Hybrid plane: Prius in the sky

TU Delft has received more than two million euros to design a hybrid-electrically driven airplane. "We will more or less copy the automobile industry."

Nasa's X-57 electric plane. Electric engines allow for better aerodynamic aircraft design. (Photo: NASA)
Nasa's X-57 electric plane. Electric engines allow for better aerodynamic aircraft design. (Photo: NASA)

Boeing has a fuel cell powered light airplane. NASA flies around with a small electric research plane. Essentially every airplane manufacturer or researcher deploys electric aircraft initiatives with the aim of emission-free quiet flight.

Not so at the TU Delft. "This is a hype," said Dr. Leo Veldhuis of the section Flight Performance and Propulsion (Faculty of Aerospace Engineering). "The last couple of years saw a remarkable increase in projects aimed at fully electric flight. This is not the way to go."

Veldhuis has bet on a different approach, namely hybrid-electric propulsion technology. His research group, together with NLR-Netherlands Aerospace Centre, has received five million euro's from the European Union to design a passenger airplane that partially flies on electricity. He has just started recruiting PhDs and postdocs and will ultimately expand his group with seven researchers.

Battery capacity

"If you want to transport many people, say one hundred, over a long distance, you cannot do that fully electrically. The battery capacity is much too small. A hybrid plane however could consume much less energy. The idea is to have a gas turbine motor generate electricity to power electric motors, just like hybrid Prius cars do. We are more or less copying the automobile industry."

Airbus also has high expectations for this technology. They already tested a tiny single passenger hybrid electric plane, the E-fan and the company is involved in this project as well. It has asked two other teams to also venture into hybrid-electric flight, specifically the DLR (Deutschen Zentrums für Luft- und Raumfahrt) and the French aerospace lab L'Onera.

Three teams

The three teams will compete with one another. According to Veldhuis: "The team that will have designed the most promising hybrid-electric passenger aircraft after four years will probably see its design tested using scaled model technology in subsequent years of research."

"The benchmark airplane is an enlarged version of the ATR-72 that should be able to carry up to 100 passengers. We will have to make sure that our plane performs better than this twin-engine turboprop airliner."

Hybrid technology will allow for lighter planes, Veldhuis argues. "Electric engines can be smaller than the turbines normally used. We can use many more of them and distribute them over the wings of the plane. As this will lead to enhanced lift, it will allow us to make the wings shorter and thus lighter."

Aerodynamic advantages

In a press release, NLR points out the aerodynamic advantages: "As the propulsion will be detached from a gas turbine motor, it is possible to position the propulsion on the wing or fuselage in a way that improves the aerodynamic properties of an aircraft."

According to NLR, hybrid-electric propulsion combined with a new aircraft configuration can reduce fuel consumption by ten percent. In the end, there will be an important trade-off between the above-mentioned advantages and the weight of the batteries.

The project of TU Delft and NLR is called Novair (Novel Aircraft Configurations and Scaled Flight Testing Instrumentation) and will run for seven years. It is part of Europe's Clean Sky undertaking. Clean Sky is the largest European research program aimed at reducing CO2 gas emissions and noise levels produced by aircraft. It is funded by the EU's Horizon 2020 program.