Turboprop vs piston question

ArrowFlyer86

ArrowFlyer86

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This is probably a dumb question... but why don't more single engines use small turbines instead? Their reliability is better, they use jet fuel (seems preferable to 100LL), they don't vibrate as much (I think?), and from what I understand they require less maintenance.

It's kept me wondering why it's fairly rare to see one (like an LX7, Lancair PropJet, Pilatus Porter PC-6 and Piper M600 -- to name a few).

Are they that much more expensive to build? Or is there something in their construction that makes them not suitable or just undesirable for more smaller airplane applications? Can their size not be scaled down to produce horsepower reasonable for most singles?
 
They burn a crap ton more fuel. The overhaul cost on the pt6 the M600 uses is 450k+

The rv10 turbine conversion burns 32 gph at 100% power. Cruise was 21.7 gph vs 11.5 for the Lyconosaurus at a similar true airspeed
 
Racerx said:
They burn a crap ton more fuel. The overhaul cost on the pt6 the M600 uses is 450k+

The rv10 turbine conversion burns 32 gph at 100% power. Cruise was 21.7 gph vs 11.5 for the Lyconosaurus and similar true airspeed
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I gotta be honest, I'd be down to buy 2x as much gas (in cruise) if it meant that my engine was super reliable and req'd substantially less mx money!
Buttttt I would definitely projectile vomit all over an overhaul invoice of 450k$ lol.
Also had no idea the Rv10 had a turbine option. That thing is already pretty quick.
 
ArrowFlyer86 said:
I gotta be honest, I'd be down to buy 2x as much gas (in cruise) if it meant that my engine was super reliable and req'd substantially less mx money!
Buttttt I would definitely projectile vomit all over an overhaul invoice of 450k$ lol.
Also had no idea the Rv10 had a turbine option. That thing is already pretty quick.
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There is no RV-10 turboprop option per se. Someone stuck one on, as anyone is free to do since it’s E-AB, but it’s far from a practical option. The E-AB world has been chasing a small turboprop option for at least 20 years and none have proven successful. The cons just simply outweigh the pros at this point.
 
As displacement and power get smaller on turbine systems you don’t actually get much of a fuel burn savings. Hence it’s never been practical. Also JetA is heavy.

Turb Aero (sp?) is building a 200hp equivalent concept.

Also, go big or go home!


Edit: I’m in for 1/2 of an MU-2. Let’s discuss tomorrow…
 
Been wondering the same thing, but using the allison/RR 250 series. They seem reasonable for the lower altitudes (suck less? :) )

The extra ea-500 made a ton of sense to me, dunno why they weren't more popular. I think a single certified turbine is superior MTBF as two pistons
 
A few things to keep in mind…
Jet fuel is heavier than a gas. Between that and the higher fuel burns, I would estimate that the turbine Maule can carry about enough fuel to fly around the pattern with a pilot on board at max gross weight.

the yellow arc on the airspeed indicator goes away…redline is at the top of the green. A lot of GA pilots don’t want to give up the speed.

In order to make a turbine economical, you’ve got to go to altitudes that require oxygen.
 
MauleSkinner said:
A few things to keep in mind…
the yellow arc on the airspeed indicator goes away…redline is at the top of the green. A lot of GA pilots don’t want to give up the speed.
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Probably another silly question, but why would your yellow arc go away?
I thought that was related to the stress on the airframe, not the type of engine you have?
 
ArrowFlyer86 said:
Probably another silly question, but why would your yellow arc go away?
I thought that was related to the stress on the airframe, not the type of engine you have?
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My understanding is that yellow arc goes away with altitude because of the effect of TAS on components’ flutter. That’s why Vans does not advise bolting a turbine onto the RV10… goes too fast and goes into that TAS range that’s bad.
 
I only have "operating" experience, not "ownership" experience of turbine engines, so this may be worth what you paid for it:

- Yes, turbines are smooth. Much smoother than the best-balanced piston engine. It's quite nice.
- Comparing apples to apples is important. A great example is the Piper Malibu Mirage (350 hp turbocharged piston) vs the Meridian (500 hp turboprop). Or better yet the Malibu Jetprop conversion. On the Malibu Guru podcast, Joe Casey often talks about cost comparisons between the piston and turboprop versions. And his conclusion is that the annual operating cost of the turboprop is only a little higher than for a piston. Although the engine burns more fuel, you go faster, and jet fuel is usually cheaper per gallon. Plus, there is far less routine engine maintenance. Of course, if you need a hot section inspection or an overhaul, that does blow your piston-sized budget away.
- But you do need pressurization to really take advantage of the turbine.
 
ArrowFlyer86 said:
it meant that my engine was super reliable and req'd substantially less mx money!
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FYI: reliability is subjective to the comparison used. As to substantially less mx money" not really. Depending on the model there might actually be more items to deal with over a set period than with a recip. Collectively its these issues that keep the turbine conversion numbers so low. Regardless a turbine vs recip comparison at the private SE airplane level is more based on personal desire than a mission upgrade from a maintenance point of view.
 
ArrowFlyer86 said:
I gotta be honest, I'd be down to buy 2x as much gas (in cruise) if it meant that my engine was super reliable and req'd substantially less mx money!
Buttttt I would definitely projectile vomit all over an overhaul invoice of 450k$ lol.
Also had no idea the Rv10 had a turbine option. That thing is already pretty quick.
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Burn twice as much of a heavier fuel to go 3knots faster. The engine is lighter. But the thirsty engine requiring a heavier fuel kind of makes it a moot point. Plus powerplant cost itself. A big nope for me.

There's turbine Bonanza and 210 stc's out there and they're both just silly. Especially when you look at the cost of the conversion. 980k for the 210 Silver eagle conversion

Besides, I kind of like the smell of Avgas. Kerosene...nope.
 
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Racerx said:
Burn twice as much of a heavier fuel
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That weight difference (6 -> 6.75) seems to be kind of a distant second concern relative to the fact that it burns so much more!

Racerx said:
Besides, I kind of like the smell of Avgas.
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its-always-sunny-charlie.gif
 
ArrowFlyer86 said:
That weight difference (6 -> 6.75) seems to be kind of a distant second concern relative to the fact that it burns so much more!


its-always-sunny-charlie.gif
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The weight compounds the problem of burning more. 120 gallons of kerosene is 90 pounds of useful lost to 120 gallons 100LL.

Love Always Sunny.

 
There’s a turbine P210 at my old base and there are/were two different turbine conversion for the Bo. My understanding is all have some payload and range challenges.
 
I wish they were less expensive, too. They're less efficient than piston, but lighter and more reliable. Even taking out the cost of design, the materials for the engines are expensive, and the machining is, from what I understand, expensive and a bit miserable. High temperature/high nickle alloys.

Maybe if someone figured out how to make little engines out of some sort of castable ceramic it could work? I know GE and others have been making some parts out of exotic ceramics for a while now, but for smaller sizes, so probably less stress on the blades, it might be simpler/cheaper. I'm basing this on small things spinning at high RPM are a lot simpler to design than giant things spinning at high RPM.
 
It would be great, but the physics and economics aren't there.

At 150 - 300 HP ranges, the lower weight and size per HP of a turbine just isn't that much vs a piston. But it's a lot more expensive.

But with larger power requirements - such as in a King Air - the size and weight of a piston gets too large to be economic. You have to add more and more size and weight for smaller and smaller returns on power increases with pistons. Not so with turbines. And there is a practical upper limit on the maximum power a piston can ever deliver, which is below what you can get out of turbine.

I saw a video of a guy at Sun n Fun (?) promoting a small turbine. But the cost was twice as much as a piston. He didn't mention what the overhaul cost was.
 
My uncle flew a Tuebine Bo for a guy, for about 3 or 4 years in the late 90s. My first logged 0.25 is a pattern around Austin Mueler field, with his instructor while my uncle was getting checked out on the plane. So I have turbine time before my solo. My uncle told me it was pretty nice at 17k but the red line was always keeping it back. O2 was required to get any reasonable benefit and was a cost loser vs piston. But the owner didn't care of cost so it was fun for a while on someone else's dime.
 
I mean, if you guys want to burn jet fuel and be more efficient there's always the diesel option..

The DA-62 *twin* (the big one, not the smaller 42 twin) will carry seven people at 180 knots while burning 9 gallons per hour combined!

Turbines are cool as hell!

But they simply don't scale down and size well. The sonex jet is a great example of this
 
Tantalum said:
I mean, if you guys want to burn jet fuel and be more efficient there's always the diesel option..

The DA-62 *twin* (the big one, not the smaller 42 twin) will carry seven people at 180 knots while burning 9 gallons per hour combined!

Turbines are cool as hell!

But they simply don't scale down and size well. The sonex jet is a great example of this
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Not according to Diamond. 16.7gal/hr combined at 12k gets you 180KTAS. Even 60% power is listed at 11.8gph combined.

I like the concept of the FADEC engines. Turnkey mixture management is long overdue in this recreational space. Too bad it's too expensive to ever be enjoyed by us fixed timing magneto poors. *shrugs*
 
At the risk of stating the obvious, whether a turbine is an efficient or effective solution for any airplane depends upon the mission. Keep in mind, the Meridian is nothing but the Mirage airframe with a turbine engine strapped on.

Without question, the scheduled maintenance of a turbine engine is substantially higher than the costs of maintaining a piston engine. The tolerances of turbine engines are much smaller and, even though there are fewer moving parts, they are much more expensive parts. But the times between overhaul are also much greater on turbines.

I went from a turbo Aztec to a Saratoga II TC, which was great for trips without passengers up to about 500 miles or if I was willing to make a stop, trips up to 700 miles.

I started looking at other options to deal with longer trips and a growing family, including multiple piston solutions. I looked at some larger piston twins and the Mirage as potential options. What really settled me on the turbine options is my trip to Sun Aviation, which specialized in Meridian and MIrage maintenance. I became friendly with the guys that actually maintained the airplanes and they convinced me that the Meridian was the way to go.

The overall feeling of the people that worked on the airplanes is that you are asking for a lot out of a piston engine to produce sufficient power to achieve speed, run the electrical system and the pressurization. The PT6, on the other hand, is lounging around on the Meridian because it has been substantially derated. They told me that I will likely require at least two overhauls on a piston before you even need to think about a hot section on the turbine.

On that advice, I got a Meridian and flew it for seven years. It was a great way to get around for business and to do my charitable flying. In addition, it wasn’t until the Meridian that my family used the airplane for serious travel.

I agree with some of the comments above that the fuel burn and operating costs of a turbine don’t make a ton of sense if you have short missions and lack the ability to get into the flight levels. Keep in mind that the engine is operating at about 55% at idle power, so it is always burning some fuel. But for those missions that are between 300 and 700 miles, strapping on a turbine and going into the Flight Levels is a great way to go.

Abram Finkelstein
N685AS
 
It comes down to the fact Turbines only make sense if you need to fly high or need the HP a piston can’t provide. For everything else a piston is just a better option. You can do a lot of overhauls for the price of a turbine and the fuel you’d feed it.
 
I sold my 421B and bought a 425 about 9 years ago, the 425 burns more fuel, but is much faster and the fuel is cheaper. In my typical trips from FTY to Destin, I saved about 10-15 minutes each way. I burned about 15 gallons more fuel, I figured fuel cost was about $60 more per trip. Turbines are very reliable and seldom break, but when they do, hang on to your wallet!!
 
But the Celera 500L goes 400 KTAS on a piston engine. :)
 
It you do the math, the cost of a Lycoming/CMI engine which is roughly comparable to a RR250 for example; over the life of the engines. You will find the total costs are roughly equivalent (depends on assumptions).
However, the life of the RR250 with a standard extension program I believe is 5000 hours. How many IO-540s would you go through by then?
Very few owners of small piston planes keep the plane long enough to ever consider making the higher capital costs for turbines financial worth while.

And this does not even bring up the actual airplane considerations.

Tim
 
Not an engineer, but my understanding is hot metal tech requires super-sophisticated and expensive design and production methods - heck, the Chinese still aren't making reliable, first rate jet engines for fighters yet. I imagine sinking tons of $$$$ into developing a lower cost, smaller turbine doesn't make sense. I mean it might be technically possible, but the market might not justify the up front costs? My impression is most of the production turbines out now have had a long history of design, development, and refinement - they've evolved, as opposed to hitting the market successfully with a from-scratch design.
 
tspear said:
It you do the math, the cost of a Lycoming/CMI engine which is roughly comparable to a RR250 for example; over the life of the engines. You will find the total costs are roughly equivalent (depends on assumptions).
However, the life of the RR250 with a standard extension program I believe is 5000 hours. How many IO-540s would you go through by then?
Very few owners of small piston planes keep the plane long enough to ever consider making the higher capital costs for turbines financial worth while.

And this does not even bring up the actual airplane considerations.

Tim
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An RR250 goes for three times the amount of an IO-540. That’s not even a newer version. That’s an old C20. We used used to get overhauled 250 C47Bs for around $200K. A good one these days is twice that amount. Just the ECU will go for over $80K and a fuel pump (HMU) around $40-50K.
 
Turbines are great but not for very small airplanes - hard to find places to stuff in enough fuel. And conversions suffer from IAS limitations as noted here. They also operate a a different price point, and while over 5000 hours they may pencil out as integrally cheaper, most owner-pilots are looking at cash flow only.


On the other hand, spill some diesel aka jet-A on you fueling and you’ll wish you were gasoline powered. Ask me how I know…
 
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