Autorotations

[DroneDog]

I post this as I don't know

My current thinking is that as most helicopters can't select negative pitch an autorotation is dependant of main rotor velocity or rpm being held by the rotors inertia hence it is important to bottom the pitch ASAP to conserve momentum. But the guy in this video suggests something else, he states at certain angles of pitch part of the disk is windmilling as the helicopter descends and at the same time part of the disk is providing lift and an autorotation is a balancing act between the two to control your decent.

You need to go to 3 min into the video before he gets to the meat of the story.

Is that how it works?
BTqu9iMiPIU

[paco]

What people call autorotation is actually windmilling anyway, as the tail rotor etc are being driven as well. The only parts of the main rotor that are truly in autorotation are the neutral points between the driven/driving/stalled sections. That said, changing the size of the driving section with collective to control the descent is one aspect - but most people I suspect only use the collective to keep the RPM in the green range.

phil

[PDR1]

Yes. Just research Autogyros (http://www.jefflewis.net/autogyros.html) (it's the same thing).

PDR

[hueyracer]

The reason why you want to leave a little bit of pitch in an autorotation is to prevent an overspeeding of the main rotor.

Basically, one can say:
The higher the pitch, the lower the rotor speed..(up to the point where the rotor just stops)....

[NickLappos]

The rotor design (in this case mostly its local pitch angle, set by twist and rigging) is a compromise. For ideal autorotation, the twist and collective rigging could be one way and for hover efficiency quite another. The bottom collective is set by the ability to successfully enter and stay in autorotation, usually at the worst case state for the approved envelope. Worst case is light weight, low density altitude, where the trimmed steady low-collective auto rpm is lowest. If the aircraft is mis-rigged and the bottom pitch is a bit too high, the auto rpm might slide below the green arc. Why not have massive collective pitch range to allow lots of bottom pitch and also lots of high collective? This is one of the most expensive and heavy items in the designer's tool bag. Collective pitch range is also main shaft (mast) length, and also servo length and collective control rigging. Masts and servos and controls are among the most expensive and heavy components, so every degree of collective range is hard-won.
Many helos have a maintenance check to confirm proper rigging, others have enough margin in the rigging procedure to assure same without actually checking.
Naval helos that have to glue themselves to a tossing deck often have a few degrees of negative collective pitch so they stick themselves down. You can tell this by the fact that as you land, you can watch the torque go back up as the collective gets to the bottom.
These naval machines have the strong propensity to overspeed in auto, of course.
What affects auto rpm? Gross weight, the higher the weight, the faster rpm builds and trims. Density altitude, the higher the altitude, the faster the rpm builds. Airspeed, the faster the speed above the Vy, the less rpm the aircraft trims at.Pitch attitude rate, a push over (bunt) causes a strong rpm decay while a pull up causes an rpm build up.

[Arm out the window]

Great summary from Nick - you're getting the good gen from someone who really knows there, Drone Dog.

[Ascend Charlie]

The diagram of the disc from above only showed the hover - the 2 circles were concentric. In forward flight, the inner circle moves to the left, and also goes further left with pitch. Too far left, (too fast, too much collective) it is all drag and you lose RRPM in a serious way. Stall flick spin crash burn die.

[SFIM]

Mr Charlie

the 2 circles were concentric

Err, no they weren't, it was offset to the left, see at 3:56