Low-G pushovers

January 29, 2010 by Tim McAdams

A two-blade or semi-rigid rotor system (such as the Robinson or some Bell series helicopters) is susceptible to a phenomenon called mast bumping. To avoid mast bumping it is important to fully understand the limitations and performance capability of this type of rotor system.

In order to produce thrust a helicopter’s rotor system must be loaded. Controlled by the cyclic, the swash plate changes the pitch angle on each blade separately. This creates an imbalance of thrust across the rotor disc forcing the disc to tilt, which causes the helicopter to roll or pitch in the desired direction.

Pushing the cyclic forward following a rapid climb or even in level flight places the helicopter in a low G (feeling of weightlessness) flight condition. In this unloaded condition rotor thrust is reduced and the helicopter is nose low and tail high. With the tail rotor now above the helicopter’s center of mass, the tail rotor thrust applies a right rolling moment to the fuselage (in a counter-clockwise turning rotor system). This moment causes the fuselage to roll right and the instinctive reaction is to counter it with left cyclic. However, with no rotor thrust there is no lateral control available to stop the right roll and the rotor hub can contact the mast. If contact is severe enough it will result in a mast failure and/or blade contact with the fuselage.

In order to recover the rotor must be reloaded before left cyclic will stop the right roll. To reload the rotor immediately apply gentle aft cyclic and when the weightless feeling stops, use lateral cyclic to correct the right roll.

The best practice is to exercise caution when in turbulent air and always use great care to avoid putting the helicopter in a low-G condition.


21 Responses to “Low-G pushovers”

  1. Ehud Gavron Says:

    As a recent (August) rotorcraft certificate holder I did most of my training in the R-22 and the R-44, The first thing they covered (prior to even the demo-flight) was SFAR 73, which requires awareness training for mast-bumping and low-G conditions. My instructors at Voyager here in Tucson drilled the “always slight aft-cyclic in turbulence or low-g” over and over and over again.

    P.S. I have an unrelated question you or the other readers might be able to answer. Doing a zero-speed autorotation changes the angle of attack of the rotor disc from doing one at e.g. 75kt. I was asked why this does not pose a problem and my gut-level answer is that because we’re dumping the collective and not trying to use power, there’s no question of stalling the blades, and the driving region is perfectly able to spin up the rotor. (I know it does this from a practical view; I just want to understand the aerodynamics).

  2. Brendan Fitzpatrick Says:

    That’s why I like the fully articulated Schweizer 300 – plus my own cyclic and a little space between me and the passenger. See some of my flights on youtube – search pdxflyerZ.

  3. Don Arnold Says:

    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.

  4. Sid Wood Says:

    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?

  5. Chuck W Says:

    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…

  6. Chuck W Says:

    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…

  7. Snake Driver - Says:

    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…

  8. Peter McBride Says:

    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.

  9. John H Says:

    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.

  10. Ehud Gavron Says:

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

  11. John H Says:

    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.

  12. Bastian Says:

    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.

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