If there was a turboprop engine with a decent SFC, perhaps only 30% worse than a piston engine, then an unpressurised turboprop would make as much sense as an unpressurised piston.
Buit I don’t think there is such an engine, or ever will be.
It’s been tried before – see the Grob 140
Grob went bust anyway but that one was done as a military trainer, built to a high standard and to withstand a lot of G, and would have been expensive. They were talking about €1.5M at the time so it would have been €2M+ in reality. Diamond seem to be aiming for the retail market with this one but I think they are pushing the “fixed gear = low maintenance costs and low insurance” thing way too far. If this can do say 200kt IAS, the gear is going to be costing them a LOT of fuel.
The engine seem to be from Ukrainian origin: Motor Sich AI-450S turboshaft engine.
I can’t find any specs on it.
There is an engine listed, model AI-450M1 which is a multi-purpose helicopter engine and might be comparable since it is the same “AI-450” and offers the same HP in cruise as the Diamond video mentions (300hp). 
I’ve flown the Grob 140 TP.
A wonderful airplane. A 4 seat TP with 4 doors, great visibility, fully aerobatic … But not the right IFR traveling machine without a pressurized cabin. They only made the one prototype AFAIK …
Peter,
Here’s your unpressurized turboprop, it’s even capable for aerobatics:
http://www.planecheck.com/eu/index.asp?ent=da&id=26102&cor=y
Money you save over a Jetprop will buy lots of Jet-A.
Peter,
I don’t really know the numbers in terms of SFC, but the Rolls-Royce turbine in the Silver Eagle burns only about 21 GPH in cruise.
For some reason, nobody else ever tried to put this one into a more modern certified airframe. The only exception is the Extra 500, which pretty much failed on the market (too ugly).
Very cool aircraft! I’d love to have one of those, but I guess family will veto against 3rd plane
If there was a turboprop engine with a decent SFC
0.31kg/(hp*h) is about 0.68lb/(hp*h), typical piston aviation engines seem to be around 0.41lb/(hp*h), so it’s only 65% more 8-)
I think the issue with TPs is always the same.
Once Marketing see the effect which a $300k TP engine (as compared with a $50k IO540 etc) has on the selling price, their brains go into overdrive in the direction of “what goodies can we stuff into the aircraft to make it look worth the money” to the customer who has $1.5M+. And since they all come from the same culture (where you lay out the red carpet and fancy sandwiches for any prospective customer) they always arrive at the same list:
And they are probably right – for that market sector.
But I think there are other markets.
The 1st one makes sense for the mission profile necessary to optimise fuel economy (always flying FL200+) but the rest are just stuff thrown in to arrive at a marketable package. Apart from maximising TAS (MPG), there is no need to fly higher than the weather, which means FL100 (the general minimum Eurocontrol IFR level) on a nice day, and up to FL200 to stay VMC on top in the absence of fronts (which can be done easily with cannulas). To stay above warm fronts you need FL250 and that needs a mask but how often do you do that?
With jet fuel being so much cheaper and so much more available, plus the reliability advantage of a TP, I am sure there is a market, but thus far every prospective attacker has aimed at the same bloated formula. And got …. exactly nowhere. The Kestrel is going nowhere. Epic likewise (well, homebuilt only).
The PA46 Jetprop is successful because it is an off the shelf PA46 with a mod, brand new at $1.5M or so.
The TBM is successful because they got in at the right time with a very good product and captured the owner-flown TP market.
The PC12 is a different market; mostly non owner flown, high utility value aircraft.
Then you get special-purpose stuff like the Caravan which has been very successful in that niche, and that had to be a TP despite being unpressurised and expensive because there is no piston engine big enough (well there are but they would look ridiculous).
I think a sub-2T unpressurised TP, properly engineered from the ground up to use the TP engine (i.e. big fuel tanks, to deliver a TB20-like 1300nm zero-fuel range), with full TKS, 400HP, would sell very well. It should do ~180kt IAS so awesome TAS high up if you like the cannulas. One reason I think there is a market is because the prices of stuff like a fully loaded SR22 are astronomical, yet people buy them.
Jetprop, TBM and PC12 all have a pressurised cabin.
There is no point to have a turboprop without pressurisation as ideal conditions for a TP is FL250+.
The only exception is the Extra 500, which pretty much failed on the market (too ugly).
I wouldn’t say too ugly, but very unpractical.
You need to be a pilot with max 1,65m everything above will not work.
I just could not sit in it and move the controls and I’m just 1,90m. The Dutch person who tried to get in next was 2,05 and there was not a hope in hell to fit in.
