Power moves

The direct-drive, air-cooled, spark-ignition piston engines we’ve relied on for decades face growing competition from alternatives promising lower costs, noise, and environmental impact.  The electric-powered Pipistrel Velis Electro is on the cutting edge of light aircraft powerplant development, and other innovations include refinements to spark ignition, diesel, and turbine technologies. That all adds up to more choices for what powers the new aircraft on the market today (see Y2K Trainers)

Originally a snowmobile engine manufacturer, Rotax began producing two-stroke engines for ultralights in the 1980s. Once the light sport aircraft category was created in 2004, the four-stroke 912 series became the engine of choice for new aircraft flooding that market. Rotax engines are designed to run on unleaded mogas and have a low fuel burn, a high power-to-weight ratio, and features such as electronic ignition and automatic fuel/air mixture adjustment. The engine turns twice as fast as conventional direct-drive engines, which helps keep the engine compact but requires a reduction gearbox to the propeller; and a combination of liquid and air cooling keeps temperatures low. Rotax has since branched out into certified aircraft and scaled up to models with up to 160 horsepower.

In many other parts of the world, jet fuel is more widely available than 100LL avgas, and you’ll see diesel-engine or “Jet A” versions of many models from European manufacturers. Aircraft diesel cycle engines, also known as compression-ignition engines, ignite the fuel-air mixture using compression rather than spark plugs and run on jet fuel. I recently flew the Tecnam Gran Lusso, a luxury version of the P2010 powered by a Continental CD-170 diesel engine. The 170-horsepower diesel engine in the Gran Lusso is heavier than its 180-horsepower avgas-burning counterpart, but it burns 5.2 gallons per hour in comparison to 10. It also uses a single power lever—no mixture control—and is noticeably quieter than its spark-ignition counterpart.

One of the latest innovations in propulsion isn’t widely available yet and won’t change anything about the airplane except a sticker. General Aviation Modifications Inc. (GAMI) is working to commercialize a high-octane unleaded fuel that was authorized for use in the piston airplane fleet in 2022. Airplanes with low-compression engines like the Cessna 172s and Van’s RV–12s I fly don’t need high-octane fuel, but for airplanes like the Beechcraft Bonanza, using a lower grade fuel than intended could cause detonation—and catastrophic engine failure. Today’s avgas contains small amounts of lead additive to boost the octane of the fuel, and a handful of companies have been working on the technical challenge of creating a lead-free alternative; the GA community has committed to phasing out lead from aviation fuel by 2030. AOPA is currently demonstrating the performance of unleaded fuels by fueling a twin-engine Beechcraft Baron with avgas for one engine and unleaded fuel for the other.

Turbine engines are more reliable than reciprocating engines, but they’re only in the realm of multi-million-dollar aircraft. That could change with the introduction of French manufacturer Turbotech’s 100- to 160-horsepower engines. These tiny turboprops recover heat that’s normally lost in exhaust gases and reinject it into the combustion chamber for reduced fuel burn. JMB Aircraft and Bristell Aircraft are working on Turbotech-powered models of their light GA aircraft; Bristell projects a fuel consumption of about 6.6 gph.

In an exciting regulatory development, turbine aircraft could soon be certified as light sport aircraft. AOPA has long pushed to expand the LSA definition and reduce its operating limitations, and in 2023 the FAA delivered a proposal to overhaul the rules governing LSAs. Under the proposal, performance-based criteria would replace prescriptive rules on powerplant type and takeoff weight, opening the door to further innovations in light GA.

The 1920s and 1930s were the Golden Age of aviation. The 1960s and 1970s were boom times for GA. I’m not sure what history writers will say about the 2000s, but I can’t wait to find out. 

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