Just to be clear, I thought for the longest time electric aircraft would simply never happen, at least on a large scale. Batteries are simply too heavy and not energy-dense enough, and nothing is more critical on an aircraft than weight. The famous adage of Colin Chapman - “add lightness” - is daily edict for the aerospace engineer. Anything that adds unnecessary mass is a sin, and compared to a good old air-cooled aluminim-block internal combustion engine and 200 gallons of AvGas a battery just can’t compete. Lately, however, I’ve been giving it some thought and I’ve become a convert - hybrid and electric aircraft will indeed be the savior of air travel.
The Past Hybrid That Could Be the Future of Aviation
The basic definition of the word “hybrid” means any two things coming together to form a single entity. While in the automotive world the word “hybrid” now comes to specifically denote an electric motor and battery pack paired with an internal combustion engine, technically any two motor types operating a single powertrain can be a hybrid engine, and most of the world’s trains and ships have been operating on such setups for decades. The idea of a highly fuel-efficient engine is also thought of as a modern problem; however, even in the immediate postwar period, a time defined by ever-increasing displacement V8s, fuel efficiency had always been an aim of aerospace engineers. Enter the Napier Nomad.
When jet engines were still a new thing engineers often combined jets and piston engines in the most straightforward fashion, putting both in the same aircraft. The idea was that each engine would overcome the deficiencies of the other - piston engines provided take-off power and economical cruise as early jets lacked great acceleration capability, while the jet would provide raw speed. Each engine was still very much separate, with the jet sticking straight out of an otherwise very normal-looking propeller airplane. The Napier Nomad was different as it was aiming for different goals. It combined a jet engine and a piston engine into a single engine, at first each engine driving different propellers but later versions acting in concert as a single powerplant. Unlike other types of “hybrids” of the period, both engines operated all the time, producing power simultaneously - nor would the jet engine be providing power with direct thrust, instead acting as a turboprop.
There was a specific reason why this novel hybrid setup was tested by Napier & Sons: just like how a piston engine and jet engine operate differently in raw performance, they also operate differently from an economic standpoint. To save the headache of trying to explain the technical minutia, let’s just say that a jet engine is cheaper to run than a piston engine during certain portions of the flight, and vice-versa. If an airplane could suddenly switch from jet engines to piston engines during some phases of the flight, and then back to jets for others, significant fuel savings can be had. The Napier Nomad tried to accomplish just that, with the jet engine and piston engine trying to help overcome the economic deficiencies of each other for all flight phases. By adopting a diesel piston engine, the Nomad also ensured that both engines could run off a common fuel source. According to our handy-dandy and certainly trustworthy guide in Wikipedia, the Nomad claimed some of the highest fuel efficiency numbers of any aero-engine ever put to the air up to that point. However, airliners were increasingly more interested in pure jets and their raw speed advantage over the antiquated look of having spinning blades attached to the wings, and market interest in the Nomad shrunk to oblivion as operators prefered the snazzy jets and turboprops instead.
Today, airlines are always interested in anything that can result in fuel savings. Perhaps the time is right, then, to bust out the old Napier Nomad concept once again. Take for example the Q-400, the most popular turboprop airliner in the U.S. and broadly representative of turboprop airliners in general. A future variant can have its PW150 engines supplimented by, say, Cummins inline-six diesel engines modified to operate off Jet-A fuel. Like the Napier Nomad, the entine setup can be combined into a single powerpack to install cleanly in the aircraft’s nacelles (the “pods” the engines attach to) to avoid cutting into the passenger cabin. And on top of that, a cleanly and intelligently-installed battery pack (concievably lining the belly of the aircraft, for example) can send additional power to the props for a triple-hybrid setup. It would be hard to nail down the specific fuel savings, but one has to surmize that it would be a massive improvement over current single engine-type aircraft.
Large-Scale Hybrid Aircraft for Green Travel
I had previous wrote that jet engine and airframe technology has hit serious limits of diminishing returns that may make it questionable regarding additional advances in fuel economy and carbon footprint reductions. I still believe that to be true - but the success of hybrids and electric cars in the automotive world have convinced me that improvements in battery energy density can only go upwards, and that jet-eletric hybrid aircraft with vastly reduced carbon footprints compared to current airliners may be forthcoming in a short while, if not already technically possible.
A hybrid jet engine could work similar to a turboprop in that it contributes both direct thrust and power supply to a battery/electric motor setup that would power fans to provide the aircraft’s main means of propulsion. Rather than increasingly huge hi-bypass jet engines as seen on 777s and A380s, a pair or even just one relatively small engine could provide all the power necessary with the associated fuel savings.
How Hybrids and Electrics can save Hobby Aviation
I had also written previously about how the fuel crisis can hit General Aviation hard enough to effectively destroy it. This is no exaggeration - because AvGas is a specific formula that’s effectively been banned from production due to what goes into the formula and the processes needed to make it (it’s basically leaded gasoline), once the world’s supply runs out, that’s it, you’re grounded. Battery-powered aircraft obviously wouldn’t have that problem.
While Airbus has demonstrated an all-electric aircraft suitable for sport or hobby aviation, it likely represents a dead-end for many of the reasons why some early jet aircraft failed - it relies too much on what’s becoming an obsolete paradigm; a paradigm made obsolete by the very technologies it’s demonstrating. However, an sport aircraft designed around an entirely new paradaigm - one with many evenly-distributed electric motors, such as what Chris Clarke had previously written about, not only would be a more efficient and safer aircraft but the energy efficiency from the physical arrangement alone alleviates many of the energy-density concerns of commercial battery technology. Before the technology matures to the point where we can have 250-400 MPH electric aircraft (such as what’s typical of high-end multi-engine piston and turboprop aircraft) we can have 100-150MPH all-electric aircraft comparable to the typical Cessna 172 Skyhawk or aircraft of lesser performance (and lighter construction) that would be suitable for the “Light Sport Aircraft” category - right now. Having these aircraft be technically feasible and rolling off the assembly line are different things, and the process is additionally slowed by bureaucratic red tape, but increased enthusiasm will at least get aircraft manufacturers like Textron and Airbus interested in further investigating these solutions and bringing them to reality.