Q for the mech minded

[Mr John]

Helicopters - awesome machines.

From my understanding, & I'm quite certain I'll be corrected if I'm wrong, helicopter's main rotors are powered by a gas turbine - basically hot gas is squeezed past some blades to spin a shaft which is mechanically connected to the mast which spins the rotors which pushes air down with enough force to lift the whole contraption and off it flies.

It seems likely to me that a percentage of the engine's energy is always going to be lost in the conversion through the turbine, gearing, bearings, blades etc.

Would it be less efficient to generate lift straight from the hot gassy bit?
Ducted down, Harrier jump jet style, through a number (um...say 6) of 'thrusters' would surely require less energy to produce the same lift. Might even require less lift due to not having to carry rotors & tail booms around too.

Any thoughts, besides get a fucking life.

Mr John

[Crashlington]

it would actually. jets of hot gasses when used to lift are highly inefficient. plus it would be extremely unstable.

[Barfridge]

I think (could be wrong) what helps a chopper is the length of the blades. Out at the tips they are moving very quickly, which is where most of the lift is generated.

I know harriers and other similar aircraft are very inefficient when hovering, and chew through obscene amounts of fuel to do so. Most of the problem lies in directing the thrust through narrow openings, there's too much back pressure for the engines etc etc

That's my random uninformed ramblings, anyone know the truth?

[likefry]

hmm doesn't really work like that.

in order to lift the craft off the ground it would need a certain amount of thrust out
say 10,000 kilograms of thrust.

to do this requires alot of energy and a big engine.

helicopter blades work like wings on a plane they provide lift by moving through the air fast.

the work required to move the blades through the air is less than the work required to provide thrust to lift the plane off the ground.

Plus it would burn the shit out of everything underneath it kind of screwing up the landing pad

[Klink]

Harrier's are terribly inefficient energy wise. They can only sit in hover for around 3mins (from memory). I think it was something to do with not enough cooling from airflow. They actually cool the engines with flowing water in the hover and they only have a certain amount of cooling that the water can do.

[thro]

What fry said.

However, there's a chopper (V22 osprey) that kinda works like you describe (avoiding the direction changes between engine and blades) - it mounts the entire engine+turbine-driven props in pods on the two wings, and tilts them.

So, in forward flight it can be like a conventional aircraft, for increased speed, etc... tilt rotors back up and it can hovver or fly like a chopper.

edit:

[xphread]

If you spread the lift over an area (to combat the instability issue as mentioned above - eg rocket style) you have the loss of distributing the gasses out and round corners/bends etc in the piping. Which would negate any saving you were after in transmission loss etc

I am fairly sure the efficiency of a jet turbine would be best in a single wide outlet rather than a distributed one so that would require multiple engines....
and failure of only one of them would be critical to stability etc...

I dont think Harriers and similar vtol methods would currently be classed as efficient!

plus there is the safety issue - I'd much rather be in a chopper with engine failure than a vertical jet propulsion system!

(Ooops - beaten by lost of other posts while I was distracted!)

[Drach]

Harriers are obscenely inefficient when hovering. Imagine getting a SCUBA tank, and letting it go, and try and have it lift off. It's damn near impossible to keep it going straight up, and you'd mostly likely be just too heavy. Liquids tend to be better at lift as they tend to be nearly incompressible, so thus retain pressure. When you blow out a gas, it compresses due to the pressure that is forcing it out, thus absorbing energy when you blow it out, a bit like suspension. I think gas turbine engines run under a huge pressure to keep the energy.

[thro]

heh. i'll also add - the V-22 isn't in service yet. During development, its had a pretty abysmal safety record - the avionics to handle flight transition such as it does are pretty hard core and "new"...

[Barfridge]

Isn't the Osprey known for killing lots of people? From memory the transition from flying to hovering is quite difficult, so landings are tricky

[xphread]

Yeah, well the saying in general aviation is
"Helicopters don't fly. ... They are just so ugly the earth repels them."

Maybe the Osprey is just that little bit nicer looking?

[thro]

explains why the a-10 is so resilient to gunfire, too

[delete_me_kthxbai]

the MV-22 Ospreys are in service already.
They have been for a couple of years now.
Just not that great a number.

Fkn awesome aircraft.

[Flyboy]

*dons aviation nerd glasses*

Okay....

Harriers are disgustingly inefficient due to the way they employ their propulsion system. They use the Pegasus engine which is a low bypass turbofan (of sorts). It has four thrust vectoring nozzles, two from the bypass fan, two from the 'hot' section. These nozzles direct the airflow from the engine for VTOL (vertical take off & landing). The Pegasus F402-RR-406 produced about 20,280 lbs of thrust and the Pegasus F402-RR-408A produced about 22,200 lbs of thrust.

Now a Harrier weighs 12,190 lbs empty (so add more for fuel, armament etc). So, to theoretically keep the Harrier hovering we need the engine to produce 12,190 lbs of down thrust which is already over 50% of the engines total capability.
I can't be assed working out the theoretical rpm/fuel/temp mathematics to get just over 50% total engine performance but I can tell you that the eninge will need to be spinning in the order of about 90% max rpm to create substantial useable thrust. At 90% rpm (not taking into account the effect of the fuel computer and required torque values) that little turbofan is sucking down about 1 litre evey second (equivalent to a Rolls Royce Trent engine at full power).
You're also losing a lot of energy to heat trying to squeeze bulk airflow through a small hole (the thrust vectoring nozzle).

A Chinook weighs 26,918 lbs and produces 4,867 shaft horsepower from each of its two T55-L-714 turboshaft engines. Again I can't be assed working out the transmission losses, torque requirements etc but the engines will be running at about 85% of max rpm once the blades are up to speed and consuming about 1 litre every 3 secs (if I remember rightly which I probably don't ) The large blade area effectivly converts this rotor head rpm into useable lift (to the order of about 50,000lbs).

In short its high velocity, low area airflow vs large area, slow velocity airflow.

Theres a lot more to it but I can't be bothered talking about high and low speed aerodynamics and fluid motion in relation to turbines and rotor blades etc. If you wanna know more, PM me.

So although theoretically its possible to be more efficent using pure jet thrust we don't have the technology to make it so. Helicopters win this time but when technology catches up, pure turbine will be the way of the future.

*removes nerd glasses*

References:
The Jet Engine by Rolls Royce

[Nero Diablo]

As said more efficent to use blades think of the rotors and a circular wing rather than two-seven individual blades.
Water cooling whilst hovering is a new one water is [email protected]#$ing heavy and to lug it around so you can hover is highly unlikely. They may hover have a water injection system that allows contingency power (WWII they called it emergency war power) for a couple of minutes then the turbine needs rebuilding. High Humidity greatly increases the perfomance of a turbine athough thin hot air kills it off as well.

Whislt you logic is probally corret the plantery gear setup to step the turbines speed of 25 000rpm plus down to probally under 1000rpm for the rotor do consume a bit of power. Earlier machines used pistion engines but turbines are usally lighter and produce more power espcailly in constant rpm applications. There is the other issue of directing that power so controlable flight can be achived. When Sikorski was working on some of the early proto types one overseers comment the only time he saw a helicopter in flight it was flying backwards. Sikorski comment this was a minor problem and he should sort it out before any production started.

Here's something to mull over to control the direction of flight forwards, backwards left right etc etc the helicopters rotor disk is tilted. But due to the gyroscopic effect the force has to be applied 90 degrees before the reaction (this is sorted out inthe controls between the pilot and the rotor disk). So when the pilot pushes forward on the stick/cyclic control to fly forward the rotor disk actually tilts left or right depending on the manufactuere and whaich way the main rotor rotates.

Helicopters You apply massive amouts of power they then defy all known laws of physics and lift off where by rights they shoud screw themselves into the ground.