Dynamic Rollover Question?
Senis Semper Fidelis
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Dynamic Rollover Question?
This is one of the things that we most hear of in small things like R22's and occasionally R44's, my question is, does this problem affect big and heavy helicopters Ie Puma's Seakings and the like, I have not seen anything connected with this problem and big Helis?
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It does indeed affect all helicopters. The higher the CofG and the smaller the udercarriage width the more likely it is. There was a Seaking accident a couple of years ago because of it. Boscombe Down doing a trial to confirm the problem also managed to roll one!!
Have heard in russia that a Mi-17 also turns an excellent barrel roll on a taxiway while too fast taxiing. Additionally to CAC Runaway words it's also depends from the windspeed and winddirection (liftvector) and the reaction of the pilot. Seems to be the best method to react with the collective (if possible) and to hold the cyclic neutral.
The problem of the smaller ships is the light weight (inertia) and the relatively high CofG (on piston helos the fuel tanks are very high compared with the bigger ships and their low fuel tanks) resulting in a much more faster rollover.
I believe the lots of accidents of that kind with uncle Franks toys staying in line with the mostly more unexperienced pilots (not at all, of course
).
If the peak of a skid gear is boring a hole on a slope it's nearly impossible to avoid a dynamic rollover.
The problem of the smaller ships is the light weight (inertia) and the relatively high CofG (on piston helos the fuel tanks are very high compared with the bigger ships and their low fuel tanks) resulting in a much more faster rollover.
I believe the lots of accidents of that kind with uncle Franks toys staying in line with the mostly more unexperienced pilots (not at all, of course
). If the peak of a skid gear is boring a hole on a slope it's nearly impossible to avoid a dynamic rollover.
Avoid imitations
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In the early 1970s a Puma was rolled over by a student pilot ground taxying to line up on the disused runway for his first solo takeoff.....
The RAF taught pilots to hold the cyclic into a turn whilst ground taxying to minimise the likelihood of adding to the stats.
It is even easier to upset the applecart if you apply the wrong (left) pedal for lift-off. The aircraft rolls very rapidly to the right as it becomes light on its wheels.
The old Pume has a few other nasties she can also deal out to the unwary pilot...
The RAF taught pilots to hold the cyclic into a turn whilst ground taxying to minimise the likelihood of adding to the stats.
It is even easier to upset the applecart if you apply the wrong (left) pedal for lift-off. The aircraft rolls very rapidly to the right as it becomes light on its wheels.
The old Pume has a few other nasties she can also deal out to the unwary pilot...
I've been hunting throught the AAIB archives for a report ab out a Super Puma which rolled while taxying at Aberdeen.
I can't find it - if I can be arsed I'll look later and post the link - the report explains precisely the mechanism which led to the rollover.
I can't find it - if I can be arsed I'll look later and post the link - the report explains precisely the mechanism which led to the rollover.
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The Boscombe Down accident was to determine that you couldn't get dynamic rollover in the Sea King - there had been an earlier accident in a training squadron....
The TPs certainly settled the question quite categorically - you can get dynamic rollover in a Sea King. It is a hot topic in the Sea King force at the moment and there are several factors that can make you more prone to rollover:
nose down slope (TR thrust vector is higher thus increasing the rolling moment about the right wheel),
crosswind from the left (reduces the amount of cyclic available to counter any roll as you have to use more left cyclic to keep the disc level)
lateral slope (the classic example of the right wheel up slope is valid but equally with left wheel up slope the resoring moment of the mass acting through the C of G is much closer to the right wheel and therefore less effective)
low fuel state (as the fuel load reduces, the vertical C of G rises reducing static stability and the resoring moment is reduced)
lateral C of G (this is not usually calculated but could easily be off to the right depending on where the crew and passengers are sitting)
poor visual cues (night ops are the obvious example where it is very difficult to see the tip path plane and therefore keep the disc level)
A typical SAR scenario might be at a hospital HLS at night after a long job (low fuel and tired crew) with the wind off to the left and an unfavourable slope (easily managed if you land with the cabin door facing the best direction for patient offload and then shut down). Forgetting to engage the ASE would just be the icing on the cake!
Prevention is better than cure so keep the dics level during take off!
PS grabbing a handful of lever to try and break ground contact before the rotors impact just will not work no matter how many people kid themselves that it will.
nose down slope (TR thrust vector is higher thus increasing the rolling moment about the right wheel),
crosswind from the left (reduces the amount of cyclic available to counter any roll as you have to use more left cyclic to keep the disc level)
lateral slope (the classic example of the right wheel up slope is valid but equally with left wheel up slope the resoring moment of the mass acting through the C of G is much closer to the right wheel and therefore less effective)
low fuel state (as the fuel load reduces, the vertical C of G rises reducing static stability and the resoring moment is reduced)
lateral C of G (this is not usually calculated but could easily be off to the right depending on where the crew and passengers are sitting)
poor visual cues (night ops are the obvious example where it is very difficult to see the tip path plane and therefore keep the disc level)
A typical SAR scenario might be at a hospital HLS at night after a long job (low fuel and tired crew) with the wind off to the left and an unfavourable slope (easily managed if you land with the cabin door facing the best direction for patient offload and then shut down). Forgetting to engage the ASE would just be the icing on the cake!
Prevention is better than cure so keep the dics level during take off!
PS grabbing a handful of lever to try and break ground contact before the rotors impact just will not work no matter how many people kid themselves that it will.
Avoid imitations
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Crab,
Strange it required test pilots to crash to prove for themselves what was already known for years about dynamic rollover, the knowledge was already right there in the RAF! Why didn't they just ask the Puma boys? (same scenario as Sea King but with a narrower undercarriage width).
Your last comment is very true. Raising the lever gives (a lot) more cyclic control power so if the cyclic isn't already offset to the "recovery" side, the inertia of the aircraft might not be overcome in time. Then it's "Clap hands here comes Cadburys and mind me car, Biggles".
Had DR myself once and nearly took out an army dog handler and his dog during my over enthusiastic recovery. The old Puma, having sat happily on the ground for about half a minute, suddenly slipped sideways on me, only about a yard or so, down the steep rocky slope; it was wet, muddy limestone. Then the wheels found firmer ground again and stopped very suddenly. The rest of the aircraft didn't stop and she rolled away from the slope against full inslope cyclic (not a lot available on that old bird). I pulled an armful of collective and she rapidly rolled back into the slope, the blades nearly hitting the ground. Fortunately the army had disembarked at the gallop and were outside the disc...
Crewman highly unimpressed, as he was looking through the side door at sky one second and the slope the next.
Potential nightmare stuff.
Strange it required test pilots to crash to prove for themselves what was already known for years about dynamic rollover, the knowledge was already right there in the RAF! Why didn't they just ask the Puma boys? (same scenario as Sea King but with a narrower undercarriage width).
Your last comment is very true. Raising the lever gives (a lot) more cyclic control power so if the cyclic isn't already offset to the "recovery" side, the inertia of the aircraft might not be overcome in time. Then it's "Clap hands here comes Cadburys and mind me car, Biggles".
Had DR myself once and nearly took out an army dog handler and his dog during my over enthusiastic recovery. The old Puma, having sat happily on the ground for about half a minute, suddenly slipped sideways on me, only about a yard or so, down the steep rocky slope; it was wet, muddy limestone. Then the wheels found firmer ground again and stopped very suddenly. The rest of the aircraft didn't stop and she rolled away from the slope against full inslope cyclic (not a lot available on that old bird). I pulled an armful of collective and she rapidly rolled back into the slope, the blades nearly hitting the ground. Fortunately the army had disembarked at the gallop and were outside the disc...
Crewman highly unimpressed, as he was looking through the side door at sky one second and the slope the next.
Potential nightmare stuff.
Interesting discussion! But we talk about the dynamic rollover on ground and while taxying. But more much more accidents happens on sling load operations. The cable runs from hook above the skid or other airframepoints to the load. The cyclic against the bank and raising collective is deadly...
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The complete answer to dynamic rollover?
Not so.
You'd have even less cyclic authority if one of the skids were stuck/snagged. What you have there is the complete answer to STATIC rollover - an item of keen interest to the Puma drivers.
You'd have even less cyclic authority if one of the skids were stuck/snagged. What you have there is the complete answer to STATIC rollover - an item of keen interest to the Puma drivers.
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Dynamic rollover
Buitenzorg. You have my undevided attention! I understand satic rollover as for example, Heli on floor, full right cyclic, up with collective, over she goes. Dynamic rollover, three feet off floor, moving to right, skid hooks in wire, not enough left cyclic to stop movement,over she goes. surely rollover whether static or dynamic is a combination of force in a certain direction resisted by a greater force. I have in mind a pole vaulter who may well lift himself twenty feet using a fourteen foot pole, but would not have sufficient energy to achieve this with a twenty four foot pole
As usual, correct me if I'm wrong experts; here goes:
Static Rollover - helicopter has rolled to such an angle that a line straight down from the CG is outside the skids / wheels, and it falls over.
Dynamic Rollover - helicopter is picking up to the hover. Either from lateral cyclic input, or a skid getting caught on something, a roll develops. Once a certain angle of roll is exceeded, even full opposite cyclic can't tilt the lift vector back past the vertical, so any lift from the rotor is pro-roll. The only option available is to dump the collective and hope that the static rollover angle hasn't been exceeded.
Sound reasonable?
Static Rollover - helicopter has rolled to such an angle that a line straight down from the CG is outside the skids / wheels, and it falls over.
Dynamic Rollover - helicopter is picking up to the hover. Either from lateral cyclic input, or a skid getting caught on something, a roll develops. Once a certain angle of roll is exceeded, even full opposite cyclic can't tilt the lift vector back past the vertical, so any lift from the rotor is pro-roll. The only option available is to dump the collective and hope that the static rollover angle hasn't been exceeded.
Sound reasonable?
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Left out of the discussions above is the critical role that the cyclic control power has in eliminating/preventing dynamic roll-over. The old helo operating guidelines (and the training manuals) were written when the teetering rotor was common, and when thrust control was the only real control the cyclic generated. This is also true of the low offset controls of the Super Puma family.
More modern control systems produce (literally) tons of roll control at flat pitch, and so make it much more rare that a dynamic rollover condition could be reached. This involves the flapping hinge offset of the rotor, and its ability to produce head moment regardless of the thrust the rotor is producing.
Of course, the vertical cg and the landing gear spread are also contributers, but I believe it is quite rare to experience roll-over in most Boelkows, and in the Black Hawk, for example.
More modern control systems produce (literally) tons of roll control at flat pitch, and so make it much more rare that a dynamic rollover condition could be reached. This involves the flapping hinge offset of the rotor, and its ability to produce head moment regardless of the thrust the rotor is producing.
Of course, the vertical cg and the landing gear spread are also contributers, but I believe it is quite rare to experience roll-over in most Boelkows, and in the Black Hawk, for example.
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Dear all, don't be tempted to confuse the issue between STATIC and DYNAMIC roll over.
The various definitions of static roll over are quite correct, the C of G is positioned in such a way (either through landing on a slope, towing the helicopter onto a slope or taxiing too quickly) that the helicopter literally falls over!
A helicpter can be committed to a crash due to dynamic roll over at an angle much less than that required for static roll over. As has already been mentioned, the rolling moment that is generated and the ability to counter it with cyclic is what matters. The only role that the collective plays is to help generate the rolling moment, lowering the collective is the most important corrective action.
It is possible to get dynamic roll over on a flat surface! A Bell 212 rolled over in Canada due to one skid being frozen to the hard standing it was parked on, the other was exposed to the sun and had thawed out. As the pilot pulled in power the old girl flipped onto her side, if ever there was an advert to apply collective slowly then this is it.
As mentioned the Puma family are likely candidates for both of the above due to the high C of G, however, ALL helicopters are liable to roll over if not handled correctly.
Anyway, that's enough from me; I'm off to teach yaw/roll divergence and rotor droop in the Puma.......
The various definitions of static roll over are quite correct, the C of G is positioned in such a way (either through landing on a slope, towing the helicopter onto a slope or taxiing too quickly) that the helicopter literally falls over!
A helicpter can be committed to a crash due to dynamic roll over at an angle much less than that required for static roll over. As has already been mentioned, the rolling moment that is generated and the ability to counter it with cyclic is what matters. The only role that the collective plays is to help generate the rolling moment, lowering the collective is the most important corrective action.
It is possible to get dynamic roll over on a flat surface! A Bell 212 rolled over in Canada due to one skid being frozen to the hard standing it was parked on, the other was exposed to the sun and had thawed out. As the pilot pulled in power the old girl flipped onto her side, if ever there was an advert to apply collective slowly then this is it.
As mentioned the Puma family are likely candidates for both of the above due to the high C of G, however, ALL helicopters are liable to roll over if not handled correctly.
Anyway, that's enough from me; I'm off to teach yaw/roll divergence and rotor droop in the Puma.......