I'm willing to bet an efficient gas turbine running a generator and having an electric motor drive the prop, would be much lighter than any combustion engine.
Let's aim for 160 hp. A brushless motor for a C172 would weigh about 15kg (yes, the deliver about 7-10Kw/Kg). A generator that could provide such power would weight about the same (as a motor is a generator in reverse), so add another 15kg. Add electronic controls for both and you have maybe 10kgs on top of that. How much does a gas turbine weigh that can produce about 160 shaft horsepowers? Well, the venerable helicopter turbine the Allison C18 delivered 317 shp on 70kg (140lbs), which would make it a fair assumption that half that power output could reduce the weight to about 50kgs. Let's add it up: 15+15+10+50=90kg. Now, that's less than any combustion engine.
So why isn't this available?
Well, you could as well ask yourself why a PT6 costs 10 times more than a combustion engine when it's simpler and cheaper to manufacture? Nothing makes sense in aviation, that's why. No normal amounts of logic apply, because the regulatory organs have a stranglehold on the market. It's an oligopoly. Want an alternative to that PT6 in your TBM? You're out of luck - it's been certified with that engine and that engine only and now they have you over a barrell. They can charge whatever they want.
They will fight electric tooth and nails. GE, RR, P&W will bring out everything they got and the FAA and EASA will listen. The revolution will not happen within the "big" industry, military or certified planes, it will happen with the grassroots, EAA, homebuilts and nutjobs in sheds. Just like it's always done - you look at most big inventions in the flying industry and it's almost always from the small enthusiasts and tinkerers in garages. There would be no composite 787 Dreamliner without the Windecker, there would be no B2 Stealth Bomber without the Horten brothers etc, etc.
Last edited by AdamFrisch; 15th March 2011 at 23:36.