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Downwash question

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Downwash question

Old 20th Nov 2019, 21:11
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Downwash question

Can anybody tell me if there would be any rotor downwash from a pair of stationary Apaches. They were waiting at the threshold, as i came in to land in a PA28, there was a strong wind blowing in the direction from the Apache's towards my landing point, and i experienced turbulence and windshear. I'm not sure if this would be due to the Apaches, or just the strong wind conditions, any thoughts would be greatly appreciated.
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Old 20th Nov 2019, 21:27
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Depends if they’re holding ground taxi power sitting on the brakes. Even then, the symptoms you describe seem a bit severe.
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Old 21st Nov 2019, 06:27
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If they were on the ground, the downwash would be negligible - if they were in the hover then it might have been an issue.

The turbulence and windshear would be due to the strong wind you mention.
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Old 21st Nov 2019, 07:02
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Not necessarily downwash, but turbulence, yes.

Anything upwind, such as a tree or building will give you turbulence.

Wind turbines are well known for their turbulence and they are deriving thrust, not imparting it.

And don't overlook short finals/ flare/ float is the most vulnerable phase of flight.
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Old 21st Nov 2019, 08:55
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Many thanks, they are big machines, and I think you are all correct, it was turbulence, ATC maybe should have held them further back the taxiway with those wind conditions.
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Old 21st Nov 2019, 13:44
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A 200' wind turbine side on isn't the same as a 60' rotor edge on and a helicopter that is designed to fly through the air with a minimum of drag is unlikely to generate a great deal of turbulence when at flat pitch on the ground.
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Old 21st Nov 2019, 15:46
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Well,
even if the collective is flat pitch - if one of the pilots is lazy on the pedals, the tail rotor can produce quite a bit of wind.
I blew a (rotten) wooden fence with the power of a fenestron ;-)
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Old 22nd Nov 2019, 10:40
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Originally Posted by Flying Bull View Post
Well,
even if the collective is flat pitch - if one of the pilots is lazy on the pedals, the tail rotor can produce quite a bit of wind.
I blew a (rotten) wooden fence with the power of a fenestron ;-)
Presumably you were "lazy on the pedals" !!
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Old 22nd Nov 2019, 11:33
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Originally Posted by Flying Bull View Post
Well,
even if the collective is flat pitch - if one of the pilots is lazy on the pedals, the tail rotor can produce quite a bit of wind.
I blew a (rotten) wooden fence with the power of a fenestron ;-)
should only be one pilot on the pedals, and why did you have so much pedal in to do that while you are sitting on the ground?
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Old 22nd Nov 2019, 12:08
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Originally Posted by Davey Emcee View Post
Presumably you were "lazy on the pedals" !!
Well, not really, I wanted to turn (wheeled helicopter)

@ rottenjohn - if someone stands on the brakes its a little bit different then normal - depending on the pedal design and your seating position, you need to stretch the leg/toes to get proper braking action.
Assume it is at the holing point of a runway, you´re normally turning a little bit to reach that position - hence uneven braking/pedal position is a possibility
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Old 22nd Nov 2019, 15:05
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But again, that is a rotor side on, doing what is designed for - producing thrust - not one edge on at flat pitch.
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Old 22nd Nov 2019, 15:50
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Originally Posted by [email protected] View Post
But again, that is a rotor side on, doing what is designed for - producing thrust - not one edge on at flat pitch.
The tail rotor produces thrust if you put pedal in, flat pitch on the collective or not.
Have seen someone blown away passing to close to the tail - lucky for him, he was on the right side. Was an ground accident to happen....
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Old 22nd Nov 2019, 16:28
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Originally Posted by Flying Bull View Post
The tail rotor produces thrust if you put pedal in, flat pitch on the collective or not.
Have seen someone blown away passing to close to the tail - lucky for him, he was on the right side. Was an ground accident to happen....
I think that’s what Crab was saying.

in a Super Puma you can roll the aircraft on its side with too much right pedal!
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Old 22nd Nov 2019, 17:50
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Quite. You blew away a fence because your TR was producing thrust - the OP was about turbulence from a MR at flat pitch
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Old 22nd Nov 2019, 18:02
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Originally Posted by [email protected] View Post
Quite. You blew away a fence because your TR was producing thrust - the OP was about turbulence from a MR at flat pitch
A plank driver asked, wether it is possible to have turbulence due to nearby helicopters.
Not understanding all the factors - he assumes, it must have been downwash.
I just added another possibility - which might not spring into mind helicopter pilots operating smaler helicopters - but heavy "birds" ;-) ahh helicopters need a lot of anti torque - which is produced by the tail rotor.
Even a slight foot forward, which won´t move or turn the helicopter will produce quite a lot of wind from the big fan at the back.
Understandable now?
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Old 23rd Nov 2019, 08:57
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Even a slight foot forward, which won´t move or turn the helicopter will produce quite a lot of wind from the big fan at the back.
Understandable now?
I quite understand what you are trying to 'mansplain' to another helicopter pilot BUT a wheeled helicopter like an Apache will turn with very little pedal if the tailwheel is unlocked and, since they appear to have been waiting to line up on the runway and would ground taxi to do that, it is extremely unlikely that the tailwheel would be locked again and even more unlikely that they would sit with pedal applied and the tailwheel locked.

Yes, a TR will produce thrust on the ground but in the scenario posted by the FW pilot - the turbulence he experienced would not have been due to the helicopters.
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Old 23rd Nov 2019, 11:09
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Would you bet your life on that?
May be they were already light on the Wheels, ready to line up immediately after the landing aircraft?
Thus making „a little bit of wind“?
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Old 23rd Nov 2019, 11:53
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I refer to my earlier answer but even then it would only be about 10% Tq above MPOG. Symptoms still seem too severe for the AH to have a demonstrable effect.
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Old 23rd Nov 2019, 15:24
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May be they were already light on the Wheels, ready to line up immediately after the landing aircraft?
And maybe the turbulence pixies had eaten beans for dinner
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Old 23rd Nov 2019, 17:39
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I call bullshit on the helicopter turbulence ������
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