Krylian.
Quote: However, if I found myself in an auto with the NR going down through 80% I think I would still pull back gently on the cyclic.
If you've got the lever down and your Nr is still decaying you should come back and haunt your engineer! (or mechanic if you live in USofA). Helicopters should be trimmed to maintain autorotation with the lever down and should do to the point where the rotor stalls.
The aerodynamics of rotor stall are roughly this. There are 3 regions of the autorotating blade starting at the mast. First there is a stalled area which contributes drag to the rotor system. Moving out there is an area of autorotation where the lift vector drives the rotor. Moving out further there is an area of drag. As the angle of attack increases due to falling Nr the boundaries move outwards. At normal Nr the stalled area at the root is small and insignificant but as Nr drops this area gets bigger so that eventually drag is greater than autorotative force so the rotor stalls. In forward flight the retreating blade stalls more than the advancing blade so roll to the left is likely in a R22 as catastrophy looms.
lets assume an arbitrary rotor stall speed of 75% in stable autorotation. at this point the angle of attack is critical. Any increase will cause the rotor to stall. Pulling back on the stick makes no sense at all. You will die. At 80% there is a little latitude for aft cyclic. At 90% quite a lot and at 100% you can haul back on it with near impunity.
***The nearer Nr is to rotor stall, the less safe aft cyclic or increased load factor get. Simple as that.***
Aft cyclic causes an increase in angle of attack as the aircraft pitches aft. Check it in your PPL book. (mine is Norman Bailey's Helicopter pilots manual, P100).
I totally agree that if you don't stall the rotor all the right things will happen. Just be careful.
No one really knows what the dead pilots of all the crashed R22s did last. I suspect some of them did a big flare with low Nr because they thought it would increase Nr. Imagine their surprise when the rotor stalled.
Think about it.
FDA
You posted while I was answering Krylian.
You are asking all the right questions.
Question: What is the angle of attack of the blades at full down collective, 60 knots forward speed and 80% RPM versus 104%
Answer: A lot greater. cant give you numbers but airspeed is a lot lower. As airspeed decreases angle of attack increases.
Question: Could an aft cyclic stall the advancing blade? Lets say at full down, 60 knots, level attitude and 80% RRPM the AOA is 14 degrees. Add in two degrees to the advancing blade from the cyclic input and it would exceed a theoretical critical angle of attack of 16 degrees.
Answer Yes. And as the aircraft pitches aft add in the angle the mast tilts aft too.
Question:What factor does inflow/ROD play? You can't get 14 degrees of AOA on blades who's pitch is low without it.
Answer: Tricky one but basically increased ROD causes increased AOA. Inflow during a flare in auto comes from further below the Plane of rotation so increases AOA.
No numbers because I'm not good at that but the basic principles are good for understanding. Hope this helps.
The more I think about this the clearer it becomes. You say Robinson recomend no flare. My explanation explains why.
I would tentatively suggest, based on my crackpot theories that after an engine failure, if you find Nr very low, i.e. less than 80% you should:
1) Get the lever down. RRPM should stabilise. Do not flare. do not make any big cyclic movements.
2) Watch the tacho - it should stabilise and start to rise.
3) when it does, very carefully apply aft cyclic. watching RRPM.
If RRPM rises maintain aft cyclic and recover as normal.
Heres the really controversial bit.
If any left roll or fall in RRPM is detected apply slight forward cyclic until RRPM stabilises.
If after lowering the lever, RRPM continues to fall, the rotor has started to stall. Left roll is likely. flare will push the disc further into the stall. Judicious forward cyclic to reduce AOA is your only chance.
It is better to arrive at the the ground with 80% RRPM than 0%
Shoot me down on aerodynamic theory. Not because this is not what you were taught.
Last edited by Gaseous; 8th January 2005 at 01:24.