A daft question about helicopters

[18-Wheeler]

I watched a documentary of some sort a couple of days ago about autogyros, with Ken Wallis, and I got to wondering if something like that could be used with regular turbine helicopters?

I mean in the unfortunate event of losing the tail rotor, if you could decouple the turbine engine and rev it right up to use the thrust from it to either prolong the glide or perhaps even have some level flight, to give you options as to where to land. The rotor would stay spun-up just like a regular autogyro, etc.

But I suspect there wouldn't be anywhere near enough thrust to get any useful forwards speed ... ?

[weave1376]

losing the tail rotor or losing power to the tail rotor are two different things... I suspect that you mean the latter.
To give my two cents worth; there is no significant thrust to speak of and "reving it up" is not an option.

Regards

[Farmer 1]

You seem to be under the impression that the tail rotor pushes the aircraft forward. It does not. The sole reason for having a tail rotor is to counteract the torque reaction between the main rotor blades and the rest of the aircraft.

The tail rotor takes its driving force from the engines, so losing it actually causes an instant increase in engine rpm, because it is now doing less work. Unfortunately, you have lost the device which is stopping the aircraft spinning in the opposite direction to the main rotor blades. What the pilot must do - instantly - is reduce engine power dramatically, perhaps completely.

Isaac Newton had something to say on the subject.

[18-Wheeler]

You seem to be under the impression that the tail rotor pushes the aircraft forward.

No, and that wasn't the question.



The sole reason for having a tail rotor is to counteract the torque reaction between the main rotor blades and the rest of the aircraft.

Yes I already know that.

[Farmer 1]

Then I'm missing the point of your question, 18-Wheeler.

Decouple the engine from what? It's already decoupled from the tail rotor, as you've lost it, which leaves the main rotor, and the only way to decouple from that is to enter autorotation, thereby cancelling all your thrust.

This is where gravity rears its ugly head.

[Tail-take-off]

18 wheeler

I understand what you are suggesting. The problem is turbine helicopter engines are designed drive the gearbox. The "jetblast" is usually directed towards an area where it will cause minimum problems to ground operations & not necessarily directly rearwards as you would need for propulsion.

You could of course design a gear box where you disconnect the drive from the main transmission & direct is to a big propellor on the back as with an autogyro. However if you are going to these lengths it would probably be better just to have a reseve tail rotor.

As with any design the risk of system failures is evaluated against cost & weight. Having a back up system for the tailrotor would be a huge penalty for a very rare failure.

[Farmer 1]

The penny drops, I think. Well done, TTO.

18-Wheeler,

No. The engines are normally working at over 90% in the cruise, and the forward thrust at that power setting is hardly worth diddly squat. If it were possible to decouple the engine from the main rotors, by the time you had reached a point where the engines were wrecked due to overspeeding, you'd still have only diddly squat and a bit of forward thrust.

By that time anyway, the overspeed protection would have hopefully done its job and shut down the engine.

[18-Wheeler]

Ta guys.
As I thought, there's nowhere near enough thrust to make any real difference.

[Cough]

Sorry to tap into this thread, but a quick related question...

What is more efficient in the cruise, heli or autogyro?

Cough (For whom spinnythings above your head is a mystery!)

[SNS3Guppy]

Efficient in terms of what?

[Graybeard]

Helicopter design has to be the most frustrating endeavor. Everything you try to do is counteracted by an opposing force.

The McDouglas/Boeing NOTAR light helicopter has been around quite awhile now. Don't know how successful it is. Rather than a tail rotor, it ports engine exhaust into the tail boom, where it is directed at 90 degrees to the boom. It might be on wiki.

Saw a video demo of a heli recently that has counterrotating rotors and a large prop on the tail to provide forward thrust. It looked good.

Boeing has bought a company that has developed a UAV heli. It has conventional rotors. It's called the A160.

GB

[Will Fraser]

Fly Chinook and moot the TR. There was a man who bult a Heli with an airfoil as a tail boom, it provided counter lift to balance Mr. Newton's Law.

[tartare]

A further daft question along this thread.
Can someone explain how the gas turbine in a light helicopter like an MD500 actually drives the main and tail rotors?
Is there a physical link via a reduction gearbox and a clutch that allows it to diconnect in the event of auto-rotation, or do the hot exhaust gases from the turbine act on a paddle type arrangement which is linked to the rotors, in the same way that some turboprops drive a propellor?

[Senior Pilot]

I may have misunderstood the questions here, but there are some fundamentals about helicopters that may have been missed.

Following a tail rotor failure, or TR drive failure, the helicopter can still be controlled under certain circumstances. If the airspeed is sufficient there will be a large proportion of the anti torque requirement provided by the vertical stabilisers, and the pilot will be able to fly to a running landing on a suitable surface.

Turbine thrust is quite slight: if it were not for the exhaust temperature, on most helicopters you could easily put your hand into the exhaust and hold it there, the flow is so low. Westland modified the Gems on their World Speed record holder (Lynx) to use some of the residual thrust, but from memory it was quite low.

"Free power turbines" are used in helicopters, whereby the air is effectively a fluid clutch between the power turbine and the free power turbine. As the airflow passes through the last power turbine stage, it then hits the free power turbine, which is mechanically separate to the compressor and is driven purely by the final stage of gas. This powers the engine gearbox, which has various outputs including a drive shaft to the main rotor gearbox.

In the event of a power failure, helicopters enter autorotation and the engines are disconnected via the freewheel unit in the drive train. The main and tail rotor gearboxes are mechanically connected, and the airflow through the main rotors keep the rotors turning. Autorotation is essentially energy management, where three types of stored energy (airspeed, height and rotor RPM) are managed to control the descent until the helicopter is flown to a suitable landing area, and a near zero speed touch down achieved by sacrificing the stored energy.

Simple, really ;)

[18-Wheeler]

Thanks for the excellent explanation, SP.