Lu,
While there is no mechanical spring acting on the stationary rotor, a delta-3 hinge creates an aerodynamic spring that comes into play when the helicopter rotor is turning and the blades are moving fast enough to produce lift. The formula I mentioned earlier can include this (though I didn't include it), where the delta-3 angle is subtracted from the phase lag:
delta(phi) = 90° - arctan((nu^2-1)/(gamma/8)) - (delta-3 angle)
So even if a rotor has no mechanical spring, the delta hinge can reduce the phase lag to less than 90°
As you recall, Hookes law states (something to the effect) that a spring applies a restoring force proportional to the deflection. If you have a rotor with a delta hinge and the rotor is turning, as the blade is perturbed upward the angle of attack is thus reduced (made more negative) because of the coupling, and the blade is aerodynamically forced downward. This in essence, is a spring.
Until I looked around, I wasn't sure if this was the case in your examples, but I found a page that shows both the rotor head of a JetRanger, and that of an R-22.
http://www.ai.mit.edu/projects/cbcl/...h/mr_semi.html
While the photos aren't exactly lined up, you can see that the point at which the horn attaches to the pitch links is on a line through the teetering hinge on the 206 (the photo doesn't show this well but I'm guessing this is the case). This would mean that there is no flap-pitch coupling and the phase lag is exactly 90°. However, if you look at the R-22, the delta-3 angle (between the teetering hinge axis and the line running through the vertex of the teetering and feathering axes out to the pitch link attachment) is greater than zero, and quite possibly 18°.
Next time I go out to fly an R-22 I'll have someone move the stick directly forward and watch from outside to see in which direction the rotor disc tilts. I'm guessing it will be in the exact same direction as stick movement.
I understand your argument Lu, and have read all your posts on this forum. I've also looked at your diagrams. I commend your tenacity but think you haven't looked at all factors that affect a dynamic rotor system--it's not quite as simple as a motionless one.
I'm learning and thus cannot be 100% certain of the validity of my assumptions. A little knowledge is often dangerous... However, I firmly believe the aforementioned text is worthy of citing--Dr Johnson has probably the most expertise in the science and engineering behind rotary-wing flight.
- Kyrilian