Dave,
Taken me a while to convince myself but i agree that d3 is not the same thing as
90` - PhaseAngle
Looking at the
R22 M/R hub from this superb
Rotorhead Image Site it can be seen that the pitch link does not have to be a vertical installation. It could attach to swashplate 90` ahead of blade while the pitch horn is only 72' ahead of blade.
I also agree with you about +ve d3 (pitch cone coupling) reducing the response of the rotor to cyclic input. This probably explains the possibility for reduced "pilot gain factor" in R22 hovering (ie subconciously filtering out fast cyclic inputs (~0.5 sec time constant)).
What your
d3 web page does not directly account for is the effective hinge offset of a rigid rotor. The set up in this case would be the flap hinge tipwise of the pitch bearing, but with the pitchlink at the root. The closest would be the twisted spline geometry ( Method C/ ), with most rigid rotors having no pitch link coupling.
The practical upshot is that in a rigid rotor head phase angle is as per the Prouty calculations (which consider Lock number - my original prompt), and rigid rotor d3 could only be achieved by the pitch cone coupling in the flexural compliance of the blade.
I definately can understand why alter the phase to compensate for coning in forward flight at 1g loading. However, this is the bit i don't get, unless considered for a rotor which is being "forced" by non aerodynamic/gyroscopic forces:
Originally Posted by Frank Robinson
In a steady no-wind hover, when forward cyclic pitch is applied, the 90-degree rotor disc will end up tilted in the forward direction, but if no lateral cyclic is applied, the rotor disc will have some lateral tilt while the rotor disc is tilting forward, sometimes referred to as "wee-wa." This occurs because while the rotor disc is tilting, the forward blade has a downward velocity and the aft blade has an upward velocity. This increases the angle-of-attack of the forward blade causing it to climb, and reduces the angle-of-attack of the aft blade causing it to dive.
So perhaps a reasonable explanation might be:
Originally Posted by Chuck Beaty
Mr. Robinson is not only president and chief engineer of Robinson Helicopter but is also chief promoter and head salesman. When forging dies for blade grips and pitch arms are bought and paid for, it's sometimes expedient to make theory fit practice.
That said the R22 is a superb flying machine, so this is really more an academic exercise in understanding rotorcraft dynamics.
Mart