Comments on: Low-G pushovers

By: Bastian

Bastian — Wed, 03 Mar 2010 06:26:17 +0000

Ehud, forward speed in an autorotation is actually more complex as far as the dynamics of the rotor disc than a zero-speed auto. In a zero-speed auto the rotor disc has a constant angle of attack at all areas of the disc and the sizes of the stalled, driving, and driven regions are constant thoughout the disc. When you are autorotating with forward speed however the slower airflow over the retreating blade causes it to flap downwards which increases it’s angle of attack (and equalizes lift) and creates a larger stalled region and a smaller driven region. On the advancing blade the stalled region becomes smaller and driven region larger.

By: John H

John H — Sun, 07 Feb 2010 20:04:56 +0000

Ehud, after my posting I got out my “Principles” book and re-read the chapters on the flare and autorotation. To answer my own question, the advancing/retreating blade issue at flare is no more significant than it is in normal, level, powered flight. The flare, using the driven region of the disc, reduces inflow from the top of the disc (because more relative wind is applied to the bottom), just like in ground effect. Reduce inflow, if you work out the vectors, gives you more lift with less power. It also applies a rearward component to the total reaction to slow down your forward speed, just as in powered flight. The difference in autorotation is that the “power” is coming from the driven region instead of the engine. So it’s about reduced inflow.

By: Ehud Gavron

Ehud Gavron — Sat, 06 Feb 2010 17:36:59 +0000

John – thank you! Principles of Helicopter Flight has been One-Clicked Good explanation… and thanks for the referral!
E

By: John H

John H — Sat, 06 Feb 2010 16:37:36 +0000

I think I can answer Ehud’s question. The forward speed in the autorotation is only needed as an energy reserve when you need to flare at the end. At other times, the driving region of the advancing blade’s velocity due to its rotation is way higher than any due to forward translation. The high, horizontal, rotational velocity gives it a more nearly horizontal angle of attack, which is what you want for the least drag. Besides, the forward translation has a slight _negative_ effect on the retreating side of the disc, and so if it were to be any benefit on the advancing side, it would have to be countered on the retreating side.

Indeed, I’ve had an instructor demonstrate reverse flight in an autorotation, in order to reach a spot that was otherwise too steep. As long as you get your 50 knots or more before the flare, you’re fine.

About that flare, I think what happens is that you immediately put a huge angle of attack on the disc by pitching up (not by using collective). That makes a lot of lift to slow your descent rate, but it makes a lot of drag, too. The forward speed, I think, is the energy to overcome that drag. I’m not completely sure, frankly, how the flare works, because the retreating blade gets a large negative AOA, but we know it does. As an airplane pilot, too, I can tell you that flare feels just like an airplane.

I don’t know if you have “Principles of Helicopter Flight” by Wagtendonk, but that goes into great detail on aerodynamics. It’s really interesting.

By: Peter McBride

Peter McBride — Fri, 05 Feb 2010 20:29:16 +0000

I’ve been flying R22′s for many years and at present own three of them. Virtually all of my helicopter flying is without a passenger herding livestock or game, There are quite a few of us commercial pilots who do this type of work and to say the least, we make many, many abrupt maneuvers in a day’s flying. None of us recallls encountering mast bumping in R22′s. Of course we avoid maneuvers that will chop the tail boom. If bumping occurs, there will be several bumps since the rotor is turning over 600 rpm and time passes before one can react to the situation. A second or two is a long time. The helicopter that I’ve flown that was prone to mast bumping was the Hiller 12 model, both D & E. We used them for the same purpose as the R22′s that we now use but any pilot doing our work soon learned how to avoid the situation. The R22 is a “solid” aricraft.

By: Snake Driver -

Snake Driver - — Fri, 05 Feb 2010 17:05:37 +0000

The title is a misnomer – It’s not “mast bumping” – it’s “mast bump” – it only takes one “bump” and you’re glide angle resembles that of a a crowbar. Those of us flying Bell two-blade systems (JetRanger, Huey, Cobra) are quite familiar with the danger of large cyclic inputs in a low-G state.
The inside of the blade yoke flaps too far down, and contacts the mast. The inevitable result is a separated mast. I lost two buddies to this problem – one during training, when he was making a high-angle approach to a confined area, and the instructor slapped the stick to the side (it was the wrong CAL site). He jumped and lived long enough to tell the tale.
The other buddy was flying a AH-1J solo in a formation on an admin move to MCAS Yuma, when a safety-nut came loose from the bottom of his cyclic control and he lost all lateral control (he missed the loose nut on pre-flight.) From 8000 ft altitude, he had 45 seconds to tell us what happened before impact. I should have been his co-pilot, but I had S4 duties in the rear. My pre-flight went from 30 minutes to over an hour…

By: Chuck W

Chuck W — Fri, 05 Feb 2010 16:34:49 +0000

Ehud,
I will take a stab at your question. We all know how a plane flies and that the wing is moving forward at the proper angle to produce lift. If one slows the wing, an increase of angle of attack is needed to produce lift. if we continue to slow down, the point comes when the angle of attack become critical and the stall occurs. Now as for the rotorwing, it is just that the wing is in motion by rotation, thus the term “Sling-wing Pilots”. We need the same lift and do so by using our engine power spinning the wings while remainder of the airframe can stand still. With the rpm the “relative wind”, from the blades point of view, is within the angle of attack and you answered your question with the collective down, relative wind is in a state to drive the rotor at the cost of altitude. However with that high vertical speed straight down recoverery would be impossible from altitude until we add some additional energy with forward speed back to 65kts prior to flare. In short, the speed of the rotating wings vs the lift demanded (lack of drag) allows for blades to continue spinning by being driven with the excess energy. That may be clear as mud, the blades are the same as wings of a plane in a descent, they are just moving alot faster thorough the air and see it differently than straight up due to that speed. A difficult thing to explain because we aren’t sitting on the blades…

By: Chuck W

Chuck W — Fri, 05 Feb 2010 15:40:32 +0000

My two cents is a perference to semi-rigid teetering due to simple and a ecomicial answer to dissymmretry of lift. Brendan may perfer the fully articulating, but he has to watch for ground resonance and the point is, no free lunch. Sid, imagine a teeter toter that we played on as kids. Mast bumping occurs when the hub of which the blades are attached reaches the limit of downward travel and makes contact with the mast on which the hub is mounted. As the blades are rotating, we are talking about tons of force from centrifugal force needed to put rigidity into the blades. Once it reaches the limit the blade can flex enough to strike the tailboom or damge the mast. Yes I believe every known law of physics was needed for the helicopter to fly…

By: Sid Wood

Sid Wood — Fri, 05 Feb 2010 15:02:24 +0000

I understand this mast bumping thing is a fact of life. I am quite ignorant of the mechanics for the rotor hub and mast. I thought the the rotor hub was attached to the mast by some very heavy duty bearings. What actually bumps what? Is there some play in the bearing structure or is it some play on words for which I don’t have the code?

By: Don Arnold

Don Arnold — Fri, 05 Feb 2010 14:49:13 +0000

Back in the seventies, we flex wing weight shift (hang glider) pilots took a lot of criticism for flying aircraft that needed to stay loaded to stay in control. Those of us who survived did so by watching the weather closely. We accepted the reduced utility because you could buy, learn and fly very cheaply. Choppers ain’t cheap by any measure.