Helicopter forward propulsion only from rotor?

dohouch

there was a big family discussion about this.

One side claiming that choppers which have jets/turbines get some of their forward drive from the thrust of this power unit. The other side claiming that this turbine played no part in providing forward thrust.

Also the fact that rotors have a physical speed limit beyond which they no longer function , (cavitation ) would mean that the one can not move a chopper forward faster just by increasing the speed of the rotor.

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I'm no expert on the subject but from what ive noticed from looking at various choppers is the turbine exhaust is angled to the side as it exits the engine and so would not provide forward thrust.

I think the pitch angle of the blades is more a factor with speed than just the rotation speed of the blades.

But what do I know....:)

*Kol*

I agree. It is possible that there are helicopters that have the capability to derive some forward thrust from their turbine engines. However those turbine engines are quite small and wouldn't produce much thrust anyway.

robinph

Well the main thrust direction it has is in the vertical, in relation to the heli when its on the ground, to get over that annoying gravity thing. I thought it was just about angling the blades so that they point more forward and you'll have that power used in more of the horizontal direction instead ... aproximately

Deacon Blues

In the majority (I don't know if any which have any forward thrust from the engine, but there may be some, so I won't say there are none) of helicopters, there is minimal forward thrust from the engine. Sort of similar to the amount of thrust from your car exhaust ... it's there and measureable, but is of no significant use. They can be angled to the side which looks like it's to assist with anti torqueing, but the main reason is to prevent exhaust gasses discolouring the paintwork and causing corrosion on the boom.

The forward speed of a helicopter is limited by the speed of the blades moving through the air. You can look at it in simple terms and use rounded numbers for illustration only. Take a two bladed helicopter ... Bell Jetranger. The tips of the blade are travelling at 100 mph. The helicopter is travelling forward at 100mph. Now look at the airspeed of the blade tips. when the tip of blade 1 is fully aft, on top of the boom, it's moving forward (in an angular direction) at 100mph. As it starts to arc around to the side, it's airspeed increases to a peak of 200mph (adding the forward speed of the craft), and the reduces back to 100mph as it goes fully forward. As it continues it's rotation, the tip airspeed then drops to 0mph to the other side. In fact that bladed would have stalled long before this situation. These are convenient, not real numbers, but illustrate that the lift generated by blades is unbalanced due to the airspeed of the craft. The blades cannot be just speeded up, as the tips will go supersonic and introduce noise and shockwaves.

There were experimental helicopters which used thrust from seperate jet engines (usually mounted on stub wings) for thrust, but to achieve high(er) speed, the main rotor speed was reduced, and the lift came from the stub wings, again because of the problem with the retreating blade stalling. Another "solution" is the ABC, advancing blade concept, where you have two contra-rotating blade discs, so you will have an advancing blade on both sides generating equal lift. The limit to airspeed here is solely the tip speed.

High speed helicopters such as the Lynx use very complicated tip design to reduce the shockwaves generated when they go supersonic, but they are still limited to relatively lower speeds than jet aircraft.

Fishtits

When mounted on an regular jet, the output of the engines provide the thrust (push) that moves it through the air.

On a helicopter, the output of the engine(s) is used to turn the rotor.

Energy in = energy out, (ignoring efficiency)

A helicopter uses its engine energy to turn its rotors, hence sod all energy out as thrust - or exhaust thrust in other words.

This is a really simple explanation.