the coyote writes:
"And yes, I'm as concerned as you are that someone might take this speculation on board somehow and in confusion deviate from what they have been trained to do when the blood is pumping.
Stick to the training! Pprune is a great source of info, ideas, and increased awareness. How people choose to interpret that and what course of action they ultimately take is up to them eh?"
I agree. I hope that what I've put forward is simply accepted as my theory of what's going on, not to be tested by someone in no reasonable position to do so safely. The RFM and manufacturer's instruction should be trusted. Unless there is an unforseen problem (jammed/misrigged controls) the collective should simply be lowered and NR allowed to slowly come back up. It's just my nature to look at the other 'what if's', and consider what's actually going on physically. Perhaps this isn't the place to do it. But if not here, where?
Flingwing207 says:
"This is one significant difference between dropping the collective and a cyclic pushover. Believe me, the faster you get the collective down, the less Nr decay you will have."
To a point...
I think it's been said before that dumping the collective too quickly will actually slow the NR more than a less hasty reduction. Consider that when in regular lifting flight air comes down through the rotor disk at a decent clip. If you instantly reduce collective to zero and at portions of the blade the blade angle becomes negative, the air coming down on the blade will cause a quite substantial drag with negative lift (negative angle of attack). Why would you want more drag and negative lift (possible zero-g)? By more gradually reducing collective the blades don't end up pushing up at the air. You allow the airflow through the disk to change direction as the blades do, perhaps maintaining a small constant angle of attack or simply zero angle of attack, rather than a large negative one.
You also say, "In the R22 the blades do not go into negative pitch with a full-down collective, so the airframe will continue to pull down on the disk."
If this were true than what would cause the rotor to keep turning in an auto despite the drag acting along the span? The type specific info in the back of "Helicopter Performance, Stability, and Control" by Prouty states that the collective range on the R-22 is +1.5 to +14.5deg. This is likely the root angle. The twist is -7deg, which means that at less than 21% span the local blade angle is positive, but beyond that it's negative (-5.5deg at the tip).
Gaseous says:
"Cyclic input alters the plane of rotation. NOT the angle of attack. (Bailey, principles of flight, p40)."
and
"forget the red herring of cyclic affecting pitch. do not confuse pitch with angle of attack. forward cyclic will not increase angle of attack on retreating blade"
I think you are over generalizing Bailey's statement. Think of what happens when you're in a hover and push forward cyclic to commence forward flight. Sure, your forward cyclic will cause the plane of rotation to tilt forward, but how? If the angle of attack doesn't change then the lift in that quadrant is unchanged and the disk won't respond.
Also consider fast forward flight. You know that the blade has a larger angle of attack on the retreating side than the advancing side because the relative airspeed acting on the blade is different. To yield the same lift on each side the higher relative airspeed on the one side (advancing) is offset by the higher angle of attack (and higher blade angle) on the other. This is due to your forward cyclic input. If the angle of attack was the same on each side the lower lift due to the slower relative airspeed on the retreating side would cause the disk to tilt back and you'd slow down. If you brought the cyclic back to center, the blade angle and also the angle of attack on the retreating side would similarly be reduced and the disk would come down over the tail as a result. The steady state final condition would put you back in a hover.
What I think he meant was simply that the average blade angle as the blade revolves does not change, and thus the average angle of attack does not change.
Going back to my earlier calculation... You are right in your statement that blade angle does not equal angle of attack. However, they are strongly related, especially when we discuss transients. In my earlier post I made the point that I was making broad assumptions and this was one, but I stand by my earlier position that for illustrative purposes and especially for maneuvering, this is fair. It looks like my non-standard text didn't come across correctly. I'll fix that.
The change in angle of attack is exactly equal to the change in blade angle for a moment before the airflow has a chance to respond. As time goes by the angle of attack finds a new equilibrium, but for a short while their changes are the same.