Helicopter speed limitations
Discussion
This is a new thread after this one got off topic
http://www.pistonheads.com/gassing/topic.asp?h=0&a...
Is there any reason why you couldn't employ a similar concept to planes swept wings but in the rotors to avoid some of the issues with them going supersonic?
http://www.pistonheads.com/gassing/topic.asp?h=0&a...
Is there any reason why you couldn't employ a similar concept to planes swept wings but in the rotors to avoid some of the issues with them going supersonic?
I assume you mean so that the rotor disc can turn faster thus delaying retreating blade stall? If so, it's likely just limitations of blade stiffness (as swept planes need to be significantly stiffer than straight planes due to their tendancy to wash out under load), the associated mass increase of the blades accordingly and thus the significantly higher stresses carried by the hub and associated increases in gyroscopic forces.
All that said, shorter swept blades will still suffer from retreating blade stall because it's the tip speeds you're worried about. The tip speeds need to be *really fast* compared to the forward speed of the helicopter so your limiting speeds are about the same.
All that said, shorter swept blades will still suffer from retreating blade stall because it's the tip speeds you're worried about. The tip speeds need to be *really fast* compared to the forward speed of the helicopter so your limiting speeds are about the same.
Edited by jamieduff1981 on Wednesday 30th April 09:16
To a degree they do, if the look at the rotor tips of a modern helicopter they're quite different than those of older slower ones in that they have funny shaped tips that sweep back to better cope with going supersonic, eg this from a Merlin
However drag goes up massively as you approach supersonic speeds. So you need more power to spin them, the loads on the blade are higher, and importantly these days, they make a lot more noise.
The retreating blade stall is the real speed limit though. If your advancing tips are supersonic (760mph) and you're doing 200mph, then the retreating tip is 360mph backwards, but other end of that blade is doing 200mph forwards, so 1/3 of your blade is going backwards.
However drag goes up massively as you approach supersonic speeds. So you need more power to spin them, the loads on the blade are higher, and importantly these days, they make a lot more noise.
The retreating blade stall is the real speed limit though. If your advancing tips are supersonic (760mph) and you're doing 200mph, then the retreating tip is 360mph backwards, but other end of that blade is doing 200mph forwards, so 1/3 of your blade is going backwards.
Edited by RizzoTheRat on Wednesday 30th April 09:30
Several different concepts there, each with their own issues.
Contra rotating blades mean you're not relying on lift from the retreating blades, but they are passing though any shock waves generated by the advancing supersonic blade. The hub assembly is complicated (no idea on reliability but I guess more susceptible to breakages, more expensive to maintain, and heavier) but all your power is going in to lift/thrust rather than wasting a load of it with a tail rotor, which has it's advantages. You can also get a spectacular performance. The Ka-50 only has something like 10 degrees traverse on it's gun because they can rotate the whole helicopter faster than an Apache can traverse it's gun, and it can fly sideways almost as fast as it can forwards (only limited by the increased drag of the fuselage). They also claim top speeds of production aircraft not that far off the Lynx's world record, and that Lynx was stripped out and had a few modifications.
Telescopic blades I'd have thought would be a complication too far, the stresses blade are enormous. There are a few gyrocopters with folding blades (The Butterfly LLC Skycycle is fantastic idea) but they rotate at much lower speeds.
Compound helicopters seem a much more sensible way forward for higher speeds, as you're only using the rotor disk for lift at high speeds, not thrust, you'll still have the retreating blade issue but there's a lot less load on the blades so it's less of a problem.
Contra rotating blades mean you're not relying on lift from the retreating blades, but they are passing though any shock waves generated by the advancing supersonic blade. The hub assembly is complicated (no idea on reliability but I guess more susceptible to breakages, more expensive to maintain, and heavier) but all your power is going in to lift/thrust rather than wasting a load of it with a tail rotor, which has it's advantages. You can also get a spectacular performance. The Ka-50 only has something like 10 degrees traverse on it's gun because they can rotate the whole helicopter faster than an Apache can traverse it's gun, and it can fly sideways almost as fast as it can forwards (only limited by the increased drag of the fuselage). They also claim top speeds of production aircraft not that far off the Lynx's world record, and that Lynx was stripped out and had a few modifications.
Telescopic blades I'd have thought would be a complication too far, the stresses blade are enormous. There are a few gyrocopters with folding blades (The Butterfly LLC Skycycle is fantastic idea) but they rotate at much lower speeds.
Compound helicopters seem a much more sensible way forward for higher speeds, as you're only using the rotor disk for lift at high speeds, not thrust, you'll still have the retreating blade issue but there's a lot less load on the blades so it's less of a problem.
I always thought the fictional Whispercraft from Arnie's The 6th Day would be a good idea if they could be made to work, at lower speeds, take off and landing they are lift rotors then as the speed increases they stop like a straight wing to provide lift then sweep back to form a wing. A bit like a swing wing fighter.
The problem with those kind of designs though is the hub is very complex as you have to articulate the blade in helicopter mode (higher angle of attack for the retreating blade than the advancing one, and vary the angle of attack front/back to gain thrust), and I guess they'd need to be symmetrical cross section blades as one of them would be backwards when using it as a wing. Plus as you say the transition would be a bit scary, most designs that have tried to do that kind of thing have had a separate wing, but of course if the wing is under your rotor you're getting a lot of downforce on it cancelling rotor lift.
FourWheelDrift said:
I always thought the fictional Whispercraft from Arnie's The 6th Day would be a good idea if they could be made to work, at lower speeds, take off and landing they are lift rotors then as the speed increases they stop like a straight wing to provide lift then sweep back to form a wing. A bit like a swing wing fighter.
The trouble with that is that aerofoil sections which are symmetrical about 50% mean average chord are not stable. One blade would need to work effectively going forwards and backwards.Forward swept wings need to be extremely stiff or they try to fold back on themselves, so even bring the blades to a halt in forward flight would be an extreme structural challenge.
NASA experimented with a symmetrical aerofoil wing/rotor aircraft a few years ago, it stopped the rotor in flight but also had small stubby wings.
http://x-plane.org/home/lemonade/suggestion/rotwin...
http://en.wikipedia.org/wiki/Sikorsky_S-72
http://x-plane.org/home/lemonade/suggestion/rotwin...
http://en.wikipedia.org/wiki/Sikorsky_S-72
Sikorsky also played with the S-69 Advancing Blade Concept with contrarotating blades, stub wings and compound configuration. It was abandoned around 1981 due to stress and vibration before they could experiment with slowing the rotor speed. It achieved just shy of 240kts in level flight IIRC at normal rotor speed though.
I think for VTOL and speed the answer is really going to be a tiltrotor. I know the V-22 had it's problems but all these concepts of doing funny things with composite helicopter rotors once in flight have even more problems, and carrying separate lift engines seems inefficient.
Darpa's got a VTOL X-plane competition running to try and get something that'll do 350+mph and lift a 2 tonne payload.
http://www.darpa.mil/Our_Work/TTO/Programs/Vertica...
Darpa's got a VTOL X-plane competition running to try and get something that'll do 350+mph and lift a 2 tonne payload.
http://www.darpa.mil/Our_Work/TTO/Programs/Vertica...
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