13snoopy
Question: How does flaring a helicopter LOWER rotor rpm???
Answer : If it precipitates blade stall because Nr is near stall speed.
As angle of attack increases so does aerofoil stall speed.
Aft cyclic increases the angle of attack.
Turns increase load factor which also increases angle of attack.
Increased sink rate increases angle of attack.
Decreasing Nr increases angle of attack.
If the increased angle of attack raises the stall speed to above the speed of the blade, the rotor will stall followed by a catastrophic blowback. this will result in mast bumping, blade incursion into airframe and death. (sound familiar to readers of R22 NTSB/AIBB reports??)
Attitude is irrelevant to maintaining Nr.
Angle of attack is what is important.
Take the following fixed wing analogy. If flying along at 5Kts above stall speed in a Cessna. Pull back on the stick hard and you will instantly stall. Push the stick forward to decrease angle of attack and you are flying again. No problem. Try this in an R22 with Nr at 5% above stall speed and you dont get the option of pushing the stick forward because the stall is non recoverable.
As long as the blades do not stall all the expected things happen in response to turns and aft cyclic. We all know what is supposed to happen - and it does when we practice it at 90%and above Nr.
Coning angles increase as Nr decrease or load factor increases. As coning angles increase, disc radius decreases, which tends to increase Nr. Great. The problem is there are mechanical limits on coning angle.
The point is we don't practice low RRPM recovery at 70-75% Nr in an R22. You only do it for real and if you get it wrong you die.
Where this is relevant to this thread is that if an R22 pilot has an engine failure and1.3 seconds later finds Nr at 75-80%, thinks Oh SH**T, and does a big flare as trained, it will be the last thing he does. The question is would keeping the stick still or slight forward cyclic have kept the rotor turning at such low Nr??
RRPM is life. Stall it and its all over.
Regards
Last edited by Gaseous; 7th January 2005 at 16:23.