Originally Posted by
FlightOops
Surely, the POR is the plane described 90 deg perpendicular to the AXIS of rotation (i.e. a vertical axis extending through the mast).
Correct.
Originally Posted by
FlightOops
The TPP is the plane described by the rotor tips - nothing "Construct" about it TorqueTalk (lets discount the effects of tracking / out of track blades)
The reason the tip path plane is described as a construct is that there is usually not a single plane in which the path of the tips lies. The "higher harmonics" that have been mentioned earlier in this thread explain this phenomenon.
For example a second order harmonic can result in the tips being higher at the fore and aft positions than at the left and right positions, hence no single plane will pass through the tips at all locations.
The TPP "construct" basically involves finding the plane from which the average displacement of the tips is zero - i.e. the blades go above the plane as much as they go below the plane.
Originally Posted by
FlightOops
With cyclic input applied (and the effects of Hookes Joint, Flapping etc), the the TPP can describe a plane which is NOT parallel to the POR.
It is for this reason that the two descriptions (POR and TPP) exist otherwise they would be described by the same definition ... mange tout, non ?
It is not just cyclic input that can cause the tip path plane to differ from the plane of rotation, translational velocity can cause it to do the same thing (this is flapback).
The third axis - that Dave mentioned - is the control axis/plane. With zero translational velocity, this corresponds to the TPP, and differs from the axis/plane of rotation as determined by cyclic input. With translational velocity, the TPP will tilt back away from thr direction of motion.
So in forward flight, your control axis/plane will be tilted forward (forward stick trim at speed), but flapback will cause the TPP to be tilted back towards the plane of rotation.
Daniel