View Full Version : Light Helicopters and Tail Rotor Rolling Couple
India Four Two
30th May 2012, 15:03
I posted this question on the Questions forum:
On an AH&N thread,Tiger mate described the tail rotor rolling couple:
Any Borneo Whirlwind veterans here? (http://www.pprune.org/aviation-history-nostalgia/485933-any-borneo-whirlwind-veterans-here.html#post7210919)
This is something I hadn't heard of before and did some research on the subject, including looking at a three-view of the only helicopter I have flown - a Bell 47.
Some of the information I saw indicated that the ideal tail rotor position for avoiding a rolling couple, is level with the rotor head. However, all of the light helicopters that I looked at, including the Bell 47, have the tail rotor significantly below the rotor head.
How is the rolling couple handled in these light helicopters?
I42
and I received one reply:
Trim!
And if you use the Rotorheads - PPRuNe Forums (http://www.pprune.org/rotorheads-23/) section you'll probably get some links to illustrative graphics.
Links or explanations anyone? :)
Mcmppl2atplh
30th May 2012, 18:08
Just because they still fly with the tail rotor below the main rotor does not mean the problem is handled, this is the reason that for example in an r22 you get the whole left skid low thing!!
Just because they still fly with the tail rotor below the main rotor does not mean the problem is handled, this is the reason that for example in an r22 you get the whole left skid low thing!!
I have a bridge for sale if you are interested!
John R81
31st May 2012, 21:16
Take a ruler and place one finger either side, in line with each other. When you push, the two forces cancel each other out absolutely, and the ruler stays still.
Now move your fingers apart on the ruler. When you push, the ruler will turn.
That's what happens when the main rotor hub (one force) is not in the same plane as the tail rotor hub (the counter force).
The ruler spins round completely because the force you exert is massive compared to any counter force. In the helicopter, the rolling couple caused by the difference in height between the main and tail rotor hub causes a rotation of the helicopter which is resisted by gravity acting on the mass of the helicopter. The rolling couple moves the mass of the helicopter away from verticle but once it matches the gravitational pull on the mass of the helicopter harmony is achieved, and the helicopter simply flies one-skid-low.
Which skid low depends on whether the main rotor spins clockwise or counter-clockwise.
Simples!
Tail rotor drift occurs because the TR has to produce an anti-torque force to oppose that produced by turning the main rotor - on an R22 rotation (anticlockwise viewed from above) the gearbox pushes the rotor round one way and tries to turn the fuselage around the other way (equal and opposite reaction).
The TR produces a force in an anti-clockwise direction to oppose the clockwise rotation of the fuselage but in doing so creates a drift to the right for the whole aircraft - this is opposed by left cyclic tilting the MR thrust and giving the left wheel/skid low hover attitude - there is now a couple about the C of G between the TR thrust and the horizontal component of MR thrust.
The mass of the aircraft acting about the C of G along with the vertical component of MR thrust act as an opposing couple and the resultant attitude is a balance of the two couples.
The canting of the tail boom to elevate the TR is a design issue depending on what the aircraft is designed for - aircraft designed to operate mainly in the hover will have a raised TR so that the MR hub (horizontal component of MR thrust) and the TR are reasonably aligned in the hover giving a more level cabin - aircraft designed for high speed cruising will tend to have a straight tail boom so the TR is aligned with the MR hub when the attitude is nose down for forward flight. This is a simplisitic and not definitive answer based on older helicopter designs since modern aerodynamic fixes (canted TRs, clever horizontal stabilisers etc) mean that it is not such a clear cut division.
Ray Prouty often states that with helicopter design 'what you gain in the hover you lose in forward flight and vice versa'.
India Four Two
1st Jun 2012, 14:37
crab,
Thanks for the detailed response. I first saw it early in the morning and decided I had better wait until later when I was properly awake, before trying to understand it ;)
However, you mentioned something in your reply, which I had read elsewhere and I don't understand.
but in doing so creates a drift to the right for the whole aircraft
How does the tail rotor thrust cause the fuselage to drift to the right, when the force is being applied so far aft of the centre of gravity?
India Four Two - although the TR is producing a turning moment about the rotor mast, it is also producing a translating force sideways because the aircraft isn't fixed in space, it is just hung under the MR and it doesn't take much to move it in any direction. That force is always there, even in forward flight such that the aircraft is always sideslipping (to a small degree) even in the cruise - it is called inherent sideslip.
India Four Two
1st Jun 2012, 18:34
crab,
I still don't understand it and I was going to ask if you would recommend Shawn Coyle's book, but I see you've already done that in another thread. :)
in this same forum...................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.
Bridge withdrawn from market.
rogerer
1st Jun 2012, 19:19
Try to understand Newton's third law, and you'll understand it all...
Arm out the window
1st Jun 2012, 21:28
At the time, I remember questioning Nick's logic about the height of the TR hub vs the MR head not being important. He is obviously a smart and respected bloke, but when you look at the forces involved it seems clear to me there is an effect.
Using a US-direction of rotation helicopter as an example, the source of the two opposing thrust forces that produce the rolling couple are the TR (force to the right as viewed from the rear of the aircraft), and the MR head (leftward component of MR thrust to counterbalance the TR thrust, stopping the aircraft drifting to the right).
If those two forces were applied to the same vertical point, directly opposing one another, there would be no roll, as they wouldn't form a couple. Once they are split vertically, there's a couple and therefore a roll.
The roll continues until the opposing couple (vertical MR thrust balanced by increasing lateral displacement of the C of G from under the rotor head) grows big enough to counterbalance the first-mentioned couple.
The C of G vertical position is important because it affects how quickly that couple will vary with roll angle, but the TR/MR hub lateral displacement from one another is, I believe, equally important.
Arm - Quite right
But then there may still be a (small) roll couple at the head for non-teetering types - since a horizontal MR force component would still be required and might (depending on rigging) still generate a moment requiring a CoG displacement.
John R81
3rd Jun 2012, 19:15
A twin motorcycle has a rolling couple caused by the offset of the crankshaft as each cylinder moves out / back together. So if a difference of a couple of inches creates a couple in a motorcyle engine, it will also create a couple when the MRG and TRG centers are not in a single plane.
The influence of that couple might not be large, and it does not account for tail rotor drift; but my understanding is that it will be there.
DennisK
3rd Jun 2012, 20:58
As an FI, I've always found it simpler to explain a basic principle of engineering physics ... that it isn't possible to offset a couple (torque) with a force (T/R) without a residual force. This residual force drifts the whole assembly parallel to and in the direction of the T/R component ... T/R drift. To correct the T/R drift the designer simply tilts the mast which causes the 'one skid low' attitude. Safe flying to all. Dennis K.
AoTW - whatever height you place the TR, it won't affect the fact that the TR thrust is pushing the aircraft right - the roll is only a factor of the MR being tilted left to stop the TR drift. Imagine if the TR was on a very high tail, above the level of the MR - it would still cause TR drift but any resultant couple might reduce (but not negate) the amount of left roll required to compensate for the TR drift.
India Four Two - the aircraft is not fixed to the ground - if it were, say on a spike protruding from the bottom of the fuselage, the TR thrust would only produce a rotation. Since the aircraft is untethered, the pure thrust it generates not only acts about the C of G to counter MR Tq but drifts the whole aircraft to the right as well.
India Four Two
4th Jun 2012, 16:04
Hello All,
Thanks for the interesting replies. It does appear that there seem to be significant differences of opinion, both here and in the online literature, about how the force created by the tail rotor affects the helicopter, apart from counter-acting the main-rotor torque. One of the main issues seems to be the point about which the rolling couple is acting. Some say the main rotor hub, others the centre of gravity.
crab, [edit: I see DennisK also makes the same point].
I understand what you are saying and I appreciate that the right-drift must be a real effect, although I have to say I didn't notice it in my 25 minutes in the Bell 47 - I was concentrating too hard!
The difficulty I have is reconciling the idea of right-drift, which implies a force acting through the C of G, with the fact that the tail-rotor force is being applied several metres aft of the C of G.
Try to understand Newton's third law, and you'll understand it all... rogerer, not a very useful contribution to the thread. For your benefit, I should point out that I have understood Newton's third law and his other ones (;)) since I was first taught them, 46 years ago, and this is the reason I am having difficulties with the physics of the situation.
India Four Two - if you had selected a wings level attitude in the hover, you would have appreciated the effect of TR drift.
It is always good fun to take a FW pilot and teach them to hover, for them the wings level attitude is the norm and it does not compute when you tell them the hover attitude is different and to stay still they need a left/right wing (delete as appropriate to direction of MR rotation) low plus some nose up to maintain position.
SimFlightTest
4th Jun 2012, 18:06
A force, regardless of where applied on a body, can always be thought of as acting through the body's center of mass along with a corresponding moment (or couple) that is proportional to the distance from the body's center of mass to the actual point where the force is applied.
Arm out the window
5th Jun 2012, 06:34
Crab, your argument sounds convincing, but to take the example of a teetering-head machine (ie no direct rolling moment applied to the mast by the rotor head - fuselage simply hangs pendulously), I would think the situation for a very high tail rotor would work like this:
1. Tail rotor drift still present as usual.
2. Left cyclic applied to cancel drift.
3. There is now a vertically-displaced couple between the rotor head side force on the mast (left) and the tail rotor side force (right). Tail rotor force is above rotor head, so the resultant fuselage roll is to the right! It will stop rolling when the C of G is displaced far enough laterally (leftward from under the mast head this time) to counterbalance the afore-mentioned couple. Obviously we now end up in the whacky situation where the fuselage has an angle of bank to the right, but the disc is angled to the left to stop the drift. Probably one good reason why they don't make them this way.
To put it another way, I'm saying that 'normal' mast head above T/R hub configuration will give you a left-skid low hover (nothwithstanding strange loading, crosswinds etc). If they are both at the same position vertically, there shouldn't be a roll. If the T/R hub was above the mast, I reckon it would roll right.
This is going purely on the relative positions of the forces applied to the system, and therefore the couples they form. The only forces are main rotor thrust, tail rotor thrust and weight, so to quote Marjie from the movie Fargo, 'I don't rightly agree with your police work, Vern!'
A bit of a bone of contention. Not saying I'm right ... perhaps I just can't see how I'm wrong, yet!
AoTW - I think you are completely correct if we are only talking about a teetering head helicopter - in that case the height of the TR is important.
Chaps,
Your physics is completely flawed!
Regardless of where you put the horizontal vector it remains the same.
The action is the drift caused by the horizontal vector from the TR.
The reaction to maintain equilibrium (i.e no drift) is from the main rotor and it reacts around the vertical centre of gravity.
Simple question - tell me where you get the extra energy to make it tilt further around the centre of gravity?
Answers on the back of a postage stamp please.
AoTW - even with a teetering head helo and a TR that is above the MR hub, I don't agree that you will have a right roll as you always have to tilt the disc to the left to oppose the TR drift - I think the best you can expect is laterally level fuselage in that condition. The disc is 'flying' left and will tend to drag the fuselage with it.
My 'completely correct' statement was a result of too much celebrating the Jubilee last night and a somewhat fuzzy head this morning.
Arm out the window
5th Jun 2012, 20:49
Glad you had a good time, Crab!
I still think I'm right, as I can't see any other way to resolve the force diagram.
I admit it sounds a bit counter-intuitive, but if you put the forces at the appropriate vertical positions, look at the couples they form and see which way the torque from those couples would act, it seems to me they'll do as I pointed out before.
Yes, the MR anti-drift force must act left, and the TR drift force acts right, but if you reverse their vertical points of action the roll is also reversed. When they're acting in line (hubs at same height), no roll.
Weight can be said to act through the C of G, but that doesn't imply the aircraft must roll about it all the time, as some posters imply.
RVDT, the reaction from the main rotor, as you call it, is a force applied at the rotor head (for the teetering head case I was describing), not through the C of G.
Send return stamp, over! (All in good fun, though).
Arm out the window
6th Jun 2012, 03:30
http://i1150.photobucket.com/albums/o607/eacham1/TRRoll2.jpg
A couple of rough diagrams, maybe not enough colours for CFS but you get the idea.
If we're going to talk about physics, this is what you'd put up as a force diagram - the forces, and where they act.
The picture on the right is obviously the silly case where the tail rotor is mounted way above the rotor head, the one on the left the conventional setup. In both cases, two couples form in equilibrium.
An unopposed couple causes a roll, in these cases the MR and TR side forces. The roll will continue until the opposing couple is big enough, ie the one formed between MR thrust vertical component and weight. As the roll progresses, the vertical thrust line of those two forces splits until the moment arm between them is big enough so the opposing torque they create stops the roll.
I can't see it working any other way, but am happy to consider opposing arguments, postage stamp or otherwise.
AoTW - but the horizontal component of MR thrust will be large enough to oppose the TR thrust, even if the TR is very high since the aim is to counter TR drift so you just put more left cyclic in. Then, the horizontal forces cancel each other out and you are left with a level fuselage. It's like putting a fat kid nearer the fulcrum of a see-saw to balance a lighter kid further away. Apologies to any fat kids who may be offended;)
Arm out the window
6th Jun 2012, 08:28
True, Crab, the magnitude of the opposing forces will be equal no matter where they are positioned vertically, but it's the relative vertical positions that produce the rolling couple.
When the main and tail rotor hubs are equal in height, there'll be no roll, but when the opposing forces are split vertically, there'll be a roll in the appropriate direction which will continue until the opposing couple (the weight/vertical MR thrust couple) grows big enough to counterbalance - when the C of G has been sufficiently displaced from under the head by the roll.
You only put in enough left cyclic to counter the tail rotor drift force, which is the same no matter what vertical position the tail rotor is at. So those forces are essentially fixed, but their by-product (which varies with relative vertical position) is a roll.
The fuselage will only be level when the side forces are at that co-height point. Anywhere else, there'll be a roll angle.
But we know that doesn't happen - for example when you push the nose down to transition to forward flight (thereby raising the TR) you don't get a roll to the right even with excessive amounts of nose down.
So somewhere along the line the physics is incomplete since the theory doesn't match the reality.
Arm out the window
6th Jun 2012, 10:29
But we know that doesn't happen - for example when you push the nose down to transition to forward flight (thereby raising the TR) you don't get a roll to the right even with excessive amounts of nose down.
Well, that's a dynamic situation with all sorts of stuff going on - collective, cyclic and pedal inputs all being made by looking out the front and reacting to a sight picture, inflow roll, flapback etc, so it's a fair bit different to a stable hover situation which is what the discussion was based on.
hence as Nick Lappos said in a previous post some 10 years ago, the placement of the T/R relative to the M/R head is only slight.
Fair enough, the diagrams are not to scale, but they illustrate the forces and where they act. Nick didn't say the effect was slight, he said it had nothing to do with it, which is what I'm at odds with.
A force diagram is pretty straightforward, as are force couples. If we're having trouble with this idea, then does anyone have dramas with thrust line in a fixed wing, for example?
A high mounted engine with respect to the centre of pressure (drag wise, whatever the aerodynamicists call the point where drag is said to act, vertically speaking) will cause a nose-down pitch when power is added. If the engine is lower than that point, you'll get a pitch up. Same sort of thing.
Well, that's a dynamic situation with all sorts of stuff going on - collective, cyclic and pedal inputs all being made by looking out the front and reacting to a sight picture, inflow roll, flapback etc, so it's a fair bit different to a stable hover situation which is what the discussion was based on. Not necessarily, imagine a 100' still air OGE hover, then gently push the dose down just enough to get the TR above the MR (easier in a helo with a cranked tail). Only a small change in cyclic attitude, no change in power and before flapback and inflow roll can take effect - if your couples are correct the aircraft must roll right as the TR passes the height of the MR and I don't think it will.
Arm out the window
6th Jun 2012, 21:26
if your couples are correct the aircraft must roll right as the TR passes the height of the MR
Yes, that's the crux of my argument, Crab. Whether it really happens that way I've never tested, and in the end it's not a biggie. I still don't see why the force diagrams aren't correct, but perhaps I'll never know...
The placement of the T/R relative to the M/R head has nothing to with the rolling force generated by the T/R.
That doesn't affect the magnitude of the force generated by the TR, but the separation of the forces in the couple does affect the torque generated (increasing separation, or moment arm, providing more torque), which again is a crucial point in my (perhaps misguided!) argument.
Cheers all, we must look a bit like a bunch of old bearded blokes arguing about verses in the Koran or something.
oTW - even with a teetering head helo and a TR that is above the MR hub, I don't agree that you will have a right roll as you always have to tilt the disc to the left to oppose the TR drift - I think the best you can expect is laterally level fuselage in that condition. The disc is 'flying' left and will tend to drag the fuselage with it.
the fuselage will be level except for the moment caused by the height of the T/R thrust not being in the same plane as the Torque which must be opposed.
In this example height of T/R wrt the disk is the only thing influencing the roll required to counter that 'mismatch'
The point is : you could measure the 'heights' of the horizontal components from the CoG - but the height of the CoG cancels out so it becomes the height of the T/R horizontal vector compared with the height of the plane in which the torque is delivered.
So a T/R hypothetically above the plane of the disk will lead to the counter intuitive roll direction... (in a teetering rotorhead)
Head coupling might modify that since to produce a M/R horizontal component which is equal and opposite to T/R thrust some roll moment from that will occur.
Arm out the window
7th Jun 2012, 00:39
Sounds like we're in agreeance, AnFI, though my aim is more to make sure I understand the balance of forces and moments clearly than to prove someone else wrong!
With a semi-rigid or rigid head, as you say, the leftward inclination of the rotor disc to provide the side force will also be associated with an anticlockwise torque applied to the mast head (as seen from the rear of the aircraft), which I think would increase the leftward roll angle a bit. Maybe!
Whilst AoTW's physics seem correct, I can't find anything in any of my books to corroborate the theory - in most, such a situation isn't covered at all (including Prouty). The US Army manual shows the limit of TR roll to be a level fuselage on a teetering head helo (205) with a high placed TR with all other configurations giving left roll.
AoTW may be completely correct and this may be why there don't seem to be any helicopters designed this way yet there are lots of clever ways (offset rotor mast, modifications to the mixing unit etc) of trying to reduce the amount of TR roll to the left in 'normal' helicopters.
Arm out the window
7th Jun 2012, 05:01
I have a lot of affection for the 205, the Huey being my first operational type, and it generally used to sit a bit nose up in the hover (MR hub a few feet above the level of the TR hub) but in the cruise the two would come back into line.
I don't know what the designers were after with the high-mounted tail rotor, maybe just to keep it out of the grass, but it would have mitigated the tail rotor roll somewhat no doubt. Even then it was almost always left skid low in the hover unless you had some strange loading going on or someone hanging off the hoist.
I think to get it any higher would be structurally impractical, bring the main rotor too close to the boom or have an infeasibly long upswept part on the rear of the boom, not to mention the weight penalty.
Happy days!
The whole point about understanding Newton et al is that all of a sudden you don't need to learn each phenomenon from a text book. There's a framework from which other behaviors and characteristics are self evident. This understanding enables pilots to fly sympathetically.
The thought of you doing this in a Lynx:When performing loops or backflips where a maximum cyclic displacement and high rate of application is required, the amount of left cyclic required to keep the 'wings' level is quite noticeable. from your position of lack of understanding highlights the need for tough airframes.
AoTW - the raising of the TR on the 205 will be to do with giving a useable range of Longitudinal C of G so that with a lot of troops in the back you don't have too much left roll as well as giving ground clearance at the back end.
Aerobot
7th Jun 2012, 19:46
That's what I like about PPRuNe. You can learn so much here. Where I come from the Tail Rotor Rolling Couple is illegal in every state except Nevada, and there they make you pay extra for it. ;)
Arm out the window
8th Jun 2012, 08:47
I want to see what was so flawed about our physics, RVDT!
AotW, the more I look at it the more I think you are right and one of the reasons we don't normally see this situation is that most teetering head helos (205, 206, R22) have the MR mounted very high on a long rotor mast so that the TR is very unlikely to ever get above the MR (except in extreme attitudes).
Now I don't know about the 205 but I vaguely seem to remember on a 206 that the MRGB is tilted slightly to the left to compensate for TR drift, possibly because the 205 had a raised TR boom but the 206 doesn't.
Who would be a helicopter designer?
JohnDixson
8th Jun 2012, 14:29
Aotw,
I suspect that you are correct re the UH-1 series designs and TR position. A huge factor for a tactical machine ( that is, keeping the tail rotor away from terra firma ), and the reason that the UH-60 has a tail wheel whose crash load design is the same as the mains.
With regard to the 205 shaft tilt, it is five degrees and is up a couple from the UH-1B which preceded. big improvement in drag at cruise vs the B model. I always thought that the reduced 2P vibration in the UH-1D/H was flapping related and for sure the flapping in a D/H at cruise ( and loaded ) was far lower that in the B. Maybe someone from Bell can comment and /or correct that supposition.
With regard to optimizing hover attitude, two models come to mind that employed lateral shaft tilt to accomplish that goal: The Sikorsky S-64 Skycrane and the Kaman UH-2 Seasprite. Both were successful in accomplishing a level hover, but there were other impacts on overall handling such that none of the modern designers ( from any OEM ) seem to think its worth it. Count me in that number.
After returning from flying a Cobra in Vietnem, Nick Lappos graduated second in his class at Georgia Tech: pay attention to his comments re free body diagrams.
Thanks,
John Dixson
Arm out the window
8th Jun 2012, 22:16
Thanks, John!
I'm sure Nick knows more about helicopters than I'll ever learn, and if you're the same bloke I just googled with impressive credentials in the industry, no doubt you do as well.
Re the 205 mast tilt, are you talking about an offset to the left to counteract TR roll, as we've been discussing? I never knew that - just shows you how many little things there can be about a type that you don't necessarily learn at operator level.
MightyGem
8th Jun 2012, 22:46
Where I come from the Tail Rotor Rolling Couple is illegal in every state except Nevada, and there they make you pay extra for it.
Do you mind!! Almost spilt my tea over the keyboard! :D
Aerobot
8th Jun 2012, 23:18
Higher praise a dumb joke cannot expect. ;) Thank you.
JohnDixson
9th Jun 2012, 01:53
Aotw,
Sorry for not being clearer. Was referring to forward shaft tilt, as your post talked about the 205 being nose- up in a hover.
Thanks,
John Dixson