Re: Phase angle etc. Let me explain about the Lynx.
The rotor head is rigid in the flapping axis and dragging axis, but not the feathering axis; hence it is often called a semi-rigid head. For all intents and purposes we can treat it as rigid with respect to the aerodynamics of flapping.
When a cyclic pitch change is input by the pilot, the blades flap with respect to the control axis. The displacement of the blades is creates a bending moment on the rigid head, which in turn creates a restoring force that is a function of the displacement. This is a key point, as the restoring force is 180 deg out of phase with the DISPLACEMENT. The result of this restoring force is that the control input must be made later, i.e. phase lag is reduced below 90 degrees. See
http://www.av8.org.uk/phaselag.htm for a graphical proof.
This brings a problem. While the restoring force is constant for a given displacement, the aerodynamic forces resulting from cyclic displacement will vary with density altitude, in effect the system phase lag will vary with density altitude. This is obviously a problem for the designer, who must settle on an average phase lag when choosing the rotor system rigging angle, hence any difference between the actual density altitude and the design altitude will induce a control cross couple.
Next up come all up mass. As the AUM changes so the total power required changes and thus the coning angle changes. This will affect the restoring force and thus in this case it follows that AUM will have an effect on phase lag.
Next up, blades. If you change the blade characteristics then the aerodynamic performance of the blades will change. This will change the relationship between the bending moment and aerodynamic moments. Once again, therefore, a change in blade performance will change the phase lag.
Now, take the lynx, first envisaged will an all up mass of around 4200Kg, now cleared to over 5200kg, that’s nearly a 20% change in max AUM, imagine the effect on phase lag. Next add new BERB tipped blades with different aerodynamic characteristics, imagine what happens to phase lag.
So, the Lynx was correctly rigged to operate with a less than 90 degree phase angle. Changes in the max AUM, blades and differences from the design density altitude will all induce a small cross couple. It is not a problem ASE in or out.
Next up, acceleration cross-couple. Despite the above, in a dynamic control environment, i.e. after a control input but before the system has found equilibrium the aerodynamic moments are greater than the bending moments, for all sorts of reasons, one being because of the effects of airframe inertial forces fed back to the head. In short, if a HIGH rate demand is made then the acceleration and inertial forces will be out of phase, hence a control cross-couple can be induced (imagine that in a rapid accelerative state the head reverts to a phase lag of close to 90 degrees instead of 72ish). This means that, because of the rigging angle is designed for normal control input rates, if the pilot makes rapid, large cyclic pitch inputs a large couple can be induced. For example in a wingover left a quick forward check on the cyclic can induce a rapid roll left. This is not a problem because this is not a normal flight regime and can be easily avoided by lower RATE control inputs. Lynx pilots are taught to apply gentle control inputs in normal operations, as long as you move the stick gently you can demand rapid manoeuvres with large cyclic displacements without encountering the effects of acceleration cross-couples.
GA