But in aviation we want high revs, to get the maximum power out of a minimal engine weight.
Not necessarily. The prop blade tips can’t go anywhere near mach1, hence our ~2500rpm engines, which requires a lot of torque (at ~250HP) which requires big pistons, which requires a physically large engine, but the engine is simple, and due to the low rpm a lot of things are not critical so e.g. you can comfortably use pushrods.
Higher rpm also doesn’t give you a better SFC. You get high pumping losses (and general engine friction etc) in proportion to how fast you are thrashing everything around.
As the Rotax 912 illustrates, reduction gears are worth their weight today even in small engines,
It could also be that is why Rotax are stuck in the very small engine market 
If I was in their business I would be seriously looking at doing an engine which is IO540 dimensionally and mounting compatible (like the SMA diesel).
as they were in yesterday’s larger Jumo, Merlin, and the multi-row US’an radials.
Those engines however needed copious amounts of attention, often after every flight. The old P&W radials use to get through cylinders like they were spark plugs.
Those engines however needed copious amounts of attention, often after every flight. The old P&W radials use to get through cylinders like they were spark plugs.
Not so much the Jumo 205, it had an excellent reputation for its reliability. Lufthansa used it in scheduled service. That was the 1930s, very early after the first powered flight. Only 30 years before that the car with combustion engine was invented.
If I was in their business I would be seriously looking at doing an engine which is IO540 dimensionally and mounting compatible (like the SMA diesel).
Why not use the SMA engine? It’s there today and development costs have been written off mostly. Rotax had a 6 cylinder aero engine program with 225-275 HP ca 2006. They canned it. It’s incredibly expensive to develop an engine. Mercedes-Benz typically spends north of 1b€ on a new engine which is just an iteration over something they have and that’s a company with some experience.
There is a reason the SMA has gone almost nowhere. I don’t know what it is – maybe the totally crazy price?
The retrofit diesels all seem to be priced by a bunch of people sitting around a table and working out a case for a busy FTO doing 500hrs/year, and then going just 20% under the avgas (the status quo) option.
That is how most business is done. Nobody wants to bomb the market.
With a diesel in a PA28 you can do flights like this (that really happened, but I am not saying any more).
There are a few Thielert retrofitted PA28 and C172 in Europe, but I have never heard anything about their operation, whether they are good or bad. Does anyone have any first hand experience with these versions? I’ve only flown DA40/42 with diesels so it would be interesting to know how they compare. The DA40D is really a three place aircraft, which is what I would expect for a PA28 or C172 diesel as well, so they should be closely matched.
I flew with a student today in our PA28 and there is one thing that I really do not like with most spam cans. There’s no space! As a trainer the PA28 does ok, but the lack of elbow room really is annoying. Especially when one or both occupants are rather large (in this case exacerbated by bulky clothes), an hour feels like 14 in a torture chamber.
With modern aerodynamics and construction techniques it should be relatively easy to build a very roomy and comfortable trainer that cruises 100kts on 20-25 lph, or if you throw in a diesel, perhaps 15-20 lph. Where are they?
There is a reason the SMA has gone almost nowhere.
It’s still ongoing. You know how slow our industry is. The first version of the engine did not deliver on its promises (anaemic at FL100) and the 2nd generation is still rather new. The company is backed by SNECMA (one of the largest turbine makers in the world) and they have an OEM deal with Cessna which is hopefully going to result in a shipping product soon. A 6 cylinder version is under development (why should I doubt that when they have already developed and certified two engines?). Durability seems to be excellent, from the start it had a TBO longer than Lyco/Conti and it’s just the start.
Not all bleak there. The price is a chicken egg problem which they will have to solve. That and power at altitude are my main two concerns.
Let’s start a company, buy the TB20 TC from Socata, license the SMA engine and kick ass. The goal is to outsell Mooney. 
If I had to invest money in the development of aero engines for light aircraft (say motorglider, microlight, and single engine tourers, maybe even light twins) I would not spend a single Euro on combustion engines. The future is the electrical motor, supplied with electricity from a fuel cell, e.g. Methanol-air powered. The overall efficiency is in the order of 80% (as opposed to the <50% of the St*i*rling engine, the most efficient combustion engine), it is small, leightweigt and absolutely clean, the only source of noise is the propeller. Methanol can be obtained from oil, natural gas or biomass. It is very easy to build in some redundancy, e.g. two motors driving the propeller, an extra battery that can sustain flight for some time in case of fuel exhaustion or fuel cell failure. Smaller aircraft that are only used for daylight VFR flying can have solar cells on their wings that supply a large amount of the required electricity, making for a real “green” aeroplane.
Smaller aircraft that are only used for daylight VFR flying can have solar cells on their wings that supply a large amount of the required electricity, making for a real “green” aeroplane.
That is hardly enough power for the GPS. In Germany peak solar energy is 1000W/m2 and solar cell efficiency about 15% while a 4 seat aircraft needs about 70kW for cruise.
While I’m all with you, I think the car industry should be the one to come up with these technologies and the airplane industry can adapt them. Volkswagen & Co can spend hundreds of millions to improve the rear mirror by 2% so they have the R&D funds to come up with new technology.
buy the TB20 TC from Socata, license the SMA engine and kick ass
That’s been tried; recently by people I can’t talk about. I recall hearing that Socata are unwilling to talk…
Also the deal would have to include the purchase of the spares operation which is worth a few million €. Unless you get that, you have nothing.
Then you need to get EASA145 and FAR145 approvals, and EASA21 so you can in-house signoff some mods. The first two can be transferred from any previous holder even if it’s an empty hangar now. The last one is harder.
So maybe €10M to get started.
6 cylinder version is under development
Still IO540 compatible size?
I think the 4-cylinder SMA is the right model for a TB20 sized airframe. From the looks of it, it should fit. It does fit the C182 which also has a (T)IO-540. The 6-cylinder engine will use the same cylinders/pistons, a principle that we are all familiar with…
That’s been tried; recently by people I can’t talk about. I recall hearing that Socata are unwilling to talk…
Nobody has come up with a good plan how to increase the total size of the GA market. As long as that doesn’t happen, there is no business to be made. Cirrus as the market leader is just an OK business, nothing more. Cessna as the other big player isn’t probably even a business at all. Growing by taking away somebody’s market share is much harder than growing with the market. Cirrus as market leader is worth $200m, Tesla as a complete underdog with not much unique technology is worth billions, more than half of the European car industry combined. It’s all about growing markets and totally addressable market. Only when people think that something is going to explode in size — like the personal jets a few years ago — will we see significant investment.
Those engines however needed copious amounts of attention, often after every flight. The old P&W radials use to get through cylinders like they were spark plugs.
In actuallty, P&W radials in the size ranges under discussion are pretty reliable. A good example is the R-985 450 HP Pratt – 18,000 of them were flying reliably on the 9,000 Beech 18s produced every day and night for decades. Another good example is the R-670 220 HP Continental – I know people flying these commercially today, every day, giving sightseeing tours. They don’t need copious attention, or attention after every flight, or anything close. It was when they were trying to make 2,000 HP plus out of them that they exceeded their basic capability. Up to nine cylinders and 1000 HP or so they work fine, just as horizontally opposed engines work well up to about 400 HP.
If I were looking to improve IC engines relative to those in current service I’d be looking at engine anciliaries and controls with the aim of slightly improving efficiency and substantially reducing workload. The basic engines are techncially pretty good, assuming a similar quality of fuel, and I don’t believe the real market is willing to pay hugely for owning Diesel engines. I think they will remain an expensive specialty product for those with poor fuel availability.
It’ll be a good thing when SMA sells their first engine to a OEM airframe manufacturer for delivery to a retail customer, but I don’t see it happening in the immediate future. Cessna seems to have stalled, as they previously did with the Thielert-powered 172 project. Maybe this time they’ll find a solution.
