February 18, 2022, ©. Leeham News: Having covered batterie-based electric aircraft in the last three articles, we now move to the next ideas to improve our aircraft’s efficiency and burn less carbon-based fuels, hybrids.
A word of caution first. The words Sustainable, Electric, and Hybrid are, because of their success in ground transportation, virtually a must in any news release from anyone involved in aviation these days. We will unveil what is behind all this and what is real and what’s Greenwashing.
It used to be when a major portion of the propulsive power of a vehicle comes from an electric motor, like for the Toyota Prius, Figure 1 (for the Prius it’s 100%). While this used to be the case for aeronautical vehicles, it’s no more.
Today, with the Sustainability value embodied in the word Hybrid, anything that has the tiniest electric motor involved somewhere in the aircraft’s propulsion, even if it contributes perhaps a percent or two, is labeled as “ using Hybrid Technology.”
We have explained why the concept works for cars. These energy hogs (they use 5-7% of the energy in the fuel) waste all the energy used for acceleration when braking for the next stoplight. The Hybrid instead brakes with the generator and regenerates the energy to the battery. That this is the case shows by hybrids being super efficient as town taxis, whereas, on the highway, they show the same mpg as a regular car.
As we have no stoplights in the sky the regeneration of energy to a battery doesn’t work (and no, you can’t regenerate energy in the descent, we come to that).
Back to Civil Air Transport and the Greenwashing going on today.
When Boeing launched the 787 as “more electric’ in 2003, it would today be “using hybrid technology.” By this, I’m not accusing Boeing of doing it (it hasn’t done it); I’m just giving examples of what things used to be called and what’s the marketing hype of today.
It’s where an electric motor is substantially involved in powering the thrust-producing part, the propeller or fan.
A Hybrid is not when the engine’s starter generator is used to generate power to an electric system. Starter generators have been around for 100 years or more, have always worked as a motor to start the aircraft from a battery or ground electric source and then, once the engine runs, change role to a generator for the aircraft. It then supplies the aircraft’s system with electricity (an electric motor can serve as a generator, you change some electric fields and done). This is nothing new, it’s been done this way on small aircraft since the beginning of aviation.
This hasn’t been called “hybrid technology” during these 100 years, but it is today. We won’t call this a hybrid, and others shouldn’t. What we call a hybrid is the following:
A propulsive concept where the electric motor supplies an important part of the power to a fan our propeller as in Figure 1.
Figure 2 shows the principle differences between turbofan/turboprop propulsion, battery-based, and hybrid propulsion concepts. It also shows the typical path projects take as they work through the plus and minuses of these concepts as a replacement for today’s turbofan/turboprops.
A serial hybrid often results when a project starts as a battery aircraft and the design team realizes that the range/endurance is not practical. Most projects start as battery-based, then migrate to battery plus “range extender” to fix the problems we saw with the Alice and ES-19.
The range for such aircraft (pilots always think in flying time/endurance, weather/wind then convert this to range) is not enough for everyday use. The risks for pilots and passengers are too high that a flight runs out of Alternate or Go Around energy. What battery entrepreneurs don’t understand is that experienced pilots NEVER fly with only regulatory reserves; they always add on top, and often they add a lot. Real weather is too unpredictable for you to rely on a forecast. Risking passengers’ life because “we fly Sustainably” is not on.
A range extender is harder to make work than it seems. Project after project abandons the idea because when all the safety precautions (failure mode and effects analysis, etc) are worked through, the project now has two propulsion systems. The result is, the Hybrid is heavier, more complex, and most times not better in fuel economy than the turbofan/turboprop propulsion it shall replace.
Often hybrid projects start with a serial hybrid, then discover it doesn’t bring enough improvement over the turbofans/turboprops it shall replace. The next stop is a parallel hybrid.
A downsized gas turbine is working in parallel with the electric motor feed by a battery system for takeoff and climb. During cruise, the gas turbine generates the power for the fan/propeller.
This concept assumes a downsized gas turbine, that doesn’t have to be sized for takeoff and climb, will be more economical than one sized for a mission’s varying loads. We will check if this is the case.
There is a final case, the embedded starter/generator, that we will look at, Figure 3.
By an enlarged embedded starter motor cutting in and helping the gas turbine during transients (increasing/decreasing power), the gas turbine can be optimized harder. While this is valid technology and has been studied and tested for decades, it’s questionable if it is to be called hybrid technology. But it is today.
We will look at all these concepts in the following Corners and analyze if and how these contribute to lower emissions.