Anyone flown (or fly) the Huey?
Low-G pushover's in Huey or Cobra
Could someone with lot's of hours in the Huey or the Cobra, please tell me from experience whether they roll right in a low-g push over, or whether you just lose control power.
Many Thanks
CRAN
Many Thanks
CRAN
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Huey Tail Rotor
As any D model Huey pilot will tell you, she's not too good hot & high. This is because the tail rotor is on the wrong side (left - and trying to pull air through the vertical fin). It also spins down at the front, away from the rotorwash, losing even more effectiveness.
My question is why? Was this a design flaw? Vietnam photos circa 1972 show formations of Hueys, with a mix of left & right tail rotors and as, with the more modern 212, they spin up at the front.
The Bell 47 spins the same as the early Hueys but it of course has no vertical fin to impede the airflow.... as such, I wonder if they just copied the design, without much thought?
Similarly the Lynx tail rotor spins down at the front whilst the earlier Sea King spins up....... pourquoi?
Any venerable Huey design gurus out there?
My question is why? Was this a design flaw? Vietnam photos circa 1972 show formations of Hueys, with a mix of left & right tail rotors and as, with the more modern 212, they spin up at the front.
The Bell 47 spins the same as the early Hueys but it of course has no vertical fin to impede the airflow.... as such, I wonder if they just copied the design, without much thought?
Similarly the Lynx tail rotor spins down at the front whilst the earlier Sea King spins up....... pourquoi?
Any venerable Huey design gurus out there?
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The direction of spin is a contributor to the issue of tail rotor thrust, but it becomes important only when a limited marginal design is fielded. The power capability of the tail gearboxes, the pitch range of the tail rotor, its rpm and blade chord all conspire to make the maximum thrust what it is. When the tail rotor proves to be marginal, the easy fix is to flip it over and add that few percent of improved thrust back, since no components are changed (as long as the gearbox maintains adequate lube when upside down.) Any other substantive change to make more thrust usually means the entire tail rotor has to be changed, an expensive proposition.
In other words, flipping the tail rotor makes an unacceptable tail rotor now marginally better. Actually fixing it by creating more thrust would be better yet, but much more expensive.
In other words, flipping the tail rotor makes an unacceptable tail rotor now marginally better. Actually fixing it by creating more thrust would be better yet, but much more expensive.
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I was in a H model Huey, in a 4000+ft/min updraft (I dont know exacty how much it was as the VSI only goes to 4).... when 1. it suddenly stopped or 2. we hit a downdraft.
We were well less than 0 G, as witnessed by the bags of cargo that hit the roof then 'floated' forward.
The acft pitched to 50+ degrees nose down and rotated rapidly to the right. (From memory it wasnt a violent or bumpy rotation, like 'jack stall' or RBS)
Needless to say, flew home to change my shorts!
We were well less than 0 G, as witnessed by the bags of cargo that hit the roof then 'floated' forward.
The acft pitched to 50+ degrees nose down and rotated rapidly to the right. (From memory it wasnt a violent or bumpy rotation, like 'jack stall' or RBS)
Needless to say, flew home to change my shorts!
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Looks like they put the tail rotor back on the "wrong side".
See:
http://www.bellhelicopter.com/conten...upgrades2.html
See:
http://www.bellhelicopter.com/conten...upgrades2.html
CRAN :
I'm not a Huey driver ( hey - anyone out there willing to change that ? ), but R22s suffer in exactly the same way.
From memory, under negative G there will be an uncommanded roll right in a helicopter with an underslung rotor system and counter clockwise rotating main blades. And you probably know this bit but the instinctive left cyclic to counter the roll will reduce the mast-hub clearance even further . . .
I'm not a Huey driver ( hey - anyone out there willing to change that ? ), but R22s suffer in exactly the same way.
From memory, under negative G there will be an uncommanded roll right in a helicopter with an underslung rotor system and counter clockwise rotating main blades. And you probably know this bit but the instinctive left cyclic to counter the roll will reduce the mast-hub clearance even further . . .
Once you've had the mast strike, it rolls in all sorts of directions. Then you hit the ground.
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There was a technical paper on tail rotor design way back in the 1950s that said top turning aft was the best way to go.
Why were the various machines made the other way round? Could be anything from not having to re-qualify a new gearbox to not-invented-here syndrome.
The Lynx changed tail rotor direction of rotation and it made a world of difference to handling, performance and noise.
All the civil Bell 205 series appears to have the tail rotor top going aft.
Why were the various machines made the other way round? Could be anything from not having to re-qualify a new gearbox to not-invented-here syndrome.
The Lynx changed tail rotor direction of rotation and it made a world of difference to handling, performance and noise.
All the civil Bell 205 series appears to have the tail rotor top going aft.
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1. Jackstall occurs when the aerodynamic loads on the blades overpower the hydraulic jacks controlling the swashplate, causing control reversal. Cause and symptoms similar to RBS.
2. The roll during zero - g conditions is due to the sideways push of the tail rotor creating a rolling moment about the head (think back to tail rotor drift and roll u studied at groundschool).
3. Anything less than +ve g and helicopters definitely do not mix
2. The roll during zero - g conditions is due to the sideways push of the tail rotor creating a rolling moment about the head (think back to tail rotor drift and roll u studied at groundschool).
3. Anything less than +ve g and helicopters definitely do not mix
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I agree with the "DON'T DO IT"...
I hit similar up/down draughts in the sierra nevadas in Spain on a ferry flight back to base last year, the only thing that happened to my 205 was loss of RPM! All the horns and lights went on and frightened the c... out of me but instinct luckily took over and I dropped a bit of collective and all was well.
"Helis and -G don't mix" Well not exactly, you can play a little bit with rigid rotor systems but very gently, but not with teetering systems....NO WAY!
I hit similar up/down draughts in the sierra nevadas in Spain on a ferry flight back to base last year, the only thing that happened to my 205 was loss of RPM! All the horns and lights went on and frightened the c... out of me but instinct luckily took over and I dropped a bit of collective and all was well.
"Helis and -G don't mix" Well not exactly, you can play a little bit with rigid rotor systems but very gently, but not with teetering systems....NO WAY!
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All the civil 205's have the tail rotor on the right turning lower blade to the front.
I had a thread on this a while back and got some good info on it, worth checking back to mid Sept.
I had a thread on this a while back and got some good info on it, worth checking back to mid Sept.
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The way I understand it [from verbal stories only] is that the Huey tailrotors were on the left side because of a US army requirement. Apparently the Army boffins thought that it was safer to have the T/R 'pushing' itself on to the output shaft in case of damage occurring to the retaining hardware when being shot at.
I came across a vague reference to this fact, saying that Bell modified the Huey T/Rs to the RH side but was then directed by the Army to return them back to the L/H side.
I came across a vague reference to this fact, saying that Bell modified the Huey T/Rs to the RH side but was then directed by the Army to return them back to the L/H side.
That's not to say that you can't throw them around a bit, but smooth manoeuvres with positive g are the go; it's also not too much of a drama to throw the collective down fairly quickly with a central-ish cyclic; I just wouldn't push forward on the cyclic at any more than what you would describe as a gentle rate.
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Centralize has it a bit off, let me discuss:
Jack stall is caused by rotor stall. The airfoil on the blade shifts its moment to pitch the nose of the blade down at stall, just as the wing of an airplane retrims in stall, and the nose drops. This stall sends large forces into the helicopter's rotor system, trying to force the servos backwards. If the servos are of marginal design, they can be over powered by the stalled blade, and this backdrive is felt by the pilot, in fact, the helicopter gets a mind of its own and is actually out of control. This jack stall is the natural product of the combination of a stalled rotor and a marginal servo system.
Low g is a problem for teetering/underslung/semi-rigid rotors, but is duck soup for other designs. The explanation is that the rotor does not control the helicopter purely by tilting the lift, as is often said in helicopter primers. The moment produced by the rotor is more powerful than the lift tilt, but this factor is almost never explained.
In an articulated system with hinge offset of 4%, about minus 1/2 g is where the controls have little effect, and for rigid high offset rotors, it can be minus 2g's or so before they have no effect.
Nick
Jack stall is caused by rotor stall. The airfoil on the blade shifts its moment to pitch the nose of the blade down at stall, just as the wing of an airplane retrims in stall, and the nose drops. This stall sends large forces into the helicopter's rotor system, trying to force the servos backwards. If the servos are of marginal design, they can be over powered by the stalled blade, and this backdrive is felt by the pilot, in fact, the helicopter gets a mind of its own and is actually out of control. This jack stall is the natural product of the combination of a stalled rotor and a marginal servo system.
Low g is a problem for teetering/underslung/semi-rigid rotors, but is duck soup for other designs. The explanation is that the rotor does not control the helicopter purely by tilting the lift, as is often said in helicopter primers. The moment produced by the rotor is more powerful than the lift tilt, but this factor is almost never explained.
In an articulated system with hinge offset of 4%, about minus 1/2 g is where the controls have little effect, and for rigid high offset rotors, it can be minus 2g's or so before they have no effect.
Nick
jack-stall
Nick--have to disagree,a little!
I think most early helo`s reached RBS before reaching jack-stall,and usually pitched up and rolled unless you reduced collective-quickly!That said you could demo jack stall by winding RRPM back towards the power-on limit,and you would feel it on the controls( in the S-55 and S-58(WWIND& WX).In the Gazelle, the flight controls were developed so that the a/c could be manoeuvred hard without fear of reaching blade -stall,by designing the hydraulic control system to limit its output,and give a tactile feedback to the pilot that he was at his control limit,or nearly so,and could adjust to compensate,without fear of "stalling",and whilst flying "eye-balls-out"; an admirable aid to any tactical helo pilot,manoeuvring hard even close to the ground.Pity it had to be the pragmatic FFrench who devised the system.!!On the early prototype Gazelles,-001,01,02,the system was superb,you could pull nearly 2 1/2-3G,and as you reached the limit the a/c would roll gently upright,in left and right turns,all nice and safe,even when in-the-weeds."Great",we said," well have a couple of hundred of those".Unfortunately, by the time the a/c got to prod. std., the tailboom had been lowered,the cabin had reduced stiffness,due to the door for the winch,the u/c was now different as you could get grnd. resonance,due to the door and tailboom mods.,and the control -jack manufacturer had changed, and that is why at jack-stall the Gazelle rolls the same way at the limit in both left and right turns!!That said ,if you`ve not tried one ,Nick,go and do it; it`s like getting out of your Olds/Chevy/Plymouth,and getting into a Ferrari,or BMW M-class,or if you may be a Harley-D "biker",get a ride on a Kwik-Kwacker ZX-9RR,or at a lower level,first night with a new mistress- so I`ve been told!!
I think most early helo`s reached RBS before reaching jack-stall,and usually pitched up and rolled unless you reduced collective-quickly!That said you could demo jack stall by winding RRPM back towards the power-on limit,and you would feel it on the controls( in the S-55 and S-58(WWIND& WX).In the Gazelle, the flight controls were developed so that the a/c could be manoeuvred hard without fear of reaching blade -stall,by designing the hydraulic control system to limit its output,and give a tactile feedback to the pilot that he was at his control limit,or nearly so,and could adjust to compensate,without fear of "stalling",and whilst flying "eye-balls-out"; an admirable aid to any tactical helo pilot,manoeuvring hard even close to the ground.Pity it had to be the pragmatic FFrench who devised the system.!!On the early prototype Gazelles,-001,01,02,the system was superb,you could pull nearly 2 1/2-3G,and as you reached the limit the a/c would roll gently upright,in left and right turns,all nice and safe,even when in-the-weeds."Great",we said," well have a couple of hundred of those".Unfortunately, by the time the a/c got to prod. std., the tailboom had been lowered,the cabin had reduced stiffness,due to the door for the winch,the u/c was now different as you could get grnd. resonance,due to the door and tailboom mods.,and the control -jack manufacturer had changed, and that is why at jack-stall the Gazelle rolls the same way at the limit in both left and right turns!!That said ,if you`ve not tried one ,Nick,go and do it; it`s like getting out of your Olds/Chevy/Plymouth,and getting into a Ferrari,or BMW M-class,or if you may be a Harley-D "biker",get a ride on a Kwik-Kwacker ZX-9RR,or at a lower level,first night with a new mistress- so I`ve been told!!
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sycamore,
Good point of view, and great story about the Gazelle, although I do disagree in that a loss of control is a very poor way to prevent stall (could be the tree you are avoiding is worse for your rotor than some stall induced loads on the controls, for example).
The point I am sticking to is that jack stall is Caused by rotor stall, because the blade pitching moment increases due to rotor stall. Yes, one might start earlier than another but they are both part of the same physical situation.
Look at the lift and moment plots on this paper (Cn is the normal force, like CL, and Cm is the blade pitching moment):
http://ho.seas.ucla.edu/AeroMEMS_htm...es/Control.PDF
Good point of view, and great story about the Gazelle, although I do disagree in that a loss of control is a very poor way to prevent stall (could be the tree you are avoiding is worse for your rotor than some stall induced loads on the controls, for example).
The point I am sticking to is that jack stall is Caused by rotor stall, because the blade pitching moment increases due to rotor stall. Yes, one might start earlier than another but they are both part of the same physical situation.
Look at the lift and moment plots on this paper (Cn is the normal force, like CL, and Cm is the blade pitching moment):
http://ho.seas.ucla.edu/AeroMEMS_htm...es/Control.PDF