Dynamic Rollover. What is it?

Carol

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Carol
Heard of a local helicopter pilot having one of these. Not being a wing-flinger I can't picture in my mind what it is. Can any of you shed any light on this?
 
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This is similar to what causes a sailboat to capsize. What we're talking about here is rolling moment.
 
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If for some reason a helicopter exceeds a critical angle of bank, the roll can not be corrected, and the helicopter rolls over. Common factors leading to excessive bank angles are takeoff/landing on the side of a slope, and catching a skid on the ground.
 
Nav8tor said:
If for some reason a helicopter exceeds a critical angle of bank, the roll can not be corrected, and the helicopter rolls over. Common factors leading to excessive bank angles are takeoff/landing on the side of a slope, and catching a skid on the ground.

Thanks, guys. I googled it. Should have done that first rather than put you to all that trouble.

The pilot is injured but not severely. (Played golf that afternoon.)
 
Weight distribution is much more important as it affects stability. The diagrams below illustrate this point.

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Firstly consider the hull section (a) which is through a ship at rest in the water. Point B is the centroid of the displaced volume and is known as the centre of buoyancy. The resulting force exerted on the hull by the surrounding water is the force F which passes vertically upwards through B. This must be equal to the weight of the ship W for the ship to float. The weight of the ship W acts vertically downwards from the centre of gravity G. As the two forces F and W are equal and opposite and as they are perfectly aligned with each other this is a stable system and there is no movement.

Now consider the diagram (b). The hull has rolled over. Because of the shape of the hull the centre of buoyancy has now moved to a new position B and the vertical buoyant force F has moved with it. G cannot move provided the internal weights of the ship do not move (i.e. that the cargo/ballast does not shift). The resulting force W therefore is offset from force F by a horizontal distance Gz (known as the righting lever). As the two equal and opposite forces F and W are not aligned this is an unstable system and therefore an anti-clockwise rotation is set up which rotates the hull back to the stable condition. The righting force is the result of the weight x righting lever which therefore decreases as the angle decreases. The point M where the vertical line through the centre of buoyancy intersects the centreline of the hull is known as the metacentre. The height h of this point above the centre of gravity G is known as the metacentric height. For a hull to float upright M must always be above G. In practice for most hull forms the position of M, and as G is fixed therefore the metacentric height, is virtually constant for angles of roll up to 20o. In practice therefore points G and M are fixed and are a property of the particular vessel. The greater the metacentric height the greater the righting level for any given angle of roll and the righting force resisting roll and therefore the greater the stability.

Finally, diagram (c) shows an inherently unstable hull which has G above M and therefore will capsize. This is the typical hull form of a racing sail boat and illustrates why they capsize when they loose their fin and ballast weight.


What is not mentioned above is that a longer fin or increased ballast weight would increase the dynamic stability. On a helicopter the placement of the skids is as important as the depth of the fin and wt of the ballast on a sailboat. A much greater force would be required to counteract skids which were placed further outboard. Also, lower skids (not as tall) would have much the same effect except that method is greatly restricted because it is less practical.
 
Carol said:
Thanks, guys. I googled it. Should have done that first rather than put you to all that trouble.

The pilot is injured but not severely. (Played golf that afternoon.)

No trouble at all, it is enjoyable to crack the books. I have a link to an interactive graph on displacement hull dynamic stability somewhere around here. You get to place various amounts of weights at various inboard or outboard points and heights above/below Center of Bouyancy and witness the results. Quite educational.
 
Nav8tor said:
If for some reason a helicopter exceeds a critical angle of bank, the roll can not be corrected, and the helicopter rolls over. Common factors leading to excessive bank angles are takeoff/landing on the side of a slope, and catching a skid on the ground.

I would think on a helicopter there is some component of torsional force involved in the 'capsize'. Think of it as 'rounding up' in a sailboat. If not enough counter force is applied quickly enough it will lead to a pitch pole which may lead to a a sort of head long sideways capsize. Of course, as you say, you'll likely be beyonde that point already because the righting moment is too far gone.
 
Dynamic Rollover is not at all like capsizing a boat. Yes the helo has a CG, but the helo does not pivot around this CG. It pivots around the mast/rotor connection. Dynamic rollover also has nothing to with slope angle. That is called static rollover. Dynamic rollover occurs when the helo pivots laterally around a point other than the mast/rotor connection. Typically (99.9%) this point is the skids/or wheels, on a small rock, a taxiway light or the hardest to figure out for the pilot is the skid stuck in soft ground. And the correction for this is actually not opposite correction it is down collective right now. Not in a second RIGHT NOW!!!!. The trouble with trying opposite correction (say you are rolling over to the right) is the components of thrust may be working so far against you that when the rotor should be pulling you back left it will actually be pushing you over to the right. Remember the rotor system is beating gravity into submission and is pushing the helicopter away from earth (at least at this point) not pulling it into the air.
Chris
 
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Nav8tor said:
How about the helicopter. Was there much damage?
The typical dynamic rollover is a destroyed bird. The rotor is producing full (or nearly full) power as this takes place on pick-up and set down. When the helo rolls the main strike the ground once the mains are ripped off the helo will spin horizontally around its ground contact point and the tail rotor will strike the ground and rip the tail rotor off and the tail boom. Usually a total loss.
Chris
 
Chris Connor said:
And the correction for this is actually not opposite correction it is down collective right now. Not in a second RIGHT NOW!!!!.
Chris
That's not necessarily true. An attempt at a slope landing where the slope is too steep will result in a rollover if you don't immediately RAISE the collective when you realize that you ran out of cyclic.
 
Nav8tor said:
How about the helicopter. Was there much damage?

I haven't heard yet but I imagine Chris is right.
 
RotaryWingBob said:
That's not necessarily true. An attempt at a slope landing where the slope is too steep will result in a rollover if you don't immediately RAISE the collective when you realize that you ran out of cyclic.
Bob,
Yes you are right, but I don't think that it technically a "Dynamic Rollover". I believe that is considered a static rollover. Although it is a dynamic event, the heicopter would have rolled over with the engine off. The difference is that the rolling moment has not exceeded the critical angle. I agree completely with the corrective action you suggest for critical angle recovery caused by slope, not an obstruction.
Chris
 
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Chris Connor said:
Bob,
Yes you are right, but I don't think that it technically a "Dynamic Rollover". I believe that is considered a static rollover. Although it is a dynamic event, the heicopter would have rolled over with the engine off. The difference is that the rolling moment has not exceeded the critical angle. I agree completely with the corrective action you suggest for critical angle recovery caused by slope, not an obstruction.
Chris
Chris, the Army Rotary Wing Flight manual describes this situation as a dynamic rollover (page 4-8), because you have one skid on the ground and it acts as a pivot point. That's also the way my primary CFI taught it, but then he's a retired Army pilot.

The FAA Rotorcraft Flying Handbook implies the same thing in that it says that you need an understanding of dynamic rollovers before attempting slope landings.

The definition of a dynamic rollover that I was taught was that it required a pivot point, in this case the uphill skid, lift and weight equal, which they are in this case, and exceeding the critical angle. So for my money, anyway, I think this is a dynamic rollover situation. YMMV :dunno:
 
Nav8tor said:
If for some reason a helicopter exceeds a critical angle of bank, the roll can not be corrected, and the helicopter rolls over. Common factors leading to excessive bank angles are takeoff/landing on the side of a slope, and catching a skid on the ground.

I don't think it's directly a function of "bank angle" at least as we fixed wing pilots relate to that. My understanding is that there has to be something restricting the body of the helicopter from moving to create a dynamic rollover event. Also I expect that you could cause the same problem about the lateral (pitch) axis if for example you caught a skid under a pipe and tried to power away forward.

There is a very similar situation for fixed wing pilots of tailwheel aircraft. If you open the throttle with the brakes on or the main wheels up against an obstruction (or stuck in the mud etc) you can easily bring the tail up far enough that no matter what you do next you will roll onto the nose.
 
RotaryWingBob said:
Chris, the Army Rotary Wing Flight manual describes this situation as a dynamic rollover (page 4-8), because you have one skid on the ground and it acts as a pivot point. That's also the way my primary CFI taught it, but then he's a retired Army pilot.

Ok Bob. Yes what you are describing here is definitely Dynamic Rollover. I made an assumption when I read your other post. Since you mentioned hitting the cyclic stop I assumed your roll was to the down hill side.
Chris
 
lancefisher said:
I don't think it's directly a function of "bank angle" at least as we fixed wing pilots relate to that. My understanding is that there has to be something restricting the body of the helicopter from moving to create a dynamic rollover event. Also I expect that you could cause the same problem about the lateral (pitch) axis if for example you caught a skid under a pipe and tried to power away forward.
Lance, I've never heard the term bank angle applied in this situation either, but I guess it does sort of work in this sense:

We're taught to land on a slope by coming to a hover parallel to the slope (that is, pointed neither up nor down), and then descending until the uphill skid touches. At that point you have a delicate balancing act. Lateral cyclic towards the slope will cause the helicopter to tip torwards it, and away from the slope does the reverse. In this sense the cyclic causes the helicopter to "bank" in a similar way that it will in normal flight (control inputs behave differently since it's not in normal flight). The trick from this point is to increase the cyclic input into the slope as you lower collective, basically to get a soft touchdown. If you slam the collective down, you might very well get a dynamic rollover with the downhill skid now acting as a pivot point.
 
lancefisher said:
I don't think it's directly a function of "bank angle" at least as we fixed wing pilots relate to that. My understanding is that there has to be something restricting the body of the helicopter from moving to create a dynamic rollover event. Also I expect that you could cause the same problem about the lateral (pitch) axis if for example you caught a skid under a pipe and tried to power away forward.

There is a very similar situation for fixed wing pilots of tailwheel aircraft. If you open the throttle with the brakes on or the main wheels up against an obstruction (or stuck in the mud etc) you can easily bring the tail up far enough that no matter what you do next you will roll onto the nose.


True. Its a function of angle of the rotor disk. I was putting it in terms that are familiar to fixed wing pilots. I think its more likely to happen in roll, because in pitch there is more stability due to the mass of the fuselage, tailboom, empenage etc.
 
Nav8tor said:
True. Its a function of angle of the rotor disk. I was putting it in terms that are familiar to fixed wing pilots. I think its more likely to happen in roll, because in pitch there is more stability due to the mass of the fuselage, tailboom, empenage etc.

I would think it has less to do with mass and more to do with arm from center of mass.
 
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