Future of short-haul flights: Electric planes

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Domestic short-haul civil aviation has not taken off as desired essentially because of lack of demand, which is a function of pricing. With today’s combustion engine, fuel-powered aircraft, it is not economical to fly short distances. For safety, aircraft must have at least one standby engine, dead-weighting the aircraft. This problem gets bigger with the seating capacity.

Today’s commercial aircraft are built for long hauls. Almost all of them typically have a range of 5,000-10,000 km and flying 300-350 km has cost implications. Fuel price fluctuations add to the uncertainty.

There are two technical solutions for short-haul, sub-regional civil aviation: Electric planes, whether as vertical take-off and landing (VTOL) aircraft or otherwise. VTOL, however, requires many rotors capable of running at variable rpms, to provide thrust and balance for lift. But airborne, they are dead-weight. Technically it is possible to change the axis of the rotors from vertical to horizontal, turning them into propellers. But that doesn’t help either. Propulsion is needed for neutralising ‘drag’, the frictional resistance of wind; normally, drag is a tenth of lift — which means that if you need 10 rotors to provide lift during take-off, you’d need only one to overcome the drag. So, even the rotors as propellers is a lot of dead-weight.

The other solution, therefore, is electric planes that take off and land conventionally.

However, the battery technology is yet to mature; energy densities of batteries should increase by an order of magnitude. Second, you’d need to design completely new aircraft — merely replacing engines and fuel tanks of existing aircraft with batteries and motors will not work.

The best of today’s lithium-ion batteries have energy densities greater than 200 Whr/kg, inadequate for a reasonable range of an aircraft. Theoretically, metal-air batteries can store much more energy — 8,000-12,000 Whr/kg; 1,000-1,200 Whr/kg could be realisable. With zinc-air, aluminum-air and solid electrolytes (see article at the top) we could get these energy densitiesv—but that’s a few years away and best suited for short-haul civil aviation.

Even the currently available batteries allow for designing electric planes for a 200-300 km range; with future batteries, it could be significantly more.

Electric planes can drive down costs with their lower redundancies, unlike conventional aircraft — with the ‘distributed electric propulsion’, an aircraft can have several motors. For example, the X-57, NASA’s under-development electric plane, has 14 motors — seven on each wing. Each motor can be adjusted individually, depending on flight conditions.

Not many of the motors will fail, especially in a short-haul flight — so the level of redundancy needed is much lower compared to conventional aircraft with only two large combustion engines.

Besides, the distributed electric propulsion needs to provide lift only to balance the aircraft weight. As such, the surface area of the wings need not be large, like in conventional aircraft. Consequently, the wings could be sleeker, reducing the drag and power requirement.

Furthermore, motor technology is also improving with better power-to-weight ratio. Even five years ago, a motor of 5 kW/kg was a big deal, but today they go up to 13 kW/kg. This also helps in electric aviation.

Add to these the opportunities arising out of material science for lightweight aircraft, such as the use of composites, and you see electric aviation getting closer to reality.

The Directorate General of Civil Aviation has identified 449 airports in India, of which only a hundred are operational. Small electric planes, which typically need a runway of 200-300 metres, can provide use-case for the others too. Where even this length is not available, it is possible to think of circular runways.

In sum, electric aviation has the potential to make air tickets affordable, spurring demand. That is when one will see UDAN — regional air connectivity — truly flying.

The author is a professor of Aerospace Engineering, IIT Madras



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