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Turboprop engines are more fuel efficient1, lighter for the same power, mechanically simpler and consequently more reliable. They are also slightly easier to operate (no need to fiddle with mixture) and burn cheaper fuel. As a result they replaced piston engines in all transport aircraft long ago. So why didn't they find their way into small GA planes (basically everything above 8-10 seats uses turboprops, but in the 4-seat category, nothing does)?

1Actually looks like I was mistaken. Reciprocating engines seem to be more efficient.

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Similar Question. –  Farhan Apr 21 at 13:15
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If I remember correctly my lectures on propulsion, turbines are more efficient than piston at higher altitude and thus not that much more interesting for small GA planes... –  Ludovic C. Apr 21 at 13:17
    
@LudovicC. I thought the high altitude was the jet vs. propeller part (not turbine vs. piston), and the reason why many modern airliners have turbofan rather than pure jet engines. I believe a lot of helicopters use turbine engines, and I think they're quite happy flying low, or am I missing something? –  falstro Apr 21 at 13:30
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@LudovicC.: That's not efficiency (energy delivered per unit of fuel) but power. Maximal power declines slower with altitude for turbine engines. But since it also declines for propeller, turboprops are usually limited to about FL250 anyway (while turbojets/turbofans happily operate above FL400). But turbines should be more efficient (burn less fuel for the power they are delivering) at all altitudes. –  Jan Hudec Apr 21 at 14:43
    
@Jan Hudec: Don't tell that to the crews of airplanes like the Tu-95! Turboprops can fly as high as subsonic jets if they are designed properly. The power over altitude behavior is the same, only at high Mach numbers the props put an earlier limit to the maximum speed. Think of a turboprop as a jet with a large, geared, unducted fan. –  Peter Kämpf Apr 22 at 12:39

3 Answers 3

up vote 2 down vote accepted

First, piston engines are more efficient than turboprops, so their operational cost are lower. This also means that the system mass (engine plus fuel) for a trip is lower once you go beyond small ranges. In a helicopter, the engine mass is more important, because average flight times are much shorter, so you find many helicopters with turbo engines and only few with pistons.

Next, there is a well developed infrastructure for maintenance on those pistons, and new turboprops would need new, expensive infrastructure (trained people, tools, spare parts ...)

And then there is simply no incentive to develop a new GA engine. The expense to get it certified is too large for the rather small market. This is the same as for Diesel engines.

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Efficient in general, or efficient at lower altitudes? –  egid Apr 21 at 23:21
    
I don't think the development argument applies here. Back when the current engines and aircraft were developed, turbines already dominated the higher power market. –  Jan Hudec Apr 22 at 4:47
    
... but of course the first argument is enough. It seems reciprocating engines are indeed more efficient. (Diesel engines are even more efficient than spark-ignited ones, but they are also quite a bit heavier). –  Jan Hudec Apr 22 at 4:52
    
@egid: Efficient in general, if a sufficient turbocharger is fitted. The more the engine is charged, however, the more the thermodynamic cycles of piston and turbo engines converge. –  Peter Kämpf Apr 22 at 9:39
    
@Jan Hudec: Turbo engines make more sense in the higher power settings simply because the airplanes with more powerful engines fly faster. With a piston engine, the power is constant over airspeed, so thrust goes with 1/v. Turbo machines can make better use of the kinetic energy of the airflow, so their thrust drops less with speed. Once you want to fly at Mach 0.6 or faster, piston engines don't cut it anymore. At lower speeds, turbines never were a real option, except for helicopters. –  Peter Kämpf Apr 22 at 9:51

This is probably due to certification inertia, at least in some part:
A Piper Archer is certificated with a specific engine type (If you scroll down to page 21 of the TCDS you'll see it's a "Lycoming O-360-A4M"). Piper can essentially make as many Archers as they want under their production certificate as long as they conform the type certificate.

Changing the engine is a major change to the type certificate, and would require more certification test flights, paperwork, and ultimately money from the manufacturer, and sales would be uncertain (because it would be different -- A flight school flying 50 planes that are all powered by Lycoming O-360s is not going to want to take on one turbine-powered plane, it would be a maintenance headache).

It's worth noting that for some portion of the avgas-burning GA fleet efficiency gains similar to (or better than) turboprops can be achieved by converting from old-style Lycoming and Continental engines (which are effectively 1960s engine technology at best) to engines like the Rotax 912 with modern electronic ignition and engine management systems. An 80HP Rotax would pull a Piper Cub along just as well as an 80HP Continental - The barrier is again in certification (and cost to modify existing aircraft).


On the technical end of things, a turbine engine is overkill for most small GA planes. We're talking about aircraft that usually require 100-300 horsepower (and if we're considering light GA training aircraft you're usually below 200 horsepower). The popular PT6 turboprop engine starts around 500 shaft horsepower and goes up from there.

Also while a turbine engine is a relatively simple device a turboprop gets to be pretty complex -- The turbine section is still spinning at turbine speeds, and while this isn't a problem for a jet engine on a turboprop it could easily make the propeller tips reach supersonic speeds (the "Loud and Inefficient" operating range). This necessitates a reduction gearbox to produce an appropriate shaft speed, which in turn necessitates maintenance of the gearbox.

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No, I don't buy the argument about certification. If that was the case, the newer designs would be certified with turbines from the start. And "newer" does not have to be all that new, since turbines dominate the higher power market at least 60 years. Or we'd at least see some conversions. But there are none, so there has to be a technical reason why turbines with less than ~150-200 kW don't exist. And it's not just gearbox. Most reciprocating engines are geared as well. –  Jan Hudec Apr 22 at 4:42
    
@JanHudec You can "buy" whatever argument you like - In reality it's probably the confluence of several :-) Specifically Re: certification though, the simple fact is certificating a major aircraft component (like an engine) is an involved process: Irrespective of any possible technical impediments, a new 200-hp turboprop engine would still need to go through the FAR 33 (engine certification) process in order to be used in a certificated aircraft, even a "clean-sheet" design built for that engine. That's lots of money and time for an uncertain return on investment for the engine company... –  voretaq7 Apr 22 at 4:57
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The engine currently in use had to go through that certification process and it did so when turbine engines already dominated the higher power market. So back then a decision was made to stick with reciprocating engine for low power uses and that decision was not affected by certification because either variant had to be certified. –  Jan Hudec Apr 22 at 8:39
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@JanHudec most, if not all, of the powerful GA piston engines (large 4- and 6-cylinder designs), are developments of engines that have been around since the 1940s or 50s. There is no way that turbine engines dominated that time period. –  egid Apr 22 at 15:25

First, for a given power rating, a turbine engine will be a lot lighter than a piston engine. Think about what that does to the aircraft's balance: the nose gets a LOT longer, not to accommodate the hardware put to put the weight where it needs to be. Of course, this only applies to conversions.

Second, a turbine engine will cost a lot more than a piston engine. Smaller turbines cost more per unit of power than large ones, so overall you will be paying more for the same result. The engine might use less fuel over it's lifetime but you still have to pay for it up front.

Third, while turbines in general require less maintenance, when something does have a problem the bill is not for the fainthearted. And you can cook a turbine pretty easily.

So in total it just comes down to money. If you really want one, Soloy sells a conversion kit for a Cessna 206 that will put a 450hp Rolls-Royce turbine in the nose and make you the envy of every other light aircraft owner in the region. The kit starts at $275,000. Plus engine. So you are looking at over half a million to modify an airframe that costs a bit less than that. If you have that much spare cash, pick up a matching MTT Superbike with an only slightly smaller turbine. As Jay Leno said, it's "the best ever motorcycle for shutting up the Harley guys."

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