h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

While there are no doubt some great advantages to coaxial contra-rotating main rotors, I'm intrigued to see a claim that attack helicoptors exist capable of reaching 600mph in forward flight. Well, other than being shipped inside a cargo plane

The only attack helicopters with coaxial rotors I know of are the Kamov KA-50 & KA-52, the former of which is reckoned to have a dive speed of maybe 390km/h. I'm seeing a figure of 249mph quoted for fastest helicoptor: is this is the Westland Lynx with the modified rotor tips? Is that in level flight? I've also got a slightly lower figure for the never-production Lockheed Cheyenne (which had a push-prop on the tail).

I was under the impression that the limiting factor was the forward-sweeping rotor tips exceeding mach 1: is this not the case?

Disclaimer: I have never flown a real helicoptor (in fact, I've only flown _in_ a helicoptor twice), but I've crashed plenty of virtual ones.

A russian boy who lived with our family for two months half a year ago once showed me a picture of a russian attackhelicopter (in the air) with two rotors that was built at his hometown. 1000 km/h is a high speed, faster than for intance stealthbombers, but I belive it's possible with the twinrotordesign.

Yes but why do you believe it to be possible? If current production twin rotor designs yield similar top speeds to single rotor designs, then what changes gives you the leap to over 2.5 times that speed?

That american twinrotor helicopters does not go faster than ordinary choppers does not have to mean that the russian's does not. I don't see any physical limit under the speed of sound.
In general they seem to go for high performance rather than price/performance ratio. Just look at their fighter airplanes that is not far from mach 3. That's alot faster than any fighter here in the western world. Try to hunt down one of those with a lowsignature airplane that won't break the soundbarrier. But then they come along with a pricetag...

Sorry, maybe I should have said: the Ka-50 & Ka-52 are built by Kamov, a Russian company who specialize in coaxial rotor designs.

If I might summarize your argument, it seems to be "if you pay more money you can build something faster". This is a gross oversimplification of the situation: building a mach 3 aircraft requires significant additional design considerations to those of building a mach 2 aircraft. You don't just plug in a bigger engine and it goes faster (at least not _that_ much faster).

Building helicopter that can fly 2.5 times as fast as the fastest currently noted in the record books (250mph = 400km/h) requires many new factors to be taken into account. These will result in radical changes in design and mechanism. I want to know what possible changes could be made to achieve those ends. Without any indications of those (let alone plausible ones) why should that part of the article go unchallenged?

Finally, I know it's petty and rather off the point, but a phrase like "Try to hunt down one of those with a lowsignature airplane that won't break the soundbarrier." seem to indicate that you have little idea of what an F-117 is for. Hint: they don't carry air-to-air weapons and they don't have a radar.

suppose your right

That F is confusing though. But when you say it, it certainly makes sense to assume they are for bombing considering their specs. But why did they make it an f instead of a b?

Check this link out for reasons why a helicopter can not go that fast http://www.helis.com/howflies/maxspeed.htm

Spoke with a friend last night who flies Pumas in the Royal Air Force. On a helicopter with contra-rotating rotors the rotors will still stall in the same way as on a single rotor helicopter on the backwards motion of the rotor. Two rotors rotating in opposite directions still have to obey the aerodynamic laws. Each rotor is independant so far as the rules of aerodynamics are concerned!!

Actually, the limiting factor to forward speed is called "retreating blade stall".

Although the results of one blade reaching Mach 1 will definitely have a very limiting effect on the flight, they are usually not mentioned as the main cause for the relatively slow top speeds of todays helicopters.

"Retreating blade stall" occurs when the blade moving with the relative wind, caused by the forward motion of the helicopter, reaches a condition where the lift production across the blade practically ceases. There are actually different areas across the blade with different lift production but let's just make it easy and say the blade moving backwards stops producing lift.

This will result in a very violent reaction of the helicopter, because one half of the rotor disk (an imaginary disk created by the rotating rotor blades) produces a lot of lift, here the relative wind adds to the speed of the rotor blade, and the retreating blade doesn't create any lift at all. The helicopter will then pitch up and roll to the left.

A situation I would rather aviod. Usually an increase in vibrations will alert the pilot of the increasing stall condition and therefore measures can be taken not to get into it.

I hope this helps a bit.

Lightning

and having two contra-rotating rotors will make no difference as each rotor will still experience retreating blade stall.

Fatman
ex Air Cadet of 9 years.

Absolutely. Might even lead to more problems since this condition also results in extreme blade flapping, and with counter-rotating disks... You'd probably have a mid-air with yourself if the one blade flapping down hits the one blade flapping up.

Lightning

Hi, its a very interesting conversation that you have. what i was wondering was that two superimposed counter-rotating disks will produce an equivalent disk of uniform lift over the chopper. The only problem will be that there will be immense stress produced on the central vertical shaft for the two main rotors. There is no reason to belive that the half disk generating a lift effect will be worse for two disks.

Secondly, a flap up and flap down collision of blades is prevented simply having the blades of the rotor evenly spaced out on the two rotors such that no two blades are one on top of the other.

However it seems more likely that super sonic flight is difficult because of the shock wave from the forward movement of the chopper, i can only guess that materials that can handle the pressures produced by a mach of forward speed, plus the speed of the tip of the rotor will be very rare.

Even if you evenly space out the rotorblades, if they are counterrotating there will be points in space where one blade moves over the other.

The next problem is that with blades breaking the sound barrier you have part of the blade below the speed of sound (since the relative speed near the mast is comparatively slow) and a portion of the blade above. I don't think there's a material available to handle that kind of stress for a longer period of time.

And aerodynamics differ quite a bit on the opposite sides of the sound barrier so the reaction to control inputs would be unpredictable at least, if not uncontrollable.

Lightning.

Sorry, very silly of me to not see the problem of collision of blades in a twin main rotor helicopter. What i just wanted to point out is that i have seen on the idiot box certain machines that use a V shape to support two rotor shafts. The two rotors move in tilted discs and are so designed that the blades from the two rotors can cross each other. That is what i had in mind when i talked of rotor blades to avoid collision by flapping up/down. I can see that i was totally unclear in my writing.

Totally agree with your view on the stresses on the blades. However, i think that jet turbines, with their extremely high rotation angular velocities may offer materials that work on blades that are partially beyond mach 1. This is just a conjecture, i really have no background on aerospace engineering.

Ok, I know the design you're talking about. They won't help us much since they have to rotate in the same direction since this is the only way to have them move without colliding.

I'm not sure that the materials used in turbine engines are suitable for a rotorblade. It is true that they are very stress resistant and can cope with extremely high temperatures and all but they have a big disadvantage: no flexibility whatsoever.

The aerodynamic forces on a rotorblade force it to bend up and down in addition to the different ways of attaching it to the rotor mast that already provide a possibility of movement up and down and also on some systems fore and aft. A completely rigid material would probably break before bending. Also the price would be impossibly high.

Lightning

Its not just the rotor blades that need to be able to flex - many modern helos use a flexible metal joint at the hub to get round the mechanical complexities of articulated hubs.