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Old 13th Jan 2020, 16:47
  #6 (permalink)  
Paul Cantrell
Join Date: Nov 2007
Location: Massachusetts
Age: 64
Posts: 171
Originally Posted by helimutt View Post
Under Discussion heading.

The third paragraph :- yes reducing collective reduces pitch angle and then the rotor rpm can recover (not engine power recover. surely the available engine power is already trying to recover the Nr but is unable to) Once drag reduces due to decreased pitch angle, then the available engine power can return Nr to within normal operating parameters. Surely not “spin at full speed”?

I'm having a little problem understanding your point about engine power, but for those that haven't flown pistons in a while I'll remind you that unlike turbine helicopters, in a piston helicopter available engine horsepower is directly related to RPM. If you have let your Nr decay by 10%, you have lost 10% of your available horsepower. It's a double whammy, that at the very time the rotor system is demanding more power, you have less available from the engine.

So, the original statement is correct: if you find yourself in a low RPM situation in a piston helicopter, you are also in a low horsepower situation. Lowering collective will recover both Nr and available horsepower ( let's ignore any talk of derated piston for now ).

Apologies if I just didn't understand what you were getting at.

BTW, the high inertia Enstrom rotor system makes things much worse here. For all of our complaints of the low inertia R22 rotor, recovery from low rotor RPM is quick and easy in that aircraft. The Enstrom, while a magnificent machine for autorotation, can be very time consuming to recover rotor RPM, and it's not that difficult to get yourself into the corner of the envelope where you are going to hit the ground before you can get flying RPM back... As in, if you lower the collective you will descend and hit the ground before the engine can get that heavy rotor system back up to speed, and if you don't lower collective it will stop flying and hit the ground. Nothing unique to the Enstrom, but especially in the "A" model Enstrom an interesting combination of low horsepower and high inertia.

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