# turboprop At 31kW (42hp), the fuel consumption was just 15.2 lph.

### Help Support HomeBuiltAirplanes.com:

#### rv7charlie

##### Well-Known Member
In terms we barbarians can understand, that's a bit over 4 gallons per hour for 42 HP. While a significant achievement in (tiny) turbine terms, that's still as bad or worse than a piston-ported 2-stroke piston engine. And I suspect that it's a tad more expensive to purchase, too (by maybe 2 or 3 orders of magnitude). No doubt the guys operating Hellfire missile haulers (Predators) will be excited because they can get rid of their gasoline supplies, but I don't see one in my future.

Charlie

#### berridos

##### Well-Known Member
a 115 kg TG-R55 system (engine + fuel) carries the same energy as 1000 kg of Lithium-Ion batteries

#### kubark42

##### Well-Known Member
that's a bit over 4 gallons (15L) per hour for 42 HP(31kW). While a significant achievement in (tiny) turbine terms, that's still as bad or worse than a piston-ported 2-stroke piston engine.
All things being equal, turbines will always be less efficient than pistons, for the simple reason that the Otto Cycle (ICE) is a higher efficiency cycle than the Brayton (turbine). This has to do with the thermodynamic impact of using a constant pressure process instead of a constant volume process.

(Comparison of P-V and T-S diagram of Various Cycles tells the tale, but if you've never looked at a temperature vs. entropy diagram it probably just looks like gobbledygook)

Last edited:

#### TFF

##### Well-Known Member
If you look at companies that sell both piston and turbine helicopters concurrently, you find this. Turbine is double price of the piston, and burns double fuel. Generally gearboxes and blades are the same or slightly modified but not screaming different. So where is the performance gain from? More horsepower in a smaller unit for sure, but it’s not all about the horsepower, really it’s weight. The difference between a Lycoming and a small turbine is equal to one FAA person weight at that size dynamic.

#### Riggerrob

##### Well-Known Member
Military users are increasingly converting to turbines for two reasons: logistics and damage-control.

Only using jet fuel means one fewer logistics line item to stock, ship, etc. Since fuel is a high-use, consumable store, cost of delivery can be quite high. It can also be a nuisance to stock two different types of fuel. A single gallon of diesel fuel costs thousands of dollars by the time it is trucked to a remote US Army post high in Afghan mountains, after you consider the cost of mine-clearing, drivers' food, sleep, etc.
Aviation gasoline has disappearred from many regions like Canada's High Arctic and great swaths of Africa, requiring commercial operators to convert even their smallest airplanes to burn jet fuel.

Damage control is a big risk for ships at sea. Since both diesel fuel and jet fuel are less flammable than gasoline, they are at lower risk of destroying the ship by accidental fire. Jet fuel is also has a higher flash-temperature, making it more difficult for enemy to ignite.

Last edited:

#### BoKu

##### Pundit
HBA Supporter
...While a significant achievement in (tiny) turbine terms, that's still as bad or worse than a piston-ported 2-stroke piston engine...
Fuel consumption is only a tiny portion of the whole here.

Having raced GP roadbikes for four years of my misspent youth, I have either a respectful loathing, or a loathing respect for two-stroke motors. They offer incredible power per unit weight at the expense of short TBOs, tempermental operation, high vibration, and many other peculiarities.

I would gladly accommodate the higher fuel flows of a turbine in exchange for their smoothness, durability, and reliability. The fact that they drink diesel or Jet-A makes it that much sweeter, since there is no ethanol to dissolve composite fuel tanks or plastic fuel system parts.

#### Riggerrob

##### Well-Known Member
Turbotech's main claim seems to be an integral heat exchanger that uses exhaust gases to pre-heat air entering the combustion chamber. This heat recuperator is basically an extended annular combustion chamber with much longer walls to improve heat transfer. They further improve heat transfer by adding thousands of micro tubes. Turbotech claims to be able to use almost 30 percent of the energy - from jet fuel - to turn a shaft.
Previous to this, heat recuperators were so big and heavy that they were only installed on ships or M1 Abrams main battle tanks.

#### berridos

##### Well-Known Member
Those iconel tubes have to be pricey.

#### rv7charlie

I suppose my earlier statement was a bit vague; I only mentioned 2-strokes as a tool to show how bad (worse than a 2 stroke) the fuel consumption really is for these new 'efficient' turbines. Power/weight didn't look that attractive either, for the smaller versions. You could do as well in pwr/wt with a 4 cyl 4 stroke Yamaha snow/water craft engine, and halve the fuel burn. The real kicker is going to be price. If you can buy one for less than $50,000, I'll be very surprised. I try not to say 'never' about tech, but turbines just don't scale *down* well, for a variety of reasons. This one isn't the 1st effort to use a recuperating system with a small turbine, and while they are better with than without, they are still not that great in efficiency (even if the claims can be realized in practice). As I said earlier, I'm sure the Predator operators will be happy (giving up range to lose the gasoline requirement), but how many of us mere mortals will pay$50K-$100K for a sub-100 HP engine? If they would (could) sell a 150-200 HP version for anywhere near the Lyc price point, they would have a market in GA, but I'm not holding my breath for that. Charlie #### Geraldc ##### Well-Known Member As I said earlier, I'm sure the Predator operators will be happy (giving up range to lose the gasoline requirement), but how many of us mere mortals will pay$50K-\$100K for a sub-100 HP engine? If they would (could) sell a 150-200 HP version for anywhere near the Lyc price point, they would have a market in GA, but I'm not holding my breath for that.
As piston airliners got replaced by jets maybe this could be the future for us.

#### akwrencher

##### Well-Known Member
HBA Supporter
I believe I read that it was 120shp
I wouldn't expect it to be more efficient than a piston engine, but if it gets close, and is reliable, that would be huge. Price point? Who knows. I'm not holding my breath. But it's still cool. I hope they pull it off. Maybe in 20 years I can afford a mid time one.

#### sming

##### Well-Known Member
Those iconel tubes have to be pricey.
Why pay for tubes when you could 3d print and pay by the pound of the stuff?? IIRC, their heat exchanger was due to that kind technological advancement and was the phd subject of one of the founders

#### Riggerrob

##### Well-Known Member
I believe I read that it was 120shp
I wouldn't expect it to be more efficient than a piston engine, but if it gets close, and is reliable, that would be huge. Price point? Who knows. I'm not holding my breath. But it's still cool. I hope they pull it off. Maybe in 20 years I can afford a mid time one.
20 years from now, 97 octane gasoline will be so rightfully expensive .... and scarce ....

#### Riggerrob

##### Well-Known Member
Why pay for tubes when you could 3d print and pay by the pound of the stuff?? IIRC, their heat exchanger was due to that kind technological advancement and was the phd subject of one of the founders
Good point!
3D printers are most valuable when printing tiny components made of expensive materials.

#### slociviccoupe

##### Well-Known Member
There are guys that are really tallented machinists and have great understanding of gas turbine engines on the gast turbines wwb site and facebook page making all sizes of gas turbine engines. For the hp range you just have to size it right. They are making gas producers with 100mm turbocharger components that make about 200hp shaft piwer using allison c20 ngv and turbines as free power turbines. The gas producer is almost easy its getting the free power to work correctly that is proving difficult. But in my reading, research and watching these guys build gas turbines none seem to be building for efficiency. They all have very high fuel flows but they are moving lots of air and msking lits of noise and some with up to 250-300 pounds of thrust from the gas producer alone. Not bad for an 8-10" diameter that is no longer than 12" and weighs no more than 40 pounds.

#### berridos

##### Well-Known Member
Turbotech's main claim seems to be an integral heat exchanger that uses exhaust gases to pre-heat air entering the combustion chamber. This heat recuperator is basically an extended annular combustion chamber with much longer walls to improve heat transfer. They further improve heat transfer by adding thousands of micro tubes. Turbotech claims to be able to use almost 30 percent of the energy - from jet fuel - to turn a shaft.
Previous to this, heat recuperators were so big and heavy that they were only installed on ships or M1 Abrams main battle tanks.
Rover used a regenerator in their turbine car.
So did the STP Indy car.
And Chrysler's turbine effort.

#### aeromomentum

##### Well-Known Member
While a prop can have much higher efficiency due to the much higher mass flow, I do not understand the efforts to develop small turboprops. Compared to a Diesel running nearly the same fuel type, the small turboprop will burn about twice the fuel for the same power offsetting the weight savings (if any) of the engine. Maybe the efforts (money) would be better spent on developing a small Diesel with more cylinders for reduced vibration. Maybe a 9 cylinder Deltic style. On the turbine front, maybe a higher bypass version of the Williams EJ-22 would be more useful.

If anyone has a few million to spare, I am open to developing a ~300hp 9 cylinder Deltic style Diesel designed for cost effective low quantity production.