Diesel powered 337

[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

[hharney]

The Super Skymaster with TSIO-520 rated at 310 hp has the same structural limitations as the standard 337 airframe that it was modified from. The advantages of the increased hp is climb. The higher speed at altitude is true airspeed but the calibrated airspeed is still within structural limits. The Super is about 20-25 kt faster at altitude with a cruise of about 220 kt TAS. The positive was 2500 FPM climb but the extra weight of the engines eroded the useful load terribly.

The 73 and newer airframes were substantially different from the pre-73 models. Cessna also changed the wing spar in the H model vs the G model. There were gross wgt increase mods but all landing weight requirements remained the same except for some proprietary STC's. I am not aware of anyone attempting to increase the structural load limits. It would be very expensive (if any possible) and there just isn't enough of an audience to justify it.

[Mayhemxpc]

I spoke with the Delta Hawk people at OSH. They are focusing on their smaller 160 hp engine which they say delivers the equivalent of 180 hp performance. No idea how they measured that. They seem to have a "pie in the sky bye and bye" attitude towards larger engines, and in particular the 337. TF Hawk was depending on Delta Hawk producing their engine according to its original program. (I know the president of TF Hawk.) As a result, even though they had everything else ready to go, they had to suspend everything because there was no engine, and no realistic delivery date for one. Sad.

That and they had very little in the way of solid interest from prospective buyers. Last I talked to him about that particular subject, the cost of a fully re-built TF Hawk O-2/C337 was not going to be that different than a Caravan. (The USAF has modified C-208's to launch HELLFIRE missiles for the Iraqi Air Force.)

Bottom line: A good idea has money attached to it. If they don't see enough potential sales interest, there won't be much development.

[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.