Here is Orion's comment, big diameter vs small oneI don't see a problem in a long, unsupported shaft if you have a big diameter. A 5" thick, 10' long carbon shaft, with a wall thickness in the order of 1/4th of an inch doesn't seem unrealistic
This might be useful too http://ibis.experimentals.de/downloads/torsionalvibration.pdforion said:There isn't a lot written on the torsional characteristics of this type of prop driveline, mainly because there really aren't that many airplanes that are configured in this manner. In looking at the problem, you have two choices: One, you can design a very rigid system, which is then coupled to the engine through a flexible coupling designed with a low frequency visco-elastic characteristic. Your shaft would most likely have to be something like a graphite wrapped aluminum unit (very rigid and very light) - these used to be manufactured by Spicer but I'm sure there are others.
One coupling manufacturer is Vulcan, which I think is an Austrian company (or at least they handle a line of Austrian produced couplings). The difficulty though is finding one that will work in your application. Most of these couplings are designed for the high magnitude impulses generated by Diesel engines. For aircraft, the unit does not to be that big but finding a small light one will be somewhat a challenge.
The second approach to this is to have a torsionally flexible system, such as is often used for powering the tail rotor of a helicopter. Here they use a small diameter shaft, suspended by a series of bearings along its length. The small diameter has very low torsional stiffness characteristics and thus operates below the range of the system's natural frequency and most of its harmonics.