The X-57 experimental airplane developed by NASA is set to fly for the first time this year. It has an impressive 14 propellers along its wings and is powered entirely by electricity. This is great considering that we have to move away from fossil fuels yet our demand for aviation is increasing. But how close will NASA’s aircraft get us to this goal? Finding an alternative to aviation fuel like kerosene will be important if we want to continue flying. The X-57 uses lithium batteries to drive the electric motors for its thrusters. But the energy you get from batteries, relative to their weight, is 50 times less than you get from aviation fuel.
The X-57 is a modified, four-seat, Italian-built Tecnam P2006T aircraft. It relies on a combination of lots of propellers, small motors, and many batteries spread throughout an aircraft, which is known as “distributed propulsion”. This approach represents an exciting area of research and development that can be found in many experimental electric aircraft designs.
What’s different about the X-57 is that the wings have been completely redesigned along with propellers to optimize the air flow around them. When a propeller is not needed, its blades can be folded back to reduce drag. Propeller technology is generally reborn. Designs are becoming not only more efficient, but also less noisy and more economical.
Propeller scan speed and pitch angle can also be changed during flight to adapt to different aircraft speeds required for takeoff, landing and cruising. Air density varies with altitude and affects the thrust obtained from the propeller. Now that we can make propellers that work effectively at all altitudes and speeds, we can really make the most of the energy stored in batteries.
Newer designs, such as the first 11-bladed propeller (on the Piper Cheyenne plane), could achieve much greater thrust even at higher air densities. Some aircraft also use “vector thrust” by allowing the motor and propeller to rotate, which gives the option of vertical takeoff and landing. These aircraft may be more akin to helicopters than planes, and could mean that traditional airports with long runways and large terminals will be a thing of the past.
battery technology
The X-57 uses off-the-shelf lithium-ion batteries. This is because the project is primarily addressing the potential of new propeller and wing configurations, rather than developing the correct battery. But it will be a significant challenge for electric aircraft developers to overcome. Lithium batteries are by far the best we have, but they’re still heavy. Lithium metal is also dangerous because it catches fire easily.
There are advantages to using batteries. Their weight remains constant throughout flight, meaning they do not need to be stored in the wings as aviation fuel traditionally has been. With liquid fuel, the weight of the aircraft is significantly reduced as fuel is consumed and keeping the fuel in the wings ensures that the balance of the aircraft is not changed.
However, it’s really the energy density – the amount of energy in a battery compared to its weight or size – that matters. New advances are constantly being made, such as batteries built on the basis of quantum technology. But while these charge faster than normal batteries they will not replace lithium batteries and are unlikely to change the prospects for electric flight. We are truly looking forward to a revolution in battery technology, providing an energy density comparable to that of aviation fuel.
Is the X-57 the future?
With a range of about 160 km and a flight time of about one hour, the X-57 is not expected to be a replacement technology for long-range flight. At least not directly. Instead, short-hop flights with ten or so passengers are a good and potentially promising target for early, battery-powered flights. Hydrogen-powered aircraft are also of great interest because the energy density of hydrogen is approximately three times higher than that of conventional aviation fuel.
But hydrogen is a gas and it needs to be stored in pressurized fuel tanks to reduce its volume. This would require a complete rethinking of the aircraft’s design. Some work has been done with hydrogen stored as a liquid at -253 °C. Hydrogen for aviation is therefore exciting, but probably impractical.
Synthetic fuels are ready to go as a substitute for aviation fuel at a cost. Perhaps as technologies develop they will become cheaper, but it is still likely that the cost of flight will increase as we move away from fossil fuels.
Batteries will almost certainly power our short-haul flights in the near future, and if battery technology is revolutionized, the future of aviation will change completely. Eventually, we will be faced with an ultimatum: either we figure out how to make planes that don’t require fossil fuels, or we stop flying.