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June 01, 2007

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Big D

I really like the crane--despite being 75 knots slower, the 40 ton limit handles the weight bloat of FCS and allows the Marine version (which ISTR is 35 tons for the heavy vehicles) to be carried as well. Plus, it can accomodate the entire crew and/or some supplies internally. I'm questioning whether it's true ABC, or just a standard coax, with that speed and rotor clearance, though.

Two big questions... how far, and how much?

Also, a really stupid question... is there anything particularly stupid with the idea of large, hollow composite blades that channel jet exhaust directly through them and skip the transmission? It seems like that would solve a few things.

Big D

Yeah, stupid moment... they're both 20t max, not 40t.

Bill Sweetman

Particularly the "skip the transmission" bit...
I've never seen this really analyzed, but it seems to me that one challenge is matching the necessary airflow to a low-drag rotor, without hot and heavy ducting. Hence the Fairey Rotodyne (with burners in the tips) and the Hiller proposals for super-giant helicopters with turbojets in rotor-tip pods. But Groen Brothers says that their Heliplane concept (which IIRC does not use tip burners) is scalable.
And I think you're right - that crane is a coaxial with a normal articulated rotor.

Big D

Well, what if you cheat?

Use a turbofan instead of a turbine, and duct the mixed air (or even just the bypass air) into the rotor head. That should reduce the heat the rotors have to take.

Then, you could actually turn it into a jet-powered gyroplane once you got up to speed, just by redirecting the thrust back where it's supposed to go and letting the rotors spin freely. I don't know drag calculations, but wouldn't good modern turbofans offset a high-lift/high-drag composite rotor to some extent?

The end result might look ugly--think of a cross between the JHL-HSL design above and a AN-72--but at least without any hard numbers to back it up, it sounds neat.

Outlaw13

The attack helicopter of the "future" in the upper right hand corner of the picture looks suspiciously like a Lockheed AH-56. Great Scott Marty!

Bill Sweetman

The problem is speed. Even with the rotor operating in minimal-lift autorotation (like a Rotodyne flying at cruise speed) the tip of your advancing blade is going to be moving pretty fast. This means the blade has to have a reasonably low thickness/chord ratio, which makes it very hard to stuff a lot of airflow through it. The Rotodyne got around this by pumping high pressure air to the tips and then mixing it with fuel, but it was noisy. The actual nature of the Groen Heliplane's tipjet system has not been disclosed, but the blades don't look big enough to accommodate the mass flow of two FJ44s or even FJ33s.
CarterCopter tries to cheat by lowering the RPM to the absolute minimum necessary to stop the rotor flapping to bits. But that's hard too.
By the way, I actually saw a Rotodyne. And I built the Aurora kit of the AH-56...

Big D

So, if you use a thick blade, even if it is optimized for lift, the drag at high speed is just too much to be compensated for by raw thrust (aft, not through the rotors) from the engines? Any chance that stub wings could help offset that drag, or would they be insufficient and/or too big a penalty in hover?

Clearly, I don't know the math involved. :)

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