Rampside Capsule: Engines with Altitude

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– Posted on August 5, 2012

 As I wrap up “Oshkosh Week,” let’s take a brief look at another area where automobiling and aviation cross over: powerplants.

The ratio of power to weight is important in automobiles, but even more so in aircraft. The Wright brothers were unable to source an auto engine of sufficient horsepower and light enough weight for the 1903 Flyer, so they had their shop mechanic, Charlie Taylor, design and build one from scratch. Lacking spark plugs, a fuel pump, carburetor or even a throttle, the engine did innovate in one area: it had an aluminum block, a first for any engine used in an aircraft. This engine developed 12 hp at 1,090 rpm and weighed 170lb (77kg), which works out to about 14lb/hp.

Only 26 years after the Flyer first flew, Bernard Pietenpol published plans for his two-seat Air Camper, which could be built using locally scrounged materials and powered with the widely available (in 1929!) Model A engine. The cast-iron blocked Model A engine weighed 244lb (110kg) in aero trim and developed 35hp at 1,600 rpm, which equates to about 7lb/hp.

Plans are still available for the Air Camper, and it’s probably one of the most versatile airframes ever, with over 30 known engine types having been used by builders over the years.

After Chevrolet introduced the Corvair, Bernard built two different Piets using this engine, and really liked the performance improvement. The Corvair engine (adapted for aero use) has quite a following, with numerous horsepower upgrades and dedicated web sites for conversions. A typical installation will weigh around 220lb (100kg) and make a reliable 100-120hp, which works out to around 2lb/hp. The Corvair engine has been used in well over a dozen different aircraft types, and is a good, low-cost substitute where less than 120hp is required.

The Suzuki-sourced Chevy/Geo Tracker and Metro engines have also found a following among homebuilders. The three cylinder engine produces around 62hp at 140lb (64kg), which is about 2.26lb/hp, and the four cylinder comes in around 90hp at 168lb (76kg), or around 1.9lb/hp. The P-38 replica above is powered by a pair of belt-driven Metro engines.

One particularly interesting engine aero conversion happened not long after WWII (1946), when the Crosley COBRA (COpper – BRAzed tin) engine, making all of 25hp, was adapted for use in the prototype Mooney Mite. In auto trim, the engine weighed about 133lb (60kg), for a power to weight ratio of 5.32hp/lb.

The example shown above has been repowered with a Lycoming engine and was involved in a crash in 1950. The owner still has the factory receipt for the aircraft rebuild, which cost $1,182.00.

One would think the COBRA engine would be better suited to aero than auto use, given that an aero engine runs for long periods at constant power settings (which were the design criteria for the COBRA), but the engine proved unreliable in the Mite, and the twelve (or possibly seven – sources disagree) initial production aircraft were all repowered to Lycoming or Continental aero engines, which greatly improved performance and reliability.

 

Taking a look at the boxer “four” family, the Subaru engine is another popular power option for homebuilders. With the EA81 engine weighing in at around 185lb (84kg), they make a reliable 70hp, or 2.64lb/hp. There’s a huge fan base for these engines, with a ready supply of parts and cores to work from.

Finally, we’ll take a short look at the auto engine that’s probably been adapted to the most number of airframes over the years, the aircooled VW Type 1 engine. There’s such a variety of upgrades to these engines, it’s really hard to pin down a typical weight and hp rating.

I’ll use this WWI Nieuport 11 replica (which belongs to my friend Dick Starks of the world-renowned Kansas City Dawn Patrol), powered by an 1,835cc Type I engine as an example. These will typically weigh in around 150-160lb (73kg) and make around 60hp or so, or 2.5lb/hp. The Type I engine can be pushed up close to 2,500cc and 80+hp. Coupled with a reduction drive and a honkin’ big prop, it will really pull you along!

At the other end of the VW spectrum are “half-VW” engines, weighing in at a featherweight 87lb (39.4kg) and making a shade over 35hp, or 2.49lb/hp. This particular installation is in a legal FAA Part 103 ultralight (the aircraft empty weight must be 254lb or less).

In this Capsule, I’ve only tried to cover the more popular auto-to-aero engine conversions, and other than this brief mention, haven’t considered the reverse angle of aero-to-auto conversions such as the Franklin engine used in the Tucker.

Aircraft builders, and particularly homebuilders are always searching for the right balance of low weight, good horsepower, reliability and economy. Cost is a factor, too, especially as you get closer to the ultralight scale aircraft, but as my flying buddy always says, “I didn’t get into aviation to save money!”

For many, an auto engine conversion is just the ticket.