Just to throw a little wild hair response in, imagine the convergence
of these technologies:

Sonex e-Flight Initiative: http://aeroconversions.com/e-flight/
(DC Electric Motor for aircraft)
--or, if you prefer AC power--
http://www.teslamotors.com
(AC electric motor, could be adopted for aircraft)

AND

http://news-service.stanford.edu/new...re-010908.html
(New Nano-Wire Battery Technology)

In the near-term, we're probably looking at smaller (80 - 150hp
equivalent) motors running for a few hours at a time. But in 10 - 15
years I think that battery and electric-motor technology will be at a
point where towing would be possible... Here's hoping, at least!

Take care,

--Noel
P.S. My Dad's a Nuclear Engineer at a power-plant (former Navy
Submariner on nuclear-powered subs); but I don't see that process
fitting inside an engine cowling anytime soon! *chuckle* I know I'm
asking for some strange responses by saying this, but I hope folks
will soon realize that the small amount of long-term nuclear waste is
FAR less damaging to the environment than all of the bad side-effects
of fossil/bio-fuels. Europe got it right in switching to more Nuclear
power (and now with photovoltaics and other good stuff), unlike our
good ol' USA... If we ever want to have Hydrogen Fuel Cells, large-
scale electric power for vehicles, or large-scale electrolysis
(desalination of sea-water to produce clean drinking water), Nuclear
Power is the most efficient system; and the only economically-viable
way to do so with current technology...