To: Dave Jackson
“Lu; please note that the above is for a simple teetering rotor and its 90-degree phase angle. Any delta3 or flapping hinge offset will reduce the phase lag to below 90-degrees”.
This may be true relative to the calculation being applied to a simple teetering rotorhead. Although there is no offset hinge on the teetering rotorhead there is a delta 3 pitch coupling that occurs when the blade teeters. This coupling tends to restore the pitch in the blade moving forward and takes pitch out of the blade moving backwards very much like a tail rotor. Again referring to your statement about an offset hinge reducing the phase angle to a level below 90-degrees. This to may be true but in the design of almost all rotor systems the designers assume a 90-degree phase angle and they design the control system to accommodate that 90-degree phase angle. If the rotor system does no respond to direct control input and flies as if the phase angle is 88 or 92 –degrees the pilot will simply adjust his cyclic input to result in forward flight as opposed to flying slightly to the left or right.
The certification of rotorcraft stipulates that it is acceptable to have a few degrees of coupling but the direction of flight must be in the same sense of control input.