UK AAIB December 2007
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Just read the A109 report. 
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Well done that man for getting it down safely - must have been a real "new pants" moment. It is that sort of result, in those sort of conditions, that the rest of us aspire to being able to achieve; but hope never to have to find out!
If you frequent these parts - well done again!
.Well done that man for getting it down safely - must have been a real "new pants" moment. It is that sort of result, in those sort of conditions, that the rest of us aspire to being able to achieve; but hope never to have to find out!
If you frequent these parts - well done again!
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The following quoted from the R22 report:
"In a still-air hover the amount of power required is reduced due to the effect of increased air pressure or ‘ground cushion’ below the rotor disk. This effect reduces as airflow, or wind, increases. At the time of the accident G-UNYT would have been close to its maximum weight. In the hover, with a wind of 20-25 kt, it is possible that, at this weight, there would have been a negligible margin of power."
Does this sound blatantly wrong to anyone else? First off, anyone foolhardy enough to quote the old "cushion of increased pressure" idea on this forum would be looking at a sound thrashing from the aerodynamics gurus here. Why is this debunked myth showing up in an official report?
Furthermore, can we get a show of hands from anyone who thinks that hovering in a 25kt wind would take MORE power than in still air? That wind speed is above ETL range for a Robbo, even fully loaded, resulting in greater efficiency of the rotor system and thus LESS power would be required to hover.
Do I need a better understanding of the basics, or do the AAIB need to check their facts?
"In a still-air hover the amount of power required is reduced due to the effect of increased air pressure or ‘ground cushion’ below the rotor disk. This effect reduces as airflow, or wind, increases. At the time of the accident G-UNYT would have been close to its maximum weight. In the hover, with a wind of 20-25 kt, it is possible that, at this weight, there would have been a negligible margin of power."
Does this sound blatantly wrong to anyone else? First off, anyone foolhardy enough to quote the old "cushion of increased pressure" idea on this forum would be looking at a sound thrashing from the aerodynamics gurus here. Why is this debunked myth showing up in an official report?
Furthermore, can we get a show of hands from anyone who thinks that hovering in a 25kt wind would take MORE power than in still air? That wind speed is above ETL range for a Robbo, even fully loaded, resulting in greater efficiency of the rotor system and thus LESS power would be required to hover.
Do I need a better understanding of the basics, or do the AAIB need to check their facts?
I think you're right there SpinningSnowBird.
The decrease in induced flow due to 25kts of wind creating clean air would be far greater than the decrease created by hovering in nil wind ground effect.
The decrease in induced flow due to 25kts of wind creating clean air would be far greater than the decrease created by hovering in nil wind ground effect.
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This 'debunked myth' is still the basis of the explanation given to military crews in the UK!
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If anyone fancies a go at writing these reports the AAIB are recruiting
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This 'debunked myth' is still the basis of the explanation given to military crews in
So what is the actual explanation for Ground Cushion?
Been searching through many many threads and didn't see a different explanation to the one I was taught all those years ago at Strawberry.
Just curious.....
Been searching through many many threads and didn't see a different explanation to the one I was taught all those years ago at Strawberry.
Just curious.....
I think Shawbury got it right for you and me both
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Saint Evil said: So what is the actual explanation for Ground Cushion?
Let's start with a simple answer, courtesy of Nick Lappos from this thread.
"For the record, the effect is not due to a "ground cushion" or "air pressure" or "a bubble of air" under the helo. The ground bends the air flow coming off the rotor, and this bend makes the rotor blades work at a lower angle to produce the same lift. The lower angle needs less power, so we see the ground effect."
For further reading, here are some threads where this issue has been discussed and the pressure bubble folks (sorry mil guys) have been sent home with tails firmly between their legs.
Helicopter Urban Myths #11 (Post 69 and onwards)
Ground effect/Urban myth
Varying degrees of IGE
OGE Hover, Service ceiling
Let's start with a simple answer, courtesy of Nick Lappos from this thread.
"For the record, the effect is not due to a "ground cushion" or "air pressure" or "a bubble of air" under the helo. The ground bends the air flow coming off the rotor, and this bend makes the rotor blades work at a lower angle to produce the same lift. The lower angle needs less power, so we see the ground effect."
For further reading, here are some threads where this issue has been discussed and the pressure bubble folks (sorry mil guys) have been sent home with tails firmly between their legs.
Helicopter Urban Myths #11 (Post 69 and onwards)
Ground effect/Urban myth
Varying degrees of IGE
OGE Hover, Service ceiling
