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Hanging one side low

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Hanging one side low

Old 27th Aug 2002, 23:49
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Hanging one side low

Why does a helicopter with a fully articulated head (AH64) hang one side low? What does the panel think? Tail rotor drift (lateral tendency), tail rotor roll, something else? I'm interested in your views.
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Old 28th Aug 2002, 03:23
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Jeep, how's the course going? I'm surpised that you've got the time and energy to PPRuNe

Because the t/r being lower than the main rotor forms a couple. Move the t/r level with the m/r(which it tends to in forward flight), and things level out. Well that's my theory anyway.
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Old 28th Aug 2002, 06:30
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From what I understand, the tail rotor wants to push/roll the helicopter a certain way (to the right on CCW rotating blade helicopters), so the pilot has to counteract with the opposite direction cyclic (left in CCW helicopters). This explains why Hueys (for example) hover left skid low, and A-Stars hover right skid low.

Just what I've picked up over time..

Mike
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Old 28th Aug 2002, 10:47
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ill have a go!

in a hover, the helicopter will hang left skid low on an anticlockwise blade rotation heli.
so you put the fatty on the right to balance the tail rotor roll.
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Old 28th Aug 2002, 10:56
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and again:

aswell, if you have 1 passenger and they want to go on one side (to take photo's) the lateral balance will be noticable in hover and in flight. and while in flight if you centre the balance ball the aircraft will level but you will be crabing in the wind. i think the hover balance ball reading would be the correct in flight setting for the cabin being strait into wind??
any idears?
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Old 28th Aug 2002, 11:23
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Now let's really confuse the issue....in the Alouette III, the mast is inclined to correct that....and the aircraft flies a half ball off....and most operators put a piece of yarn on the canopy for trim indications.....guess pilots cannot get used to flying one wing low or something. Also the attitude indicator shows a wing low attitude when in level balanced flight.
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Old 28th Aug 2002, 13:08
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ON the SA365C they tilted the artificial horizon to compensate for the built in lean!!!!!!!
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Old 28th Aug 2002, 20:15
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Jeep
What a question to ask, as if you didn't know!!
MG has the UK answer sewn up there, you an A2 yet MG or is all that planking getting in the way?
However, in the States as every patriot will tell you, if it doesn't say its a coupling effect in 'their' fundamentals of flight, then it aint true, dont forget me old mate, you need a reference for everything you say!!!
If you look in F of F page 5-26 para 1, the US explanation. Sorry to be so sad!
email on its way.
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Old 28th Aug 2002, 23:10
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Pretty simple concept - main rotor torque wanting to twist the body of the helicopter in the opposite direction is countered by a fan on the end of a stick - your tail rotor - which provides a sideways force.

In the absence of another force pushing the other way laterally, the machine will drift sideways. To counter that, we put in some lateral cyclic, tilting the lift vector opposite to the direction the tail rotor wants to push.

This, as mentioned above, creates a couple; high (talking about vertical position, not magnitude) main rotor force one way and lower tail rotor force the other way.
That makes the helicopter roll until this couple is exactly balanced by another one, that is, the couple between gravity pulling down on the centre of gravity, and the upward force on the 'suspension point', ie the centre of the mast.

Something like that, anyhow!
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Old 29th Aug 2002, 04:42
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Arm out the window has it almost perfectly right. For US convention helicopters (rotor turns CCW from the top):

The tail rotor is pushing the aircraft to the right, because it has a pure thrust. The aircraft does not lean to the left until the pilot sees the right drift and cancels it with some left bank.

The height of the rotor head has nothing to do with it, the height of the tail rotor relative to the head has nothing to do with it either. The height of the tail rotor relative to the cg of the aircraft (the vertical cg) is important, however. A high tail rotor cancels some of the needed left roll, a low tail rotor, on the centerline, needs more left roll.
Articulated, semi-rigid and rigid rotors all have the left roll tendency, the type of head has very little effect on the issue.
 
Old 29th Aug 2002, 11:05
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sassless

cant you balance it with weight in the oposite side?

you- >The height of the rotor head has nothing to do with it, the height of the tail rotor relative to the head has nothing to do with it either.

are you sure? if you raised the tail rotor above the rotor hub would'nt the effect be to change the low side over?



there was a problem in my last post the part begining with 'you' was to nick lappos.
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Old 29th Aug 2002, 15:31
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A helicopter with CCW rotor rotation will usually hover with the left skid low. Tail rotor thrust is to the right, known as translating tendency, the copter wants to move to the right. The pilot applies left cyclic pressure to stop the movement, the horizontal component of lift to the left cancels out the tail rotor thrust to the right.

Some manufactures will offset the mast to the left by one or two degrees to counteract the translating tendency, Enstrom uses an offset while Schweizer does not.
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Old 29th Aug 2002, 18:15
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Well, you could put the lateral CG way over to the right side, & it might hover level, but you'd still have lateral CG issues in cruise flight. Why bother?

I don't see how putting the tail rotor above or below the main rotor could reverse the thrust of the tail rotor. It's thrust that tries to move the helicopter laterally, regardless of the height of the center of thrust. Having the tail rotor above the main rotor is not uncommon.
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Old 29th Aug 2002, 18:20
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Bloody simple really;
It's a fat pilot plus lunch box.
If he/she is a thin pilot then the fat pilot left his/her lunchbox in the cab on which ever side it leans.
If pilot has eaten lunchbox then it's the left over tupperware box plus crisp packet and a half eaten apple core (leans less).
If pilot is thin and lunchbox is not present then it must be something else in the boot i.e. crate of beer left over from last do.
If crate is empty the the glass bottles are too thick, change brewery.
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Old 30th Aug 2002, 01:23
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GLSNightPilot

it doesnt change the tailrotor thrust, just the way it rolls the cabin.
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Old 30th Aug 2002, 11:39
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You could have left the fuel hose in.....

Phil
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Old 30th Aug 2002, 14:49
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Dubious logic - Nick

Two attitudes here:
Fuselage attitude
Disc attitude

Disc must run at an attitude to cancel translating tendancy

Fuselage attitude will be such that cg is directly below thrust unless modified by t/r thrust not being in the same horizontal plane as the disc (in which case a cg/thrust couple is required).

Rotor head matters (a bit)
In Rigid and Fully Articulated cases another (much over debated) couple also modifies the fuselage attitude - which does not apply in the pendulous type rotor heads.




Incidentally Nick is the only person I have seen correctly qualifying CW and CCW with "when viewed from above/below" - they are meaningless terms without. - Similar to: "captain the left hand engine is on fire"
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Old 30th Aug 2002, 16:32
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Cool

Seems to me, anytime I look up through the skylight the blades are turning CCW on my Bell. Now if you are on the other side of the equator does it spin the opposite way ???

Cheers

Randy_G

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Old 31st Aug 2002, 04:49
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Q Max,
You bring an interesting increase in depth to the discussion, and are some what correct, in that the type of rotor head does affect the angle, and the disk angle can be different than the fuselage angle, but the idea that the thrust must pass through the CG is actually quite a bit off, unless you are flying a teetering rotor helicopter. The old idea that thrust is the only product of the rotor is brought about by the fact that all the old training manuals assume a teetering rotor, and were written for a simpler time!

A few threads ago I discussed the moment produced by an articulated rotor, and illustrated the concept of how any cyclic flapping produces a powerful moment that is transmitted through the mast to help control the aircraftr. The moment produced is quite large, and for articulated and rigid systems, produces a clear mismatch between the line of action of the rotor thrust and the cg.

In a nutshell, most helos tend to have a fuselage tilt that approximates the disk tilt (usually a bit less fuselage tilt, due to the head moment due to flapping). Rigid rotors have the most, and teetering rotors have the least.

Thanks, Q Max for opening the discussion!!
 
Old 31st Aug 2002, 08:58
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Vorticey, I've flown several models over the years, certainly not every one ever built, but I've never flown one that hovered with the rotor OR the cabin rolled any direction but against the tail rotor thrust, to one extent or another, unless a strong crosswind countered it.
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