June 7, 2019, ©. Leeham News: In last week’s Corner I wrote: “The reason electric cars work and airliners don’t is the Sky lacks Stoplights“.
The discussion was part of my previous series on Electric aircraft, but it was in the comment section. Here is a more exhaustive run through of the main reasons.
The key reason electric cars with a battery energy source work and airliners don’t is that the normal car is an energy hog in our daily use and is less sensitive to weight.
Car: We burn energy when accelerating the car from a standstill after every stop, be it for stoplights or traffic jams. Then after a while, we brake the car to a slower speed or to a stop with our friction brakes, wasting the energy we used to get to speed. It repeats over and over again.
The electric car recovers the built-up motion energy when slowing down, by it making it less wasteful with energy than the normal car.
Airliner: The airliner accelerates once from standstill on the runway to takeoff speed and then gradually to cruise speed during the climb. When descending the investment in potential energy during the climb is recovered. It overcomes the drag during the descent.
A minimum of energy waste characterizes the airliner operation. All energy is used to overcome drag. The potential energy from cruise height is to a large part recovered during the descent before landing. So there is no energy waste to gain for an electric airliner compared with a combustion one.
Car: The car is less sensitive to the added weight of batteries or hybrid propulsion than an aircraft.
A normal 1,500kg car when driving at 100km/h (60mph) has a tire rolling resistance of 220N/50lbf and an aerodynamic drag of 220N/50lbf. If we substitute the fuel with the typical Tesla battery pack of 65kWh the car weighs 2,000kg. Drag stays the same with tire rolling resistance due to weight now at 294N/66lbf.
Aircraft: If we take a similar loaded weight aircraft, like the Cessna 206 with five passengers, drag due to weight (induced drag) is 440N at 100kts. If we substitute fuel for a Tesla 480kg battery pack the induced drag increases to 780N. The substituted fuel has an energy content of 2,600 kWh whereas the pack has 65kWh but we use the pack weight of 480kg to make the weight increase comparable.
Electric cars can recover the wasteful brake energy lost if many situations. They can, therefore, successfully compete with our energy-wasting combustion cars.
There is little energy waste in a modern airliner’s operation. Electric airliners, therefore, have no energy waste to recover.
The drag increase due to weight for the Car is 33% when going from combustion to electric. The airliner’s drag due to weight, induced drag, increases with 78% when we convert our aircraft to electric with the Tesla battery pack. As noted we would cut range to a fraction of the fuel based aircraft. We need a much larger battery pack to keep range, making the induced drag increase much larger (induced drag scales to weight squared).
Aircraft are very sensitive to weight, cars are not. Weight is the aircraft designer’s arch enemy. This is ignored/forgotten by those proposing battery-based or hybrid transport aircraft.