Sustainable Passenger Air Travel

by Wayne Lanier 

Sustainable air travel is on its way. Electric aircraft have not only been built and flown, but they are expected to inherit the same advantages found in electric automobiles.

Greater efficiency, greater reliability, quieter operation and lower cost in volume production, operation, and maintenance. So, how would all this work and how do we get from this concept to a passenger fleet of such airplanes?

Electric Flight “Comes of Age”

Traditionally, new aircraft designs “come of age” with a successful flight around the world. In July 2016, the Solar Impulse landed in Abu Dhabi after a 26,000-mile flight around the world.  Four solar-powered electric motors turned Solar Impulse’s propellers.

Solar powered plane ‘Solar Impulse 2’, piloted by Swiss adventurer Bertrand Piccard, flys over the Golden Gate bridge in San Francisco, after a flight from Hawaii (Photo by Jean Revillard via Getty Images)

For those who were out to see it, the Solar Impulse flew over the Golden Gate Bridge on its way east from Hawaii. I saw it from the end of Pier 39, at a great distance through my binoculars.

Certainly, this experimental phase of sustainable flight has been successful. But don’t pack your bags for a sustainable passenger flight just yet. The average speed of the Solar Impulse was just 45 mph. It is, essentially, a giant propeller-driven glider.

Small, privately-owned aircraft have been successfully converted to electric power for many years. For example, consider the very popular Piper J-3 Cub. Its gasoline engine is 65 horsepower and runs at 2,350 rpm. Piper Aircraft built over 20,000 J-3 Cubs between 1937 and 1947.  Most are still in service (the result of strict FAA requirements for very regular maintenance).

The Piper gasoline engine weighs 170 pounds. GE and other companies make DC (direct current) electric motors in the horsepower and rpm range of the Piper’s engine. The physical design is simpler. The engines weigh less than the Piper gasoline engine, and they are far more reliable. So, as you might expect, with modest engineering, electric motors can replace the original engines of this and similar small aircraft. And, among the experimental aircraft folks, they have been.

The Barrier to Sustainable Flight: A Potent Alternative Fuel Source

There are, however, three big problems standing in the way of sustainable passenger flights: fuel, fuel and fuel.

First, aviation fuel is, essentially, cheap kerosene. It has an enormous energy content per unit of weight; a 20-to-1 advantage over present-day rechargeable batteries. At issue is not “bang for the buck,” but “bang for the fuel volume/weight.”

Second, as the plane burns its fuel it gets lighter, and weight is everything in aircraft efficiency. This decrease in weight shapes all aspects of aviation, from take-off issues to high-altitude flight advantages and to the construction of landing runways. Indeed, if the landing weight of large passenger aircraft becomes the same as the take-off weight, all landing runways would have to be rebuilt to stronger and more expensive construction standards.

Turbojet Engine

Third, the most efficient aircraft engine for high speed flight is the turbojet. The turbojet compresses a fuel-air mixture and burns it at the front-end to spin the turbine. The turbine then drives the resulting compressed carbon dioxide (CO²) out the back of the jet to push the airplane. Designs of an equivalent electric motor driven compressor/jet are still “in the works.”

So, how much CO² does a turbojet spit out into the atmosphere? Using a standard calculation of passenger-space share in this carbon exhaust, every economy passenger taking a round-trip flight from San Francisco to New York and back is responsible for 2.32 metric tons of atmospheric CO². When you fly first class, the cost triples to almost seven (7) tons of CO² (since you take up more space and space equals weight). By comparison, the average total yearly household carbon footprint in California is about 21.5 metric tons of CO².

Sustainable Air Travel Is On the Horizon

Is there any hope at all for regular, practical, sustainable passenger flight? Well, Forbes Magazine recently published the following article: “Hybrid-Electric Passenger Jet Gets 100-Plane Launch Order from JetSuite.” See the article: https://www.forbes.com/sites/michaelgoldstein/2018/05/22/hybrid-electric-passenger-jet-gets-100-plane-launch-order-from-jetsuite/#70d3bb435c08

Production orders are not made on the basis of vague hope. The concept behind this order is a sort of “air hybrid,” with a hybrid electric/gas turbine powertrain, which they expect to upgrade to full electric as the technology improves.

Apparently, they will pack the batteries in the wings and use a hybrid power unit generating 1,300 horsepower (1,000-kilowatts), driving two ducted fan engines, along with something called a “range extender” gas-only turbine mounted in the rear of the aircraft. This aircraft will carry 12 passengers and the delivery date is 2022. The cruise speed specified is 340 mph and the range is 1,000-miles. The estimated seat-cost per mile will be about 8-cents.

JetSuite claims they have been working with FAA for the last three years on small commercial passenger electric aircraft standards for certification.

Courtesy of Rossmoor News, January 30, 2019. Email Wayne Lanier at waynelanier-phd@gmail.com

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