Diesel powered 337
 

I was told today that someone has converted the front engine of a 337 to diesel somewhere in the northeast USA and is flying it under a restricted category. Has anyone heard of this who could elaborate and does anyone know of any plans to retrofit diesels on a 337?

Rick Gardner
www.caribbeanskytours.com

I too would be very interested to hear more on this subject. I have been doing some research on the topic and would assume they used the Centurion 4.0 by Thielert in Germany. That is the 350hp V8 water cooled engine with the gear reduction unit that runs on Jet A. I know it is not safe to assume anything but this is the only diesel I could find that was close to big enough. They already have kits to convert 206's, 340's, 414's, and 421's, as well as Beech Duke B60's, so the front end of a 337 should not be much of a stretch.

They claim more static thrust for the HP than conventional because of the higher torque and to be burning about 12gph (per engine) which could get 350hp 337s running for less than $100/hour in fuel up in the 300mph neighborhood. They are however almost 200lb heavier each than the TSIO550's, not to mention 4" wider, 6" taller, and 3" shorter but I don't know if any of those numbers include the gear reduction unit, and I am not entirely sure how their water cooling system fits in the cowl or how much more weight that adds to the package. I also found it interesting they offer no TBO as they are supposed to be replaced not rebuilt every 2400hours. They also take mixture and prop control and hand them over to the computer which like most new technology sounds great when it works.

Anyone know anything more?

I would rather have a small turbine running then a diesel.

Delta Hawk Diesel Engines, http://www.deltahawkengines.com/archives.shtml
N1700M is the 337E.

Delta Hawk has been testing for several years now. There should be more info on the board about this. Also, at the SOAPA meeting in Kalamazoo 2008 there was a presentation about this work that DH is doing. It pays to attend.

From what I can see on their web site it looks like a Diesel engine will be more expensive than a regular engine. But will the overhaul time be about the same?

Does anyone know, when Thule-River talked about installing SMA diesels in a Skymasters, did they have to perform major modifications to the cowling like SMA had to do with the 182? Is there enough cooling in the Skymaster to dissipate the heat?

Karl

Front
 

The front engine on a skymaster was a test bed for Continental. You may have read that Continental was sold to the Chinese. One of thing that was the impetus was additional funding for their diesel engine development.

Thule River/Super Skyrockets, looked at the SMA Diesel for the Skymaster. It was intended to increase their market for modifications. I had the good fortune to be at Osh when Marianne ( Mary Ann ?? ) the owner of Super Skyrockets spoke to the Skymaster group. It was a meeting put together by Kevin McKenzie. In any event, the SMA engine conversion was intended for the normally aspirated Skymasters. That's because the certified ceiling for the engine was 10.5K. The project never got off the ground for several reasons. First, SMA was slow to produce and respond. The Skymaster conversion was not tops on their list. Second, SMA never produced an engine designed for pusher engine installations. I spoke at length with the folks at Super Skyrocket, about the time I needed a new rear engine, and that was their response. Interestingly enough, Continental does not differentiate between tractor engine and pusher engines. Third, Super Skyrockets was looking for additional markets for their business (the Riley Conversions were not selling real well), additional conversions they could do. They did a considerable amount of work with a variety of "New Engine Technology" companies, and I think they simply got worn out trying to come up with something that was certified, or certifiable. They have since closed their doors, and it's not likely that anyone else will take up the mantle to install SMA engines in a Skymaster.

Delta Hawk, on their website, had a date for certification. That date has come and gone. Super Skyrockets looked at the Delta Hawk engine, and in fact commissioned a study for engine mounts. There was a really cool solid model of the front engine mount for the Delta Hawk engine, on the Super Skyrocket site, for a while. It had the NASTRAN load analysis associated with it. Really cool looking.

Thanks for the information Larry.

There were a couple issues I didn't like about the SMA diesels on the C182. 1. The extensive cowl modifications 2. I saw a video where they were demonstrating it in flight and they couldn't pull power all the way to idle incase of a flame out. Didn't like the sound of that, what if I need to go around or make a missed approach, 3. I like the idea of FADEC, but still don't like the idea of battery back up. What if the entire electrical system fails (I've had it happen at night) and your over water. The magneto system would allow you to continue to fly until you exhaust your fuel.

I was hoping that deltahawk would have the certified engines going by now, but I'm no longer holding my breath about it. I don't know how the sale of TCM to Technify will affect us in the long term, but I'm not feeling too doomed and gloomed about it either.

Karl

Update
 

I don't know where they are getting the engines, but apparently TFHAWK is serious about a diesel Skymaster. I say I don't know, because Delta Hawk Engines is still in the testing phase. It could be that TF is working with them. In any event, I have learned that they are going to produce, and soon, what they call the "Golden Hawk" conversion. Pricing is expected to be about $340K, including new props and all mods. Some of the mods make a lot of sense. They say they will have engine block heaters and fuel heat. Heating the fuel is a good thing, because Jet fuel tends to gel, and attract moisture. Those jets who have fuel heaters don't need Prist added to the fuel. Since Prist is expensive, the fuel heaters are a good thing. They also say they are going to have dual electrical systems, another good thing. If you don't have the Gear Door STC, they want to to put it on. I have mixed feelings about that, but a lot of people have them. I remember Super Skyrockets saying they did not approve, because the wheels could collect ice. I know that when we cleaned Jerry's plane, his wheels tended to collect OIL. New engines would solve that issue. They also talk about electrical heated wing de-ice. I have seen this, and I have seen this removed. I think it would be up to you.

They will also put on the Horton STOL kit, and spoilers. Both good things to have. I have flown Herb's with the Horton, and it really helps on take off and landing. Jerry has spoilers, and those are great for getting down. There will be cowling mods, of course, and an MVP engine analyzer.

The price seems steep, but I don't think it is. When I flew Owen's SkyMonster, he was talking $350K for that mod, and that didn't include all the things that TFHAWK is talking about.

Changing engines is a big undertaking, because not only do you have the cost of the engines, you also need a newly designed engine mount, and new cowlings.

Being able to run Jet A should appeal to a lot of folks, especially when you consider that in some parts of the world, 100LL is virtually unattainable. When Paul and Mary flew up the Amazon, then needed to buy 100LL in 55 gallon drums, and have them delivered to their planned fuel stop. If they were burning Jet A, it would have been readily available.

When I get more information, I'll pass it along.

More Information
 

I have learned that as part of the certification process for the Delta Hawk engine, they are flying the engine in their Skymaster.

They also anticipate having certification this year.

According to their website:
Some of their modifications are not applicable to civilian use (like armoring the crew seats). Not to mention the tip tanks that some folks just don't want.

Still, great to see SOMEBODY will be doing it!

I kind of wonder what they'd charge just to replace the engines/mounts/props?

Here is what I have learned:

You can expect the Golden Hawk conversion to be in the range of $340,000, based on using an existing customer-owned Skymaster that has no major defects and is moderately low time (i.e., like 2,500 to 3,500 TTAF) since zero-timing it is an option to add and above the base price. That includes a wide range of modifications, so it isn't just the engines that you get.

(Of note, this is not far from the Riley Rocket II pricing which they say "starts at $250,000" but some say in reality is more like $300,000 once you add in the equivalent modifications package that the Golden Hawk includes as standard.)

The standard Golden Hawk modifications package includes the following in the base price: new Hartzell propellers that have been custom-engineered and designed for the aircraft and engine; electrical strip tape anti-icing on wing leading edges and props; the gear door STC; dual electrical systems; an updated electrically actuated hydraulic system; fuel heat and engine block heat (for cold weather and high altitudes); modified cowlings, wrap-around front windshield without the central support; Horton STOL; an MVP-50P avionics system that is customized for the aircraft and engines; plus wing spoilers to allow the plane to descend more rapidly, and more.

All Golden Hawk aircraft will undergo an extensive 101-point inspection program, including the wing spar inspection that has been a widely discussed issue in the forum -- so this is a good way to get the inspection done and a major transformational upgrade at the same time.

Performance is essentially the same or better than the military version, which gives (based on the TFHawk.com website): more than 12 hours endurance on just 166 gallons of either Jet A, Russian TS-1 or diesel truck fuel from a regular truck stop, or any mixture thereof with no changes to engine settings; normal operating altitudes of up to 25,000 feet; no loss of power/manifold pressure up to FL185; and climb rates that are out of this world.

The plane can fly non-stop from Kitty Hawk to San Diego.

The company will offer the conversion in both non-pressurized and pressurized versions, though they state that the PRESSURIZED version may be preferred because of the typical high altitude operating profiles. Of note, this means that for the first time in Skymaster history, the heavier pressurized version of the plane (the P337) is superior! With the diesel conversion, there is sufficient power in the engines so that the extra weight of the P isn't a problem. Thus the Golden Hawk converts the old dog of a pressurized Skymaster into a little high altitude sportscar, giving the passengers extra comfort without having to wear masks or tube-based O2 in the plane. This will inject some life back into the Skymaster market, no doubt.


They are looking for some qualifying aircraft to convert..........don't forget the checkbook.

Diesel powered 337
 

I have been interested for some time in the possibilty of installing diesel engines in a 337. For a time it appeared that the SMA four stroke 230-235HP engine was the only candidate and it appears to have been successful in the Cessna 182. The "improved" version of the SMA engine has just received EASA certification and this amongst other things raises the service ceiling, which was a problem with the original engine. There are apparently plans for SMA to bring out a 280HP model in due course. Tule River raised the suggestion of developing an STC for fitting SMA engines to the 337 a while back, but I do not believe they have taken the idea any further.

Thielert/Centurion have put on hold their plans for their 300+HP version of their engine and in any event the experience of users in the UK of the Thielert product has not been good.

The Deltahawk engine as a two stroke has always looked interesting, offering a better power to weight ratio than the competition. It can also burn motor diesel, unlike the SMA engine which is limited to Jet A or Jet A1. I have looked at the TF Hawk website setting out their plans for the 337 Goldenhawk with Deltahawk engines, but it is not clear how far they have got with that project. My main reservation about this is that the four cylinder Deltahawk engine will when certified, probably later this year, have a maximum power output of 200HP. I wonder if this is really sufficient to power a 337, particularly the heavier pressurised models. I would be interested to hear views on that. Deltahawk plan in due course to bring out 6 and 8 cylinder versions of their engine, offering power up to 400HP, but that is likely to be some time away.

Horsepower is not everything.

Check out the diesels on the Diamond Twin Star DA42 (yes, they are Thielerts, but follow along for a moment). They are 135HP each, not even the original Piper Apache had engines that wimpy!

But talk about fuel economy - that puppy cruises on roughly 10gph (5 per side) of Jet A and does it quickly. While the 337 retrofits will never be as aerodynamic or as light as the DA42s they still could be reasonably fast and fairly fuel efficient.

The Golden Hawk also is amazingly quiet -- another result of its original military design goals. The new Hartzell props provide lower noise signature and the engines are so quiet that it is said that it makes less noise on a take off roll than a Cessna 150 taxiing by on the ramp. Skymasters are usually noisy inside, so this will be welcome -- I don't know if the plane's ability to land nearly silently at midnight on a desert strip in the middle of nowhere half a world away is of interest to anyone, though....

TF Hawk said that the costs of operating the Golden Hawk Skymaster are extraordinarily low -- first, the engine maintenance is less than half of an IO-360 powered aircraft; second, the fuel is much cheaper than 100LL; third it burns less than half the fuel per hour and, when at altitude maybe even 1/4 the fuel; and fourth, it flies higher and faster TAS as a result, so you get there quicker. Based on that, the Golden Hawk is likely that your flying costs will go down by as much as $200 to $300 per hour or even more, depending on where you live and the price of fuel at your airport (I imagine that this conversion will be very interesting for European customers, where fuel prices are four to five times higher than they are in the USA).

Thinking about it, this means that if you fly 300 hours a year, the conversion will pay for itself in less than four hours as a result of the cost savings you will enjoy.

More to come.... I am arranging a phone call in the next week or so with one of the top team at TF Hawk, where I intend to confirm this and learn more yet, which I will pass on. When I discussed this with them, the company did state to me that the above is fairly accurate, though they're not confirming anything yet pending the launch of a new civilian conversion announcement on their website. They did confirm that they will deliver an aircraft in as little as six months from date of order and that they can handle up to six aircraft at a time in the shop. This is their first venture into the civilian market, despite nearly four years of development work underway for exclusive military uses, so they are excited about it.

Morne and Herb

Thank you for your comments. I can certainly see enormous benefits of diesel aero engines in terms of both performance and economy. I suspect that they are likely to be increasingly seen in GA. This applies even more in Europe than in the USA, in view of the stratospheric cost of 100LL avgas here.

The Golden Hawk project certainly looks very interesting and I will look out for updates on the TF Hawk website about their proposed civilian version of the aircraft. Herb - have you had your phone call yet with TF Hawk? I will certainly consider this option for the 337 I am looking for. My only reservation at this stage is engine power, which will be lower than currently available for any of the 337 varients. I imagine that this will not be a problem at altitude as diesel engines retain maximum power at greater altitude than the avgas fuelled equivalent, but in terms of take off performance do you consider the Deltahawk limit of 200 HP will be a significant factor? My concern is based upon operating from a relatively short grass strip.

Many thanks for your advice and information.

Neil

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.

Oshkosh
 

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.

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

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?

"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.

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.

Skymaster Frames
 

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

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

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.

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.

Has anything new happened since August?

Turbines burn a huge amount of fuel.
 

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.

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

You are almost certainly correct.
 

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.