Forum: Rotary Wing Aircraft

I've never heard of retreating blade pitch issues. What I have heard of is "retreating blade stalls". In forward flight, there is something called dissymmetry of lift. It's where the advancing blade produces more lift by virtue of its TAS than the retreating blade. If you get going too fast the advancing blade with have so much more lift you'll go into an uncommanded roll.

"The eggbeater, only with the rotors separated out much further, might do it. Minimal camber. Counter rotating each rotor. Amazing complexity. Exotic materials...."
This has already been done in the CH-47 and CH-46. Both aircraft use counter-rotating, inter-meshing rotor systems to defeat retreating blade stall. The CH-47 is the fastest helicopter the US military operates.

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Nice pics and cool concepts here.

The advancing blade is almost just as big of a problem as the retreating blade stall.
Most rotor blade tips are built to withstand the stress of local supersonic speed when the rotor is in the advancing phase of rotation, however as you keep increasing the speeds the shock waves advance from the tip of the rotor (where you can easily make it very strong w/out being too heavy) down the blade to nearer the middle span where there aren't many easy solutions to keep the blade strong enough and light enough to allow a light transmission/powerplant. 'Weird' harmonics start to take place along the length of the blade span that defy standard remedies and greatly shorten the lifespan of the materials both of the blade, the rotor bearings and pitch control arm joints. I am not privy to the real reasons the Army canceled the Comanche, but I strongly suspect that what played a role was that the materials when subjected to high speed strain were not passing longevity testing.

The dual rotor is part of the solution as it is shortening the blades and placing the high speed bad effects on both sides of the aircraft instead of on a single side. NOTE: This does not actually solve the problems, just masks the effects of those problems on your overall stability.

I would think that if you could find some way to both slow, AND retract the main rotor blades themselves (think of telescoping lenses) as you increase forward speed and transition lift to other fixed wing surfaces you would be better off pushing back (not eliminating) those evil demons that wait beyond 300mph.
That is a complex solution but hey --- if it were easy it would have already been done by now.

DARPA has a disc-rotor concept that does that:
http://www.youtube.com/watch?v=pg6LuwyNIxk

count_to_10 wrote:

DARPA has a disc-rotor concept that does that:
http://www.youtube.com/watch?v=pg6LuwyNIxk

Well shucks! Embarassed

If it were me developing that craft (which it ISNT), I would make a few changes to the design that my tiny brain sees as reducing unneeded risk.

Why not bring in those wing engines/fans closer to the airframe? It could crowd side door ingress/egress abit but you reduce the main rotor dual drive shaft complexity/length. You also greatly reduce the engine failure recovery problems. I can't tell if the engines are located above the airframe or inside the fans. Anyway. Put the loading area in the back. This is just a test craft anyway right?

Can the wing fans be disabled when in hover mode?

Why not use a conventional tail rotor and boom? Well worn path to follow for helo flighht control development.

That retracting blade disk is actually less complex than what I was thinking. Well played Boeing!

I think it is still a computer model as opposed to a physical prototype. It has to have some way of pushing itself forward, and also needs a way to counter torque.

Anyone know how fast the X-49 Speedhawk has gotten up to?

I saw a video that claimed 165 knots.

Just about every helicopter in the modern US military arsenal can reach that speed.

count_to_10 wrote:

There was some issue about the V-22 not being able to use a side winch, ruling it out for SAR.

This is old, I admit, but V-22 winches from the belly or the tail. It does have a pretty potent downwash, true. But then, USAF picked the H-47(!) to be their CSAR bird, until that program collapsed.

Something like the subsequently announced V-280 would be a better fit. Also, unlike other high speed concepts, there's no prop spinning close to the ground for someone to walk into. always a big concern on SAR and EMS missions.

Forum: Rotary Wing Aircraft

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