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Vertical Autorotational Descent

Could you help me refresh my knowledge on the aerodynamics during a VERTICAL, zero-forward speed autorotational descent in any helicopter.
I've gotten myself confused. A vertical descent rate of 2000 FPM is only about 23 MPH, and if the blade tip speed is 350-400 MPH,
it looks like it can be done.

I had convinced myself that the blades would become stalled by the inflow from under, and now I am wondering why I had thought that.
A 23 MPH inflow from below wouldn't change the angle of attack of a 400 MPH blade but 3 degrees or so???
I requested this because I want to have the right information before I participate in a discussion on this topic, on another forum, and I realized my thoughts were wrong.
In Primary, those crusty old Pan-Am CFI's demonstrated zig-zagging autorotations where they did 10 seconds in forward flight followed by nose up, then followed by 10 seconds
autorotating backwards. They repeated this 4 or 5 times on the way down.

They never demonstrated a zero speed vertical autorotational descent. I have never done one, and have never had one demonstrated to me.

Fast forward to 1990. While I was getting a Robinson SFAR signoff, I watched as Frank Robinson showed 2 people dying when their R-22 fell almost vertically from
altitude to the ground because the ROTOR HAD STALLED due to low RPM.

For some reason, in the time since then, I had incorrectly convinced myself that their rotor had stalled during a near vertical auto.

I was about to join the discussion on RotaryForum about this so I questioned my correctness first. Now I'm almost positive
that a helicopter CAN be autorotated VERTICALLY with ZERO AIRSPEED, even though the rate of descent will be extremely high.

Some of the guys on there say they have done VERTICAL autos down to the H/V Chart and THEN nosed down to best auto speed.
I started questioning my thinking that a vertical auto from altitude was bad, but now, when I imagine a 20-30 MPH vertical sink rate
when rotor tip speed is 400-500+ MPH, it doesn't seem THAT extreme. It doesn't seem that a vertical induced flow, from directly below,
of 20-30 would put the blades beyond their stall angle of attack?
BRYAN best thing to do is go get some vertical time then you would understand better..shouldn't tell any one info if you haven't done one......lets be safe abought autos critical thing there for wrong info....
Dang Tom, Isn't that what I was trying to achieve by asking?
Dang yeah, sounded like your asking to me Bryan! Now I'm interested in the answers from the crew... keeping im mind i'll may have to adjust everything for my inverted NZ compass Smile when u push collective, i pull??
Richard, NZL  /// C? Test Dummy \\\  "I used to be a hang-glider pilot. Yeah, I remember that... day!"
I don't think I'd ever try a vertical auto, but I'm interested in the answer as well.

I know when I was talking to Mark T. a long time ago he said he didn't think we'd have enough energy to slow our descent with rotor RPM alone, we need that forward speed and the flare as well.
We were talking about it because I've seen guys do it with R/C helicopters, but things are different there, they're free to wind the rotor speed as high as they want and physics are different when the craft is so small.

At the Flyin year before last, we all saw John Snider start a vertical auto from a hover at about 300 ft (my guess) then push forward, flare and set it down perfectly.
He made it look easy, but then he always does.
youtube has a nice one in a 300c .just type in vertical autorotation 300c on youtube..and john does do a nice vertical auto never seen one like that.
Just to be clear, John pushed forward so he had some forward speed for the flare.
My guess is that by the time he did the flare he was at 45 mph or so.
Keep in mind guys, I am talking about a rotor system being ABLE to maintain RPM as it descends vertically. I'm calling that "AUTOROTATION." I am NOT talking about a maneuver that involves a vertical descent to near the ground! This is also called an "AUTOROTATION."
Bryan, I'm pretty certain that a vertical auto is OK. Like you I was taught forwards and backwards autos, and I even tried doing a vertical auto in an S61, to achieve a steep approach. Worked just fine. (but footnote: do not try this at low altitude, that great pig takes forever to pick up speed again!)

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