Porsche didn’t realize their overhead cams would make the engine too wide, and affect aerodynamics. Lycomings are made to be most efficient at lower RPM, and there is no benefit to having more than two valves per jug. Direct drive saves the weight of a PSRU. Pushrod heads are fine.
Typically, the PSRU handles thrust forces.
For superb material on alternative engines in home built aircraft see.
http://www.contactmagazine.com/
a Mazda rotary is my choice for my homebuilt aircraft:
A Corvair is also an excellent choice
http://www.flycorvair.com/
I’m sorry if anything I say has been said, but I’m too lazy to read 102 replies…
the Subaru boxer engine was originaly designed and intended for aircraft use. Thats’ why it’s all aluminum, and has a split case design instead of a cast block. Fuji Heavy Industries just used what they had at the time. But, the original subaru design has changed a lot up to the 2.5L you want to use. In the cars, the 2.5L is known for head gasket issues so it may not be a good choice. The earlier 2.2L EJ22, engines in the 90-94 Legacies may be a better one, but it is less powerful. It’s also a non-interference engine so if a timing belt breaks the valves won’t slam into the pistons, and in some cases, the engine can run on two cylinders for a short time before the engine gives out(there are two timing belts). That may be enough to get you down safely. Also, the earlier(90-94) engines have a plug on the back of one head to accept a distributor(maybe adapt a magneto) from the non-US versions or early Ford Escort engines, so you can ditch the ECU and harness and still run the engine with a carburetor. You can find more about Subaru engines here, the Ulimate Subaru Message Board, http://www.ultimatesubaru.org/forum/index.php
Good luck.
I would opt for the 2000cc Honda Gold Wing engine. It’s water cooled, extremely reliable, probably the smoothest running engine of the lot and is extremely powerful for the weight. It’s fuel injected so carb ice is not a problem. It’s also compact but, unfortunately, expensive.
I haven’t been able to find a reference to “the Subaru boxer engine was originaly designed and intended for aircraft use” - do you have one? The only reference I found was to a plane built by Fuji Heavy Industry that used Lycomings.
Wasnt the Subaru originally based on the VW which was supposedly based on a aircraft engine design?-Kevin
I don’t think the overhead cam watercooled Subaru shares anything with the overhead valve aircooled VW flat four except being a flat four. And I didn’t know the VW flat four had any aircraft roots. It was designed by F. Porsche before WWII for Hitler, I’m not aware of any aircraft use at that time.
It turns out VW’s Beetle, with its rear-mounted aircooled flat four, was inspired by the advanced Tatras of the early 30s, with aircooled engines in the rear. VW eventually paid Tatra some fairly big $$ after WWII as compensation for copying these major elements of Tatra’s design.
What about a Saab engine. Weren’t they originally airplane engines?
Nah, they used several other companies’ engines, I think the 4 had long-ago roots with Triumph cars in England. Saab did start out as a division of the aviation company, but not the engines.
Could very well be that you are 100% correct its been so long since I have read anything about VDubs-did you know that the front suspension on the VW gave Ferry more trouble then any other component he designed?(according to my increasingly falliable memory)-Kevin (of course Adolf Hitler was the impetus for the “Folks-wagon”)
I heard the phone call and I would look for another engine.
A 5000 RPM engine, geared down to turn a propeller at 2000-2500 RPM strikes me as a noisye and short lived machine. The Subaru turning at a more stately 2500 RPM or so would undoubtedly last for a dare I say lifetime, but I wouldn’t want the weight or complexity of a liquid cooled engine in a small aircraft.
I think Ed should seek advice from a competent source. Allow me to hasten to add that I certainly am not so qualified. May I respectfully suggest Ed contact Experimental Aircraft Assn for ideas.
Would not recommend a Mazda rotary conversion unless you just are fascinated by that kind of engine. Wankels have high fuel consumption (in cars as well as aircraft), consume oil to lubricate the apex seals, and have very loud, very hot exhaust necessitating an effective, probably heavy muffler. The compact size and light weight are offset by the need for propeller speed reduction unit (PSRU). PSRU is only 90-95% efficient and is another expensive, heavy component requiring inspection and repair.
This may not be an issue for the typical homebuilder, but how many of the auto-engine conversions, wankel or piston, are capable of attaining 2000 hours of use before overhaul? The Lycoming 320 and 360 series engines can do this reliably, if maintained and flown properly.
Hi Ed I’m an A&P Mechanic. The two things that you should consider are stall speed and odds. What is the stall speed of your plane? Can you land it safely if the engine has catastrophic failure at altitude? Odds, the differences in these two engines (from a reliability standpoint) is moving parts. The Subaru has a cooling system, ignition, and fuel injection components that could fail. These parts don’t exist on the aircraft engine. The odds of one of these parts causing a catastrophic engine failure on take-off are very low. The odds are low at altitude also, but I would always be aware of all airfields and pastures in the area. Hope this helps
As an A&P, I can say that auto engines are designed for much less stress and reliability than an aircraft engine.
An auto engine is designed to put out full power a few seconds at a time (accelerating to speed), then put out 15% power for the rest of the time (cruising at speed).
An aircraft engine is designed to put out 100% power for a few minutes at a time (takeoff and climb-out) then 85% power the rest of the time (cruise).
Add to that the different conditions an aircraft engine performs under (high altitude/reduced air pressure, icing issues.)
Add to THAT the inherent reliability of an aircraft engine. It will. Not. Stop. Short of throwing a rod, blowing a valve, or starving it of fuel, that is. It has redundant spark plugs, magnetos that don’t need a working battery, and much lower compression for less issues with overheating. Also, the aviation engine is likely air-cooled, while the Subaru is liquid cooled. No coolant doesn’t do anything to the aircraft engine, while the Subaru will hand-grenade. Not exactly the thing you WANT to have happen to you during your climb-out.
Have you considered?: The aircraft engine is designed in numerous special ways to run sucessfully in many different orientations. Because an aircraft can experience G forces in any direction, systems are in place to keep the important fluids (fuel, cooling, oil, etc…) flowing smoothly. In some cases critical systems are redundant. In other cases aerospace standards dictate the quality systems used in the manufacturing of the engine components. When it comes to flight, the failure of a component means death. The standards are written because people have actually died. Buy the AS certified engine.
from a non-pilot: I read with interest about a chap in CO who had a small subaru homebuilt and private mountain top landing strip. Most interesting was his stories of how he could reconfigure the cabin with a board of plywood and sleep in it to go aero camping. more than once he landed in a remote small town, taxied to the nearest gas station and filled up at the pump! Way cool! that’s why I thought the subaru engine is a plus! Couldn’t find the pages anymore (recall it was titled “see what I fly”).
My brother build a plane from a kit, not sure of spelling, Very-Ezee is how it sounds.
He started with a VW engine to save money. After it died several times on the taxiway, he put in a certified aircraft engine.
I know it’s experimental, and all that, but you are risking your life Don’t do it. When you are in that thing, and spiraling toward the trees and rocks, you will wish you had spent the extre money.
Ed is totally on the right track in considering an auto engine for his experimental aircraft. Thousands of new aircraft are built each year by amateurs and they are very safe and efficient. In fact, nearly all progress in personal aircraft has taken place in this market segment. Furthermore, it was aircraft homebuilders who were responsible for the widespread use of GPS systems for navigation by private citizens.
Auto engines are far more robust than aircraft engines because they have to put up with knucklehead drivers. Most of the power of a car engine is RESERVE power, power that you never use, except when you floor it to get into traffic, or climb steep hills. Even then, you will only use part of the reserve. There is very little strain on an aircraft engine at 75% power, because in a reciprocating engine, it is the increase and decreases of speed that cause them to fail (yes, decreasing speed will shock cool the cylinders on air cooled engines). Air cooled engines are rarely used in autos because they cannot take the abuse. Until recently, however, the specific power (horsepower per pound of engine) of care engines was too high to make them a good choice as an aircraft power plant, especially when you have to add a cooling system weighing around forty pounds. But continued advances in automotive engines and cooling systems has brought specific power to the point of being competitive with aircraft engines.
I have owned a number of airplanes, and aircraft engines are very high maintenance and the parts are outrageous—not because they are better, but because they are CERTIFIED. The problem with the certifications is that the standards themselves are over fifty years old. As a former automotive engineer, I can assure you that an auto engine will run for 6000 hours without failure, whereas an aircraft engine will not run more than a hundred hours before it requires maintenance.
One big problem with using auto engines in aircraft has been the fact that most experimenters use junkyard engines that are already worn out. The second major problem is the horsepower issue. The rated horsepower of an aircraft engine is it’s maximum allowable power, which will normally come at 2800 RPMs. The rated power of an auto engine is normally over 6600 RPMs. So when you look for a 150 HP engine for an airplane, you need to do some torque calculations and find the engine that will deliver 150 HP at 2800 RPMs. Instead, converters seem intent on doing the piston speed calculation and operate a car engine at 5000 RPMs (popular on airboats in Florida). This is a bad idea. Car engines have much shorter strokes than aircraft engines because of the reserve power issue. Because of limits on propeller speeds (won’t go into that issue), the propeller has to be kept below 3000 RPM, so the converters want to use a PSRU (prop speed reduction unit) to bring the 5000 RPM engine down to 3000 RPM. This is where conversions get into trouble with cooling and the extra weight of the PSRU. This is also where I think you are leaving the trail of proven technology and venturing into the unknown, something to think twice about with an airplane. If you choose an auto engine, use a direct drive installation, not one with a PSRU.
All the nonsense about Alaska Airlines, etc., is irrelevant. Power is power. All engineering comes down to the details of the application. One last comment about the use of auto engines. While auto engines can run avgas (but you can’t do this in your car because of the lead in the Avgas), the reverse is not recommended (although you may hear otherwise). Furthermore, if you choose to use an auto engine, make sure you always use fuel free of ethanol.
Exactly Right. Use A Certified, Manufactured Aircraft Engine. Hey, While You’re At It, Use A Manufactured, Certified Airframe And Buy Them As A Matched Set !
“There are old pilots and there are bold pilots, but no old, bold pilots.”
CSA