The Rapture Aircraft Turbine Bug

[Geoff_H]

21 minutes ago, Flightrite said:

Apart from the costs and different operating requirements it's always been difficult to design, produce & market a small GT in the 200HP range, that size powerplant is already well & truly entrenched in the industry with time proven cheap (compared to GT's) workable designs.

The Allison 250 series GT's have been very successful in the low 300's to mid 400HP range but costs are way beyond most light A/C operators budget to equip or re-equip their planes/fleets. Saw just the other day replacement compressor sections for same around $50-85K! Ouch! The two biggest "ouch's" for GT's are FOD & hot starts! Ive got more hrs behind GT's than I care to remember, just glad I wasn't paying for them:-)

I'd love to see a 200 HP GT and maybe I will before I hang up my flying boots (hurry up please I ain't getting any younger) but for now $$45-55K is still doable for a recip that any donkey can operate:-)

Search Micro Turbine.  They are available, reliable, and can do 200hp.  They are industrial so have a bit of conversation. I couldn't afford them. 

https://en.wikipedia.org/wiki/Microturbine

[Rastus]

12 hours ago, Marty_d said:

Bit concerned about the effect the name might have on the more religious friends of pilots.  I can just see their faces when the pilot says "I'll be taken up in the Rapture tomorrow!"

Jonker JS3 Rapture 😎

[RFguy]

What's the efficiency of the 200HP with the recuperator ?

25% + ?

 

25% plus efficienc y , I might consider it say at the 120-150HP level at 50k$

 

How does turbine efficiency change as the percentage of rated thrust ? IE if I run it at 50% of the rated thrust, is the efficiency in the toilet ?

[Geoff_H]

For those who don't know how a gas turbine is limited. Stoica metric combustion of the fuel produces a temperature higher than any gas turbine blade known at the moment can withstand. To ring the temperature of the gas entering the first stage turbine the mixture is over Ed air acts as a cooling agent. The higher your blade can withstand the less air you have to pump in the compressor so the higher the temperature the higher the efficiency. Also affecting efficiency is the pressure at which the compressor generates the ones I worked on we had a pressure of 250 psi full stop this is just like the compression ratio of a piston engine the higher the pressure the more efficient. Out of a typical gas turbine the oxygen in the exhaust gas will be around 16%.  The  material of blades the get around 30 to 40% efficiency is exotic the manufacturer is also exotic Pratt & Whitney have a duct up the centre of the blade with small holes radiating out to the leading-edge so that a small layer of cool air runs along the leading edge of the blade. These materials are exotic the burner I found more expensive than the actual blading. For a 60 megawatt gas turbine used for electrical generation the rehabilitated cost of burners for the machine was $5000000 the cost of blades for the same machine was around $5000000.  The control system of a gas turbine or run with as much fuel as you can get in before you reach the turbine inlet temperature Max for blade life.  All expensive stuff.

[Geoff_H]

For those who don't know how a gas turbine is limited. Stiometric combustion of the fuel produces a temperature higher than any gas turbine blade known at the moment can withstand. To bring the temperature of the gas entering the first stage turbine the mixture is over aired.   Airr acts as a cooling agent. The higher your blade can withstand the less air you have to pump in the compressor so the higher the temperature, the higher the efficiency. Also affecting efficiency is the pressure at which the compressor generates, the GT worked on had a compressor discharge pressure of 250 psi. The pressure is  just like the compression ratio of a piston engine the higher the pressure the more efficient. Out of a typical gas turbine the oxygen in the exhaust gas will be around 16%. As this oxygen just gets taken in and heated and exhausted at quite high temperature, a lot of energy that could be going to the output shaft is just wasted.

The  material of blades the get around 30 to 40% efficiency is exotic the manufacturer is also exotic Pratt & Whitney have a duct up the centre of the blade with small holes radiating out to the leading-edge so that a small layer of cool air runs along the leading edge of the blade. These materials are exotic the burner I found more expensive than the actual blading. For a 60 megawatt gas turbine used for electrical generation the rehabilitated cost of burners for the machine was $5000000 the cost of blades for the same machine was around $5000000.  The control system of a gas turbine or run with as much fuel as you can get in before you reach the turbine inlet temperature Max for blade life.  All expensive stuff.

[Geoff_H]

Sorry second time is edited a little better.  Voice to text is something that I still have to master.

[Geoff_H]

1 hour ago, RFguy said:

What's the efficiency of the 200HP with the recuperator ?

25% + ?

 

25% plus efficienc y , I might consider it say at the 120-150HP level at 50k$

 

How does turbine efficiency change as the percentage of rated thrust ? IE if I run it at 50% of the rated thrust, is the efficiency in the toilet ?

Efficiency often depends upon the design.  Our turbine had moving pitch blades that made compression variable. That changed efficiency.  But not sure of aircraft GTs air flow control.

[TurbAero]

1 hour ago, RFguy said:

What's the efficiency of the 200HP with the recuperator ?

25% + ?

 

25% plus efficienc y , I might consider it say at the 120-150HP level at 50k$

 

How does turbine efficiency change as the percentage of rated thrust ? IE if I run it at 50% of the rated thrust, is the efficiency in the toilet ?

We anticipate that the Specific Fuel Consumption (SFC) of the Talon (the name for our 200hp recuperated turboprop) will be around 0.56 lbs/hp/hr at 10,000' ISA conditions when the engine is operating in the 150-180hp range.  So 150hp at 10,000' will yield a fuel flow of 12.6gph/48 litres p.h. of JetA. 

 

To compare the Talon to a 200hp IO-360 at the same altitude, the IO-360 will be running at WOT and be producing 142hp maximum with a fuel flow of 11.8gph/45 litres p.h. of the more expensive Avgas.  The cost of fuel for those cruise conditions is in favour of the turbine, due to the difference in price between JetA and Avgas.  Also, our Talon engine will still have 189hp available at 10,000' versus the IO-360's 142hp maximum.

 

As power is reduced below 150hp, the SFC (fuel efficiency) of the Talon will deteriorate so if the power is reduced to 120hp in the circuit for example, the fuel flow may be around 9gph/34 litres p.h.  The Jabiru 3300 owners will know what their engine burns at sea level and WOT.  I suspect that the 200hp turbine will be burning more fuel at that power setting but that power setting is not where the engine is being optimised.  We can only do so much...

 

RFguy, our 120hp variant which will be optimised for the 100-120hp power band and at a lower level will potentially reach your acceptable efficiency/price point targets.

[RFguy]

Hi, thanks for the comprehensive answers

What is the exhaust noise of your turbine like compared to a well muffled O-360 ?

I guess the biggest difference is the spectral energy density at higher freqs. (I am a electronics engineer) .

Yes,  3300 jabiru engine 9.5 gph / 36 lph at sea level  WOT, which doesnt really matter as it isnt spending too much time in that operating condition. 
It runs as you probably know in a rich quadrant of the mixture setting at WOT .
cruise (114 KCAS)  at sea level is 26 lph, ~ 90hp.

 

Will be interested to see the 120hp.

 

 

 

 

Edited August 16, 2021 by RFguy

[TurbAero]

8 minutes ago, RFguy said:

Hi, thanks for the comprehensive answers

What is the exhaust noise of your turbine like compared to a well muffled O-360 ?

I guess the biggest difference is the spectral energy density at higher freqs. (I am a electronics engineer) .

We haven't yet predicted the noise levels but in 6 months time, I'll let you know!!  Given that the recuperator is reducing the amount of energy that exits the engine via the exhaust by quite a significant amount, we are hoping that the engine will be a little quieter than your typical small gas turbine.  That said, I am definitely not an acoustics engineer so I may be way off the mark!!

[RFguy]

Understood.

compared to your average Alison 250 screamer.   I've worked alot in noise cancellation acoustics etc, since the turbine is spectral energy is tightly concentrated and predictable (RPM, pressure)  there exists good possibilities and probabilities, that an effective  passive (or active) acoustic  canceller/reducer could be viable and significantly reduce noise. (not just venting upward) 

[Ian]

Hi,

Generally in the GA market turbines aren't popular because their efficiency doesn't match that of their piston driven brethren, especially during part load conditions. As Geoff pointed out this is limited by the compression ratio and the temperature in the burner, turbine inlet area. You can gain some additional efficiency using a recuperator but it's fundamental physics that limits turbine efficiency. Turbines can be efficient however this is at the cost of exotic techniques and materials and generally only happens in large engines.

Piston engines for all their faults efficiently provide high compression and high combustion temperatures especially with diesel engines, however average temperatures are low as combustion is intermittent. Piston engines also tend to run more efficiently under part load scenarios whereas turbine engines tank under these scenarios. So really you need two engines for takeoff and cruise on a single engine.

I would like to know more about this engine, especially what the pressure ratio is and what the combustion/turbine temperature thresholds are, and if there is a working prototype. Continuous combustion is a hard engineering nut to crack compared to intermittent processes. If as you suggested you're using commodity materials I'm struggling to see how this can be achieved, but I hope that you succeed.

There's a nice article on BFSC on wikipedia where they've collected information on lots of engine. Notice how the smaller turbine engines efficiencies tend to suck.

Another key feature is that WW2 piston aviation engine efficiencies still beat the best very best turbine engines. I occasionally wonder what they could do with modern injectors and turbocharger technology.

 

 

[Geoff_H]

"Another key feature is that WW2 piston aviation engine efficiencies still beat the best very best turbine engines. I occasionally wonder what they could do with modern injectors and turbocharger technology."

 

GE have produced a 50+% efficient engine.  Admittedly it had inlet cooling, water injection and steam cooling of the inlet....stationary engine too heavy to fly lol

[Geoff_H]

I have thought that the dual cycle cycle piston  engines offer better hope.  , Mercedes get 400hp from a 2litre engine with quite high efficiency, around same as cheaper GT designns.

[Flightrite]

With the cost of fuel these days any reaction donk is expensive to operate!

I love turbines to drive behind, when I’m not paying for them👍
 

[Ian]

I meant to say "Aviaiton Turbines" 😉

Stationary turbines and marine turbines have achieved greater efficiencies as shown in the embedded link. It is an interesting time for engines as most research dollars will be going into electric.

An indication of how difficult it is to make turbines efficient is this article where a rotary engine is embedded in a turbofan type engine. As rotary car drivers know they're thirsty but it would appear that they can provide a better combustors than a can. Not that is was a successful concept. 

[Geoff_H]

I have worked with the trent engine range listed in the brake fuel efficiency stated above.  Great engine with multiple shafts.  Their are a range of Trent's. The one I worked with was a stationary derivative of the Airbus 380 engines.  They have just made a Trent 1000, Rolls Royce have been testing it using renewable style energy fuels.

The article leaves out the ABB come Siemens Multiple burner turbine.  Much better efficiency.  

[Ian]

So the multiple burner turbine gains efficiency by a second or third burner stage between the turbines to keep the peak temperatures down but the average temperature higher, improving efficiency?

Out of curiosity, what are the physical dimensions of such a creature as it would no longer have the high bypass section just the core. Essentially if you needed a lightweight portable power source what sort a minimal footprint are you looking at if you're willing to sacrifice the secondary efficiency measures that you're normally concerned about with power generation.

[Geoff_H]

Residual oxygen on a gas turbine is about 16%.  Excess air is used to keep the tit within its maximum temperature range.  If you put another burner in the has stream coming out of the first stage turbine and take advantage of the 16% oxygen and reduced temperature of this has stream then burn additional fuel to bring the second row blades up to the same as the first stage you get additional power from the turbine without any extra compressor load. As compressors can amount to around 40% of turbine load this is a considerable advantage.

Only big units have been made for stationary applications.  I will look it up and post shortly.

I worked for Siemens in Florida in the mid 1990s designing 186MW machines when ABB first designed these GTs.  They sold them off their design, the machines were good but didn't meet the theoretical design.  The liquidated damages sent ABB very close to the wall.  Siemens jumped in bailed them out and got the design.