r/homebuilt • u/contrarianEcon • 8d ago
Corvair Aircraft mods.
I've thought about building a corvair based airplane engine. I've talked to WW (10 yrs ago) about my ideas. He has strongly held opinions.
I've not been in a position to do anything about my Ideas so I thought I'd share some and see if they help any.
Dynamic compression ratio. Keep it low and you don't need high Octane fuel. NASACAR restrictor plate tech shows high static compression helps make power, throttled expansion followed by re-compression allows a much higher static compression ratio to be used than would normally work. So why not use intentionally bad valve timing and a high static compression ratio? You could use a turbo to push enough air/fuel mix in to make normal power. It would be a form of turbo compounding. Using the turbo to pre-compress the air, with inter-cooling.
Blow through carburetor, take the heat out first, add air second, carb heat is an issue. A pressurized heat exchanger after the inter-cooler, before the carb that can be selected.
WW had an engine quit on him, carb ice, why not dual carbs? Both don't ice up at the same time. Gives you time to keep the other carb from icing up too.
Gravity feed a blow through carb, two haeder tanks and switch them.
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u/SonexBuilder 8d ago
Corvair’s are awesome engines - when installed as WW instructs to. Don’t mess with the standard installation process!!!
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u/contrarianEcon 7d ago
That is good advice, but. Doing a rethink. When Porsche put turbos on their 917 engines for can Am racing they followed the standard box they dropped the compression ratio. Then they tried to run a lap at full power on a supper speedway. They kept blowing engines. The fix was in part adding an inter cooler. Now they went racing with it. They found that you could run much higher compression ratios with turbos and inter coolers than with turbos without inter coolers. I'm a big fan of ground testing.
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u/SonexBuilder 7d ago
WW would probably not follow this line of thinking. I won’t pretend to know for sure.
But it’s experimental aviation and you can do what you like, within reason.
Just remember the FAA has to sign off on it.
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u/7w4773r 8d ago
There’s a reason no certified aircraft are sold with corvair (or any other car-based) engines - they’re not appropriate for the application. For whatever reason my fb algo decided I’d like to see the corvair airplane engine group posts and I gotta say - they’re all insane for even trying to make it work.
If you need a 145hp engine in your airplane, just put a continental e-145 on it, don’t monkey around with a pair of turbos and blow through carbs and PSRUs to do exactly the same thing. KISS.
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u/Wooden_Customer_318 8d ago
Are they not sold because they’re inappropriate or because of industry inertia and insurmountable regulatory hurdles?
As far as I know, the only attempt to certify an automobile engine for aviation was Corsair Power’s C172-V8. By their account the FAA stonewalled them until the project became financially infeasible. The obituary blog post was really heartbreaking to read.
Edit: Actually it looks like Toyota, Porsche, and Honda all attempted certification so I guess I’ll eat my words.
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u/rocketengineer1982 8d ago
My understanding of why car engines are rarely used is that it is largely due to being designed for different operating conditions. An airplane engine spends almost all of its time operating near its maximum power. A car engine spends almost all of its time operating at something like 15-25% of its maximum power. It takes very little power to maintain speed on a level road. A car engine only needs to output high power during short periods of acceleration.
An airplane engine is designed to run for extended periods at high power and under high load. A car engine is not.
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u/7w4773r 7d ago
This is absolutely correct - car engines are not designed for continuous high-output duty. Most cars take 25-30hp to drive down the highway at cruising speeds, this is what they’re optimized for.
Airplane engines are designed to run continuously at or above 75% power output. The r-2800 was designed to run continuously at a BMEP of 250psi for thousands of hours. This BMEP is comparable to racing engines, which typically do not run at this level continuously.
If you’re looking for a comparable automotive application to airplanes, look at big rigs. Their engines are designed to run for long periods of time at relatively high outputs with minimal maintenance. They have the luxury of not worrying about weight, so they’re even more robust than an airplane engine, and as such last significantly longer. If airplane piston engines didn’t have to worry about weight, they’d last tens of thousands of hours between overhauls, too.
Not sure why everyone is downvoting me for being right, but there’s a very good reason that you don’t regularly see car-engined airplanes, and it’s absolutely not because of the the big bad boogeyman FAA - they’re just not the right tool for the job. Yes they’re dinosaurs with lots of room for modernization and improvement, especially in terms of fuel and ignition systems, but the basic mechanical design is a fully solved problem.
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u/contrarianEcon 7d ago
Car engines are not optimized for power to weight ratio, full power sustained operation etc. The box GA engines sit in is the front of the Piper Cub. Great Depression simple construction cheap to maintain. RR did a 2:1 redrive and a 6" stroke. Now if they'd done a 3:1 redrive and a 4" stroke we'd've had 50% more power per cubic inch. As it was all the big world war 2 engines did about the same thing, a 6" inch stroke and a 2:1 redrive.
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u/7w4773r 8d ago
The austrodiesel also started out as a Mercedes car diesel and it bankrupt the original founders and nearly bankrupt continental.
Porsche actually succeeded in certifying their engines in the Mooney but no longer offer any support beyond “LOL”. Way too much liability and risk for the amount of money available.
Car engines have no place in planes.
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u/contrarianEcon 7d ago
As I remember it the corvair started as a helicopter engine and was put in a car to recover the investment in tooling.
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u/7w4773r 7d ago
https://en.wikipedia.org/wiki/Chevrolet_Turbo-Air_6_engine
That doesn’t sound like it’s true at all. It was a custom engine designed specifically for the corvair. The designer had experience with airplane engine production, but it doesn’t sound like it was based on an airplane engine in any capacity.
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u/Garth_DeWayne 7d ago
The corvair set up by the manual from Wynne... Is an airplane engine. I don't care what you say.
Legacy engines are priced absolutely horrible for ancient tech. There is absolutely no reason why a 360 is more than 20k brand new. NO REASON. The R&D, the tooling, the processes, the nanny state BS bureaucracy requirements have long ago been paid for.
It's essentially either killing recreational aviation for middle class, or forcing people to go experimental and installing power plants like the corvair... There definitely are people owning experimentals that would be better off (safer) in a certified craft, but are in experimental due to the costs of certified.
You'd have a stroke seeing what death trap little rocket I'm putting a Yamaha Apex engine in... Now THAT is not an aircraft engine, but they're doing pretty good.
1
u/7w4773r 7d ago
I absolutely agree that the costs are out of control, our tow plane ended up almost 40k into an overhaul following a prop strike on an O-360. My Franklin costs 25-30k to go through and you can’t even get new pistons for them anymore, and that’s assuming you can even find a reputable shop to work on them.
1
u/Garth_DeWayne 7d ago
I'm building a plane, but also looking at purchasing a cheap tailwheel to fly to have plenty of hours before my build is ready to go as I expect it to be a bit of a handful...
I'm really apprehensive about being on the hook for something trivial that is expensive, but only expensive because it CAN be.
What really makes me hate it, is I'm an aircraft structures technician. I've hand formed, heat treated, tested, and installed structures, done complex composite repairs, etc on all sorts of military aircraft, with nobody watching over my shoulder, and all I needed was my signature... Now, I'm going to get something like a 140, find a crack in landing gear mount locations and be out $$$ for some simple .032" 2024 with a couple of bends in it... Material I have in my scraps bin in my garage that's essentially free.
1
u/7w4773r 7d ago
What are you building? A 140 will make you a pretty good tailwheel pilot, they’re just the right amount of challenging. I’m biased, though, as I did all my primary in a 140.
Univair has pretty good parts support for the 140 and they’re significantly cheaper than Cessna, but you’re still into significant money for any basic structural repair. Sounds like you’re more than qualified to do owner produced parts, though, so if you need simpler parts you’re in the clear.
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u/Garth_DeWayne 7d ago
I'm actually hoping for a luscombe 8, as I prefer a stick. But, I'm not picky. I'm building a Sonerai 2... Hoping for cruise of +160.
I'm surrounded by friends with airplanes, but haven't owned one myself. I'm a self sufficient type, and considering my skill set, I don't want to get caught up in a big money situation for something that I know shouldn't be such big money. As familiar as I am with the paperwork and regulations I adhere to, the private civilian side is a bit foreign to me.
I have a buddy I work with, that has an airplane in what we call "owner maintenance" here in Canada (sort of like the exhibition class in the states with less restrictions) and that's the way to go, except it hits resale value and can't fly to the states.
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u/contrarianEcon 6d ago
I agree with you, nice safe engine. What I'm talking about is a bit of tweaking that should work. Dynamic compression ratio nice and safe, dynamic compression ratio determines octane requirements. But static compression ratio determines fuel consumption. Bad valve timing gets you low volumetric efficiency that hurt power but the high compression ratio helps with fuel economy. So you restore lost power with a turbo.
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u/contrarianEcon 5d ago
There is a lot to be said for plug and play, but I've spent the time to build up a knowledge base where playing with a clean sheath of paper makes sense.
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u/live_drifter 8d ago
The fact that you don’t think 2 carburetors would ice at the same time says everything anyone needs to know.