Light Helicopters and Tail Rotor Rolling Couple
Thread Starter
Light Helicopters and Tail Rotor Rolling Couple
I posted this question on the Questions forum:
and I received one reply:
Links or explanations anyone? 
On an AH&N thread,Tiger mate described the tail rotor rolling couple:
Any Borneo Whirlwind veterans here?
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
Any Borneo Whirlwind veterans here?
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
Trim!
And if you use the Rotorheads - PPRuNe Forums section you'll probably get some links to illustrative graphics.
And if you use the Rotorheads - PPRuNe Forums section you'll probably get some links to illustrative graphics.
Join Date: Feb 2010
Location: Uk
Posts: 11
Likes: 0
Received 0 Likes
on
0 Posts
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!!
Join Date: Jun 2008
Location: England & Scotland
Age: 63
Posts: 1,413
Likes: 0
Received 0 Likes
on
0 Posts
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!
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!
Last edited by John R81; 31st May 2012 at 21:18.
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'.
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'.
Thread Starter
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.
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?
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
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.
Thread Starter
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.
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.
Nick Lappos said about 10 years ago..................
in this same forum...................
Bridge withdrawn from market.
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.
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.
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.
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.
Join Date: Dec 2008
Location: N/A
Posts: 845
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Jun 2008
Location: England & Scotland
Age: 63
Posts: 1,413
Likes: 0
Received 0 Likes
on
0 Posts
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.
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.
Join Date: Mar 2002
Location: Kings Caple, Ross-on-Wye.orPiccots End. Hertfordshire
Posts: 458
Likes: 0
Received 0 Likes
on
0 Posts
T/R Drift
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 - 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.
Thread Starter
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.
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.
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...
) since I was first taught them, 46 years ago, and this is the reason I am having difficulties with the physics of the situation.
Last edited by India Four Two; 4th Jun 2012 at 16:24.
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.
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.
Join Date: Mar 2009
Location: South Ridge
Posts: 61
Likes: 0
Received 0 Likes
on
0 Posts
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.
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!
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.