Carter Copter Breaks Mu-1 Barrier

tyler_larson writes "Just over a week ago, Jay Carter's CarterCopter managed to break a significant rotorcraft barrier, traveling at a mu ratio of 1. This 1-to-1 ratio (sometimes called the mu-1 barrier) represents a condition where the forward speed of the craft is the same as the speed of the tip of the rotor. This means that at a certain point, the tip of the retreating blade is "standing still" relative to the wind and producing no lift, while the rest of the blade is actually moving backwards through the air. Such a condition is normally impossible for a rotorcraft, and so the forward speed of a helicopter is limited by the the speed of the rotors. This accomplishment by the CarterCopter, which some insisted couldn't be done, proves that this new craft is not subject to that limitation."

Comments

Heli-plane?

[Anonymous Coward]

Hmmm, it looks lie it blurs the line between a helicopter and an airplane. That thing has pretty darn large wings. I guess the big deal is really having the rotors not "get in the way" traveling at that speed, since the wings really are providing most if not all the lift? What happened to those experimental copters that you could actually just shut down the rotors and have them be fixed during forward flight?

mu and swimmers

[BigMike]

I was actually surprised to hear about the mu limitation concept. Swimmers for examle might actually swim faster than their handspeed through the water - an efficient swimmer miht actually take his and out of the water at a point AHEAD of where the same hand entered the water.

Re:mu and swimmers

[Fringex]

Swimmer here.

This doesn't really apply as one poaster pointed out that simply gliding through the water will allow you to have your hand exit where it entered. Infact you can have it exit beyond where it entered with simple glide. The problem is you slow down.

Hard to say if it the slow down is avoidable but I am sure through some testing you can find out. Depends what application you are putting it in. Sprinting will never be the case since that is a mroe chaotic means of racing but distance swimmers might have more luck with this.

As for kicking it does supply power, more than you would like to think. If kicking supplied next to no power you wouldn't see swimmers kicking as often as they do. A slight kick keeps your feet aloft. Rapid kicking actgually gives you fair forward propulsion.

The best example I can give you is to try and find the video of Jeff Rouse in the 1996 Olympics. At the flip turn his dolphin kick alone allowed him to increase is lead by over a body length and a half if I remember correctly. He was the last one to surface but increased his lead massively.

Kicking alone can be impressive especially underwater. In texas they had a rule that you had to surface before the second set of flags. I watched a couple guys from a competing team nearly get DQ'd because of them kicking 3/4's the length of the pool. They also won mind you.

Wait, what?

[Council]

This means that at a certain point, the tip of the retreating blade is "standing still" relative to the wind and producing no lift, while the rest of the blade is actually moving backwards through the air.

The site is dashslotted so I can't see any diagrams, but I'm having trouble picturing this. "the tip of the retreating blade is 'standing still'" made sense, but how on earth would the rest of the blade be "actually moving backward through the air"? The retreating tip stands still, but then the rest of the blade can only be moving more forward than that.

What am I missing?

Re:Wait, what?

[jcims]

They aren't saying that the rest of the rotor is going backwards relative to the velocity of the aircraft. They are saying that the airflow over the rotor wing itself is reversed, because the forward velocity of the aircraft is greater than the retreating velocity of the blade itself.

To illustrate it by an extreme example: If you just stopped the rotors completely at a position where they are perpindicular to the flight path, the wing on the 'retreating' side would be going forwards (of course), but the airflow over it would be reversed, or 'backwards', because the trailing edge of the airfoil would be the 'leading edge'.

All helicopters experience some mu, the point about mu-1 is that the entire retreating blade is subject to non-positive airspeed, and thus provides non-positive lift. In this case, rather than speeding up to mu-1, what they did was attain a velocity where the 'winglets' were providing sufficient lift to keept the craft airborne, and then carefully slowed the rotors to speed required for mu-1. Agreed it's a bit of a 'cheat', but i think it's a very impressive aircraft nontheless.

I would have to imagine that it would be possible to create a non-hybrid helicopter that could exceed mu-1 by rapidly changing the shape of the airfoil (not just the angle of attack, which is done now) as it rotates. This would require some pretty advanced materials to put up with the stress, but i don't see why it is 'impossible'.

Re:Why is this so significant?

[Dilaudid]

Apparently this makes it much easier to create helicopter/plane hybrids - combining VTOL and efficient long range flight. I imagine the significance is that they don't have to retract the rotors.

I'm not sure if this means that if you walk the street slowly twirling a pen then you would be breaking the mu barrier too - but try it, you might get famous ;)

Helicopters

[mac123]

I've heard a (traditional airplane) pilot who took a few helicopter lessons refer to it as "ten thousand components doing their best to come apart".

Re:Helicopters

[Linker3000]

I worked for a defence contractor in the UK involved in helicopter work. We had an engine specialist on site from one of the major helicopter manufacturers and he stated that he would never fly in a helicopter because it was too much of a risk!

Re:Its a bird, its a plane, its a helicopter...

[CvD]

I'm curious: how does it work in regular helicopters anyways? The left side will always have more lift than the right side during forward flight (assuming clockwise rotation). How is this compensated for? I would imagine it slanting/leaning to one side if it wasn't compensated.

no airspeed no lift...

[Anonymous Coward]

you've got a fixed wing and a rotating wing.
when the slow portion of the rotating wing stalls, the forces on that wing go all out of whack and the thing vibrates like hell (that's the technical term anyway). If you make the transition smooth, and have an alternate source of lift, the fixed wing, you can keep flying.

it's neat, but it's ASFAICS it's got little practical value.

another way to make a helicopter fast is to make a smooth transition between sub-supersonic for the fast end of the wing. SOA is still pretty effing noisy though.

Fast and smooth comes from counter rotating wings. In all cases, rotating wings are much less efficient than fixed because of the much wider airspeed envelope the operate in.

Re:Its a bird, its a plane, its a helicopter...

[Alioth]

..and is why they are so much more expensive to fly than a fixed wing aircraft with the same engine: they are incredibly maintenance intensive. Even the worst fixed-wing hangar queen doesn't need near the maintenance of its helicopter equivalent.

Why they can't use faster rotor speed

[Anonymous Coward]

They are using a slow rotor speed to allow them to get high speeds. 100kts rotor + 500kts airspeed gets near to mach 1.

Normally the lower limit is MU as the returning blade starts to go slower than the aircraft and loses lift.
The upper limit is the advancing blade reaching mach 1 which produces too much drag.

If they increase the rotor speed thay cannot get the higher speeds and normaly mach 0.5 is the upper limit.

By exceeding mu they allow themselves to exceed mach 0.5.

Impossible?

[Chris Snook]

It used to be considered impossible to sail a boat upwind, too. The world of fluid dynamics is full of weird cheats, so the word "impossible" really shouldn't be used in describing yet-unacheived feats in the field.

Re:That's not that impressive

[ValentineMSmith]

Sorry, but the blade tips are not supersonic. What you are hearing is the blade tip of a following blade hit the tip vortex of the previous blade. The reason that the Huey has such a problem with "whop whop" is that they have such nice, fat blades and make such big vortices off the tips of the blades.

See http://www.bris.ac.uk/researchreview/2003/11138152 75 for more info.

Re:That's not that impressive

[ValentineMSmith]

I Googled, but I can't for the life of me remember what keywords I chose. I knew the statement wasn't correct (and the military pilot who said so really should have known better. :) ).

And yes, those blades have a LOT of inertia. I expect that they're probably one of the easiest helos even now to do a full auto on. If you look back at one of my previous posts, I used to be an AH-64 crew chief lo these many years ago. They taught us a little bit about the areodynamics in school, but they really didn't worry too much about teaching us areodynamics. That was why they paid the warrant officers the bigbux.

If you're interested in Hueys, there is a book by Robert Mason, "Chickenhawk", that is an absolute classic about flying them in Vietnam. He talks about not only doing autorotations, but hovering into small trees (and chopping them with the blades), and kicking with the tail rotor to get enough extra torque to get a momentary burst of lift to get over obstacles.

And then, there is the Mi-24D. If you ever get a chance to see one up close and personal, their blades are almost as sturdy as the Huey, and they've got 5 of them.