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 Post subject: Re: Allison Turbine Bonanza
PostPosted: 18 Oct 2013, 10:42 
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Username Protected wrote:
Just curious, in all the RR and PT6 discussions, has anyone looked at the General Electric H80?
http://www.geaviation.com/bga/engines/h80.html

Pure press stuff, but I have seen claims of 25% more efficient than the comparable PT6.
There was a review on the Thrush and it basically kicked the PT6 versions but.
Some Russian company is planning to use it for a short hop passenger twin turboprop (think VSTOL 8-10 passengers).
I think there was also a KA conversion available now also, but I might have that confused....

Tim

============

I've been following them since they bought the czech manufacturer. Glad they didn't simply rebrand and kept the same old stuff.
If that motor is good enough for the AG boys its good enough for me.
G.E. knows turbines like no one else in the world, I am looking forward to them making something small and very affordable for G.A. at least I hope they do. I think now we know why PWC is getting off the can, I guess they had to wait till they bought a GE motor before they moved.
Maybe they saw what happened to Blackberry, if they snooze they lose.

Given a choice I'd buy GE everytime their science, research, quality is tops.
Unfortunately usually so are their prices.
:cheers:


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 Post subject: Re: Allison Turbine Bonanza
PostPosted: 18 Oct 2013, 11:55 
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Username Protected wrote:
Just curious, in all the RR and PT6 discussions, has anyone looked at the General Electric H80?
http://www.geaviation.com/bga/engines/h80.html

Pure press stuff, but I have seen claims of 25% more efficient than the comparable PT6.
There was a review on the Thrush and it basically kicked the PT6 versions but.
Some Russian company is planning to use it for a short hop passenger twin turboprop (think VSTOL 8-10 passengers).
I think there was also a KA conversion available now also, but I might have that confused....

Tim


25% more efficient would be nice. I wonder if any of these engines are meant to be flown in the FL. I know P&W have the crop duster engines and then the high altitude ones. I'd also be curious if P&W are working on more efficient smaller engines as well.

The GE H80 engine is ~110 lbs heavier than the PT6 though.


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 Post subject: Re: Allison Turbine Bonanza
PostPosted: 18 Oct 2013, 12:35 
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Username Protected wrote:
Just curious, in all the RR and PT6 discussions, has anyone looked at the General Electric H80?
http://www.geaviation.com/bga/engines/h80.html

Pure press stuff, but I have seen claims of 25% more efficient than the comparable PT6. There was a review on the Thrush and it basically kicked the PT6 versions but.
Some Russian company is planning to use it for a short hop passenger twin turboprop (think VSTOL 8-10 passengers). I think there was also a KA conversion available now also, but I might have that confused....

Tim


25% more efficient would be nice. I wonder if any of these engines are meant to be flown in the FL. I know P&W have the crop duster engines and then the high altitude ones. I'd also be curious if P&W are working on more efficient smaller engines as well.

The GE H80 engine is ~110 lbs heavier than the PT6 though.

The H80 isn't significantly better than the PT6. I was talking with some GE folks they were "going to sprinkle some pixie dust" on the engine and give it a better compressor, but when done it would just be a bit better than the comparable PT6. When they first got the engine it was a bit worse than the PT6. The data I've see is that it's marginally better but 25% better performance isn't going to happen. Anyone who says that is smoking something.

While the H80 weight is listed as being 110 lbs heavier than the comparable PT6, the published weight of the H80 includes the starter, I believe but am not sure, it could also contain a couple of other accessories like the oil tank that are not included in the PT6 weight since they are airframe mounted. The PT6 weight is bare, no starter, no oil, and no tank and plumbing. Apples to Apples , the weight is a lot closer than that, so weight isn't much of an issue. IIRC (and it's been a while since I looked closely at it) the weights were within 10 or 20 pounds of each other when comparably dressed.
GE needs to do a better job of representing the engine so that folks better understand the weight issue.

All said and done, the H80 doesn't give you anything more than the PT6. It's about as old and was a Chinese copy of the PT6 in the first place. It didn't have the life, but it was cheap, and you got what you paid for. With GE technology they might get the life up there, but it's still just comparable to a PT6, so what's the incentive??? There was an issue with getting service in the US, but that has been addressed. Still, maybe in a AG application it might make sense. In a Bo, it doesn't at all. JMHO and qualified as such.

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 Post subject: Re: Allison Turbine Bonanza
PostPosted: 18 Oct 2013, 12:59 
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Manny, do any of the free turbines come close to the single shaft turbines in terms of fuel specifics?


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 Post subject: Re: Allison Turbine Bonanza
PostPosted: 19 Oct 2013, 00:56 
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Manny, do any of the free turbines come close to the single shaft turbines in terms of fuel specifics?

Actually there are some that are a good bit better. The TPE331 has a T/O SFC of about .52 IIRC. There are free turbine engines like the T800 and T700 that are are lower than .45 if you take off the Inlet Particle Separators that suck some power and have some inlet losses. It isn't the free turbine that gives them the SFC it's more the pressure ratio but it takes some work to get there with a free turbine. Cooling the turbine helps too, but the pressure ratio is the bigger factor.

Keep in mind compressor pressure ratio drives efficiency. More PR gives better SFC.

With a free turbine you have to drive the compressor with a turbine and then you have a separate turbine that drives the output shaft. There's a limit as to how much compressor pressure ratio you can drive with a single stage axial turbine. With an uncooled turbine you can efficiently drive a compressor to about a 9:1 pressure ratio. More than that and it starts to get highly loaded and the efficiency starts to fall off. I've see a single stage uncooled turbine drive a 12:1 compressor but the overall efficiency of that engine was no better than a 9:1 machine because the turbine and compressor were overloaded and the component efficiencies were falling off. So from a practical standpoint, if you have a single stage uncooled free turbine you get up to about a 9:1 pressure ratio.

If you put a higher pressure ratio in the free turbine engine now you have to use a two stage turbine, and that costs more. In addition, the two stage turbine is lightly loaded and the efficiency is lower than if you loaded it up more. Since you are pushing the compressor at the same time if you go above 9:1 in a single stage you really need to go to a multi-stage compressor and a two stage free turbine. At 10:1 none of these are loaded very will and the efficiency isn't good, so it makes sense push the pressure ratio on up to 13 or more (like the T800) or even to as much as 20 (T700). With that higher pressure ratio you now are loading the turbine up more and the SFC has gotten a lot better because of the pressure ratio and improving component efficiency. But now you have a multi-stage compressor and a multi-stage turbine, so the costs go up, but the SFC gets better, and if you are in the military and you really need the more expensive engine it makes sense.

The single spool engine lets you put a two stage compressor in front of a three stage turbine that has reasonable stage loading and have more pressure ratio than a free turbine that has fewer stages and is less expensive. The down sides of the single spool engine are the obvious noise since the prop is now idling at 70% speed where the free turbines prop is idling at 20 or 30% speed. The other downside is that the last stage turbine sets the turbine speed and the compressor and first stage turbines are turning at a lower speed than they really want to run. This means that the diameter has to be bigger and the blade heights of the first stage of the turbine and last stage of the compressor can get small and lose efficiency. There is a limit as to how fast you can spin the last stage of a single spool engine. The parameter of area of the turbine times the speed squared (AN squared or AN^2) is a way to measure blade root stress, and at some point you just run out of rope and the speed becomes limited.

In short, the single spool system is better than the free turbine in a very narrow range of pressure ratios, say from 9:1 up to 12:1 and once you get outside of that range then the free turbine is the superior configuration.

A really good single stage 9:1 compressor with a single stage turbine and a single stage free turbine can get to an SFC of about .52 SFC, but the TPE331 has been there for a long time, and if you really did an optimized single spool machine it would likely be a 12:1 machine and be a bit better than the 331. But the single spool machine with two stages of compression and a three stage turbine (five stages overall) is more expensive than the single stage compressor driven by a single stage turbine and one stage of power turbine (three stages overall).

As you go higher in PR, say above 13 or 14, the free turbine gets better SFC, and since the shaft speed of the gets higher the compressor and turbine blade heights are taller and tip losses improve and the stage efficiency improves and at the same time the engine gets smaller. But now we are talking about an engine with two stages of compression, two stages of gas generator turbine and two stages of power turbine. So now that engine is more complex and expensive.

If you aren't greedy you can get to about a .52 SFC kind of an engine with 1-1-1 configuration and that is a sweet spot for a low cost engine. Getting better SFC than that will cost you and that probably wouldn't fly in the small end of the turbine market, but that's still about 15% better than a Series 1 PT6, so it's nothing to sneeze at and it would be less expensive engine that a TPE331. Since that provides a light compact and small engine, that's where the lower cost helo engine market has gone to over the last 10 years or so. Engines like the PW206, scaled up a bit would make a really nice turboprop and be a lot less expensive to produce than a PT6 or TPE331.

Hope this all makes sense. I've simplified it a bit, to concentrate on the key factors, and there are other factors that get involved, but if you look at the major factors that's how it all works.


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 Post subject: Re: Allison Turbine Bonanza
PostPosted: 19 Oct 2013, 10:17 
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Manny,

Thanks for a great explanation in ENGLISH no less. :D
Just curious, a while back I read about a variable vane turbines from one of the major players. I forget who; but the target market was small turboprops/turbofans for drones/missiles. I know the cost was through the roof but how would that effect the efficiency? (The article stated the goal of the engine was to provide a much broader range of power at maximum on efficiency).

Tim


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 Post subject: Re: Allison Turbine Bonanza
PostPosted: 19 Oct 2013, 10:28 
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H80 (Thrush 510G) write up.

http://www.agairupdate.com/article_deta ... l=00001194

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 Post subject: Re: Allison Turbine Bonanza
PostPosted: 19 Oct 2013, 11:11 
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Username Protected wrote:
Manny,

Thanks for a great explanation in ENGLISH no less. :D
Just curious, a while back I read about a variable vane turbines from one of the major players. I forget who; but the target market was small turboprops/turbofans for drones/missiles. I know the cost was through the roof but how would that effect the efficiency? (The article stated the goal of the engine was to provide a much broader range of power at maximum on efficiency).

Tim

Closing down the turbine nozzle area does save fuel, but you are right it is an expensive way to do it. What it does is increases the turbine inlet temperature for a given mass flow. This results in lower mass flow for the same power and that is a more efficient way to get power. The compressor moves to the left on its map and that can also slightly increase pressure ratio, but usually not much. Also moving to the left on the compressor map can move you closer to surge which is generally a more efficient compressor operating point, but you give up surge margin.

Running hotter for a given power is more efficient, but, as you noted it costs a lot to do that and it isn't worth all that much in fuel savings. If you really are looking for the last increment in fuel consumption you can do it, and if it makes a drone fly for an hour more on a 40 hour endurance, the military customer might think that was worth it.

Generally speaking, it's more productive to increase the pressure ratio all the way across the board. That is, if you increase the pressure ratio at max power the pressure ratio at part power will still be higher and you will improve part power SFC. The trick is how to get to PR's of 18-20 in a small engine that doesn't cost an arm and a leg (like T800 and T700). Then the SFC drops down to numbers like .42 lbs/hp-hr and the part power SFC is still good. There are ways to do it, but it doesn't look like a conventional engine.


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 Post subject: Re: Allison Turbine Bonanza
PostPosted: 19 Oct 2013, 11:15 
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Ok, based on Manny's replies and the Airplane Myths thread I think I need to go back to school. My computer degree is kinda useless :D

Tim


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 Post subject: Re: Allison Turbine Bonanza
PostPosted: 19 Oct 2013, 11:59 
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Username Protected wrote:
Ok, based on Manny's replies and the Airplane Myths thread I think I need to go back to school. My computer degree is kinda useless :D

Tim

It's not all that hard, but you have to get to the point where you "think like an engine".. It becomes a balance of thermodynamics, mechanical and structural design, and design to cost awareness that leads to a good product. If you have to have done it for 35 years, it sort of becomes second nature....

Not that the guys who did engines like the PT6 and Model 250 were dummies either. Far from it. They made rational decisions based on the technology and tools that they had at the time and their progress is a testament to their skills. The PT6 and Model 250 were designed in and around 1960. There were no computers or even pocket calculators, no CFD analysis, no CAD, no finite element stress analysis, no real knowledge of low cycle fatigue, and limited material property development. The fact that these engines are still in production today says they did a really good job at the time with the tools they had.

That said, this is the 21st century and the tools and capability is there to make a much better engine than a PT6 and only the economics and cost to certify such an engine is keeping it from happening.


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 Post subject: Re: Allison Turbine Bonanza
PostPosted: 19 Oct 2013, 12:24 
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Manny,

From an operational standpoint on the Allison, what do you feel is important. Backing off the temp by 14C will double the life you say. Is that the sweet spot, and what life are you doubling? I have read that running at too low a temp in a PT6 will also cause problems.

Thanks,

Mike


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 Post subject: Re: Allison Turbine Bonanza
PostPosted: 19 Oct 2013, 13:06 
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Manny,

Looking at the specific range of FJ44 powered aircraft one can not help but be impressed by the increase in efficiency when compared to older JTD engines. A similar jump in turbine efficiency would be a significant move forward.

How would you compare efficiency of the turboprops (SFC lb/hr/hp) to that of the fan engines (SFC lb/hr/lbf)? To the casual observer it looks as though the FJ44 powered jets are coming close to the same fuel efficiency on a fuel/trip basis of PW powered King Airs.


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 Post subject: Re: Allison Turbine Bonanza
PostPosted: 19 Oct 2013, 13:48 
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Username Protected wrote:
Manny,

From an operational standpoint on the Allison, what do you feel is important. Backing off the temp by 14C will double the life you say. Is that the sweet spot, and what life are you doubling? I have read that running at too low a temp in a PT6 will also cause problems.

Thanks,

Mike

What you are doubling is the life of the turbine hardware in terms of stress rupture life. As the turbine hardware accumulates hot time the vanes blades and flowpath hardware eventually deteriorate due to time at temperature. Backing down a small amount on the temperature significantly improves the life of the hot section pieces. If you look up Larson Miller parameter it will explain it further, but basically the deterioration of metal at temperature is a square of time, so a drop of 14C doubles the life and a drop of 28C quadruples the life, and drop of 42C results in 8 times the life.

Helicopter engines run a very different mission than a fixed wing application. Helicopters take off and use a high power setting for a few minutes, but then they cruise down near 60% power. Since the missions are shorter (typically an hour or less) they don't accumulate hot time like you would flying a fixed wing aircraft at a high power setting at altitude for missions that are more like 2 to 4 hours. Since the missions are short, helicopters accumulate a higher number of start/stop cycles and that actually sets their life.

Since your missions are longer you won't "cycle out" the parts as quickly. But conversely you will accumulate hot time at a much faster rate.

In a perfect world you would cycle out the turbine about the same time that you were getting deterioration from hot time and then you would replace everything at the same time. If you have to go into the engine because you are seeing performance deterioration (due to things like nozzle vanes being eaten by hot time distress) way before you cycle out the turbines it will cost you a lot more in maintenance. Conversely if you run it too cool you will be trying to decide if it is worth putting a nozzle set back into the engine if you already have 1,500 hours on it and it might just well go another 500 hours, but do you really want to be tearing back into it in another 500 hours? Probably not, so you didn't really get the value out of your hardware.

If you look at the takeoff rating temps in the TCDS for the B17 and the C20B (same engine, different gearboxes). They are both takeoff rated at 420 hp at 810C. The MCP temp is dropped about 25F for the turboprop and that doubles the life compared to the helo at max continuous. But helicopters don't fly that much at MCP and I'm just saying that hot time at MCP in the fixed wing application is going to accrue much faster than a helicopter that flies at 60% power most of the time.

If you run at max continuous all the time in longer fixed wing missions you are going to use up the hot section pieces a lot faster than is probably advisable. Backing off a small amount will improve the life at not much cost in speed and you stand a much better chance of not eating up the hot section pieces. Where and how you fly it, your mission length, your typical cruising altitude, your climb power and cruise power settings all impact the hot section life and eventually how much you pay to overhaul the engine when the time comes. You have to evaluate how you fly with how long the engine lasts and that takes data that you can only get from users. As engines get overhauled perhaps folks can contribute how they typically flew the aircraft and how long it went between hot sections and what they had to replace at how many hours.

I don't recall anyone at RR doing that and it really wasn't in their interest to compile that data and say to the customer "use these settings to optimize life". They were in the business to sell parts and the sooner you used up your engine, the more money they make. Were it my engine I'd be running somewhere between 20 and 30C short of "top of the green" to insure I wasn't abusing the hardware and wouldn't worry about it any more than that. That gives you a hot section life of between 4x and 8x that of a helo, and that's probably closer to the way the two aircraft are actually flown, so that's what I'd do. There probably is a "sweet spot" for the power setting, and were I a users with a half a dozen aircraft or more I'd figure out where it is, but frankly I doubt anybody knows for sure right now since there aren't that many turboprop users.

I'm not familiar with why the PWC recommends specific power settings. There can be a myriad of reasons, from vibration to fuel clogging and anything in between. You would have to know a particular engine to know why the would want you to not operate in a specific range and I just don't know why PWC recommends how you fly their engines.


Last edited on 19 Oct 2013, 14:54, edited 1 time in total.

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 Post subject: Re: Allison Turbine Bonanza
PostPosted: 19 Oct 2013, 13:59 
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Username Protected wrote:
Manny,

Looking at the specific range of FJ44 powered aircraft one can not help but be impressed by the increase in efficiency when compared to older JTD engines. A similar jump in turbine efficiency would be a significant move forward.

How would you compare efficiency of the turboprops (SFC lb/hr/hp) to that of the fan engines (SFC lb/hr/lbf)? To the casual observer it looks as though the FJ44 powered jets are coming close to the same fuel efficiency on a fuel/trip basis of PW powered King Airs.

Turboprops rule at speeds below around 350 kts. Above that turbofans start to compete. So at King Air speeds the aircraft should be a turboprop, and that is more efficient than a fan for the same speed and altitude.

You have to remember that altitude comes into the equation for system efficiency. The FJ44 aircraft are flying higher than the turboprops so their overall efficiency is improved by the reduction in drag that comes from a higher altitude.

Go higher and you burn less fuel for the same speed, or you can go faster for the same power and that extra speed equates to an efficiency gain.

Cabin size and payload also come into the equation. I don't think an FJ44 powered aircraft that would have mission fuel burn close to a King Air will have the same size cabin or payload that a KA does so that's a factor too.

The fact that the FJ44 is a better engine than the older JT15D is pretty obvious, but to compare with a turboprop that is flying lower to a jet that is flying higher is like comparing apples and oranges.


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 Post subject: Re: Allison Turbine Bonanza
PostPosted: 19 Oct 2013, 16:54 
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Thanks Manny,
You do a great job speaking engineering talk that is understandable to the average non engineer kinda of guy. Very educational reading for a piston guy that would some day like to fly behind a turbine.


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