Auto conversion ideas

[tsts4]

You would probably be very popular with RV-10 builders who are disenchanted with the big lycomings and continentals.

As an RV-10 builder and operator for more than 9 years and part of the RV-10 community for over 20, you're partially correct. Most new builders are disenchanted over the current pricing and more than anything, engine availability, but not the IO-540 itself which the airframe was designed around. Continentals are non-starter as are most auto conversions which have historically centered around LS1s. If any design is going to sway the RV-10 community away from Lycoming it's going to be a reliable diesel that has a price point on par with an IO-540-D4A5.

 

[moparrob66]

Yeah diesel would be the way to go for sure. I'm watching the Deltahawk and hope it succeeds, but I'll probably never afford one.

 

[AlphaPilotFlyer]

but I'll probably never afford one

Thats the problem for most new piston GA tech, it is mostly unaffordable for most. I am curious about what Deltahawk price points will be, but expect it will be much higher than a factory zero time engine….. given that Continental’s diesel conversion for c172 is north of $100k, I expect it won’t make financial sense to install on a typical 40+ year old Cessna. Diesels are tough on gear reduction boxes and most have expensive inspection or replacement schedules, as well, which further drives up op cost

 

[hindsight2020]

The legacy powerplant price points are so FUBAR'd right now for rec use, honestly it'd be worth the gamble on a 160-175hp viking or equivalent conversion for the RVs I'm looking at. Singular inflection point of cost for me after 13 years and 3 airplanes, and what kicked me out the hobby in 2023.

 

[moparrob66]

Yeah I enjoy tackling projects and solving problems, so maybe I need to buy a lathe and mill instead of a lycoming.

 

[kyleb]

I think the most likely failure for a modern auto conversion would be the fuel pump, assuming the PSRU is properly designed. Thats a pretty common failure that results in a tow to the shop, and one that cant really be prevented with proper maintenance.

And time travel will be easy assuming we have a flux capacitor.

The PSRU is the tough part. The rest of it is largely designing for durability and robustness, where critical systems (e.g. ignition) have redundancy.

 

[Dan Thomas]

Yeah I enjoy tackling projects and solving problems, so maybe I need to buy a lathe and mill instead of a lycoming.

And a crankshaft grinder, a cylinder boring machine, a cylinder honing machine, and crankcase line-boring machine, valve seat grinding equipment, piston and con rod and crankshaft forging equipment, along with the forge, centerless grinding equipment for the lifters...

It's not something easily done. The Wrights' machinist did it, and got 12 HP out of 180 pounds, and that engine wore out real quick. The first one seized on its second day's test run on the bench.

I have thousands of hours on a crankshaft grinder and cylinder rezizing machine. Both really big and heavy and expensive. By the time you get the equipment paid for, you could buy several brand-new Lycomings. Much of this stuff is getting rare, as automotive engine overhaul is a pretty much dead business now. Engines last a long time, and their overhaul outweighs the car's value by the time an overhaul is needed.

 

[moparrob66]

I'm envisioning 3 or 4 snowmobile belts with beefy bearings on the prop shaft supported in a way that belts can be easily inspected and changed without major disassembly.

Snowmobiles use one belt and dish out 100-200 horsepower. They put up with lots of high rpm abuse with cvt sheaves changing diameter constantly, shock loads of jumps and landings (sudden high rpm on jump, sudden load on landing) and the v belt surface gets plenty of heat and friction. Yes, snowmobile belts break, which is why they typically carry a spare. If im using 3-4 of these things, its unlikely that they will all fail simultaneously and a non- contact temperature sensor for each belt could be devised to give the pilot a warning of an impending failure. It might even be wise to have a tensioner on each belt to indicate the stretch. These kind of belts would probably allow enough slippage to absorb the dreaded torsional vibration.

Of course, lots of testing would be in order before flight.

 

[kyleb]

I'm envisioning 3 or 4 snowmobile belts with beefy bearings on the prop shaft supported in a way that belts can be easily inspected and changed without major disassembly.

Is the crankshaft bearing is designed for that side load? And what happens when the first belt snaps and wipes out the 2nd one? There are solutions out there, but if it was simple...

 

[hindsight2020]

again, if the MTBF is the concern, just treat the PSRU as a consumable wear item, and press. Again, at the FUBAR legacy powerplant price points we're dealing with, the entire auto engine suite can be treated as a wear item!

 

[kyleb]

again, if the MTBF is the concern, just treat the PSRU as a consumable wear item, and press. Again, at the FUBAR legacy powerplant price points we're dealing with, the entire auto engine suite can be treated as a wear item!

The challenge is that many of them (PSRU's) consume themselves in a few dozen hours. PSRU design is big boy engineering.

 

[moparrob66]

I should have said "lathe, mill and an aluminum v-6 or v-8". Id love to build the engine from scratch, but Ive rebuilt enough of them to know its more work than my lifetime will allow. I worked in a machine shop and took several semesters of automotive machining and machine tool technology at community college. Its a step towards knowing a bit of what you dont know.

 

[moparrob66]

Is the crankshaft bearing is designed for that side load? And what happens when the first belt snaps and wipes out the 2nd one? There are solutions out there, but if it was simple...

Thats an excellent point, and the system would have to be designed in anticipation of belt failure, maybe with shields between belts to guide a shredded belt away from the others. Testing will suggest the replacement interval of the belts, and monitoring the tension and temperature differentials between belts can be a good indicator. Also, watching oil pressure and metal content as well as checking crankshaft deflection at belt change intervals can help with monitoring crank bearing health, as with any powerplant.

It doesnt help that the overhaul quotes for my o-300d are $30-40k and 9-12 months out. Thats a big price to pay for 145 hp and an 1800 hour TBO.

 

[Daleandee]

I should have said "lathe, mill and an aluminum v-6 or v-8". Id love to build the engine from scratch, but Ive rebuilt enough of them to know its more work than my lifetime will allow. I worked in a machine shop and took several semesters of automotive machining and machine tool technology at community college. Its a step towards knowing a bit of what you dont know.

You could take the Jabiru approach ... start with a large chunk of aluminum and grind off everything that isn't an engine. Then send it out to the customer for field testing ...

 

[Dan Thomas]

Is the crankshaft bearing is designed for that side load?

It's not just the crank's bearing. The crank itself is being bent as it turns, and it fatigues, cracks and breaks. This is why most PSRUs have a support bearing on the outboard side of the crank pulley. The support structure for that bearing has to be extremely rigid, or it won't relieve the bending force on the crank or the radial load on the crank bearing.

 

[AV8R_87]

The crank itself is being bent as it turns, and it fatigues, cracks and breaks.

Aren't cranks already designed for that? A 4-cyl automotive crank has three main bearings, and will flex as the rods push on it. Probably better than directly attaching a prop to it, anyway.

 

[Dan Thomas]

Aren't cranks already designed for that? A 4-cyl automotive crank has three main bearings, and will flex as the rods push on it. Probably better than directly attaching a prop to it, anyway.

There are no overhung piston loads on a crank. It is supported on each side of every rod. It's the overhung loading that bends the crank, a crank that was designed to carry the flywheel or torque converter, maybe 10 or 20 pounds. It's also designed to transmit straight torsion, not side loads. A Subaru 2.2L engine, generating 130 HP at 5600 RPM, and having a five-inch pulley on the crank, is generating nearly 125 foot-pounds of torque. That torque transmitted to the periphery of a five-inch pulley generates 300 pounds of tension in the belt, and that tension directly translates to radial loading of the crank's overhung stub or flange. A larger engine makes far more tension.

A small-block Chev V-8. Two rods fit side-by-side on each rod journal, and each journal is supported by two bearings. Note the short main bearing at the left end, the flywheel end.



Here's a Lycoming four-banger. There are bearings on each side of each pair of journals. No overhung loads. Note the length of the main bearing at the left, just behind the prop flange. That's to carry the propeller's weight, but more importantly, the enormous gyroscopic precession loads generated by the prop as the airplane turns or climbs or dives. Any directional change will do it, during the change.

 

[moparrob66]

Excellent explanation. Airboats arent very popular in Idaho, so I'd like to know more about the failure modes of airboats, which frequently use a psru and big propellers. Im sure they live a hard life and a lot can be learned from them. Any of you folks from airboat country?

 

[tsts4]

I should have said "lathe, mill and an aluminum v-6 or v-8". Id love to build the engine from scratch, but Ive rebuilt enough of them to know its more work than my lifetime will allow. I worked in a machine shop and took several semesters of automotive machining and machine tool technology at community college. Its a step towards knowing a bit of what you dont know.

So this is an important distinction in the E-AB world. If you love to tinker and that’s your primary focus then your path as described is trailered made for you. If an auto or anything conversion is simply a perceived quicker or cheaper means to a flying end you will be better served buying an air cooled, direct drive engine and go fly and save the tinkering for your lawn mower.

 

[moparrob66]

Youre right, unfortunately. I bought a kit which I hope to finish next year, but I bought a 172 to fly until the kit is done. I love the 172, but the drawbacks really motivate me to go experimental. I cant wait to finish my instrument, then sell the 172!

 

[Dan Thomas]

If an auto or anything conversion is simply a perceived quicker or cheaper means to a flying end you will be better served buying an air cooled, direct drive engine and go fly and save the tinkering for your lawn mower.

Been there, done that for an E-AB owner. I designed and did the installation of a Subaru 2.L in a Glastar. By the time it was done he could have bought a factory O/H O-235 and been flying a year or more sooner, at no more cost. The Soob came with an RAF PSRU on it, but I had to build a complex engine mount and shockmount holders, a cooling system, two mufflers, a fuel system and an electrical system. Had to make major mods to the cowling.

He later sold the airplane and got half of what those airplanes were going for with the Lycoming. $60K instead of $130K.

Lycomings cost more now, but there is far more to shoehorning an auto engine into an airplane than most folks realize.

 

[AV8R_87]

There are no overhung piston loads on a crank. It is supported on each side of every rod. It's the overhung loading that bends the crank, a crank that was designed to carry the flywheel or torque converter, maybe 10 or 20 pounds.

There are 4-cylinder engines with only three main bearings, so the crankshaft is experiencing flexing loads. And they used to build 6-cylinder engines with five main bearings to have a shorter engine and crankshaft, with less torsional flexing. And while torsional vibration is a different animal, accounting for that in crankshaft design most likely gives it a good margin in bending as well.

I recall "standard" Corvair coversions don't add any additional bearings to the crankshaft (there is a mod to add another bearing to support the prop flange). And by the way, the Corvair has tho throws between each set of crank bearings.
Same with the AeroVee VW conversions.
Yet you don't hear about frequent crankshaft failures in either one.

I do agree with you that adding a front support for the crank pulley is a good idea, though. Plus, it would also integrate with the support needed for the prop pulley, keeping both in alignment, so the weight penalty of adding it would be minimal.

 

[Daleandee]

Lycomings cost more now, but there is far more to shoehorning an auto engine into an airplane than most folks realize.

Dan is certainly correct but in the experimental world there are exceptions to this rule. My Sonex has a William Wynne Corvair conversion that was built by Dan Weseman. Dan designed the FWF package including the mount and cowling. This conversion has been done a number of times and is a proven package that works very well. The engine is a aircooled, direct drive, flat six that is smooth, reliable, and gives good power (up to 125 hp). It has a 5th bearing added for prop loads.

I did a good deal of research before going this way and very closely followed others that had done this same FWF package on the same airframe. All of that to say that some auto conversions can work well but as Dan says there is a lot to be considered and I don't think I would be willing to be a "one-off" experimenter i.e. the first guy in the pool on any auto engine conversion.

 

[tsts4]

Dan is certainly correct but in the experimental world there are exceptions to this rule. My Sonex has a William Wynne Corvair conversion that was built by Dan Weseman. Dan designed the FWF package including the mount and cowling. This conversion has been done a number of times and is a proven package that works very well. The engine is a aircooled, direct drive, flat six that is smooth, reliable, and gives good power (up to 125 hp). It has a 5th bearing added for prop loads.

I did a good deal of research before going this way and very closely followed others that had done this same FWF package on the same airframe. All of that to say that some auto conversions can work well but as Dan says there is a lot to be considered and I don't think I would be willing to be a "one-off" experimenter i.e. the first guy in the pool on any auto engine conversion.

I’m certainly no expert but if I ever build the Piet I want to, it will be powered by a Wynne Corvair. As for my RV, I’d take a bullet to the head before replacing my Lycoming. Note both are air cooled, direct drive.

 

[Dana]

Snowmobiles use one belt and dish out 100-200 horsepower. They put up with lots of high rpm abuse

But they don't have to deal with torsional resonance from a large diameter undamped propeller or gyroscopic loads.

I'd like to know more about the failure modes of airboats, which frequently use a psru and big propellers.

I think a lot of the airboats using auto engines and redrives are using smaller diameter multi blade fans, compared to the larger diameter props on airplane engines used on airboats. That would reduce the torsional resonance, I imagine the gyro loads would also be a lot less in an airboat. Though that's just a guess based on pictures I've seen.

 

[Daleandee]

I’m certainly no expert but if I ever build the Piet I want to, it will be powered by a Wynne Corvair. As for my RV, I’d take a bullet to the head before replacing my Lycoming. Note both are air cooled, direct drive.

I agree on the Lycoming. William Wynne once said about his approach to converting Corvairs for flight, "we worship at the temple of Lycoming but we don't tithe at the temple of Lycoming" meaning that he wants to use as much of their approach to engines as can be applied to the Corvair conversions.

I have wanted to build a Piet and have a core engine. I'd like to build the fuse out of steel but it's been years since I've done any serious welding (I was a General Dynamics welder many moons ago) and my talents for melting metal together these days does not rise to the level of something I'd consider flying. I'm an aluminum airplane builder so building with wood would also be a learning curve for me.

 

[martym]

The Porsche PFM engine has not been mentioned. The first blush, Porsche was to design a low cost, light weight, direct drive, air cooled engine for aircraft.

I don't recall 'low cost' ever being part of Porsche PFM marketing, indeed the PFM powered Mooneys sold for a premium. I do remember advertising which claimed a "smoother running, more reliable, more powerful, easier to manage due to a single power control lever" alternative. Porsche divested themselves of that experiment as soon as they were able, surrendering the TC in 2007.

 

[Dan Thomas]

There are 4-cylinder engines with only three main bearings, so the crankshaft is experiencing flexing loads. And they used to build 6-cylinder engines with five main bearings to have a shorter engine and crankshaft, with less torsional flexing. And while torsional vibration is a different animal, accounting for that in crankshaft design most likely gives it a good margin in bending as well.

I have been around homebuilding since 1972. I remember reading about broken crankshafts in auto conversions, many of them because of the overhung, unsupported PSRU pulley loads. They do break, and no amount of wishful thinking will change that. We need to learn from the mistakes and failures of previous attempts. Why reinvent the wheel repeatedly?

Torsional vibration and bending loads are entirely separate and distinct. Engineering for one of them does not guarantee that the crank will then be strong enough for the other.

"Those who cannot remember the past are condemned to repeat it."--George Santayana, The Life of Reason, 1905

 

[hindsight2020]

Lycomings cost more now, but there is far more to shoehorning an auto engine into an airplane than most folks realize.

I don't dispute that. What I dispute the notion that it is cheaper to eat the current certified powerplant pricing vis a vis auto conversion solution, especially in light of the much vaunted "MTBF delta", which is facts not in evidence, especially in underutilized applications like recreational lawnmower flying.

To wit, I have the receipts disproving the former argument, as an expat to the hobby singularly because of it. The problem is I can't prove a negative, as the gubmint wouldn't let me slap a honda engine into the arrow to prove my point. Suggesting it would cost 30K to fabricate a mount and radiator provisions for the honda before the engine cost as sold today, is just rank canarding (pun very much intended).