Dan wrote:
Fan size, fan ratio, multiple compressor/turbine shafts, etc. This stuff is more complicated than what is seen on these toy turbines, and would/will cost way more to replicate.
I think the only real problem you can’t overcome with more money in downsizing the compressors/turbines is that the efficiency depends on the relative size of the gap between the blades and the case vs blade height. But as you have an absolute limit on how small this gap could be (0.1mm-0.3mm?), you cannot scale this one down and your SFC grows the smaller you go…
But you are right – there might be a case for a high and fast-flying small a/c, although it will require flying really high (+IFR +airways + pressurisation +$$$).
It’s not the gap; it is small scale effects versus large scale effects of gas flow… I think it is viscosity which dominates in small scales and buggers up the efficiency. Same reason why small propellers (as on e.g. quadcopters) are inefficient. Hence centrifugal compressors rather than axial compressors in small turbines. And there is another factor, discovered recently by developers of small turbines: blade erosion is relatively a much bigger thing in a small engine, because the blades are so thin.
Dan wrote:
A mini turbine, such as used on the SubSonex has to be flown at higher levels too, probably >FL200, to reach some kind of efficiency.
And that’s why it doesn’t make any (economical) sense in my opinion, compared with a “sophisticated” piston engine like the 160 hp Rotax 916. The VL3 and other planes they are planning to install small turbines are VFR (FL195-limit) and non pressurized. If you look on FR24, most non pressurized Turbo-SEP fly up to like FL180. I’ve very rarely discovered a M20K, Cessna 400, TB21, SR22T above that altitude, and I guess for good reasons. OTOH – a turbine just sounds and smells great 
It doesn’t have a turbine
You want to bet ? A turbocharger has two major components.
1 a compressor
2 a ….
Anyway, what obviously was meant was one of these small turboprop engines. There are several around. They work just fine. They drink lots of fuel, but so what, the coolness factor weighs up for it
Well, if you’re looking for perfect regardless of price / economy, you can disregard any downside of TPs : enormous fuel burn at idle, bad SFC when reducing power (despite very high efficiency at high power), conduction losses on smaller engines (high temps), manufacturing complexity and exotic materials ($800,000+ for a PT-6A)
The Lancair Evolution looks great here, although the pressurized windows are really too small for my taste.
LeSving wrote:
You want to bet ? A turbocharger has two major components.
A turbine ignites fuel in the combustion chamber in a continuous manner. It is a continuously running engine contrary to a piston engine, which runs by means of a series of single-event “explosions”. In reality turbine ignition is only quasi-continuous, so the difference from theory is not 100% reproduced in real life, but still it’s a different working principle.
A turbocharger uses exhaust gas pressure to increase manifold pressure. There is (or should be!!) no combustion in a turbocharger.
Turning to mini-sized turbines there’s actually a lot of development in this field. There are turbines available in the power range of 100 to 150 hp, if I’m not mistaken. And they seem to have comparable fuel burns. Don’t forget that a turbine can run on Diesel, AVGAS, JetFuel, or any alcohol (bio-alcohol??) you want to burn. That’s a huge improvement. Maybe we’ll even see some 250 to 300 hp turbines anytime soon. I could imagine swapping my IO-540 with a small turbine of that size… 
(if it was affordable)
LeSving wrote:
the coolness factor weighs up for it
That’s why we all fly, isn’t it…?
A turbine burning bio fuel would be acceptable despite the high fuel burn, as long as the ‘mission profile’ can manage the weight/capacity of the fuel required.
Probably only suitable as a replacement for touring/IFR GA, but still interesting because it may also add some smaller engine failure figures and longer TBO’s
A turbine ignites fuel in the combustion chamber in a continuous manner.
No. A turbine is simply a mechanical device that by rotation extracts energy from a gas or liquid and converts it into useful work.
A combustion chamber is not part of a turbine. But a turbine connected to a compressor through a shaft with a combustion chamber in between is what is normally called a turbine engine (as long those connected parts do some useful work). Remove the combustion chamber, and you have a turbo charger.
A turbine is to a turbine engine what a piston is to a piston engine, more or less, only the piston is also a compressor, which a turbine is not, and never will be.
LeSving wrote:
only the piston is also a compressor, which a turbine is not, and never will be.
No real argument, but one could view the principle from a different point of view. On most jet engines the turbine stage(s) is solid, thru its respective shaft, with the compressor stage(s).
In the example below the purple hi P turbine indirectly (no mechanical connection) drives the green lo P turbine, but each of them is directly connected thru their own shaft to their hi and lo P compressor.
