Tail rotor position
 

Hello,

I have been looking at some helicopter photos and I've noticed that some helicopters (usually military such as UH60) have tail rotors pretty above the center of gravity (CG position guessed by eye... ). Some others (usually civillian helis) have it close to the CG.

I can understand the logic of having the tail rotor close the CG level. In this position it would not create a turning moment around the longitudinal axis. How is this compensated in other helis? With digital control systems? Does for example a Uh60 have a serious rolling moment due to tail rotor if the control system is disabled?

thanks in advance

Actually the better designs have the tail rotor above the longitudanal C of G.

It is difficult to explain (& probably harder to invisage) without diagrams but I will try.

The tail rotor applies a lateral force to counter the torque reaction caused by driving the main rotors. The trouble is this force then causes the helicopter to drift in the direction of that force. This is known as tail rotor drift. To counter the drift a lateral stick in put is applied. The aircraft has now stopped drifting but is sitting one skid/wheel low due to constant lateral input.

The designers overcome this but placing the tail rotor centre of thrust above the longitudanal C of G so that it know has a rolling moment which corrects the skid low situation.

Hope this helps.:ok:

TTO
I understood what you mean,

Nickel ,

Yes I think you are right, ... any heli would have a tad moment of indecision if tail rotor controls were disabled!

Peter RB

It was always explained to me as
If a helicopter was designed to hover (Anti sub or SAR etc.) then the Tail Rotor was high to minimise Tail Rotor Roll in the hover,(in line with the C of G ish), the down side being Tail Rotor Roll in Fwd Flt.(Reverse wing stabilator used to combat this)
A cruise helicopter would have an in line Tail Rotor minimising Tail Rotor Roll in the cruise, the down side being TRR in the Hover.
I also think there is a weight and power issue as well as it seems that only the little racing helo's have the in line tail rotor.
Happy to be corrected (Where's Crab):sad:
Hope this helps.

Obstacle clearance is another good reason for a high tail rotor.

-- IFMU

Check the archive for more hot discussions on this subject. These include everything you need (or need not:uhoh:) to know

cheers YB

The rolling couple is caused, not by the c of g, but, by the difference in height between the tail rotor and the main rotor.

If the t/r was at the same height as the m/r when in the hover, the aircraft would hover skids level, but would fly one skid low when in the cruise. As it's preferable to be skids level in the cruise, the t/r is positioned appropriately. Consequently the aircraft sits one skid low in the hover.

http://img.photobucket.com/albums/v4...Gem/TRRoll.jpg

Ouch! Better dust off the fire suit there MG! Incoming.................................................... ..................................

(Psst: You theory is 100% wrong) :rolleyes:

Yes, post No. 2 has a good explanation.

Good use of colour, though! Who's going to volunteer to do the vector diagram?:confused:

For cruise it depends also on vertical stabiliser, which is generally in a high position. Main function of tail rotor is to provide yaw control, so it is best off at same height as cg. For lateral trim TR is best off as close as possible to rotor disk, which in the cruise is tilted forwards with the fuselage. It's thus a compromise.

Don't forget any development team is trying to find the most cost effective way to deliver a reliable helicopter design. A high tail rotor is aerodynamically ideal, but requires an extra gearbox. This new gearbox requires new castings, extra machining and an additional driveshaft to the tail rotor gearbox. Unless skid low hover is an operational problem, best to avoid the complexity.

There is also the problem that the horizontal stabiliser is best off out of the downwash, particularly to avoid trim changes with forward flight. A half tail plane adds weight through torsional loads in the tail. Sometimes it's easier to put tail rotor below a full horizontal stabiliser.

MG, the position of the C of G, both vertically and laterally is important here becuase any force (tail rotor thrust, main rotor thrust or weight) acts about the C of G to produce a moment.

Your QHI course diagram omits this vital information - the horizontal component of main rotor thrust and the tail rotor thrust act about the C of G to produce a rolling couple; the vertical component of rotor thrust and the weight also act about the C of G to produce a rolling couple.

The magnitude of the forces and their relative distances from the C of G determine the eventual attitude of the aircraft when they are all in balance.

But as role 1a says the positioning of the TR is partly a function of the role of the aircraft - an aircraft that will spend a lot of time in the hover is more comfortable if the cabin floor is level and as such a high TR position is often chosen. This is not a hard and fast rule as there are many other factors that a designer way have to take into account.

Most helicopters use a horizontal stabiliser to keep the fuselage relatively level longitudinally in the cruise so the TR position is less vital.

I agree. The heavier a particular aircraft is loaded, the less the amount of roll, but it is still the vertical distance between the m/r and t/r that produces the skid low attitude. If the t/r was at the same level as the m/r then that roll would be minimal. Increase the distance apart and the amount of roll will increase.

Mighty Gem is unfortunately in strong disagreement with Physics when he says TR height to MR head has ANYTHING to do with roll in a hover. Please pruners, don't try that at home!

The issue has been discussed here several times before, I have absolutely no idea why physics works for everything else on the helicopter but not for the roll attitude!

Why is this that hard, folks?

Nick, I bow to your greater knowledge. Someone obviously taught me wrong all those years ago.

Having said that, this would appear to support my argument.http://www.tc.gc.ca/CivilAviation/ge...Exercise8b.jpg

From
HERE.

MG, that's exactly how I teach. Lifting the tail rotor will produce a smaller couple between the two vectors (one pulling left and one pulling right).
It's the same when in forward flight. If you fly unbalanced your one skid is much lower than the other. That's why in the Squirrel we fly it half a ball out to keep the skids level.

You can teach it anyway you wish, and even transport Canada has it wrong.

The forces and moments act at the CG, and that is where one measures the resulting accelerations from. Because you explanation works, it seems ok (so did the ancient scheme where the planets spun around the earth!)

Having helped design the Comanche tail rotor/fantail, and evaluated where to put it based on the REAL physics (balancing left bank in a hover against tail drive shaft clearance from main blade strike) I can assure you that designers would toss their cookies at Mighty Gem's cartoon.

Where is Shawn Coyle to explain how Canada got it wrong?

Oh Shawn!

Must remember that one from VR for the next CAT with Crab
Excuse 21a:)

I think that's what they did to the EC120 by offsetting the engine to the left side if I'm not mistaken.

Not doubting that for an instant, Um... lifting...

So the flat Earth goes around the Sun, eh? :}

Just to expand the argument, why does the Chinook hover left wheel low?

If you look at helicopter like the CH-53. The tailrotor is mounted almost inline with the mainrotor. So why do they offset the vertical stabilizer to the left tilting the tailrotor to one side and almost looks like it could create a small amount of lift in the same direction as the mainrotor?

Choppie - because that is exactly what it does

Choppie,
Crab is right, the canted tail rotor on the CH-53E does provide significant lift. It serves to offset the aft CG of the baseline aircraft, since the E model added an aft engine and a big cabin plug as compared with the D model, which had a normal tail rotor configuration.

Interesting. I would guess that splitting the CH-53E cabin plug between fore and aft was considered, but rejected due to need to redesign control system linkage. I guess that's not such an issue on a fixed wing with cables or hydraulics.

I'm guessing Chook cg is a little port of centreline.

I can imagine with a huge tailrotor like that it can create quite a bit of upward thrust.

Choppie, with a 20 degree cant angle, the upward thrust is about 1200 lbs, at the tail. This is pure lift, but also helps shift the lift balance to offset 4000 lbs just aft of the mast.

Please note that the tail lift acts as pure lift at the CG, as well as a nose down rotation due to the strong moment from the tail lift. So much for the "stick the moment anywhere you'd like" school of bizarro-physics.

Love these arguments where everyone looks to find who's right when, depending on your point of view, the truth is either everybody's right or nobody's right.

Demanding a degree in physics prior to learning how to fly a helicopter is impractical, yet understanding why things happen the way the do with helicopters is necessary. To make the concepts understandable, answers are diluted quite a bit in most cases, and in some cases are complete lies. But they still get the point across.

This idea of looking at the difference in height between the tail rotor thrust and the horizontal component of main rotor thrust has huge merit. If they act at the same height, then when there is no net lateral force, there is also no net rolling moment. Thus you hover with skids level. (At least within the many approximations that have to be made.)

Is that accurate with physics? Yes, but it is far from the whole story. Just knowing the difference in height doesn't help much in designing helicopters, but it does help in understanding a small part of helicopter flight.

So is the right answer that those two forces create a moment about the center of gravity and the vector sum of those moments is the net? Well, that is a right answer, but there are others. Because of all the approximations still being made, it would be valid to call this a wrong answer as well.

What's right really depends on why you need the information. If you're designing helicopters you need way fewer lies and much more accuracy. If you're trying to learn to fly helicopters, a general understanding is sufficient.

Mainly true Matthew, the P of F that is taught in UK is essentially a convenient explantion of something we know happens but usually isn't the truth, the whole truth and nothing but the truth.

However useful simple concepts are for teaching, there is no point letting people try to convince others that these simple concepts are the whole story.

Worthwhile discussion...
 

Correct me if wrong, but what Nick is getting at is that for helicopter to hover (or cruise) total lift must balance total weight. Ship rotates about it's cg until tail rotor thrust and main rotor lift vector sum to a vertical force above the cg. All the while the pilot is trimming the cyclic to keep references/string/ball in position. It's as Matthew says, a different perspective.

At a guess Comanche low tail rotor would hover left wing low, but with high vertical stabiliser would cruise right wing low. That powerfull bearingless rotor would then sort out any pitch attitude changes, from tail rotor cant.

Is it fair to assume centre of drag acts at similar position to cg?

Actually, designers often use tricks to get quick answers.
I'll revise my earlier posts appropriately...

I have flown a few hours on the EC130 (didnt finish my conversion though), but what I did notice is that you fly with quite a bit of left pedal when in the cruise. Is this then the same because of the huge vertical stablizer like the Comanche? But I can't say that I felt the helicopter fly left skid low. Maybe I didn't notice it.

Choppie,

Generally, yes. Same reason why you fly with a bit of right pedal whilst in the cruise in a B206. (ie the vertical stab is doing the work of the TR.)

Yes that I know. But in the EC130 it actually feels strange because it's so much left pedal. What I would like to know is does it cruise with the left skid low or level?

Matthew
 

It looks to me that some scientific based reasoning would probably have cut short many of these discussions. Over and over again some topics resurface...

If you argue that simplification is OK for a pilot that -just- has to fly the thing, then that is OK with me, there are more important things to learn. As long as these pilots know that when they start making great theories, they may be way out of bounds off from what they have been thought.


d3

So what hppens when you use this tail rotor position?
http://uk.youtube.com/watch?v=Yu2CwHwxJYA
Max

Humble pie...
 

Well, this thread has knocked my thinking into shape. Read "Cyclic & Collective" by Shawn Coyle, in particular P220 covering tail rotors in advanced helicopter aerodynamics. Good read.

An important tail rotor function is to provide yaw control. If it is above or below the cg then you get a yaw-roll coupling. The vertical stabiliser can be positioned to perfectly counteract main rotor torque, but tail rotor is by necessity more of a compromise. I've stuck in a line in my earlier post.

Choppie, fly with string centred and see what ball says about trim.
Maxtork, interesting i wondered what was happening on this. 177kias is a good start, i imagine power limits level flight to 165kias. The only information i can find on X2 development is this frustratingly slow update web vid:
http://uk.youtube.com/watch?v=E-rDl8lbVcc

Sorry for the slight change in thread!
I was wondering what determines the length of moment arm for the tail rotor, If the tail rotor was further away surely you would need less thrust to counteract the main rotor coupled with less TR drift, I am aware a heli would look pretty silly with an extra long boom but you guys seem to know what you are talking about.
Hope it has not been covered before

You would also need extra long hangars...! ;)

AW139 tail rotor is angled 11 degrees from vertical to provide lift to offset aft CG which is pronounced in this aircraft.

In north sea fit with 2 pilots and with help from tail tail rotor not much more than 1000 kg of fuel can be carried without ballast in cabin, bearing in mind that it can carry I think 1676 kg with aux tank !

CF