Diesel powered 337

[robw]

We run a pair of 337s for aerial survey work and because of the huge problems getting avgas out here in Africa, we have been tracking both the alternative engine and alternative airframe possibilities for years.

First off, if HAWK are quoting $340k for their Skymaster conversion, I can’t see that they are going to get many takers. Adding in something between $100k and $200k for the basic aircraft means you are throwing around an investment figure above the half million mark and at the end of the day you still have a forty year old airplane no matter how well it has been checked out. For that sort of price you can buy a brand new out of the box Diamond DA42 twin diesel that offers comparable range and operating cost. It makes no sense.

Diesel does seem to be the inevitable way forward but (unfortunately) it is still early days. So far there have been only two certified engine manufacturers, Thielert and SMA, and they have both experienced massive technical difficulties in making their engines reliable. They do now seem to be getting on top of it but it would be naïve to expect any new manufacturer entering the market to instantly have a product that is problem free.

Of the two existing engines we much prefer the SMA. It was designed from the start as an aero engine as opposed to a converted car engine, it has been around for thirteen years, has a lower reliance on electronic wizardry and the major aerospace company behind it (Snecma) has stood solidly behind their warranties. One of the owners of a diesel 182 out here has had his engine replaced twice without quibble.

The SMA is also the closest potential fit to a 337. Compared to the IO360C it is 8” taller, 6” wider and 2” shorter without any sacrifice of horsepower. It weighs 100lbs more but this is largely offset by the lower sg of JetA and the reduced fuel burn. Some years ago we were informed by SMA that the tractor/pusher issue was not a design limitation but merely a matter of testing and certification but of course this carries a cost implication as well.

Cheap it is not. About $65k a pop the last time I looked. Throw in new props, mounts, instrumentation and multiply by two and you can’t be much shy of $180k before STC costs. A few years ago a firm in the USA quoted me $100k to do the STC work so there you are, back in the ball park of the HAWK price with all the same arguments against it.

It was a very smart move by TCM, or maybe it was the new Chinese owners, to buy the SMA technology. It could be they will be able to leapfrog over all the teething problems and they have the muscle and know-how to get a lot of engines onto the market quickly. Originally they were promising a certified, retrofittable 4 cylinder this year and a 6 cylinder in 2013. For which airframes they did not say and whether their plans will include the relatively small fleet of 337s remains to be seen but to my mind this is the only practical hope on the horizon.

[WebMaster]

I note in my iPad App for Oshkosh, that Delta Hawk has a booth. No idea what they will be showing. They are in booth 257, in the main aircraft display area.

Thanks Herb for pointing out the App.

[ngb1066]

Is there any further news from T F Hawk or Deltahawk? Both websites do not appear to have been updated for a while.

[Walter Atkinson]

Two things I don't see discussed about the diesels:

1) the HUGE hit they take in additional cooling drag which makes the fuel efficiency not nearly as good as the claims which do not take that drag into account.

2) the significant peak pressures which are transmitted to the prop in stress vibrations.

[ngb1066]

Originally Posted by Walter Atkinson

Two things I don't see discussed about the diesels:

1) the HUGE hit they take in additional cooling drag which makes the fuel efficiency not nearly as good as the claims which do not take that drag into account.

2) the significant peak pressures which are transmitted to the prop in stress vibrations.

Walter - The 235HP SMA diesel engine seems to have been operated successfully in the Cessna 182, so presumably the two problems you mention must have been addressed in some way. Do you have any further information on this?

[Walter Atkinson]

Originally Posted by ngb1066

Walter - The 235HP SMA diesel engine seems to have been operated successfully in the Cessna 182, so presumably the two problems you mention must have been addressed in some way. Do you have any further information on this?

"Seems" is the operative word in your post. The prop issue has been marginally addressed but the cooling drag issue has not been addressed at all. It's that pesky physics. For this reason, the BSFC(min) on the installation is nowhere close to the claimed number. The claimed BSFC of about .33 on a diesel is closer to .37 when the cooling drag is added in.

In my educated opinion, diesels will not offer the answer until and unless there is a serious breakthrough in combustion technology. That breakthrough is nowhere in sight on the horizon since the physics of chemical reactions is not likely to change. There will be, however, dozens of people/companies/concerns which will continue to try to make this happen for many years before finally admitting that diesels are covered by the laws of physics.

[Skymaster337B]

The best diesel fuel engines are turbines. However, a turbo prop Skymaster is so cost prohibitive. Avgas engines are here to stay...until the unelected EPA outlaws 100LL.