Diesel powered 337

[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.

[ngb1066]

Originally Posted by 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.

It depends what you mean by best. Gas turbine engines have by far the best power/weight ratio, but the fuel consumption is poor other than in the cruise at altitude. Diesel engines are heavier but with generally better fuel consumption.

[CO_Skymaster]

I've been meaning to ask this question for awhile, since I have thought about replacing my engines with turbo diesels. If I did, I would probably replace them with the 210 HP rating on my current 1966 Skymaster or more. The TCM TD300 might be rated up to 250 hp. My Vne is shown as 220 mph (190 kts) on my airspeed indicator. I know some of the turbos and pressurized 337 can go faster (the Riely Rocket has speeds of about 250 kts). My question is about the frame of the Skymaster. Does the skymaster have to go though any structure reinforcement when faster speeds are normal or when more powerful engines are place in it? Except for the fussalge (the P-version will be different), are all other structual frame components the same for skymasters?

Karl

[wybenga]

In fact the opposite is true. Your Vne is limited to the top of the green arc, generally. But if you are indicating 165 knots at 6000 and still climbing at 2000 fpm or indicating 150 knots at 20.000 you are hauling buns.

Jack

[captbilly]

Originally Posted by 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.

Turbines have serious issues with specific fuel consumption (pounds per hp per hour). A typical small aircraft turbine will burn anywhere from 0.6 - 0.8 lbs per HP per hour, a TSIO-360 perhaps 0.5, but a diesel will burn as low as 0.3-0.35 lbs/HP per hour. That means your fuel burn at the same speed will be just about double using a turbine rather than a diesel. Even if the diesel were to weigh 500 lbs, while the turbine was at 300, you could go just as far at typical loads, and max range for the diesel would be double that of a turbine.

In addition, the specific fuel consumption of a turbine gets worse as you pull back the power. So putting in 2 750HP PT-6s would give outrageously poor fuel efficiency. Part of the reason that turbine aircraft need to fly so high to get reasonable range is that at high altitudes the engines can run at near 100% power without hitting very high IAS/CAS, which would cause a huge drag penalty. Diesels can have very high specific fuel consumption over a very large range of power settings, gasoline engines are good at lower power setting but very bad at maximum power.

I would love a diesel in my Skymaster, or Cessna 414A or anything else with a prop. The ability to use Jet-A or diesel fuel, simpler engine (no ignition system, or even a 2 stroke with no reliability issues), best possible specific fuel consumption, no icing, liquid cooling (though it could be air cooled), would all be great.

[jchronic]

Good article in the current AOPA magazine summarizing the status of diesels for GA airplanes. The takeaway (for me, anyway) is that either (1) you'd have to do a lot of flying to ever amortize the conversion with fuel savings, or (2) love your airplane so much you don't care how much money you spend on it - read 'sunk cost.'

Appears to me that diesels in Skymasters will remain in the category of 'an interesting academic discussion' for the forseeable future.

Joe

[captbilly]

Originally Posted by jchronic

Good article in the current AOPA magazine summarizing the status of diesels for GA airplanes. The takeaway (for me, anyway) is that either (1) you'd have to do a lot of flying to ever amortize the conversion with fuel savings, or (2) love your airplane so much you don't care how much money you spend on it - read 'sunk cost.'

Appears to me that diesels in Skymasters will remain in the category of 'an interesting academic discussion' for the forseeable future.

Joe

I just wish that some still built a small pressurized twin that didn't cost several million dollars. Come to think of it, nobody makes a pressurized piston twin at all. I know that many people are comfortable flying behind a single engine but I think I have spent too much time flying multi-engine aircraft (much of it in 8 engine aircraft) that I just get nervous with one ( at least when the weather is bad or I am over really inhospitable terrain). To be honest, I have never had an engine failure in any aircraft except my old Skymaster, but it flew on like it was a non-event.

I love turbines, and most of my flying has been in jet aircraft, but they do suck down the gas. I remember burning more fuel taxiing to the runway in a t-38 than the total fuel capacity of my Glassair. I would love a 4-6 seat diesel pressurized twin that could fly at FL350 while burning 20gph even if it didn't have the smooth power of a turbine.