Converting car engine to aero engine

Factual Questions
1

[url=“http://boards.straightdope.com/sdmb/showthread.php?t=355056”]In this thread I mentioned an airplane that used a modified Jaguar V-12 for its powerplant.

Car engines operate at varying speeds. They might turn 1,000 rpm, go up to 5,000 rpm, down to 2,500 when you shift gears, and so on. Airplane engines run at a fairly constant speed. Slow while taxiing, say 2,400 on take-off, and then the power is set to the desired level until it’s time to land. (e.g., the pilot may use full throttle the whole flight, or he may use a reduced power setting to reduce fuel consumption. But generally the engine is running at a constant speed.)

Seems to me that running at a constant speed would be easier on the engine. Only many car engines make their maximum power at high rpm (requiring gear reduction if you want to use it to turn a prop). How would constant operation at high revs affect it?

How about lightening an engine? Is that possible, while still maintaining integrity? IIRC that Jag V-12 put out 360 hp in the airplane, but it’s a heavy engine.

(And no, I’m not building an airplane. That’s still just a pipe dream.)

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Fixing link.

3

I ran across this subject years ago, when I was reading about experimental home-built aircraft. A few designs used Volkswagen air cooled engines from old Beetles. Stoked up with superchargers, and maybe bored out to reduce weight as well as to increase power. The consensus seemed to be that they didn’t work very well as their power to weight ratio was still such that they didn’t provide enough power for the airplane in relation to their weight. VW engines were at least air-cooled, which saved weight by not having radiators and not having to carry coolant.

Motorcycle engines might work better, if you use reduction gear for the propellor. Mr. Rutan used two 600 or 650cc Suzuki or Kawasaki engines for the Pond Racer he built.

4

I know very little about this, but I know that the kitplanes that use old VW Beetle engines have them drilled for a second set of spark plugs, in case the first system fails in flight. Also, as you know, aircraft engines must be rebuilt every (what, 500 hours?) If the engine in your Jaguar coughs out, you can pull to the side of the road, but in a plane, it’s a bit trickier.

If you plan to do some aerobatics, you might think about some modifications to keep from getting drenched in hot oil and/or fuel when climbing straight up and flying upside down. There’s a lot of drama in that, I’m told. :eek:

5

I’ve only flown Lycoming-powered aircraft. They have TBOs (Time Between Overhaul) of 2,000 hours. On Contenintal engines I’ve seen TBOs of 1,500 or 1,600 hours. I’ve seen longer TBOs as well. Seems to vary according to manufacturer, specification, and use. I remember hearing that some experimental engines have 500-hour TBOs.

As for VW engines, I’ve heard there are a lot of homebuilts flying behind (or in front of, as the case may be) HAPI-modified engines.

6

I’m a little out of date, since I stopped following the homebuilt movement in earnest almost a decade ago, but at the time, auto conversions were of very iffy reliability.

There are quite a few differences between an auto application and an airplane application, which aren’t evident on first blush. For example, consider gyroscopic precession. An engine is full of rotating masses. An airplane pitches up and down constantly in ways a car does not, and when it does, it puts side loads on the bearings. In addition, if the engine is directly geared to a heavy prop, that can put loads on the engine internals that don’t exist when the engine is connected to the drivetrain through a clutch/flywheel

Then there’s the reduction gearing problem. Most auto engines are designed to make max power between 4000 and 8000 RPM. An airplane prop is most efficiently typically between 2000 and 2500. So you need a reduction gear that can handle the power, doesn’t weigh much, and fits in the profile of the nose. Not an easy task, and it adds a new failure point.

Also, airplane engines are designed with reliability as their primary goal. That’s why they still use magnetos, and have redundant magnetos. If the electrical system in an airplane fails, the engine keeps running. If the electrical system in a car fails, the engine quits.

Then there’s cooling. Airplane engines are generally air cooled, a passive cooling system that cannot fail. Auto engines require water jackets, water pumps, radiators, hoses… All potential points of failure. If you lose a water pump on the highway, or blow a fan belt, you can just pull over. In an airplane, you’re looking at an emergency landing.

Auto engines are generally more complex, with more moving parts, more electronic systems, all of which are additional failure points.

Unless great strides have been made in auto engine conversions over the past few years, I wouldn’t take my family flying in an airplane that had an auto engine conversion. I might fly it myself and take the risks, especially if it were a ‘sport’ plane that I was using to just tool around the area. I wouldn’t want to fly long cross countries in it, though.

Then there’s another issue - maintenance. If your Lycoming gives you some trouble, you can find an A&P at any airport who will look at it for you. If your auto engine starts acting up, and you’re away from home, you’re in trouble. Even if you work on it yourself, you aren’t going to have your tools and diagnostic gear with you.

I’m of the opinion that experimental airframes should have reliable, non-experimental engines in them. If you want to fly an auto conversion, put it in a Cessna 172. One major change at a time. Those guys that put experimental engines in high-performance experimental airplanes that have high stall speeds and poor soft-field characteristics are just asking for trouble.

Again, maybe everything has changed in the last 10 years. So take my opinion with that in mind.

7

VW engines do just fine, they are just not very big.

My Dad had a Funk airplane that had an inverted - converted - model B Ford car engine.

There are many different engines in use, some liquid cooled. There are folks using Harley Davidson engines. New small radial engines, new diesel engines.

In the Experimental category almost anything goes. There is no TBO at all under some circumstances.

You want to go into controlled airspace, carry passengers, fly IFR, sell plans for you bird, all kinds of things change what is allowed.

It is all about balancing power - weight - cost - frontal area and much more. What is available at the time and the state of the art at any given time. Engines are almost always the hold up on new designs. We just can’t make then small enough or powerful enough.
The average car engine can’t run at best power all the time, it is not designed to do that. It is designed for varying RPMs.

In a V-8, the diesel is a good choice for it is designed to make it’s power at an RPM that it can run at forever, is slow enough that it does not need gear reduction and runs on all kinds of fuel. But it is heavy… But if you got the wing and don’t mind the slow pace, you can do a lot with one. ( everyone wants fast and sezy. )

For the average Joe that wants to build a plane, turbines are too expensive. If you have that much $ you by a Mig-15 or something neat.

In aviation speed and sex are directly related to how much money you spend.

Ever see an three cylinder radial on an Aeronca C-3 ? About 40 HP.

Big subject.

8

Subaru engines (four- and six-cylinder boxer ) engines have also been widely used in kits. Because they are a horizontally opposed configuration (like the VW Beetle engine, except water cooled) they have little in the way of side loads; consider it a radial engine with just two pistons (doubled up, of course). The engines also hold up very well, having little off-axis loading and very small to nonexistent flywheel. Boxers run very well at higher RPMs, giving a flat torque band and good efficiency. Of course, you do have to step down the output, as Sam Stone says, but they’re easy to access for mainanence and compact enough to fit into a small airframe.

Still, Sam makes some vaild points, and these ad hoc mods and kit sets see the most frequent failures of any light aircraft. Combined with the poor gliding characteristics of most light aircraft, I have to agree that I’d rather fly in a 30 year old Piper Cub than a new Fuji Heavy-powered kite.

Stranger

9

My friends brother is building a plane in his garage. (To make it fit the wall to the kitchen came out and there is a wing hanging over the breakfast nook.) He rebuilt a Subaru engine and added a turbo. Stainless lines and all sorts of other changes. I’ve only seen photos and don’t remember any more details.

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A related question: How does an inverted oil system work?

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This should help explain that.
In a wet-sump system, like most Lycomings, basically it’s an extra pickup at the top of the engine, and a gravity valve so that when inverted, it begins taking oil from that pickup at the top of the engine (which is now submerged in oil). Of course there’s more to it, and the link I provided explains it in higher detail.

12

Your other points make sense but I don’t understand this one. Wouldn’t a car experience a lot more pitch motion/force when going up/down hills than an aircraft? And how can a bearing be strong against yaw (which a car engine must be, I think, to handle sharp turns) and waeak against pitch?

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I was told this one by an A&P years ago. Maybe he was full of it, but he basically said the pitch-roll-yaw translations that airplanes undergo all the time do not match the typical loadings that car engines have. How often do you change directions and go up a hill so fast that you ‘pull’ 2 G’s? A car on a skidpad can pull at most 1g or so, unless it’s a race car with lots of downforce acting on it. An airplane in a banked steep turn can pull a lot more than that. And when cars make tight turns, they generally do so at low RPM. In any event, I’ve heard of crank failures in auto engine conversions, and I don’t see a lot of crank failures on the road, so maybe there’s something to it.

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It occurs to me that if this were a real problem, we might see more crank failures in the sport of ‘drifting’, where cars are spun around madly at high RPM. I wonder if that’s the case?

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Car engines do not have big propellers spinning out on the ends of their crankshafts with the attendant gyroscopic forces either.

If you have a reducer, then that takes a lot of the stress but not all.

Normally, auto engines are not over built in the crankshafts where as aircraft engines are more so even though they strive for weight loss, they do more in other areas.

Auto engines of today even if not a new design , do have better materials than ever before.

Some of the dependability is going up very fast.

I don’t thing the differences are as big as they used to be.

YMMV

16

[Disclaimer] I have no clue how the prop on an airplane is attached to the engine. [/disclaimer]
If the prop were attached to the crank directly the thrust (along the axis of the crank) loads on the crank would be extreme. Auto engines don’t have much thrust loads to deal with except for stick shift cars from when the clutch is depressed. In a plane the prop would be a constant pull against the crankshaft causing constant wear on the thrust bearings. IMHO most auto thrust bearings are not up to this task long term. I would expect thrust bearing failures before main bearing failures

17

Lycoming engine cut-away.

18

Why not make the wings detachable? He must have a very understanding and accomodating family!

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My friend Mark knows this guy whose brother built a plane in his garage and had to like kock out all sorts of walls and stuff to fit the wings and then had to use like this huge chainsaw to drop the front wall of the house to taxi it out and he did that cuz it was way cheaper than like making the wings detachable and stuff.

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Not just a gyro, but consider that the moment of inertia of a two bladed prop (most common) changes as it goes from verticle to horizontal when the airplane is pitching or yawing. Thus the gyroscopic loading comes not as a steady force, but as a series of pulses at 2X the engine rpm. This tends to fatigue the end of the crank to which the prop is attached, and is a known failure mode of VW conversions if beefier cranks are not used.

It would not be at all difficult to best existing aircraft engines from an engineering standpoint. The problem comes in certifying. It’s basically economically infeasable. Even Porshe could’t make money at it.