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.:hmm:

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.:)

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 :E)

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.:)

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.:ugh:

However, this thread has made me consider the awful possibility that I might have fallen for one of them regarding tail rotor roll.:ouch:

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?

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.

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.
That is true and is one couple I am pretty sure of. But what is it balanced by? :)

All that is familiar, but it is describing the dynamic response and analysing that is way beyond my pay grade.

https://cimg5.ibsrv.net/gimg/pprune....918fabb9e9.jpg
The subject is "hanging low", but the BK seems to have them both at the same level.

But those couples/torques must be acting around a point to have created the rotation until equilibrium is reached.

AC....BK used mine as a model!:oh:

Well...size and weight perhaps.... but not cast in Brass!

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?

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.

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? ;-)

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.

Sassy, I thought that having b@lls of brass was a requirement for army pilots?? Well, tin anyway...

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?

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:)

I wondered whether to mention dynamic rollover. In this case there is a pivot and it is critical to what happens. Is this thread-drift too far now?

Can't believe I missed this thread!

Regardless of direction of rotation...
Regardless how high or low the TR output shaft is...
Regardless of how passengers are arranged...

ALL SINGLE ROTOR HELICOPTERS have a vertical propeller creating horizontal thrust that blows the aircraft sideways. The correct term is "translating tendency."
THE ONLY WAY TO STOP the sideways movement of the helicopter is to tilt the rotor tip-path-plane a little bit to the opposite side and create an equal amount of sideways thrust the other way.
All helicopters' fuselages will tilt along with and in the same direction as the rotor's tip-path-plane, some head designs more than others.

Nobody calls the rotor a 'vertical propeller' and translating tendency is a term of US origin, it is better understood when called tail rotor drift, with tail rotor roll the description of the fuselage attitude after the rotor had been tilted to counter the drift.:ok:

A Tail Rotor is in fact a vertical propeller that produces horizontal thrust. A Main Rotor is in fact a horizontal propeller that produces vertical trust. If you are wound too tightly to accept that description,
then you are part of the reason beginners find it difficult to understand the rotors, how they work, the job they do, and the side-effects they cause.

Bryan

If your springy-thingy is wound too tightly to accept my characterization, then you are part of the reason beginners have a tough time understanding helicopter rotors, their design features, how they work, the job they do, and the side-affects of it. A Tail Rotor IS in fact a vertical propeller that produces horizontal thrust, and a Main Rotor IS in fact a horizontal propeller that produces vertical thrust.

If a beginning student's first exposure to rotor systems is through this viewpoint, and as their depth of understanding grows, they continue to process ideas through this idea, they will fully grasp all the other complex principles like flap/hunt/feather, transverse flow, driven/driving/stalled regions, dissymmetry of lift, retreating blade stall, and all the other tough to grasp subjects.

Bryan

Oh dear me....next thing we know there will be colored pencil drawings required and the mandate that only CFS terminology be allowed.

It just shows where people don't actually understand a thing - they can only parrot what was on a training slide.

PDR

Too true,Sassy, and you must use the Vu-Graph for the slides, turning it off before removing one slide and putting the next one on.

I'm watching Ascend Charlie.
E86

Bryan - perhaps it is you who is wound too tight:) I didn't say the MR and TR were not propellers in the strict definition of one - just that nobody in the rotary world calls them that.

Talking about propellers when you mean rotors is surely a quick way to confuse newbies.:ok:

I've only been teaching helicopter stuff for 31 years so I probably don't have a clue.......................................

CFSH has quite a good reputation worldwide but its easy to knock if you haven't experienced it.:)

It is more likely that you have been teaching CFSH stuff for 31 years.

Well THIS 34 year helicopter guy (87-17 Royal Blue Ft. Rucker) will continue to refer to the rotors as propellers
sometimes, as well as air-pumps, egg-beaters, paddles that thump the air, a "a big fan that keeps the pilot cool."
LOL Because if it stops...watch the pilot start sweating.

In your defense, I work as an engineer in advanced composites for Meggitt, a pretty large aerospace manufacturing
company. A few of my peers at work are wound a little too tight also and they used to give me grief for my red-neck
analogies. After several years there (in my 6th) they have learned that I can often "get-through" to someone when
they can't.

Bryan

Oh jeez, I was talking when the teacher was standing behind me. I'm in trouble now.

The CFS books, I think it was the AP3456A, had some glaring faults, as do the FAA helicopter instructional documents. But if you show it on a Vu-Graph, it seems to make sense, even if it's horsefeathers.

I looked up the acronym for CFSH online. I'm sure it stands for "Chicken Follicle-Stimulating Hormone".

and Prouty, Wagtendonk, Coyle and others. The CFS H teaching is a method of teaching not the syllabus and uses the best of the best techniques gleaned from all areas of education.

You do the course and then start learning your trade - the quality of instruction comes down to the character and personality of the instructor not the need to use the right colours.

AC are you referring to the OverHead Projector when you say vu-graph?I'm too young to know what one is otherwise:)

Jim Eli - it's all about Pelicans not chickens:)