics is used to explain this statement.

For the reader not familiar with the principles of gyroscopic precession of a rotating body, I suggest that the accompanying article titled "Gyroscopic precession: what is it when does it occur and how can I calculate it'?" before continuing on lets explain some definitions.

A number of terms used in this discussion must be defined so that the explanations can be easily understood using correct terminology.

1. The rotor axis. An axis through the centre of the teeter bolt and at right angles to the plane of rotation of the rotors defined b@, the tips of the rotors during a complete rotation.

2. The rotor head axis. An axis is through the centre of the teeter bolt and in line with the rotating shaft of the rotor head bearing.

3. The flap angle. The angle between (a) and (b). It is also half the maximum angular oscillation between the blade axis and the rotor head axis. (Oscillation about the teeter bolt axis.)

4. Mast axis. 1 have defined the Mast Axis as an axis through the centre of the teeter bolt and passing through the centre of mass of the gyroplane (loaded or unloaded).

5. Fixed pitch. The angle of the chord of the rotor blades to the teeter bolt axis. (Note: not to the plane of rotation of the rotor blades

6. Blade axis. An axis parallel to the rotor blades running down the centre of the blades. (Ignoring coning. which must be taken into account when considering rotor balance, loading and rotation rate but is unimportant in the following discussion.

7. Teeter bolt axis. An axis along the length of the teeter bolt through its centre.

Some of the angles between these vectors are fixed and some vary. Those that vary during the rotation of the rotors are indicated (1) and those that vary during flight manoeuvres or wind gusts are indicated (ii). The table below summarises these relations.

For the sake of clarity we will first consider a somewhat commonly held. but totally incorrect, explanation of the way in which gyro rotors are controlled.

Say, we wish to make a right turn. the Joystick is moved to the right and the weight of the gyro (because of weight shift) allows a torque to act on the rotor head through the control rods making the rotors tilt thus banking and turning the gyro to the right.

Nothing could be further from the truth.

Reason...

Firstly, the pivoting action of the teeter bolt prevents the control rod / rotor head assembly from exerting a torque directly on the rotor blades to change their plane of rotation Secondly, if such a torque could be applied the rotor would tilt forward and not to the right since the torque is in the forward direction (see accompanying article). Since this does not happen it confirms that there is no direct torque applied to the rotor blades by the rotor head. If this were to happen the gyro would be uncontrollable.

Explanation...

To understand why the teeter bolt prevents a direct torque acting on the rotor, consider the following:

1. When the rotor head axis is tilted towards the blade axis there is free rotation about the teeter bolt axis so Clearly no torque acts on the rotor (other than the negligible amount due to bearing friction).
2. When the rotor head axis is tilted perpendicular to the blade axis it simply rotates the blades about the rotor blade axis. Again, - no torque directly acts on the plane of rotation.

Since these two movements of the rotor head axis relative to the blade axis are in orthogonal-(ie. independent) directions. it follows that no significant direct torque can be exerted on the rotors due to rotation of the rotor head. Furthermore. since the rotor head cannot produce a direct torque to change the plane of rotation of the rotors then by the action and reaction principle the rotors cannot produce a direct torque on the rotor head to rotate the airframe of the gyro as required by a true weight shift aircraft such as a hang-glider or trike.

So what is the correct explanation of how the gyroplane is made to turn to the right?

The joystick is moved to the right causing the rotor head axis to tilt to the right and out of alignment with the rotor axis. The pivoting motion of the teeter bolt causes the pitch angle of the blades to van. during the rotation. increasing the lift at the front and reducing it at the rear. The resulting large torque due to the aerodynamic effects of the change in pitch is directed to the right and causes the rotor axis (angular momentum of the rotors) to rotate to the right following the rotor head axis. As the rotor axis comes back in line with the tilted rotor head axis, the pitch angle ceases to vary and the lift from the front and rear rotor blade equalises, the torque vanishes and the rotors are tilted to the right to initiate the turn.

The above description is rather short and concise and needs further explanation.

It is well known that the action of the teeter bolt is to equalise the lift on the advancing and retarding rotors. The relation this has to gyroscopic precession will be discussed at the end of this article. What must also be understood is that the teeter bolt places a totally independent and equally important role of making the gyro controllable in flight by causing the plane of rotation of the rotors to tilt in the correct direction for turn and bank, climb and descend, again by gyroscopic precession.

It was explained earlier that, because of the teeter bolt, the rotor head cannot produce a direct torque on the plane of rotation of the rotors so the question then arises, what does? The answer is the aerodynamic effect of the air acting on the rotors. One can think of the rotors as "flying" to align the rotor axis and the rotor head axis through gyroscopic precession.

McKillip. R.M. and Chlh. M.H. Instrumented blade experiments using a light autogyro. Proceedings of the 16th European Rotorcraft Forum. Glasgow. Scotland. September 1990.