A true GA jet engine

[tartare]

Hi Guys,
could someone with the appropriate mechanical expertise explaibn why development appears to be stalled on a true GA jet engine.
Am aware of all the VLJ powerplants... what I'm talking about is something small and very simple that could power the equivalent of a C172 or C152 size aircraft... with FADEC and one button start so inexperienced GA pilots couldn't hot or wet start it.
Aside from the power lag on spool-up, it seems a jet would be much easier to operate than a piston.
Cost versus available market?
There seems to be a thrust gap between large model a/c jet engines (which can power v.small lighties like the Cri-Cri) and the small turbofans that power cruise-missile sized vehicles.
Any insights - debate to be had?

[Mad (Flt) Scientist]

Very small gas turbine engines run into efficiency issues because of the fact that the flow through the fans, combustor and turbine must end up in a thinner and thinner ring. Even as you shrink the outer case diameter, you still end up with smaller and smaller blade heights. As that happens, the amount of the blade in the boundary layer in each case gets larger and large, and ghence less efficient. Whereas a piston-type engine is far easier to "scale" because the compression is fundamentally mechanical, not as dependent on efficient aerodynamics. So small gas turbines are always going to be relatively inefficient.

Things like small RC aircraft engines, and missile engines, can put up with the efficiency loss because they want power density - the model aircraft is constrained by the desire for scale respresentation at a sensible overall size, the missile by carriage and tactical considerations. But on a vehicle like a small GA aircraft, where volume or cross-section aren't at a premium (certainly for the kind of power outputs that make sense) there's no sense in having a tiny inefficient gas turbine when you can have a more efficient piston that doesn't drive the design size anyway.

My two cents.

[AtoBsafely]

As MFS has said, I think it will be a long time before we see a jet engine in GA aircraft, because of the cost of the powerplant. Even turboprop engines are out of the range on most GA owners, in spite of their ease of operation.
I think the first step will be to have an advanced piston using FADEC and a single lever (power+mixture+rpm) control. When aircraft engine manufacturers can offer us the convenience and economy of a normal automotive engine with the reliability needed in an aircraft, that will be a huge leap forward.
Still hoping.
O.J.

PS What ARE those Honda engineers doing?

[Pugilistic Animus]

MFS as always very informative:ok:


AtoBsafely, I agree---what a dream that would be---I guess the market is still too small for such innovation---perhaps why no interest from Honda and the like

[ICT_SLB]

Only a thought but there is a class of gas turbines that are (relatively) efficient & reliable - APUs. Why not use the electrical output from the APU to drive a large propulsor or ducted fan? It would depend entirely on the size of aircraft you were looking at but my guess is that it would be a creditable choice for a four-seater.

[AtoBsafely]

Just a gut feeling:

I think the efficiency loss of turbine:electric:mechanical is too great. APU on a dash8 is 200kW = 175hp, and able to drive a "150hp" electric powerplant. I guess the cost is a couple of hundred thousand USD, and I don't want to count on it as the only power (IAS) source! Oh! for a good 100kWh battery in a forced landing!

Mind you, even 40kWh would work wonders then.

OJ

[chornedsnorkack]

I think gas turbines on road cars have been tried, and they were inefficient compared to piston engines. Gas turbines on trains are bigger, but they did not compete well either. Gas turbines on water ships... how do those perform, compared to piston diesels?

If you want an engine offering high reliability and high power density compared to piston, what about Wankel engines?

[Granite City Flyer]

http://www.innodyn.com

[IRRenewal]

Gas turbines on water ships... how do those perform, compared to piston diesels?

A well known ferry company has stopped using a turbine powered craft between Harwich and Hoek van Holland. It had twice the speed of a conventional ferry, but seven times the fuel burn. With current fuel prices they couldn't operate it economically.

[chornedsnorkack]

A well known ferry company has stopped using a turbine powered craft between Harwich and Hoek van Holland. It had twice the speed of a conventional ferry, but seven times the fuel burn.

Yes, but it is an issue with waterships. Waterships (and airships) float, so they spend absolutely no fuel when stationary. The drag of a watership increases with the square of its speed - more than that when the speed is around wave speed barrier - which means that a ship which is slow can cover the same distance in longer time with much less fuel burn.

And conversely, a ferry which had high speed thanks to powerful diesel engines would also burn much more fuel than a slow ferry with a diesel engine.

[The Flying Pram]

If you want an engine offering high reliability and high power density compared to piston, what about Wankel engines?

Unfortunately Wankel engines are rather inefficient as well. Despite the staggering sums of money that Mazda have spent over the years, the fuel consumption of their rotary powered cars is not good compared with similar power piston engines. This is despite dual injection and ignition systems. I think for the time being conventional piston engines are the best choice for light aircraft. Whilst much can be done to improve their fuel efficiency, the problem (as usual for aircraft) is the small market compared to automotive uses, and also the product liability issues.

[jezzbaldwin]

If you are hell bent on burning Jet A instead of AVGAS (and with AVGAS prices where they are, who wouldnt be), how about the current Diesel Aero Engines? You will certainly get better operating economy, longer inspection intervals (no 50 hour checks), and jet operation simplicity.

Thats just my thought, but for the time being Diesels get my vote!

[chornedsnorkack]

Unfortunately Wankel engines are rather inefficient as well. Despite the staggering sums of money that Mazda have spent over the years, the fuel consumption of their rotary powered cars is not good compared with similar power piston engines.

But still better than gas turbine powered cars. Otherwise someone would have persisted with gas turbine powered cars.

[twistedenginestarter]

Doing a bit of research, it looks as though there is no particularly good reason why there shouldn't be small turbines. The ones they were developing for cars were lower tech than your RR Trent but still produced OK mpg. They fell out of favour because of problems with accelerator response and large amounts of hot gasses being vented. Also turbines like to be run fairly flat-out - alright for pressurised air liners but not so good for pottering around sub-10000 feet. The real reason is probably the same as why such old/low tech Lycomings and Continentals have lingered for such a long time. There seems to be a major barrier to introducing anything new.

[411A]

There seems to be a major barrier to introducing anything new.

Yes, and that major barrier is COST, considering the limited units and their application.

[tartare]

That's gobsmacking.
Not a pure jet... but imagine one of those puppies in a Cessna 180!!
The ultimate bush plane...

[18-Wheeler]

I think gas turbines on road cars have been tried, and they were inefficient compared to piston engines.

Toyota jet-engined test car. I don't know anything about it sorry.

http://www.billzilla.org/toyotajet.jpg

[twistedenginestarter]

Yes, and that major barrier is COST, considering the limited units and their application.
I suspect it's more than that. I remember Porsche were developing an aeronautical version of their engines. Now they wouldn't go into that if it was simply not cost effective. They're also not short of a bob or two. They could buy VW with the change in their pocket. I suspect ridiculous barriers appear. Maybe product liability in litigation-mad America; maybe stupid FAA/JAA rules and regulations. I suppose it doesn't matter too much anymore. Boeing and Airbus can afford it and the little people can go under the radar with Rotaxes. Certainly in the UK you can't fly seriously unless you're rich and if you're rich you can afford a million or so on one of these new single-engine jets.

[Genghis the Engineer]

I worked on a study of turboprops for small aeroplanes a little while ago. It's all proprietary data (sorry) but I can give you a few bulletpoints:

- At GA speeds, piston engines are much more fuel efficient.

- But AVGAS is much more expensive than AVTUR

- As a result, you roughly break even on fuel costs per hour and on payload (the engine becomes lighter, but the amount of fuel you have to carry becomes greater). If however they start taxing AVTUR anything like AVGAS, then all bets are off and the GT engine becomes much more expensive.

- There are some small turboprops out there in use but nothing certified - they're all in use on amateur built/experimental aircraft.

- The killer however is noise. It's almost impossible with current technology to get a small enough gas turbine engine, quiet enough to be socially (or in some countries such as Germany legally) acceptable. The best fix is probably to try and deflect all the noise upwards, but nobody's really tried this yet.


Incidentally, I think that Rover (British now-dead car company) built a gas turbine engined car in the 1950s or 1960s; I'm pretty certain I've seen one in a museum somewhere, but couldn't tell you much more than that.

G

[Cyclone733]

The engines in the Diamond DA-42/DA-40s are fully FADEC and run off of Jet-A1. I think I read somewhere they are introducing a version with 400 odd Hp. Easy to use single lever per engine damn sight easier than the good old fashioned 4 levers

[chornedsnorkack]

- At GA speeds, piston engines are much more fuel efficient.

- But AVGAS is much more expensive than AVTUR

- As a result, you roughly break even on fuel costs per hour and on payload (the engine becomes lighter, but the amount of fuel you have to carry becomes greater).

Er, why does a piston have to burn benzine? A Wankel needs benzine (because of the geometry and compression ratio, diesel cycle does not work with Wankel engine), but what is wrong about a piston diesel engine?

[Genghis the Engineer]

Cyclone - those are Diesel engines: diesel fuel and gas turbine fuel are basically the same (kerosene).

G

[rigpiggy]

Wankels actually would make a pretty good diesel, as their closer to a 2stroke in function, ie higher power/weight ratio. The reasons why they are inefficient as a gas engine has to do with intake/exhaust overlap, and low compression ratio due to heat annealing the tip seal springs.
http://www.der-wankelmotor.de/Motoren/Rolls-Royce/rolls-royce.html

[The Flying Pram]

rigpiggy, thanks for the link. I seem to remember that Rolls Royce made a very large Wankel diesel many years ago to power a military tank (I think). I believe it used two stages in a type of supercharger arrangement. The other cause of poor efficiency is the large surface area to volume ratio of the combustion chamber. This causes considerable heat loss, thereby wasting much of the useful energy in the fuel.

[con-pilot]

http://i28.photobucket.com/albums/c246/con-pilot/63turbinb.jpg

The 1963 Chrysler Turbine Car.

[Charles E Taylor]

An interesting thread.

Just to prove the I.C. engine is not yet finished.

There is some very interesting work being done in the UAV engine world.

http://gltrs.grc.nasa.gov/reports/2006/TM-2006-214342.pdf

C E Taylor