Agree with mk10, with the following reservation
in order to strictly define a plane, the tip would have to follow a strikt circular path. For an number of reasons this is not so
- global higher orders of the aerodynamics resulting in higher order loadings. In the case of a R44 in forward flight there seems to be at high speed a clear second order as showed by the coning angle which is not constant during rotation (even assuming fully ridgid blades)
- vibrations of the blade resulting in bending of a 'higher order' because of the higher order excitations (cfr the rather extreme flexing of the rotor blade film that was mentioned even recently on this forum
- during transients (ie sudden control inputs), before the rotor takes a steady regime
In my simulator the tip path is defined as the average tip path, that is taking higher orders out.
Example : look at the dotted red line :
d3