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Will we all be flying diesels?

Silvaire wrote:

LeSving, thanks for the RV/Subaru link. It looks like he had fun and learned a lot, which is the purpose of experimental category aircraft, but in terms of building a better plane more generally he’d have been better off sticking with a Lycoming, installing Light Speed ignition with low MP advance, and minimizing airframe and engine weight.

Well, that’s the same as saying that cooling drag related issues are negligible for light aircraft in general, which in my opinion is the truth looking at the big picture of things. Then they are of course equally negligible for liquid cooled diesels. Both speed and power has to be fairly high to warrant the extra engineering complexity. At the same time, with a liquid cooled engine you can much more easily include these finer engineering tricks.

The elephant is the circulation
ENVA ENOP ENMO, Norway

I think the Subaru powered thing probably had more drag than a standard RV regardless of the owner’s experimentation with extra air scoops, water pipes and radiator ducting. Less thrust too, especially when the engine quits

For higher speed aircraft I’ve read cooling drag can be something like 10% of total drag, obviously depending on the aircraft. Updraft Navions and the like probably aren’t that good On production aircraft it’s led to tight cowling designs with cowl flaps for slow flight, formed an important part of successful Lopresti aerodynamic cleanup programs and so on. All of which it’s reasonable to understand made fast planes go a bit faster.

Last Edited by Silvaire at 30 Dec 04:59

The Twin Comanche is known to be a remarkable plane, and it would be nice to see a comparison to the latest DA42 version based on real data. Not only speeds but also payload. Echoing Shorrick and Peter, I’d like to see some Twincom data from those who fly them, so please chime in!

The DA42 NG data from P&P are from 2010, so that’s not the latest DA42 (the NG -VI) of which I posted performance data in post 19. By the way, the max speed is 190 KTAS at 16.000 ft at a Max Continous Power setting of 92% (I think that’s the max available power too). FF 16.8 gal/hr. A friend owns one and I have seen these numbers.

Note that the -VI also has a better payload than the -NG. Typical useful load is 550 kg. Full fuel and full anti-ice fluid that leaves 275 kg for pax and bags. So a three person plane. But 4 persons would not want to sit in a DA42 for 5-6 hours anyway..

Another thing that I think is relevant in the discussion on Diesels in the ease of use. I agree that this is something that depends on individual preferences. Some people like to tinker with mixtures, prop controls, carefully monitor temperatures and don’t mind occasional problems with hot starts, flinging stones into your prop during run ups or making a mistake once in a while by pulling the wrong lever.. (well, I did once ). I personally prefer starting and operating an engine like a car engine. Twist a key to start and single throttle to run.. Although it does become a bit boring maybe always seeing these same temperatures and fuel flows..

Also, I think we should give Aero-Diesels (whether converted from auto-diesels or clean sheet designs) a bit more time. The Thielert design has come a long way and is still evolving (like Shorrick mentioned, the clutch has been changed to a vibration damper design, or ‘two-mass flywheel’) and I am confident that the TBR of various components will continually increase. Austro has had the luck to learn from Thielert, and that engine is clearly one step ahead in maintenance cost at this point (although the weight is rather high).

And, just to prevent any possible misunderstandings on ‘vibration’. Yes, the torque pulses of a Diesel are higher so you need a clutch/damper and a prop that can take it (for instance good old wood..). But we are sitting on the other end of the engine and other Diesel flyers and myself can tell you that Thielerts/Austros are very smooth. Smoother than any Lycoming or Continental gas engine I flew behind. And, like Flyer59 mentioned, if a large Mercedes auto engine makes it to the front of a SR22 it is definitively going to be a very smooth ride too.

Private field, Mallorca, Spain

Silvaire wrote:

I think the Subaru powered thing probably had more drag than a standard RV regardless of the owner’s experimentation with extra air scoops, water pipes and radiator ducting. Less thrust too, especially when the engine quits

Meredith effect or not, it’s simply more difficult to design an efficient air cooling than an efficient liquid cooling because for liquid cooling you can put the cooler anywhere you want. In addition the cooling system must also work when the aircraft is on the ground idling. Besides, I wouldn’t consider making good baffling around an air cooled engine any bit simpler than installing a couple of water pipes. There are obvious practical advantages of keeping all engine related systems in one place, inside the cowling, but if performance and speed becomes important things will eventually change.

It was not only the P-51 who got it right. The later Me-109 also had a rather cool cooling system with varying inlet and outlet and boundary layer control:

I also wonder about that Meredith effect. Basic jet engine relations say the best efficiency is achieved when exhaust velocity equals aircraft velocity (in the opposite direction). But thrust is proportional to Ve – V0 (exhaust velocity minus free stream velocity), so Ve must always be larger than the aircraft velocity. For a cooling system you want highest efficiency, not lots of thrust. This is achieved simply by making the exhaust directed straight back at the same velocity as the airplane. But, with varying speed, temperature and density, the inlet area must also vary to minimize drag. To me, the Me-109 looks like a better solution than the P-51, it’s also much simpler design vise, and a bit simplified version could very well be used for light GA.

The elephant is the circulation
ENVA ENOP ENMO, Norway

Just came across this old thread. Has anything actually changed since 2013? Diamond are still there, the diesel retrofit business is pretty dead, and that’s about it.

Administrator
Shoreham EGKA, United Kingdom

My club just retrofitted a 2.0s Thielert to a PA28 Warrior, with pretty good results, although we did end up with a very forward CofG which still requires a bit of tweaking. Our Archer is now near engine TBO so that one will be retrofitted with a 2.0s Thielert as well. The rest of the fleet is all DR400 and DR401s Ecoflyers, factory fitted with 2.0 or 2.0s Thielerts. The only aircraft that’s not diesel is our aerobatic R2160. For us it means the diesel-ification is now almost complete. And over the last few years we’ve basically had zero issues with them.

We also got ourselves a trailer with a 900 liter fuel tank, including pump, hoses and everything. Previously the resident handler did our diesel refueling so they brought their 40.000 liter fuel truck across the airfield several times a day to fill our aircraft up with 40 to 60 liters of Jet-A each time. Now, they just need to do that maybe once every three days, delivering 900 liters in one go. So they’re happier too, and we have the ability to quickly refuel whenever we need to.

I wonder where you are based, Backpacker?

Administrator
Shoreham EGKA, United Kingdom

the diesel retrofit business is pretty dead

I don’t think so. While the club I’m a member with still is Avgas only (six aircraft), another club here in southern Germany where I have friends has retrofitted two legacy airframes. One a couple of years ago and one only recently. They are very happy and AFIK they’ll retrofit another one once the engine reaches TBO.

EDFM (Mannheim), Germany

I don’t think so either. A commercial flight school in the area is retrofitting diesels on their C172s.

ESKC (Uppsala/Sundbro), Sweden

At the beginning of this old thread @achimha (is he still there?) talked about very interesting developments in higher powered diesel engines, that may one day even surpass WWII era technology (a bit sad that we didn’t do that 60 years ago, innit?). But nothing seems to have happened in that regard…

Low-hours pilot
EDVM Hildesheim, Germany
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