GA and turbines

[Ludwig]

Following on from the MoGas thread and the ever present threat of the demise of AvGas, a lot of work is going on with diesel engined aircraft, but not a lot is said about turbines.

Are they so frightenly expensive that they are unlikley to be attractive to GA?

Instalation costs aside, surely they would be less of a maintaince issue than any piston engine. Those that know about these things, can you say how a turbine might, or might not cope with the demand of aerobatics. Could I fit one in a Pitts?
:D

[Skylark4]

I`m no expert but I think most turbines are too powerful. I was at Oshkosh last year and saw a nice conversion on an APU, I think it was the one used in the Chinook. The basic turbine, before adding pipes and the gearbox etc. seemed to be 18 inches long and a foot in diameter. The output through the prop was 300 hp.

Mike W

[LowNSlow]

The old Rover II gas turbine was used as an APU in the V bombers I think. It developed 100 bhp at the output shaft. A small and neat unit, one was fitted to an Auster and also to a Currie Wot single seater biplane (Hot Wot).

Unfortunately a turbine's specific fuel consumption is much higher than an equivalent output piston engine thus requiring bigger fuel tanks. Also the level of precision engineering required on the blades far exceeds that of a recip engine with the appropriate costs! Until there is a method of precision casting relatively cheap ceramic turbine blades which require no machining then cheap turbines are in the same level of reality as anti-gravity devices :D

[Trash Hauler]

Overhaul costs are part of the reason turbines are not too popular on GA aircraft.

To overhaul a 6 cylinder piston engine costs in the region of A$35,000. To overhaul a simple turbine starts at greater than A$100,000.

The other reason is fuel consumption. At low alttudes a turbine consumes a lot more fuel per horse power generated than a piston.

Maintenance licensing is also more difficult as all turbines have a type rating requiring specific courses where as a pistons up to a certain power (from memory 500 Kw) are under a group license and don't require specific courses.

Cheers

Trash Hauler

[Genghis the Engineer]

Also (and I agree with TH), small turbines run at very high speeds (in the order of 20-40,000 rpm), so getting them down to an acceptable noise level can be problematic.

G

[Mark 1]

Genghis,

I'm not sure I understand your argument on noise due to the high RPM.
The shaft speeds will put all the compressor and turbine blade passing frequencies effectively out of audible range.

Jet mixing noise should be low if the maximum work has been extracted by the power turbine prior to exhausting through the nozzle. And, as it scales with 7th or 8th power of jet Mach no, it is usually insignificant on turbo-props compared with pure turbo-jets where the thrust is generated with jet velocities of 300-500m/s.

Turbomachinery generated broadband noise may be relatively more significant, especially with the shorter blade chord length. I doubt that it could be dominant.

Combustion noise can only usually be heard at low power settings (e.g. the occasional hoot of taxying airliners).

That doesn't leave much but propeller noise which should be independent of the powerplant.

I don't have much experience of small turbines except hearing APUs running on the apron. Do you have any more specific knowledge of the noise sources?

I think that specific fuel consumption is probably the biggest problem at present (and cost).

M

[Genghis the Engineer]

I'm not the worlds greatest expert on small turbines, I'm mostly quoting experience. On the SAAB Gripen for example, the perceived noise level from the APU is around 4 times that of the main engine during ground checks, and the small turbine (made from a truck turbocharger unit I seem to recall) we had in the underground lab when I was at University needed 3 layers of glass AND ear defenders to become bearable. I could be wrong about RPM being the main player, but certainly all my experience is that small turbines tend to be significantly louder than large ones.

G

[Cyclic Hotline]

I think you guys have covered a lot of it here already. One of the principal reasons is the lack of small powerplants that are useable for this application. Cost is also a definite consideration and if the piston engine can do what you want, then it is generally the way most people go.

The smallest practical free turbine engine in common use is the Allison 250 series. Although it was principally designed for helicopter applications (Jet-Ranger, MD 500 etc) the availability of a compact, small and light engine developing lots of power attracted considerable attention as a replacement for various piston engine applications that simply would not physically permit a larger piston engine to be installed.

Joe Soloy figured he could install these engines in the Hiller 12 and Bell 47 helicopters, making some pretty potent machines and he created an entire business around it! Later on he realised that the same potential existed for putting more hp into fixed-wing aircraft that did not physically have the space for a larger piston engine (generally because the next size up was a radial)! As a result Soloy created and certified the 250 powered turbine Cessna 206 and 207. There have been many other mods including Cessna 210, Beech Bonanza's, etc.

Larger aircraft utilized the PT-6 and Garrett 331, generally as replacements for radial engines. Pretty much all the good radial powered aircraft ended up with a turbine engine at some time (Beaver, Otter, Beech 18, Goose, DC-3!, etc..) but perhaps we are at the limits of General Aviation now.

The penalty with these engines is the balance between performance and fuel consumption. Often some mitigation is achieved from the increased speed, allowing an equal (or improved) effective range. The ultimate decision is whether you can gain a really useful performance increase in return for the added operating cost and complexity (plus the cost of engineering and certifying the change).

Noise is not a significant issue with these engines, as they are very quiet to start with and because the engine inlets are buried deep within the cowl, any compressor noise is well muffled. Coupled to low rpm props, these aircraft are some of the quietest around, especially compared to their piston engined variants! We were once air testing a Caravan amphibian, landing in a very popular lake on the outskirts of a big city. Amongst the water-skiers and sail boats, no-one paid the slightest attention to the aircraft, as in many cases they never even heard it.

In the search for pretty low horsepower turbine power, the APU's are really all thats out there. In their normal design configuration they are designed to produce the maximum amount of power from a minimum of weight and complexity. This is achieved (typically) with a single stage centrifugal compressor - hence that ear splitting racket they make. They are also designed for 2 running speeds, On and Off! Smaller and smaller jet engines are being produced today, and there is always the possibility that someone will produce a turboprop variant at some point. I think that the diesel engines that are currently nearing certification are the real way forward for General Aviation and will end up as a retrofit replacement for many existing aircraft.

I have operated a Soloy 206 and a Garrett and Pratt powered Beaver (all on amphibs) - so how do they compare to their piston variants (in my particular application)? The 206 was WAY better than any piston 206 (including the 550 conversion), but a piston Beaver was a better machine all round - so it got replaced. The turbine Beavers really perform well, but the piston one was fine (plus you gotta love the sound of the 985)! The turbine Otter was a great machine but the Caravan beat it hands down for speed and range (but loses on ability to get out of the water) - so its all in the reckoning!

The problem for any upgrade to an aircraft is considering the cost of the modification, you automatically end up comparing performance with the next size aircraft up. Sometimes though there are limiting performance factors that require the smaller aircraft - and boy, I've seen some beautiful machines out there!

As far as aerobatics - not sure about the Allison (don't think I'd trust it for that), but the PT-6 is in the single engine military trainers, so can't see it being a problem - if you've got the money that is!!!! ;)

Anyone interested in more info;

Soloy conversions - (http://www.soloy.com/206207.html) check out that 700 hp mod - now that would make a great 206 amphib! :cool:

Some motor and application specs. (http://www.jet-engine.net/civtsspec.html)

Soloy 207 jump ship performance! (http://www.utti.org/english/)

[Captain Stable]

Reports in yesterday's Herald Tribune of a jet engine being developed in the USA for very small business aircraft, the Williams EJ22 (?). Apparently fuel consumption, size & weight are the smallest yet, power output extremely high.