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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."
the carter copter cannot hover and it is relying on a prop on the back to provide the thrust needed for forward flight. what they have achieved is limiting the flutter associated with the approach of mu = .75. So yes, the parent is right, this is no better than a harrier with a rotor instead of motorized engine exhausts.
From the FAQ;
How can the CarterCopter fly so fast and efficiently? Shouldn't the rotor slow it down?
The CarterCopter is a hybrid between an airplane and a rotorcraft. A rotor is a very efficient device for providing lift at low speeds, but its drag increases rapidly as the aircraft goes faster if it must continue to support the aircraft. In the CarterCopter, as the aircraft speeds up and the wings begin producing more of the lift, the rotor produces less lift and can slow down given the correct control input. The reduction of rotor lift and lower rpm significantly decrease the rotor drag (in fact, a three fold reduction in rpm results in approximately a 27 fold reduction in rotational drag- drag required to just spin the rotor). The rotor drag at very low rpms and low lift basically becomes a function of its area (which is relatively small compared to an airplane wing of similar gross weight) and the forward speed of the aircraft.
Wings are very efficient at high speed, but can't provide enough lift as the aircraft slows down. In most aircraft, the wings are sized significantly larger than they need to be in cruise flight so that the pilot can fly slower for landing. Most airplanes also have some type of high lift device, such as flaps, which further decrease the minimum flight speed of the aircraft, but add weight and complexity to the wing. The CarterCopter has a very simple wing, sized much smaller than a conventional aircraft of similar size, because the wing only needs to support the aircraft at high speeds.
Hmm... are you referring to the V22 Osprey?
From the sound of it he is referring to the Sikorsky X-Wing The idea was to build a conventional helicopter that had rotors who generated lift no matter how they were oriented by using compressed air that was bled over the rotor surfaces to create lift. I am no aerodynamicist but I think this concept is called a boundary layer control system (like blown flaps). The X-Wing would thus be able to take off like a Helo but could fix the rotors in place and have them act like conventional wings for high speed flight. The X-Wing was abandoned in favor of the V-22 which is a more elegant if troubled solution. I rather liked the X-Wing though it was the closest engineers ever got to creating a real world AirWolf.
Only to idiots, are orders laws.
-- Henning von Tresckow
The angle of attack of one blade is different than the other... on advance the angle/lift is lowered, and on retreat is is increased. So, the lift generated is the same although the relative wind speeds are different.
This means that the blade angle is adjusted continually as the blade rotates - that's the main reason why you see such a complicated coupling at the hub of a helicopter blade.
MadCow.
I used to have a sig, but I set it free and it never came back.
As far as I know there are some tilting mechanisms built in to the rotor, so that each time the rotor goes backwards, it is tilted a bit more, giving it more lift. At the side going forwards, the tilt is lowered. When you balance this carefully, you should get equal lift at both sides. Complex but it works, still causing lots of shaking. That's why copters need much more maintenance than planes.
I'm a MechE who did an internship at Sikorsky 3 years ago. They had an "Intro to rotorcraft" pamphlet which was rather enlightening.
What gets me the most is that fundamentally, it's an unstable flying machine. But each corrective measure yeilds a slightly lesser instability, which requires further adjustments.
Yes, each blade changes pitch during rotation. Advancing blade flattens out, while the retreating blade increases pitch. This keeps the copter level.
To generate more or less lift for altitude adjustment, there is a "collective" pitch increase or decrease in addition to the cyclic pitch adjustment.
But what I didn't understand overall was that the rotor blades do not rotate in a flat plane. They rotate in a wide "cone" whose central axis indicates the overall main rotor force vector. By changing the shape of the cone, you change the direction of the force. This is done by "flapping" each rotor blade, like a bird wing, with respect to the central hub. So, for a helicopter moving forward, a given rotor blade will swing up on the back half of it's rotation, and drop back down for the forward half of the cone. The inclined angle allows the blade's aerodynamic lift to provide a forward component of thrust. This "cone" is adjusted for whichever direction the pilot whishes to move.
The tail rotor, as most people know, provides the counter rotating force from the main rotor. But it also provides a sideways thrust, so without correction, the entire helicopter would drift sideways. So to correct for this, the main rotor blades always flap slightly on one side to counteract this effect and keep the helicopter stationary.
Rotor blades not only change pitch and flap, but they also lead and lag freely. The angle between blades as viewed from above is not always equal. The main reason is that not only do you have stall speed problems on the retreating blade, but you've got shock wave problems on the advancing blade.
It's all a tricky balancing act.
"No fair, you changed the outcome by measuring it!" - Professor Hubert J. Farnsworth