Helicopters with turbines

[salm]

Hi,
I was wondering why there weren´t any helicopters with turbines.
I googled a while and found helicopters that use turbines instead of the heck rotor. I found a couple of others that use a main rotor and a heck rotor but have an additional turbine that enhances their speed.
However there aren´t any that use a turbine like a jet fighter, as the main rotor.
Why is this so?

[AMX]

Hi,
I was wondering why there weren´t any helicopters with turbines.

Actually, most modern helicopters are powered by turbines, rather than piston engines.

I googled a while and found helicopters that use turbines instead of the heck rotor.

The English term for that is "tail rotor".

I found a couple of others that use a main rotor and a heck rotor but have an additional turbine that enhances their speed.

I'm reasonably sure that you mixed something up there.

However there aren´t any that use a turbine like a jet fighter, as the main rotor.
Why is this so?

Ah, you're looking for helicopters using jet turbines.
There aren't any because using jet engines to produce lift ("liftjets") is horribly inefficient. (Also, if you replace the main rotor with something else, it's no longer a helicopter. Try reading up on VTOL aircraft for something somewhat close to what you seem to be looking for.))

[Old Plympto]

Also, using a turbine engine to generate forward thrust faster than a certain speed will cause reverse airflow over the side of the rotor disc where the blade moves from front to back would lose lift, in which case a conventional helicopter would flip over and Considerably Awful Things would happen to it.

Turbine engines on rotorcraft are always turboshaft engines (as opposed to turbojets) and the exhaust is dumped to the side or upwards never creating any forward thrust.

[salm]

Ah, yes, i meant jet turbines. I thought turbines were allways jet turbines. Thanks for clearing that up.
I allmost thought that it would be the inefficiency. Why actually is it so inefficient?

And yeah, thanks for the VTOL keyword. That was pretty much what i was looking for.

[Feil]

I'm not up-and-up on aircraft, but I'd imagine it's the same reason wings are efficient. For a little energy, you can maintain the momentum of the rotor, which will generate lift; if the altitude and velocity of your aircraft never changes, the only energy you have to put in is enough to counter drag and friction, because you're taking advantage of air pressure and not actually doing any work. If you use a jet or a rocket or any other reaction drive, even if your aircraft doesn't move, you still have to move all the stuff you're throwing dowards at KE=.5mv^2=.5v∫Fdt by Newton's second law. Hence the fuel required for a rotor is Loss to Friction+ΔU+ΔKE, while the fuel required for a turbofan is Loss to Friction+ΔU+ΔKE+(.5v_exaust_∫F_gravity_dt)

[Old Plympto]

I googled a while and found helicopters that use turbines instead of the heck rotor.

OK if I understand that heck = tail, I would guess that you are talking about the NOTAR where some of the turbine engine exhaust is bled off to be used as thrust against the torque on the helicopter, instead of a conventional tail rotor. It's basically doing the same thing as a tail rotor.

[Beowulf]

Actually, NOTAR still dumps the turbine exhaust (semi-)uselessly into the air. It use a separate shaft driven fan to create the counter torque.

The reason why jet engines aren't used to directly create lift is because it takes alot more energy to make a small amount of air go fast downward than a large amount go slow downward.

It's possible to have an additional method to create forward thrust (turboprop), and it is a bit more efficient than a normal helicopter. These are called gyrodynes.

[Frank Hipper]

A slightly off-topic note of trivia and point of braggartry:

I did a little work on the very first NOTAR prototype at Mcdonnel-Douglas Helicopter in 1989.

[Sea Skimmer]

Ah, yes, i meant jet turbines. I thought turbines were allways jet turbines. Thanks for clearing that up.
I allmost thought that it would be the inefficiency. Why actually is it so inefficient?

Why actually is it so inefficient?

It isn’t inefficient at all. Gas turbines are only inefficient when they are running at idle; because they use nearly as much fuel at idle as they do to at full power. That makes them a poor choice (in terms of fuel economy) for something that must often stop and start like a tank or a locomotive.

When a gas turbine is running at its design power, it is more efficient then any other kind of combustion engine. Aircraft engines pretty much always run at one power setting, so in this role gas turbines are highly efficient. Turboshafts and turboprops driving rotors or propellers are both more efficient then turbojets or turbofans which use jet thrust, but they have other disadvantages.

A combined cycle gas turbine, commonly used at electrical power stations, can exceed 60% efficiency, while in comparison no diesel engine can get past about 50% efficiency and spark ignition gasoline engines are stuck at around 35%.

If you want more speed out of a helicopter, then you can either add engines providing jet thrust and lock the rotor in place, as was done on the Sikorsky S-72 ‘X-Wing’ or you can just add stub wings and a push propeller on the tail to make a compound helicopter as in the case of the AH-56 Cheyenne and Piasecki X-49A Speedhawk.

[Kanastrous]

I think the Lockheed XH-51, in compound-helicopter flight mode, used its jet engine for forward thrust...

[Old Plympto]

A slightly off-topic note of trivia and point of braggartry:

I did a little work on the very first NOTAR prototype at Mcdonnel-Douglas Helicopter in 1989.

I know quite a few people who would be happy to sit down with you and talk shop.