Position of Tail Rotor

[RotaryRat]

Hi Guys,

I'm curious about the way the tail rotor is mounted on machines with an anti-clockwise (or clockwise) system, why is it that some are mounted on the left and others are mounted on the right of the tail boom?

I would assume that a left mounted tail rotor (with an anti-clockwise system) would be more efficient due to the airflow running right to left, therefore the thrust that is generated doesn't have a verticle stabiliser to obstruct the thrust vector.

Any thoughts??

Cheers

[perfrej]

Tail rotor placement...

Well, there are a number of theories as to which side it should be on. The most affecting factor is not which side it is on, but rather wether the tail rotor blades closest to the main rotor disc (the front one) is going up through the main rotor downwash or down with it. You can see that the UH1, for instance, had it on different sides between models and finally settled for the right side of the boom. I read a bunch of papers on this, but I cannot remember wether it was up or down that was the best solution. The issue of tail boom and vertical stabiliser interference is much less of an issue, as is the position of said items in respect to disturbing air flowing into the rotor or the thrust.

Hope this sheds some light...

[NickLappos]

Good topic! Here is the current aero school of thought:

1) The bottom blade should swing forward, regardless of anything else. This is worth perhaps 3% thrust, because the blade sees increased velocity as it meets the main rotor out-wash in a steady hover.

2) The rotor does not like to be up against a wall, or a fin, so it should be held away by the shaft, and it should toss its high velocity air away from the fin for minimum blockage effect.

Thus, the rule of thumb asks for a US helicopter to have a left mounted rotor that spins clockwise as you look at it from the left side. Other configurations also work, they just aren't as efficient.

Dave Jackson would say that a coax is the best tail rotor, I know.......

[212man]

Or, you can use a fenstron and put the rotor in the middle of the fin!

[yellowbird135]

Its not only a matter of aerodynamics, engineering aspects are of equal importance. Rule of the thumb is that you want the outflow of the T/R in hover (Normally the condition that requires the highest pitch setting of the T/R) as unobstructed as possible. Another rule of the thumb is that you want the forward Tail rotor blade moving up into the M/R downwash, thus creating an increased effectivity of the total T/R thrust regardless the amount of pitch. Some helicopters like the schweizer 300/330 series have a T/R with the forward blade rotating down. I believe they made this concession to keep the design of the drive train as simple as possible.

cheers YB

[Camp Freddie]

the R22 tail rotor rotates anti clockwise when viewed from the left, whereas the R44 tail rotor rotates clockwise when viewed from the left, so I guess that makes the R22 unconventional based on the discussion above.

I believe this is because for the R22 they found it just worked better that way, or maybe it was because they used off the shelf parts not sure which.

anyhow the AW139 tail rotor is on the right hand side of the aircraft and is inclined upward at 11 degrees from vertical, and this is to provide lift to "pull" the tail up to counteract some of the aft C of G , which means that the a/c still sits at approx 10 degrees nose up in the hover even with help from the tail rotor.

so what does all this prove? I guess it means you cant generalise.

regards

CF

[Dave_Jackson]

Hi Nick,

Dave Jackson would say that a coax is the best tail rotor, I know.......

No, No. Sikorsky is now saying that.

I would say that the best place for a tail rotor is in a museum.

Dave

[Graviman]

Good topic.

I seem to remember Prouty commenting that on one model the tail rotor had been reversed to front upwards due to tail rotor vortex ring state. Don't remember the model, but that machine apparently had an uncommanded yaw rate increase during fast yaws opposite to main rotor rotation. I also remember the comment that the aerodynamicists were mystified as to why front upwards had helped!

My own thought would be that maybe opposing main rotor downwash causes a small part of the downwash to be diverted over the tail rotor. This slights flow velocity across the tail rotor dissipates the vortex (like moving cyclic forwards in VRS).

Mart

[Rigga]

Just to throw in my thinking on the Tail Rotor positions:
I believe that-
Ideally you should have no tail rotor – but few manufacturers can do that at the moment!
Ideally the TR should be a Tractor Prop – mounted to push onto the tail pylon and avoiding the airless ‘shadow’ caused by the pylon itself.
Ideally the forward rotor should be rising into the Main Rotor wash.
Ideally the TR should be mounted so as to be the same height as the MR in the hover to prevent an additional sideways moment.
I also believe both the Huey (to 205A) and the Lynx (to Mk 7) had problems with TR’s going the “wrong” way and this caused handling and vibe problems which were mainly rectified by redesign. (205A-1 and Mk9)
Rigga

[RotaryRat]

Thanks for the feedback chaps, that certainly makes more sense now

[froggy_pilot]

No tail rotor

The best is no tail rotor

Some (very) old pilots must remember the DJIN from Sud Aviation

A nice helicopter with a jet engine, they took some of the exhaust gazs through the mast to the blades tip

if somebody has a pics, please post them

[froggy_pilot]

no tail rotor

Sorry I did a small mistake, the DJINN was not built by Sud Aviation but by SNCASO, wich became later Sud Aviation
First flight in december 1953, they built 178 of this fantastic helicopter (100 for french army)
Check:http://www.simonb6.co.uk/2002-02/WSM-108CDL-Djinn.jpg and http://www.aviafrance.com/2157.htm
You will notice it has no MGB also, can't do more simple than that