Dynamic rollover

[VP-F__]

Textbook descriptions of dynamic rollover describe an aircraft normally rolling sideways. Is it theoretically an easy thing to do in a forwards direction in a wheeled helicopter, for example a poorly handled tailwheel helicopter like a Seaking/S61 with a forward C of G?

[paco]

Yes, it can happen fore and aft. Any time your helicopter has a tilted main rotor thrust vector with respect to the C of G. The danger with it is that pulling collective simply makes the sideways vector longer

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It wouldn't be a rollover the would it?:ok:

It would be a pitchover and the fuselage would probably stop it continuing forward as the nose hit the ground.

It happens laterally much easier due to the relatively narrow width of most undercarriage.

The classic texbook version has it happen on a slope but it is easy to do on level ground as well.

[VP-F__]

Ok thanks, fair enough on the terminology but the theory is the same anyway ;) Is it possible then in a pitchover to be very ham fisted (on level ground) and actually lift off while continuing to pitch over before contacting the ground with nose and avoiding total roll/pitch over. Looking at a photo of a Seaking the potential for pitchover is maybe higher than a rollover (on flat ground) given the wide wheelbase compared to the short distance from the mast to the main wheels?

[John Eacott]

Textbook descriptions of dynamic rollover describe an aircraft normally rolling sideways. Is it theoretically an easy thing to do in a forwards direction in a wheeled helicopter, for example a poorly handled tailwheel helicopter like a Seaking/S61 with a forward C of G?


Since a dynamic rollover requires a fixed undercarriage point to roll around, it is nigh on impossible for a wheeled helicopter to provide that rollover point either forwards or backwards. Sideways, yes.

Simple brake release would negate the fore/aft problem.

[Old Farang]

Interesting question. The very early R22 helicopters tail boom was basically horizontal, which put the tail rotor closer to the ground.(later machines had longer uprights in the rear airframe which increased the tail boom angle and therefore the tail rotor was higher). There was a very experienced and well regarded instructor in Australia at the time that used to teach a technique that I have never seen before or since. Forward cyclic as you increased the collective, while keeping the front of the skids on the ground. The result was the helicopter sort of "standing on its toes" while still on the ground. As the machine started to lift off the ground the cyclic was eased back and the helicopter then came into a level hover.

As he was instructing potential mustering pilots, his reasoning was that as normal operations would likely be out in the scrub, the sooner you got the tail rotor away from grass and scrub growing on the ground, the better it was for both the helicopter and the pilot. It also ensured that the helicopter was starting to move forward away from any tendency to drift sideward towards a roll over.

Edit: it's about wheeled undercarriage helicopters :ok:
SPlot

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VP-F - the Sea King is much more susceptible to rollover and I don't know of any pitchover event but plenty of rollover close calls and actual rollovers.

The problem laterally is that you need left cyclic to displace the disc and counteract TR drift - the TR is trying to push the aircraft left and that force increases as you raise the lever and add power pedal (left). If not enough left cyclic is used, the right undercarriage tries to slide but becomes a pivot point around which the rollover happens. It is important therefore to ensure the right wheel comes off the ground first during a take-off.

The height of the TR means any nose down slope take off increases its moment about the C of G and a left crosswind exacerbates the problem by requiring more left cyclic..

You don't have the same effects in pitch.

However, with the mainwheels only in contact, it is possible in turbulence to overcontrol and push the cyclic far enough forward to impact the sand filter boxes in front of the engine intakes.

[Pilot DAR]

Would this well known video be an example of dynamic pitchover?

https://www.youtube.com/watch?v=z4mGYnldOK8

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No, that is what happens when you fill the nosebay with water during a water landing and pull more power to try to get airborne again - imagine the C of G shift!

Many of us have landed a Sea King on water - the Canadians used to run an excellent course in Nova Scotia - the lesson was that if you lost the AFCS during a landing, it meant the nosebay was flooded and under no circumstances should you try to get airborne again.

[Pilot DAR]

Ah, okay. I've never flown a Sea King, though I have landed a Bell 206 in the water a few times (happily, inflated floats are hard to fill with water!). But I know some about the drag of a hull in the water, particularly moving forward under power, with a high trust line, and the landing gear down. In the case of the Russian MIL, it appears that the wheels extended when it hit the water (perhaps the pilot was unaware), so moving forward with the drag of the wheels extended under the hull would have similarities to snagging a wheel or skid on an obstruction on the surface while drifting laterally.

But yes, the C of G change from a flooded nose compartment would do it too! I have had flooded float compartments on a floatplane, though there are different opportunities to manage that a little better.

[havoc]

Have not read the book in awhile "Fatal Traps for Helicopter Pilots" but I recall the situation when a ships salvage crewmember jumped from the ship to the hovering aircraft and created a rollover situation. I believe it was a 500 and the salvage crewmember was new and forgot the briefing for the process of getting on/off the ship.

Not the normal situation as taught likely to occur on flat surfaces but the basic formula, pivot point, rolling motion, exceeding critical angle where as you wont recovery.

[John Eacott]

Ah, okay. I've never flown a Sea King, though I have landed a Bell 206 in the water a few times (happily, inflated floats are hard to fill with water!). But I know some about the drag of a hull in the water, particularly moving forward under power, with a high trust line, and the landing gear down. In the case of the Russian MIL, it appears that the wheels extended when it hit the water (perhaps the pilot was unaware), so moving forward with the drag of the wheels extended under the hull would have similarities to snagging a wheel or skid on an obstruction on the surface while drifting laterally.

But yes, the C of G change from a flooded nose compartment would do it too! I have had flooded float compartments on a floatplane, though there are different opportunities to manage that a little better.

I would suggest that the bloody great radome under the nose would have far more effect than the wheels being down :hmm: :p


I'd also hazard a guess that there was most likely a power failure following take off, and an attempt at a running takeoff rather than a lift back into the hover.