Padfield's book and a referenced report say
"... washed-out coupling, which can occur in helicopters with feedback control systems". Delta-3 must be considered a mechanical 'feedback control systems', since it pull out pitch in a relationship to the amount of flap.
This raises the circular question; was delta-3 incorporated to minimize washed-out coupling, or did the incorporation of delta-3 cause washed-out coupling?
The following two quotations are from separate posts by Chuck Beaty, a gentleman with considerable technical knowledge about rotorcraft. They provide additional information.
1/
" Delta-3 coupling binds the rotor more tightly to the mast position, perhaps a good thing for tail rotors but a bad thing for main rotors."
2/
"Subject: Wee-wa
Bramwell in "Helicopter Aerodynamics" after tossing about some fancy math, derives some fairly simple expressions for cross coupling.
The rotor tilt in the commanded direction is proportional to: 16 * (tilt rate)/(angular velocity) * (1/y)
y is the mass constant of the rotorblade (Lock #), the ratio of aerodynamic force to mass. High inertia blades have lower Lock # than the other way around. Angular velocity is that of the rotor, everything being in radians/sec.
The tilt in the crosswise direction is: (tilt rate)/(angular velocity)
What this says is that cross coupling might not be noticed in a Bell or Hiller but might be a problem with a Robinson.
Irrelevant sentence snipped.
I've met Frank Robinson and remember having a discussion with him about his use of delta-3 coupling back when he was still working on the R-22 prototype. He's a very competent engineer and wrote a number of papers about tail rotors while he was a project engineer at Bell-Textron but I suspect tail rotors gave him his fixation about delta-3 coupling where suppression of cyclic flapping is considered desirable.
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.
With a model rotor at least, if one wants to see some really nasty "wee-wa," skew the teeter hinge and observe the nutating behavior. It will go crazy with a 45 degree skew angle. Model rotors don't exhibit any detectable cross coupling because the ratio of rotor rpm to tilt rate is so high.'
Confusion runs wild. It's all very interesting.
Lu,
Why not give Chuck Beaty a call. Hell, why not give Frank Robinson a call.
