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.