Is it possible to have dual contra-rotating fixed-pitch rotors on a gyrocopter or gyroplane?
It would seem to me that it would provide a lot more lift. I am wondering if any disadvantages would outweigh the extra lift?
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You can get additional lift with a larger rotor, without going to the extra complication of having two of them. Present-day autogyros are very light machines that have more than enough with one rotor. Besides, in the case of the coaxial configuration, the centre of mass of the machine will be much higher, a very undesirable thing for landings, as any gyro pilot would attest...
Look at rc twin rotor gyros. They perform quite well. Instead of having articulated blades they are fixed and counter rotating at the end of opposing booms .The rotors are shorter and spin much faster on ground roll for takeoff. However I would make the rotors tilt fore and aft independently for pitch and roll control and use more ruder to coordinate turns. I believe this would work better than an elevator controll surface. A faster prerotation could be attained with a motorcycle starter motor at each rotor. The shorter blades won't require as much torque to get to takeoff speed . That means a shorter ground roll. The only drawback I can think of is having to make more blades .
I've long thought that a dual-rotor gyroplane, where the rotors are mounted side-by-side with a "coordination shaft" between them (such that they mesh) would be a good idea. Look at the Flettner and Kaman helicopters for examples.
It's true that gyroplanes don't need a tail rotor or multiple rotors to offset rotor torque because, since the rotor is flying in autorotation, there is little / no rotor torque to offset. There is, however, a problem with speed. And it's a problem which helicopters also have to wrestle with: retreating blade stall.
As your forward speed increases, the rotor produces more and more lift on the "advancing" side of the rotor disk but it produces less and less lift on the "retreating" side because the forward airspeed of the retreating rotor blade approaches 0. Many, modern helicopters actually reduce the pitch on the advancing side, to keep the lift from getting asymmetric, and limit forward speed. If you watch some videos of gyroplanes, especially ones which are really hauling, you'll note that the rotor disk isn't horizontal; in fact, some of them are pretty steeply "rolled," indicating very asymmetric lift on the rotor. At least one of the designs out there has the landing gear legs designed such that the one on the advancing side of the rotor is neutral on lift, but the one on the retreating side actually provides lift to help offset the asymmetric lift from the rotor.
One of Sikorsky's later designs uses a co-axial rotor (one above the other on the same mast, rotating in opposite directions). This way, as one rotor's advancing side is the other rotor's retreating side. The rotors may provide asymmetric lift but the combination of the rotors provides symmetric lift, even at higher speeds. The Russian Kamov design bureau has been doing this for decades.
The CarterCopter has small wings. It uses the rotor for takeoff and low-speed flight. As speed increases, the wings "take over" on lift and the rotor pitch changes to produce less lift, such that it can fly even if the retreating side has zero airspeed.
With meshing rotors, you could put the advancing side of the rotor disks outboard and the retreating side inboard, such that the two rotors would maintain symmetric lift as speed went up. And since the rotors would be be retreating as they spun over the fuselage, it wouldn't really matter if they produced zero lift during that part of their travel.
I would think this would a VERY GOOD idea. But I know of no instances where anyone has actually done such a thing.