Item 1229
OTHER ~
Flight Dynamics - Rotor Hub - Flap Hinge Offset (e)
Sketch:
xxx
The blade's radial coordinate
r is still measured from the center of rotation.
"If a rotor has flapping hinge offset, the phase angle is somewhat less than 90º and the flapping is not quite numerically equal to the cyclic pitch. This is because as offset is increased, the restoring moment due to centrifugal forces increases faster than the moment of inertia about the flapping hinge." ~
[Source ~ RWP1 p.447]
Prouty, (I think it was him) has stated that when the control plane (swashplate) is changed by cyclic input the tip-path-plane will realign itself with the control-plane within 1/2 of a rotor's revolution.
"For blades freely articulated at the center of rotation, or teetering rotors, the response is lagged by exactly 90-degrees in hover; for hingeless rotors, such as the Lynx and Bo105, the phase angle is about 75 to 80-degrees."
It has been stated that the conventional swashplate is an ideal control mechanism for rotors with a 90-phase angle, but it is not ideal for rotors with a phase angle less than 90.
Amount of Offset:
Articulated rotors usually have an offset
e of = 0.03 to 0.05.Articulated rotor helicopters generate about half of the moment about the center of gravity by the thrust tilt and about half by the direct hub moment.
Hingeless rotors usually have an effective offset
e of = 0.10.
To me it appears that Lynx's 75 to 80-degree phase angle is a compromise between the gyroscopic precession's 90-degrees and the aerodynamics' < 75-degrees.
For additional information see: [Source ~ HT section. 5-14 & 5-15]
For related information see;
OTHER: Flight Dynamic - General - Phase Lag
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Last Revised: August 12, 2003