Stratus EA81 Aircraft Engine

stratus 2000 ea81 conversion

“Stratus-pheric” Events

JUNE 1994

An Amazing Auto Conversion Comes of Age

EA81 stratus engine

Progress is a wonderful concept… and progress is just what has been made this year in the field of automotive engine conversions for light sport aircraft. I have always felt that what was really needed to make auto conversions a commercial reality were “Drop-In” installations for today’s aircraft that could readily fit in where otherwise normal aircraft engines used to be.

You know… call up the manu­facturer, buy the engine, bolt it on to your favorite airplane and fly… instead of having to perform a year’s worth of experimentation to guarantee that all the appropriate widgets were within tolerable celestial alignment specs… and then having to readjust everything every day to fit in with the phases of the moon.

subaru ea81 auto conversion

Nope… we need nice little bolt-em-in, idiot-proof engines that require no more maintenance (or even better, less!) than the Lycomings or Continentals that we might otherwise be using.

The other problem inherent in all this is the fact that these conversions are not exactly cheap to perform and even those that looked promising from a power and weight standpoint were not all that competitive with the costs of the used engines avail­able in (formerly) plentiful amounts. However, all that has changed… the supply of used engines is drying up – (Editor: seen any EA81s lately?).

subaru stratus cessna 150 engine

The 182lb Subaru was some 35 pounds lighter than the O-200.

Produc­tion is down, homebuilders are snapping up O-200’s, O-235’s, O-320’s, and O-360’s as fast as they can find them. And, of course, as they do so, the laws of supply and demand take effect and drive the prices up… which has set the stage for a real revolution in sport aircraft powerplants… a Subar-revolution, you might say.

Enter Reiner Hoffman and the little aero-engineering firm of Stratus, Inc. Wearing a perpetual grin and speaking with the kind of European accent that one normally associates with Peenemunde rocket scientists, Reiner has been working diligently on the next generation of automotive aero-conversions… ones that work reliably, make economic sense, and weigh less than the powerplants that they seek to replace. By all accounts, he has done just that.

Many of you may remember our initial report on the Subaru EA81 that Hoffman built for an Avid Flyer. It was his extraordinary first effort in adapting this promising powerplant-the way he thought it needed to be done. As seen in the form it took for the flighty little Avid Flyer, the EA81 is a rugged 100-hp en­gine (priced $5800).

It consists of a carefully-converted Subaru 1.8 liter engine… a mill that has gotten a very good rep from experimenters in the aero-conversion game. An all-aluminum, four-cylin­der, liquid-cooled, normally-aspirated motor, the EA81 has a 12-amp alterna­tor (up to 50 amps, optional) and elec­tric starter for easy starting and the abil­ity to power one’s favorite accessories.

Obviously, being based on an auto en­gine, many of its parts are available at your local auto parts store, making things even more affordable should you need a part or two. The Stratus EA81 comes as a com­plete aero-engine… the 182-pound weight includes the reduction drive, alternator, starter, flywheel, carbs, mounts, and exhaust manifold.

An estimated 1000-hour TBO and altitude adjusting Bing carbs make for an easy-to-use, affordable mill that should serve well and long in most aircraft applications. The engine’s reduction drive ratio of 2.2 to 1 brings the prop rpm down to 2500 rpm… a pretty normal ops range in the aviation business.

I flew the Stratus EA81 last summer in a lightly adapted Avid Flyer (the airframe seems to have taken to this engine rather well) and the performance improvements over the Rotax 912-powered bird I flew the same day were pretty remarkable. Most noticeable, though, was the smoothness of the powerplant… some­ thing that doesn’t happen by accident, let me tell you.

subaru 1800 plane install

A 1000-hour TBO and altitude adjusting Bing carbs make for an easy-to-use, affordable mill that should serve well and long in most aircraft applications.

The company claims some 500 pounds of static thrust with a 72-inch fan attached and I tend to believe that bit of braggadocio. Our flight climbed out at well over 1400 feet per minute and temps seemed to remain pretty constant throughout the initial climb. Fuel consumption, for normal cruise flight, is spec’ed out at just under 3.5 gallons per hour (and has since improved).

Our top speed was well over 120 mph and to be truthful, we could have gone a heck of a lot faster, but this was a STOL (long) wing Avid Flyer and we’d have probably blown the wings off if we’d pushed it much faster… what a screamer!

You sim­ply won’t believe the deck angle this thing assumes under a full power climb… to the moon! OK… the Avid Flyer was a great way to introduce us to the world of Subaru conversions… and the sport-plane industry was definitely going to benefit in the future.

However, Hoffman had some more interesting ideas up his sleeve to prove out his concept. For the last few months, he has been carefully adapting this conversion to one of the most popular and well-known (if not the most boring) of general aviation air­ frames—that of the elder and much-maligned Cessna 150.

Let’s face it, the Cessna 150 couldn’t possibly be hurt by any more horsepower…most of these birds have all the pizzazz of an asthmatic senior citizen. His Cessna 150 conversion fits right 182-pound Subaru was some 35 pounds lighter (a total of 215 pounds) by the time all the appropriate accessories and good­ies were attached.

After looking through the installation and under the cowling, I was dying under the same cowl used to enclose the elder 0-200 and the installation he has designed seems quite simple. Dual radiators take up position right behind the cowling’s air inlets and the rest tucks in neatly behind it.

ea81 instrument panel cessna 150

The Subaru fit into the Cessna 150 surprisingly well… and was very easy to adapt to. The biggest difference was the lack of noise and vibration.

A normal O-200 in­stallation reportedly totes up at some 250 pounds, as it sits in the Cessna, but the to fly this thing. With bad weather coming in very quickly, it was obvious that we were going to have to fly right now or risk not get­ ting in the air at all. However, with the approaching front came wind… lots of wind… lots and lots of wind.

With 20 knots blowing down the runway and 30 knot (and then some) gusts threatening us every now and then, It was obvious that the upcoming flight was going to be “interesting”. Reiner and I bundled into the 150 late that afternoon, fired up the Subaru on the first try, and I was amazed with the very minimal vibration associated with this installation.

One thing was also evident right the from the start: this was a quiet little puppy. There were no headsets in this airplane and from the sound of things (or the lack thereof) there didn’t need to be. Reiner’s engine has a belt-driven reduction system that is exceptionally smooth and the flywheel action of the belt-coupled prop made the start a very familiar affair.

In no time at all, the whole works were purring along rather well and a quick scan of the gauges yielded excel­lent results. Everything stayed very cool (especially in the cool temps prevalent that day) and a quick burst of the throttle produced a very speedy response from the Subaru.

subaru ea81 cessna auto conversion

Reiner’s engine has a belt-driven reduction system that is exceptionally smooth and the flywheel action of the belt-coupled prop made the start a very familiar affair.

Lining up at the end of Arlington’s main runway, we did a quick check of the Cessna’s single ignition system (dual ignition is in the works) and lit the little Subaru off. One thing was evident right from the start… this was not your average Cessna 150.

Initial acceleration was quite good and far exceeded the normal lack of acceleration usually experienced with the meager 150. Less than 500 feet down the runway (yes, with a good, stiff headwind), we were off and climbing at over 800 feet per minute… even better than that climb rates promised by Reiner before we took off.

The air was very rough and we saw some pretty significant extremes in terms of both positive and negative G’s. Still, the dual Bing altitude compensating carbs sucked down the go-juice sedately and without a single hiccup.

Full power fuel flows were slightly in excess of 6 gph while throttling back to cruise rpms in the range of 3800 to 4200 rpm offered fuel burns of 2.8 to 4 gph. Cruise settled into a pretty steady 110 mph TAS and could have stayed there all day.

Operat­ing temps stayed well within specs (and were, indeed, a little on the cool side). Full power did not even threaten to temp out the 100-hp Subaru and we saw speeds of over 130 mph TAS with the little Subaru EA81 singing its heart out.

There are two small rpm ranges, both below normal cruise settings that seem to offer a little bit of resonance but a little belt tuning should work that out a mite. Overall, the effect was extremely smooth and once again, I was in awe of the low sound levels that allowed Reiner and I to discuss the flight in just slightly more than conversational volumes.

As a matter of fact, there seemed to be more wind noise than engine noise. This is quite an engine. In a little over half an hour, I was as convinced as I have ever been that this is a major turning point for sport aviation.

One thing was evident right from the start… this was not your average Cessna 150. Initial acceleration was quite good and far exceeded the normal lack of acceleration usually experienced with the meager 150.

The Subaru engine cores are plentiful, adapt exceedingly well to aeronautical uses, and can be converted very economically. Even now, less strenuously developed conversions are racking up lots of hours and decent reliability records.

With better engineering and more atten­tion to the conversion process, these engines may be in a position to really set sport av on its ear… and as we cruised around the skies of Arlington, the sound that came to my ears, though muted, was very sweet indeed.

I’m going to have to give one of these things a try for my­ self… I wonder how an EA81 would fit on a SeaRey??? Hmmm….

subaru aircraft engine

An all-aluminum, four-cylinder, liquid-cooled, normally-aspirated motor, the EA81 has a 12-amp alternator (up to 50 amps, optional) and electric starter for easy starting and the ability to power one’s favorite accessories.

All in all, I could not help but be very impressed with the work done by Stratus. These folks can be justifiably proud with what they have done. The 182-pound, 100-hp Stratus EA81 can be adapted to a great number of sport aviation’s most popular sportplane kits.

Already they have an engine ready to go for aircraft like the Kitfox and Avid Flyer while the KIS, Murphy Rebel, a number of gyroplanes, and many other 100-hp aircraft seem ripe for this engine.

Full “firewall forward” packages are in devel­opment right now for the Avid Flyer and others are being planned for the Kitfox, Murphy rebel, and other popular aircraft… these packages will get you flying in minimal time since all the installation work is pre-done by the Stratus folks… for a price currently estimated at some $2400.

Further: if you don’t mind taking your certified Cessna (or other aircraft) out of the normal certified class, you can get a lot of economy out of a conversion. I wish the FAA had a better defined way of deal­ing with such conversions… putting in a brand-new Stratus sure is a lot cheaper than overhauling an O-200 and the oper­ating economics are simply too good to ignore.

However, this kind of conversion (of certified airplanes) can only be used in non-commercial applications since the en­gine and installation are not certified. Oh yes… Reiner has one of the more powerful Subaru Legacy engines right now… and is considering a number of high performance aircraft designs for this engines potential of up to 200 hp. It’s gonna get real interesting up in the Northwest, folks.

Stratus is delivering engines right now. Not quite as deluxe as the NSI alternative listed elsewhere on this site, it is the bar­ gain hunter’s best choice for a cheaper way to get Subaru motivation. Yes, the builder/installer will have to do a little more work to adapt it to most installations until the above-mentioned firewall forward pack­ ages come available.

Still, for the homebuilder or certified aircraft looking for real economy, the Stratus EA81 may be the economy king of sport aviation engines… and one of the most pleasant motors you can fly with. We’re really impressed. Highly recommended.

Reiner Hoffman Subaru Stratus

Wearing a grin and speaking with the kind of European accent that one nor­mally associates with Peenemunde rocket scientists, Reiner Hoffman has been working diligently on the next generation of automotive aero-conversions.

EDITOR: Since the 90’s there have been other EA81 engine rebuilders offering the same reliable Subaru EA81 with modifications taking it reliably WELL over 100hp. But due to short supply, most have fallen by the wayside.

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