I have a big electric r/c helicopter, and I'm aware of the big electric planes.
The difference is that most r/c pilots tend to lose concentration after a few minutes, so the very limited flight times aren't a problem. My helicopter has been souped down to fly for up to 20 minutes, but people who're into heavy duty aerobatics make do witth flight times of 4-5 minutes. Aerobatic planes and ducted fan jets might have an endurance of 10 minutes, and my Formosa will make about 20 minutes flight time.
There's no question that electric propulsion can work for larger aircraft and a number of human-carrying electrics have already flown. However, in most cases the flight times have been less than an hour, which isn't enough to do anything useful with when you take the VFR reserves into account. It's battery technology that's the limiting factor.
Again, a number of people have used model turbines to power full size aircraft, but their fuel efficiency tends to be abysmal. In the model flying world, jet power is about the 'cool' factor, and efficiency isn't important. If they were to be built using exotic materials in order to work at higher operating temperatures and work more efficiently, then they would become much more expensive.
You could probably use a model helicopter turbine to drive a propeller, though to my knowledge this has not been done.
Last edited by abgd; 17th Oct 2012 at 04:36.