Tail Rotor

[Port Strobe]

I know there's an extremely high probability I'm asking a stupid question here (don't know much about whirly birds) but if, when viewed from above with the tail at the bottom of the picture, the main rotor is turning anti-clockwise, then the tail rotor MUST be mounted on the starboard side?? If I've got my reasoning correct a counter-clockwise spinning main rotor would make the fuselage rotate clockwise if it weren't for the tail rotor there to stop it, hence the tail rotor must provide a thrust to produce a counter-clockwise moment, which would be to the right in this situation wouldn't it?

Thanks very much for your time, much appreciated.

[PPRUNE FAN#1]

You got it 99% correct, Port. The primary reason for a tail rotor is to counteract the torque of the main rotor. But a tail rotor is merely a variable pitch propellor and thus can generate thrust in either direction depending on how you set the pitch. Propellors can pull ("tractor" props like on most small aeroplanes), but they can push like a fan as well (Beech Starship, Piaggio Avanti, Cessna 337 twin). So too a tail rotor can either be a tractor or a pusher. In reality the tail rotor can be mounted on either side of the tail boom as long as it "blows" the correct way.

[BlenderPilot]

Examples of helicopters (that I know of) that have had their TR's switched from one side to the other are . . . .

Mi8, Mi17
Bell UH-1, Bell 205

Also some helicopters have changed their direction of rotation during production.

And ?

[[email protected]]

As with all things in life - it is a compromise, a puller TR gets clear airflow which is good but blows out against the tail boom/pylon which is bad (like standing on your own shoelaces and jumping) a pusher TR gets disturbed air around the tail boom/pylon but blows into clear air which is good. I am sure that St Nick (Lappos that is) has a more detailed explanation and comparison of the 2 but suffice to say that pushers outnumber pullers by quite a margin.

[bugdevheli]

Any reason apart from cost and weight for not having a smaller rotor each side of the boom. Its Christmas and ive had a drink, so maybe tomorrow this will seem like a bl***dy silly question.

[sycamore]

Bug, difficult to engineer, and the airflow problem would probably be a lot worse. Far better to have a t/r in clear air, and not mixing it around a fin---a la Sycamore.......Sorry haven`t got a piccie for you youngsters!

Crab- out of interest ,tail nos. of your cabs?

[NickLappos]

The ideal tail rotor:

Location - Close to the roll centerline of the aircraft so that the normal thrust changes that stabilize the yaw axis in forward flight do not produce any roll BUT the rotor must be protected from ground contact during the landing flare

Rotation - bottom blade forward, so that the blade moves into the natural outflow from the rotor in low hovers, thus gaining some speed and effectiveness

Distance from fin - about 1/3 diameter or more so that the fin does not block flow

Direction of thrust - toward fin (pusher prop) so that the high velocity downwash is not directed against the fin, making the rotor more efficient

Thrust - large enough to completely overcome main rotor torque, so that typical small variations in thrust caused by main rotor wash entry do not cause loss of control (mislabeled LTE)

[Ascend Charlie]

The Frogs (and Bell and Sikorsky for a brief moment) try to eliminate the problem of sucking air past the pylon or pushing air against the pylon by putting the tail rotor in a hole in the fin to get clear air both sides.

Eliminated that problem, but misses out on some of the good effects of forward flow and being in the rotor downwash.

Everything is a compromise.

[NickLappos]

Charlie,
The smaller fan inside that duct uses quite a bit more power than a larger tail rotor, and adds significant empty weight to the aircraft. It takes lots of careful engineering to get the fan-in-fin design to equal a tail rotor in efficiency. In larger than 5Ton machines, it is so significant a penalty that it becomes impractical.

[[email protected]]

Sycamore - our Mk3A Sea Kings are ZH540 to 545 - yes we do only have six and they make the remainder of the SAR forces Mk 3 s look like steam driven heaps of sh*t. Eventually we might get them all up to a common standard, probably just before they go out of service next decade.

[Port Strobe]

Thanks very much for the replies, 99% is good enough for me Fan#1! I understand what you're saying, I should've thought of that possibility about pushers/pullers. Thanks again to all that answered.