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Hanging one side low

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Hanging one side low

Old 23rd Mar 2020, 09:32
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Originally Posted by Ascend Charlie View Post
IBut does it really matter, other than giving you something to think about, and divert your mind from the lack of Lou Rawls and the closure of Dan Murphy's?
Good morning everyone. It is only the exceptional circumstances I find myself in that have tempted me out of my shell. I hope I can be forgiven for raising a bit of diverting controversy.

I have been away looking for more information and I now have some doubts about the cause of the roll in the hover.

I am sure about the physics: the balance/equilibrium of forces and couples in the hover; the need for correct terminology; the absence of a pivot.

I looked back at what Nick Lappos had said in earlier threads and he is telling us that the torque applied to an articulated or semi-rigid rotor head by a tilted rotor disc is big. I was ignorant of this in the 70s and it is something I learned here. I could not find that he explicitly said this was enough to cause the hover roll and he also said that the position of the tail rotor was significant in determining the degree of the roll. I am also largely ignorant of the very sophisticated design of modern helicopters - the last I had some small knowledge of was the Lynx.

Here is where I stand at the moment. There is tail rotor drift and this is corrected by tilting the rotor disc laterally. The mechanism by which the disc is tilted is a combination of design choices and pilot control. The design choices will be a compromise between providing for the hover and cruise. In nearly all cases tilting the disc will result in a roll couple, which is from the torque on the rotor head and/or the offset between the tail rotor and main rotor. As the roll progresses the roll couple is balanced by the couple formed between the vertical component of the rotor thrust and the weight acting through the CG.

My real doubts are about where the roll couple comes from. The hover is a state of static equilibrium and in comparison to most helicopter aerodynamics it should be relatively easy to analyse. However, I am aware that there are a lot of things contributing to the forces and couples and that Nick Lappos may know of interactions that have completely escaped me.

Lastly, I think that the simple explanation may be true for teetering rotors, but I am even questioning that.
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Old 23rd Mar 2020, 09:57
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Bah! All you experts with thousands and thousands of hours arguing the merits of which side they hang.
Last time I parked one near the hanger with the wind tumbling over the roof it was hanging left, then right, then left, then right and so on until it finally touched that hard allergic grey stuff.

Fair weather flyers the lot of you.

(tongue-firmly-in-cheek )
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Old 23rd Mar 2020, 09:58
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Originally Posted by [email protected] View Post
We have been discussing couples that create rotation of a body - how do you identify the axis of that rotation? Is it along the line between (in this case as in ACs diagram) MR head and TR and, if so, where along that line?
I am ok on this one, I hope. A couple is a vector - it has direction and magnitude. The direction for a couple is parallel to the axis of rotation. It does not have a point of application as such. - it is acting on the whole body. I think that this is actually a tricky thing to conceptualise. I might also point out that this is statics, with forces and couples in equilibrium. With dynamics the situation is much more complex.

I am pretty sure that it is ok to apply statics to a hovering helicopter, but I am even having some doubts about that.
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Old 23rd Mar 2020, 10:24
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Originally Posted by Ascend Charlie View Post
Many years ago, Nick put out a list of Helicopter Urban Myths,
I do not know how long ago that was, but when I started teaching PoF in the 70s they were still quite widespread. Even Shawberry was turning out QHIs with traces of them. Their teaching materials seemed to be derived ultimately from the US helicopter manual of the 50s which was rife with them.

I tried to eliminate those I new of and met with a little resistance from the beefers as a result.

However, this thread has made me consider the awful possibility that I might have fallen for one of them regarding tail rotor roll.
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Old 23rd Mar 2020, 14:50
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Syd, thank you for your answers but surely there has to be an axis of rotation if the body rotates.

Consider just the translating tendency of the TR thrust before the disc is tilted to compensate - that lateral force must act either through or at some distance from the vertical C of G. If it acts through it the body will simply move laterally but if it acts at some distance (ie TR vertically separated from the vertical C of G, it must also cause a rotation.

If my sentence is correct then applying lateral cyclic gives a force acting at the rotor head which, again, if not aligned with the vertical C of G, will produce a rolling moment as well as a lateral force to oppose TR drift.

Thoughts?
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Old 23rd Mar 2020, 14:55
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As far as rotor design goes - with a teetering head, the fuselage just hangs under the rotor so you tilt the disc and the fuselage obediently follows it. When you add hinges (physical or virtual) you allow the blade flapping to exert a force on the rotor head which is fixed to the fuselage - the results are the same but the mechanisms are different.

The Lynx, with it's titanium flapping forging had an effective hinge offset of around 17% if memory serves (I used to instruct on it). The bigger this offset, the quicker the fuselage reaction to any cyclic input and the greater control power you are deemed to have. Having rolled, looped and back-flipped it, I know it is VERY responsive.
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Old 24th Mar 2020, 10:41
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Originally Posted by [email protected] View Post
Syd, thank you for your answers but surely there has to be an axis of rotation if the body rotates.
The essence of a helicopter in the hover is that it does not move or rotate - it is in static equilibrium - and there is no axis of rotation. It means that the sum of linear forces is zero and the turning effects (couples/torques) are balanced.

When analysing this you must be sure that you have identified all the linear forces and all the couples. It is my ability to do the latter that has thrown doubts in my mind.
If my sentence is correct then applying lateral cyclic gives a force acting at the rotor head which, again, if not aligned with the vertical C of G, will produce a rolling moment as well as a lateral force to oppose TR drift.
That is true and is one couple I am pretty sure of. But what is it balanced by?
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Old 24th Mar 2020, 10:45
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Originally Posted by [email protected] View Post
As far as rotor design goes - with a teetering head, the fuselage just hangs under the rotor so you tilt the disc and the fuselage obediently follows it. When you add hinges (physical or virtual) you allow the blade flapping to exert a force on the rotor head which is fixed to the fuselage - the results are the same but the mechanisms are different.

The Lynx, with it's titanium flapping forging had an effective hinge offset of around 17% if memory serves (I used to instruct on it). The bigger this offset, the quicker the fuselage reaction to any cyclic input and the greater control power you are deemed to have. Having rolled, looped and back-flipped it, I know it is VERY responsive.
All that is familiar, but it is describing the dynamic response and analysing that is way beyond my pay grade.
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Old 24th Mar 2020, 11:07
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The subject is "hanging low", but the BK seems to have them both at the same level.
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Old 24th Mar 2020, 12:05
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The essence of a helicopter in the hover is that it does not move or rotate - it is in static equilibrium - and there is no axis of rotation. It means that the sum of linear forces is zero and the turning effects (couples/torques) are balanced.
But those couples/torques must be acting around a point to have created the rotation until equilibrium is reached.
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Old 24th Mar 2020, 12:52
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AC....BK used mine as a model!

Well...size and weight perhaps.... but not cast in Brass!
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Old 24th Mar 2020, 14:02
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Originally Posted by [email protected] View Post
But those couples/torques must be acting around a point to have created the rotation until equilibrium is reached.
Here is a secret. Physicists prefer to deal with steady state conditions because they are relatively easy to analyse. They ignore how the steady steady was arrived at. Sometimes you can fudge the transition from one steady state to another by assuming that all states in between are also steady states. In the case of a helicopter you might be going from resting on the ground to the hover. Following your own dictum, the pilot sees what the aircraft does and makes control movements to achieve what is required from moment to moment. The helicopter changes its attitude quite slowly and we can say that it never really rolls around any point as such. Nice, eh?
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Old 24th Mar 2020, 14:46
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So are you saying that I am correct when the aircraft is moving but you are correct when the aircraft is static?
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Old 24th Mar 2020, 15:24
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Originally Posted by [email protected] View Post
So are you saying that I am correct when the aircraft is moving but you are correct when the aircraft is static?
When the aircraft is moving all bets are off as far as I am concerned. What happens does depend on the position of the CG but also in the Centre of Inertia, which is probably as far as this forum should go.
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Old 24th Mar 2020, 16:44
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Originally Posted by HissingSyd View Post
When the aircraft is moving all bets are off as far as I am concerned. What happens does depend on the position of the CG but also in the Centre of Inertia, which is probably as far as this forum should go.
No - don't rock the boat! But if you did, which point would it be rotating about?..... ;-)
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Old 24th Mar 2020, 16:54
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Originally Posted by 212man View Post
No - don't rock the boat! But if you did, which point would it be rotating about?..... ;-)
Don't forget I am also ex-RN. Are you posing a problem of ship stability with CG and Centre of Flotation? ;-)
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Old 24th Mar 2020, 18:59
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Try not to think of this as rotation about a point in 3-space. In the static equilibrium case, you can pick any point you want, and the most logical (giving simplest equations) is the rotor hub. Rotor disc is tilted with respect to the hub. Fuselage is hanging with respect to the hub, pushed one way by the tail rotor, and the other way by the CG.
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Old 25th Mar 2020, 02:07
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AC....BK used mine as a model!

Well...size and weight perhaps.... but not cast in Brass!
Sassy, I thought that having [email protected] of brass was a requirement for army pilots?? Well, tin anyway...
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Old 25th Mar 2020, 03:06
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Gentlemen,scholars, engineers, aviators, five pages of discussion about a fairly simple free body diagram but without concurrence as to a solution,and now,we are to proceed into an allegorical treatment of the relationship between the MBB Pendab and Army Pilot physiology?
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Old 25th Mar 2020, 06:27
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It would seem that physics isn't much use where helicopters are concerned since they never stay still once running so static diagrams don't help.

ISTR the people that investigated the 2 Sea King dynamic rollovers (one at St Magwan and the other brilliantly reproduced at Boscombe Down) on level ground were very interested in rolling moments from MR and TR and especially their position relative to the vertical C of G
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