The precession theory for the rotor does not hold any water. Yes maybe as a ‘basic’ analogy/ comparison, but that is as far as it goes. To make the disc change attitude, you don’t just apply a force 90deg prior to its desired deflection. Consider the swash plate as a ‘swash plate’ and the blades as individual wings. All that the swash plate is doing is changing the AoA/ pitch of the ‘wings’ therefore causing them to increase/ decrease lift. Because they are attached to the rotor head, the blades flap up/ down respectively. This journey does not simply start 90 deg prior, but a full 180 deg prior.
Consider this. In order to get the blade to fly low over the front of the aircraft and high over the rear – you apply fwd cyclic. This will, in blades that turn counter clockwise, tilt the swash plate to the right (starboard)….As the blade leaves the 6 o’clock position (tail) pitch is slowly, but progressively being taken off – assume that it starts with neutral pitch. The blade starts to lose AoA = lift…therefore starts to flap down. As the journey of the blade continues to the 3 o’clock pos (starboard), the MAX amount of pitch is removed = MAX RATE of flap down. Towards the 12 o’clock pos (nose), note that the blade is still continuing to flap down, but at a ‘sustained rate’ because the amount of pitch is returning to neutral. Note – this blade will continue to flap down until positive pitch is applied. Funnily enough, this is what happens once the blade passes through the 12 o’clock. The swash plate now starts to apply pitch onto the blades, therefore the blades start to climb.
This is easily demonstrated in the hangar. Apply fwd cyclic and slowly rotate the disc and follow the journey of a blade from the tail to the nose. You will find that as the blade travels, the amount of pitch subjected to the blades is directly proportional to the swash plate. You don’t even need a swash plate – ANY rotor system.
Now, with a GENERAL helicopter, you would assume that with FWD cyclic applied, the blade over the 12 o’clock pos would have neutral pitch (indeed it shold not be effected) – and where you apply the phase lag offset – the max amount of pitch removed. As soon as the blade moves from the 12, the pitch 'should' start to increase again….Well, in the Lynx it is different. As the blade is moved through the 12, it is clear that it is still being subjected to pitch. It is not until 15deg past the 12, that the pitch becomes neutral. Therefore, the longitudinal axis does not run as we would expect - 12-6 o’clock. So AFT cyclic not only produces a nose up component, but also a roll to the right.. This can also be seen when applying collective pitch in fwd flight. One would expect the nose to ‘just’ flap back when raising the collective (try using precession to explain that one) and vice versa for lowering the collective. It doesn’t – you also get a roll, therefore confirming the theory. But you already know this.
As for the AFCS being advanced, believe me it is not. Maybe 25 yrs ago….It simply compares your control inputs with its vertical gyros and applies direct control inputs to the hydraulic servos.
[This message has been edited by Baque Flip (edited 20 December 2000).]