Blimey Jeep and you an A1 an'all, fancy not knowing simple stuff like that!!!

No, I am taking the p*** mate, good luck with the bag, it doesn't sound much fun- especially for an old knacker like you!!!!

Flygunz, you are just too anal for remembering the page ref in the PoF book, I will just go and look in the copy PD gave me to see if you are right!

one-hung low
 

The reason the fuselage hangs low is primarily determined by the position of the tail rotor in relation to the a/c`s CofG,Longitudinal,lateral and vertical. The moments derived by the t/r in producing "t/r drift" or "translating effect" have to be opposed by the moments produced by the rotor,and it doesn`t matter what type of rotor it is,rigid,semi-rigid,teetering,etc,those factors relate to sensitivity,response,and control power.If you are in a hover at fully fwd.c of g,and note the a/c attitude,then go to fully aft and hover, the a/c attitude will be more "left side down"(for CCW rotor rotn.)If you can now change the lateral CofG,you will now see an even greater variation in attitudes,and finally ,if you can find the vertical CofG and change it from it`s normal position,usually fairly high on modern helos,and use depleted uranium to lower it sufficiently(even as an u/s load-rigidly attached) and always using the same a/c weight,you can draw a matrix of attitude/weight(K)/CofG(lat/long/vert.).From the known a/c attitudes ,the position of the t/r relative to the a/c CofG can be ascertained and should show that an aft,high and left CofG will give the most adverse left roll!Of course ,if you are French and dress the other way,then you`ll roll right,and if you`re Oz then use a mirror-This ,of course could all be b*****ks,as this Chilean red is slipping down too easily,and I might have it all wrong!!!! :cool: :cool: :cool:

Why helicopter takes off one side first?
 

I´m helicopter pilot in Brazil and CFI.
When I´m teaching a new student, they keep asking me why a Robinson helicopter takes off the right side first.
I lot of friends here, explain that the reason is ´cause the main tank is at the right side, so the aircraft is havier in this side. But I really dont believe in it. The tank is so near to the main rotor shaft that I dont believe that the small amount of weight can change so drastically th CG.

When you have to taxi the helicopter and you are in a low fuel condition, the R66 for example, seems to have this condition maximised.

At second, I thought It could be as a result of tail rotor thrust, so to compensate this force, the main rotor shaft is tilted a little to the right. But this is just a guest. And I couldnt find anything in the maintenance manual that could validate my guest.

This morning I was in a Helicenter here and I was watching a Squirrel (Eurocopter AS-350) landing and it was easy to see the same thing happening.

Well, I still think that it has something to do with the engineering stuff, not just a matter of weight.
Can someone help me with this and explain why the helicopter takes off and land one side first?

Or rather one skid hanging low, depending on which way the rotors spin. Helicopters using anticlockwise rotation will have the tail rotor thrust slightly out of alignment with the main rotor, because they are set to be level with each other in the cruise. As a result, the TR thrust line is below the level of the MR hub and will produce a couple that makes the left skid lower in the hover - the reverse for clockwise rotating rotors.

Should an anticlockwise rotating helicopter end up hovering right skid low, you've left the refuelling hose in :)

Phil

From the Helicopter Flying Handbook (FAA-H-8083-21B) Chapter 9 states:
"Helicopters usually hover left side low due to the tail rotor thrust being counteracted by the main rotor tilt."

Left side low implies a anticlockwise rotation of the main rotor.

Brilliant. Hours wasted trying to understand that just in case the feared Trapper asks me and you explain it it half a dozen lines. I hate you.

Brought to you by your friendly FAA

Page 2-15, Translating Tendency

Hmm...am I the only one who thinks an instructor should know that?

Maybe he should go here:
https://www.pprune.org/rotorheads/63...ght-books.html

really?

https://www.pprune.org/rotorheads/64...tml#post614480

You have two opposing Moments.
The first is the T/R thrust, acting to the right and lower than the rotor head, and the other half of the moment is the horizontal component of the main rotor thrust, which has been set at an angle to oppose the drift effect. This first moment tries to roll the aircraft to the left.

Opposing that, once the aircraft rolls a little bit, is the CG, which is no longer in line with the vertical component of the rotor thrust. It acts straight down, and is off set to the right from the main rotor thrust, acting straight up. This moment tries to roll the aircraft right.

The two moments fight a little, the more the T/R moment tries to roll to the left, the more the CG moment is offset and opposes that roll.

(
https://cimg8.ibsrv.net/gimg/pprune....cd828a4a07.jpg

I can't believe we're up to 50 posts on something as simple as tail rotor roll. As I recall, it took Uncle Dennis about 5 mins on day 2 at Ternhill, shortly before he rode his bike into the lake..

Did anyone bring up Transmission mounting that differs from a true vertical to the Airframe?

Memory serves me the Alouette III had a 3.5 Degree tilt to it.

Did anyone bring up Transmission mounting that differs from a true vertical to the Airframe?

Memory serves me the Alouette III had a 3.5 Degree tilt to it.

If you’re a flight instructor and you don’t know why this happens, perhaps you should be hanging up your instructor wings

...but hang them left wing low...

and Kaman did something completely different with offset transmission ans servo flaps.

The thread is only up to 50 posts because 2 threads have been merged together.

When I first flew the R22 20 years ago it hung left skid low. Then I flew it a couple of years ago and it hung right skid low. Now I only fly the AS350 and my conscience remains clear.

Hanging one side low sounds like my testicles:E

The right roll in ACs diagram (opposing TR roll) is a result of the arm between the action of the TR thrust and the vertical position of the C of G. All forces act around the C of G whether it be vertical, horizontal or lateral. I believe that was the point Nick Lappos was making 18 years ago.

That is not how physics works. The C of G is only relevant as the point through which the weight (gravity) can be considered to act - hence the name. Otherwise there are just the equal and opposite couples and there is nothing whatever to say about forces acting around any point.
I am amazed by the the amount of discussion this has generated. When I was teaching this to student pilots in the 70s it was a five minute item at the end of the lesson on the design and function of tail rotors/fenestrons. The summary is a two-sentence paragraph with a diagram very like Ascend Charlie's.

Not sure you are right about that - you need a point for a force to work around or opposing forces to create a couple - a pivot point essentially, and that is the C of G.

Force x arm = moment

I've taught the same as AC to student pilots since the 80s but it is a simplistic and easily explainable version of the truth - as Lofty Marshall used to say- P of F is just a simple explanation of things we know happen, it isn't scientific or mathematical proof.

No, you do not. There is no pivot with a helicopter in the hover. A couple is a couple is a couple.
And every couple has its moment, it does not have a pivot.

This is a matter of static equilibrium and pretty straightforward. If things begin to move then it starts to get complicated and inertia brings the CoG into consideration. Then:

SAS: “Did anyone bring up Transmission mounting that differs from a true vertical to the Airframe?”

The S-64 with 3 degrees left. If SA still owned the design,I’d not be surprised to learn that the ensuing discussions were still ongoing.

I think the pivot point is the rotor hub. That's why the weight (considered at the CG) can provide an offsetting force.

Rotor thrust is acting from the hub, not the CG. When the T/R thrust tilts the fuselage left, the CG is no longer under the hub, and a restoring moment is created. The 2 moments are continually playing games with each other.

You clearly understand what is going on, but what you have written above could still be confusing. An accurate explanation must use the terminology correctly.
When introducing levers in elementary science, a physics teacher as I was, will only use moments about a fixed pivot/fulcrum. That is as far as Galileo went.
This makes the calculations easy to understand and completely hides the existence of the couples in equilibrium. If someone's education goes no further they will continue to think in those terms when faced with rotational effects and be forever looking for a pivot. That is not your situation, but it is that of many that have posted on this topic, including Nick Lappos.
A couple is a rotational effect, a vector, and its size is a moment. They are not the same thing once you advance from the simplifications of elementary science.
If you replace 'moment' with 'couple' above all is well, except that 'are continually playing games with each other' is not a decent substitute for 'are in equilibrium' or 'are in balance'.
I'm sorry to be picky, but these things matter to me. :8

Well, I also have a science degree with a major in physics, but when dealing with helicopter pilots, I reduce things to the minimum to help understanding. They are simple folk, as you might be aware...

However, the degree was 50 years ago, and I had forgotten the subtle differences between moments and couples.

So, to put things right, we have a couple of couples coupling in the back of the helicopter. Better?

Maybe, but some simplifications are possibly counter-productive.

Much the same as mine, but I have been back to school for a second career since. :)

Every couple has its moment, as I said above.

I think Nick's education did progress a little further...…..:eek:

I am certain that it did.

Mighty Gem and Arm out the window provided simple and accurate explanations at #2 ad #9, only for Nick Lappos to confuse things with incorrect statements at #10 and #26. Q max attempted to put things right at #17, #29 and #38, but the seeds of doubt were already sewn.

Some wind up being a climatic event too which is not always a good thing.

Ah,”simple folk”. Hard to place Mr. Lappos in that group. Second in his class at Georgia Tech,but more to the point, over three decades or so at Sikorsky,I’ve been at any number of meetings with Nick and our resident PhD’s and in those discussions dear friends, Mr Lappos won/loss record remains unbeaten. His main problem was that he was often more than a tad ahead of everyone on the subject at hand,so there was some misunderstanding as a result. Count me in that crowd on occasion.

I am sitting here isolating myself, so I am up for the challenge, in a friendly way.:)

While I have been lurking here, I have read back over a number of threads to which NL has contributed and I have assured myself of his very high credentials. I, too, have been impressed by his lucid explanations and I understand how he came to earn guru status. But gurus can be mistaken. This is just plain wrong:

and your experience of testing and designing helicopters is????

Don't get me wrong, I'm not equipped for an academic argument with you on physics but I have flown helicopters for 38 years and the height of the TR from the MR really makes no difference to the hover attitude.

Nil, of course, but I did once read a lot of information from Westland, Boscombe Down and what I think is now the US DCIT. I also had some access to the Sikorsky manual mentioned in the parallel thread.
Just to clarify that you are thinking of the design position of the tail rotor relative to the main rotor? NL himself says this about that: :)
The attitude of a helicopter in the hover is tail low. In level forward flight it is more or less horizontal. The vertical distance from the line of the tail rotor thrust to the rotor head changes. It is that separation that is most relevant to tail rotor roll, although it is not the sole contribution, as a rehearsal of this thread will point out.

When I said that we helicopter drivers are "simple folk" I wasn't casting nasturtiums at Nick or John. I was referring to the way that things need to be simplified for students to understand them, but the simplification makes the student believe the wrong thing.

Many years ago, Nick put out a list of Helicopter Urban Myths, and among the myths were the misconceptions such as:

"The rotor system IS a gyroscope", rather than "it sometimes behaves LIKE a gyroscope."
"There is an area of high pressure air under the hovering disc, and this makes a cushion of high pressure air that the aircraft sits on". Bong! Wrong!
"Flapping to equality is happening all the time, so the advancing blade is flapping UP to equalise the lift."
O. M. G.......

Are your observations limited to a teetering rotor head like the R-22 or generalized to include the articulated/hingeless head with significant hinge offset? It seems an H-60/S-92/EC-145 arraignment doesn't put the T/R thrust line very far below the MRH in hover.

OK Syd, a genuine question for my education:

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?

My point about height of TR having little effect is that I have flown helicopters with both high and low TR (of both clockwise and anti-clockwise rotation and differing head designs) yet the roll angle in the hover is always between 2 and 4 degrees with that upper limit being on a semi rigid MRH with a large effective hinge offset.

Interesting question, Herr Krebs.

The left-rotation couple has the rotor hub at one end, and the tail rotor at the other - well above the CG. No idea where the axis would be.
The right-rotation couple has the rotor hub at one end, and the CG at the other end. I don't think the axis could be at the extremity, at the CG.

But 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?