Turbine engine an alternative for small aircraft?
 

As discussed, the availability of AVGAS will be a problem in future. Alternatives are sought in diesel and electrical engined aircraft.

Why is the turbine engine no option for smaller aircraft (e.g. C172, PA28)? I've seen some small jet engines (designed for gliders).

Is it because of the relative slow speeds (< 150kts) of the aircraft?
Is it because of the relative low altitudes (< FL100)?

I was just wondering.

Low speeds.

Turboprops and piston engines break even on fuel consumption per distance at around 140 knots, below that there's usually a piston engine advantage.

Turbofans and pure jet engines need 250kn+ to start to be efficient compared to turboprop and piston engines.

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It is potentially a very good option.

However the cost is prohibitive for most - several £100K at the very best, and turbines are not efficient low level so you probably immediately restrict the market to the touring instrument rated pilot market. That is a very small community and then they need to be high net worth individuals. Unfortunately all of that doesnt add up to the greatest incentive to create a new turbine single.

On the other hand diesel power plants (as developed for Diamond aircraft) achieve the benefits of using Jet and are efficient low level. Its unlikely they will ever provide the speed of a turbine and there are some issues an the high FLs but for the reasons given above that is not the volume GA market.

I suspect as diesels improve they will replace Avgas engines, but I also suspect it is very unlikely turbines can be made cheap enough to offer a viable alternative for the majority.

Out of interest why will AVGAS be a problem in the future? Refineries choosing not to produce it any more?

Thanks.

And what about a electrical engine with a turbinegenerator which runs on Jet-A1 (I believe there is a car build with this principle).

Governments don't like the lead content, for environmental reasons.

All engines have compromise elements.

They can produce power over a wide rev range, but not be so efficient at doing that, or produce power within a relatively small rev range, with greater efficiency.

Turbines, particularly, are examples of the latter. They're a bit like "peaky" two stroke engines. Very narrow power band, fairly inefficient (and gutless) outside that power band.

Since a lot of lighties spend significant portions of their operation at other than near-maximum power, the inefficiency becomes significant.

Add to that the inefficiency of running the jet at low levels and speeds, and it compounds.

This is exactly why we will see gensets/APU's (possibly turbines) providing electric charge to a battery and have the prime mover, i.e. the thing spinning the prop, be an electric motor. My prediction is that this will happen much before we'll see smaller turbines. It's going that way with cars rapidly already (Chevy Volt, Prius, Fisker etc). Electric motors are brilliant prime movers with almost no drawbacks: much higher power-to-weight ratio than turbines, almost no moving parts, TBO only limited by bearing life, don't need oxygen and can therefore fly high etc.

Would the narrow efficient band matter with a wobbly prop?

My understanding was that small turbines were inherently inefficient because of the gap between the blade tips and the outer casing, which becomes proportionately bigger as the turbine gets smaller.

I agree with Adam that there may be a role for turbines in hybrid aircraft - you can do a lot of fun things with electric motors, but the idea of battery powered aircraft has always seemed to me to be a case of 'greenwashing'. If you wanted to spend money on reducing carbon emissions, there are doubtless much better ways of doing it. The other advantage of a hybrid system is that you get electric reliability - which can be phenomenal - leaving a fair amount of time to complete a go-around or find a better field, in the event of an engine failure.

I agree that the idea of using an electric system as an intermediary is not a great one, though I know this has been done in bicycles with efficiencies as great as 80% (i.e. pedal into a dynamo that powers a hub motor in the back wheel, rather than using a chain).

I have often thought that the ideal would be to combine the engine and electric motor, so that the motor could play a triple role as a starter motor, dynamo, and to increase peak power output during take-off or aerobatic maneuvers. This way, the engine would feed power directly into the propeller without needing to convert it to electrical energy and back again. You could operate the petrol engine at 100% power throughout the cruise, and optimise it for efficiency at this setting.

Some of the newer lithium polymer batteries have astonishing power densities. For example, a battery weighing a kilogram might be able to supply over 30 volts and 200 amps for a minute or so - about 10 horsepower. Brushless motors have a very good power to weight ratio too - my 3 horsepower motor weighs about 700g. So you can get large transient amounts of additional power with a relatively modest increase in weight.

This would be more important for cars than for aircraft though, because the difference between cruise and peak power output is greater for cars.

Already been done.
Back in the 60s, a Chipmunk was re-engined with a turboprop of about 90shp (I think).
Reg was G-ATTS. It ran at pretty much constant rpm and speed was controlled by prop pitch. I remember the flight test report saying if you reduced to ground idle in flight, it was like running into a brick wall!
Take a look at De Havilland Canada DHC-1 Turbo Chipmunk 21, G-ATTS, Hants and Sussex Aviation

The main issue with small turboprops is fuel efficiency. At best they use about 0.7-0.8lb/BHP.hr whereas a Lycoming is about 0.45 and a good turbo diesel can get down to about 0.33. That's about 20% overall energy efficiency for the turboprop against 30-40% for pistons. Electric powerplants can get nearer to 90%, but need much higher energy storage densities to compete. There are several promising technologies in development so expect advances before long.Turbofans and pure jets are even worse below about 0.4 Mn and can be very noisy.

You can trade some of that high fuel consumption for the light-weight powerplant, high altitude performance, reliability, fuel availability and low flamibility.

With the high initial cost and all those issues, it simply doesn't make sense in our end of the GA market.

There was a recent attempt to make a small turboprop with the and RV-4, but development costs and problems killed that. Cheapest options at the moment are offerings like the Walter 601 at about £80k.

Take a look at what is going on the world of model aircraft (R/C) ... almost at the stage where electric is the norm, due to massive improvements in both motor and battery technology; incredible power:weight and at reasonable cost. Can only be a matter of time before it is scaled up to full size. In fact there are 1/4 and even 1/3 scale (of full size) models already flying electric.

Also many model turbines flying as well, though not turbo-prop, maybe that's the next development. Model turbines flying at £1200-1300, surely scaling up could result in a financially viable alternative?

I have a big electric r/c helicopter, and I'm aware of the big electric planes.

The difference is that most r/c pilots tend to lose concentration after a few minutes, so the very limited flight times aren't a problem. My helicopter has been souped down to fly for up to 20 minutes, but people who're into heavy duty aerobatics make do witth flight times of 4-5 minutes. Aerobatic planes and ducted fan jets might have an endurance of 10 minutes, and my Formosa will make about 20 minutes flight time.

There's no question that electric propulsion can work for larger aircraft and a number of human-carrying electrics have already flown. However, in most cases the flight times have been less than an hour, which isn't enough to do anything useful with when you take the VFR reserves into account. It's battery technology that's the limiting factor.

Again, a number of people have used model turbines to power full size aircraft, but their fuel efficiency tends to be abysmal. In the model flying world, jet power is about the 'cool' factor, and efficiency isn't important. If they were to be built using exotic materials in order to work at higher operating temperatures and work more efficiently, then they would become much more expensive.

You could probably use a model helicopter turbine to drive a propeller, though to my knowledge this has not been done.

RC model turbo-props already exist so no need to modify a heli engine. As well as the cool factor RC jets also have more realistic crashes. Ie, fireball ;)

Have a look on youtube at rc king air for an example of turbo-prop

It's not only the lead content (which despite the "low lead" designation is allegedly higher than it ever was in any leaded automotive fuel....), it's the required costly infrastructure and logistics (e.g. quality control, dedicated supply vehicles and hardware) for a product were worldwide sales are dwindling to such an extent that it is almost not profitable to produce and market.

I think the fact that alot of small aircraft especially trainers do a lot of small short flights and turbines maintenance schedule is with regards to start ups and shut downs plays a part

Avgas is a 'leaded' fuel, seen as very 'not' environmentally friendly. All manufacturers looking to produce a "lead free" option. No Avgas produced in UK these days.

Forget electric powered aircraft its not even on the horizon without some fundamental new technology to either store the energy or to generate the energy.

(I mean in the sense of a aircraft delivery even vaguely similar performance to current combustion engines).

Sadly to hope otherwise is cloud cuckoo land. ;)

Re: model turboprops, if you're nearby be sure to pop into the Midlands Model Engineering Exhibition (Today through Sunday). A large stand just inside the entrance doors is loaded with numerous model turbines and staffed by some highly knowledgeable people who understand subjects like efficiency. One project is a working turbo-electric locomotive in 5" gauge!

PS edited to add it's 15 mins from Wellesbourne.