I know from MS Flight Sim that helicopters are extremely "touchy"...don't know if that's the case in real life...or to what extent it is...but I get the general feeling that it is.
The control inputs and results in Flight Sim are fairly different from the real thing, especially the R.22. In the real thing, the controls are more sensitive, but you have a much better feel for what’s happening.
I just re-read your post and have a couple comments/questions
quote:
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In the R.22, what happens is the tail rotor (still producing thrust) starts to roll the airframe to the right pretty quickly, while the rotor disk does not roll!
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Now how exactly does the tail-rotor roll the helicopter? I thought the tail rotor only affected yaw...isn't the rotor right about the center of mass of the helicopter so it would not really produce any rolling moments? It would have to be higher or lower than the center of mass to cause a roll.
When you do a cyclic pushover, the nose goes down and the tail goes up. This puts the tail rotor’s thrust “line” above the CG – so the thrust rolls the helicopter. This is really only an issue with a 2-bladed rotor like the Bell 207, Huey, R.22, R.44 and so on.
And when you push the cyclic forward, the disk is unloaded cause...you gain airspeed which helps the engine rotate the rotor?
In this case, “unloaded” means the
weight of the helicopter is not “hanging” from the rotor. The rotor is still taking power from the engine to turn it.
And about the RPM/stall phenomenon...
does that have to do with the pitch from the collective which causes variations in RPM (due to drag and load on the engine)...
The two most common scenarios for low-RPM stall are:
1 – Carb ice: The engine is losing it’s ability to make power, but the pilot doesn’t notice because the governor keeps opening the throttle to compensate. When the ice gets bad enough, the RPM starts to droop, and the helicopter starts to descend. If you’ve been sleeping at the sticks, you can still save it by lowering collective to maintain RRPM – as long as you have RRPM, you are alive, even if you are descending. If you have the altitude, you might be able to get the carb heat on before the engine stops, clear the ice, and continue on. If the engine stops, you autorotate to the ground.
However, if you ignore the CAT gauge and the decaying RPM and raise collective, it is as you said, the rapidly increasing drag will cause the RPM to droop rapidly, just like…
2 – Engine failure: The engine stops and the pilot does not lower collective to maintain RRPM. The rotor is slowing, the helicopter is descending, and…
…no matter how you got there, once you hit a critical combination of descent rate and low RRPM, you cannot recover, even if you bottom the collective. The angle of attack (and resulting drag) is just too great.
so say you are at 104% Ng (RRPM and engine RPM)... Now say all of a sudden you want to climb...pretty rapidly...so you increase collective...and thus all your blade rotors' pitch increases...they take a bigger bite from the air...increase drag, increase lift.
Now increased drag means higher load on th engine which was at 104%...now say it dropped to 90% due to the load...the engine is loaded...
Now does this mean you can't possibly increase the Ng back to 104% anymore? or anymore than 90%? due to the drag from the pitch of the blades? So the engine can't do it anymore?
In the R.22, the Nr will not drop until the governor has opened the throttle fully. If the rotor drag is still higher than the maximum power output of the engine, the Nr will droop. The only way to recover Nr is to lower the collective.
A friend of mine had the unfortunate experience of being a passenger in an R.22 where the pilot did just that. The helicopter was well past max gross so it wouldn’t pick up. The pilot pulled close to full collective all at once – the inertia in the rotor provided the added power to get the helicopter about 20’ in the air. But then there they were, with the Nr heading south and nothing to do but wait for the inevitable return to earth. The pilot compounded the error by not lowering the collective to attempt to gain some Nr so they could cushion the landing, so it was a hard landing indeed, followed by a rollover. Luckily, nobody injured.
So obviously if this is the case...there is a limit, like you said maybe 80% where the RPM is very low...and the engine cant' take it anymore...
the thing is, I would think small pitch, 104% would produce same lift as 90% and larger pitch...
so I'm still a little confused I guess
actually, it sounds like you get the idea – the higher the RRPM, the higher the airspeed over the rotor blade, so the less pitch/AOA is required, result? Higher RRPM = less drag for same lift.