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Thinking Out of the Box Regarding Electric Systems for Aircraft
By Brett Hahn, EAA 647675
October 10, 2018 — I was really interested in the Word of the Month article “More Electric Aircraft,” in the June issue of Bits and Pieces, which referred to aircraft being more electrified, rather than more aircraft being electrically propelled.
Since 2008 I have driven a third-generation Toyota Camry hybrid, and after a decade of ownership and 160,000 reliable miles I can state it is one of the most efficient and best-engineered cars available. I bought the deluxe version for only US$25,000.
It uses electric air conditioning, steering, and braking in addition to the electric motor and regeneration system.
It averages 37 mpg, and I drive like a maniac up and down the 80 mph speed limit superhighways here in New Mexico. It uses a very economical four-cylinder gas engine, and the electric motor boosts it, making it feel like a powerful V-6 on steep mountain roads. It has plenty of power.
It would be a huge paradigm shift to adapt this 20-year-old technology into GA aircraft.
- Electric air conditioning to cool the cockpit during ground operations? Low-wing aircraft such as RVs are pretty hot under that canopy, I hear.
- Nose wheel electric motor for taxi and in and out. No engine noise and no prop spinning. Safer too.
- Electric nose wheel/tail wheel for assisted ground roll acceleration. Many aircraft have very poor acceleration from 0 to 30 knots. Electric motors have awesome torque and could really get the aircraft moving quickly until the prop bites — how about 0-30 knots in 2-3 seconds?
- We have to get away from the ancient concept of single-engine aircraft, especially since the loss of the powerplant results in a quick trip to the ground.
- Imagine a push-me/pull-you inline airframe similar to the Cessna 337. Put a gas engine like a 100-hp Rotax up front, and a 40-hp/30 kW electric (Boost) motor in back. This would provide a total of 140 hp. Both powerplants are used to take off and climb to altitude. Once level, the electric motor power lever is pulled back to flight idle, or if a folding prop is used, shut off. You cruise and land on the gas engine. If the gas engine fails, power up the “aft fan” and cruise in level flight to the nearest airport.
- It’s possible that a Toyota Prius hybrid engine/electric motor could be adapted for aircraft use. Both powerplants are already integrated and could drive a single propeller. The new Prius, at US$25,000, costs less than a Lycoming O-320 engine alone.
The energy density of a battery pack is still very far away from what a gallon of gasoline contains, but they are improving all the time.
We have had more than 110 years of engineers all around the planet devoting research and development to get to where we are today with internal combustion engines. Right now after a century of effort, the best automotive internal combustion engines are maybe 30 percent efficient. That means that for every 30 litres of gas you buy only about 9 litres provide thrust. The heat-energy from the other 21 litres is blown out from the radiator or cooling fins and the exhaust system.
Electric motors convert more than 90 percent of the battery energy into work spinning a prop. That means that out of 1,000 watts available only 100 watts is waste heat.
This is why, even after an electric motor has been running for hours, you can touch the motor case with your hand.The electric motor case will be hot, maybe 70 degrees Celsius, but not the 200-plus degrees of a cylinder head or the 700-degree exhaust gas temperature. If you’re interested in the latest news on automotive motor efficiency, you might like to check out this article.