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Real Coaxial Helicopters
Posted Saturday April 14, 2007 at 5:45 pm
(3 years ago)
I've been thinking a little bit about coaxial helicopters (and how much I want one). Here's a video of one in action. The typical helicopters everyone is familiar with have one main rotor that provides lift. The problem is that the turning rotor generates torque that acts on the body of the helicopter. This would cause the helicopter to spin uncontrollably if it weren't for the tail rotor, which counteracts this torque and can be used to turn the helicopter (yaw) as well.
A coaxial helicopter has two sets of rotors and one main shaft. The two sets of rotors spin in opposite directions, thus counteracting each other's torque and producing a net torque of zero on the body of the helicopter. That means no tail rotor is necessary.
Why is that good? (1) The torque cancellation makes the helicopter inherently more stable—not as stable as an airplane, but stable as helicopters go. (2) All power is used to generate lift, whereas the tail rotor "steals" up to a quarter of a normal helicopter's power output depending on the type of flying. (3) When a normal helicopter is flying forward, the advancing blade generates more lift than the retreating blade. That's not a problem if there's a set of blades turning each direction (i.e., there's always an advancing blade on both the left and right). But don't take my word for it—read up about the Ka-50 Russian attack helicopter that's faster and more maneuverable than than any US military helicopter.
So I've been wondering: why doesn't anybody make a "real" remote-controlled coaxial helicopter? There are some semi-functional versions that are used as beginner's helicopters. While these take full advantage of the stability offered by a coax, they are designed to be self-stabilizing: great for learning, but not for aggressive flight. Somebody really ought to make one with performance in mind, though.
I got to thinking about how you could have two sets of blades turning in opposite directions. The beginner's coaxials use two motors, but it'd be better to route the power from one motor to both rotors at the same time. I came up with this as a potential solution (however not a viable solution as it turns out since the orange idler gear doesn't act as a reduction gear):
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| Coaxial Helicopter Shaft |
But then I came across a superior solution by someone who's thought about it a lot more than me:
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| Coax Gears with Labels |
In terms of how the swash plates work, for collective control (vertical thrust), both the upper and lower swash plates move up or down together to change the angle of attack on all rotors at the same time. For cyclic control (left/right and forward/backward), both swash plates tilt at the same time to change the angle of attack at certain points along the blades' rotation. In other words, for collective and cyclic, the two rotor sets could be linked to just one swash plate that affects both the upper and lower blades in the same manner.
However, to yaw (turn clockwise/counterclockwise, the old job of the tail rotor) requires a little ingenuity. It can be accomplished by increasing or decreasing the torque on either set of rotors. This could be accomplished by "speeding up" one set of rotors, but that's slower than simply changing the collective one one set.
But it gets more complicated than that: changing the collective of just one rotor is going to generate an undesired increase or decrease in lift. So yes, the helicopter would yaw, but it also move vertically at the same time (unintended consequence). The solution is to change both rotors in opposite directions. This increases torque on one rotor and decreases it on the other, but the net blade angle of attack stays constant. Therefore, the helicopter yaws without a change in net lift.
Implementing that can be solved by using a sliding axis as shown in orange in the picture below. (I didn't draw this picture, for the record. It came from RunRyder.com.)
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| Coax Swash Controls |
So now that I understand how it works, and why I want one, I've just got to wait for somebody to start selling them for a reasonable price. :)
And if you're really bored, here's an online discussion I started about the whole thing over at RCGroups.
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