Hybrid aircraft offers new trajectory for greener air travel, quieter planes

Commercial aviation has relied on liquid hydrocarbon fuels ever since Wright brothers invented flight and successfully flew in 1903.
Liquid hydrocarbons have been a key enabler of commercial flights over the last century, owing to their high energy content, easy handling, global availability and affordable costs.
But the need for greenhouse gas emission reductions along with recent progress in battery technology for automobiles has generated strong interest in electric propulsion in aviation.
On the drawing board is an aircraft that uses jet fuel as the primary energy source and then switches over to electric motors for propulsion during several phases of flight.
After hybrid cars, now it is the turn of hybrid planes. Electric flying is gradually becoming a reality and we can now foresee a future that is not exclusively dependent on jet fuel.
As work on fully electric aircraft continues, hybrid-electric aircraft could become a reality and take off to the skies over the coming decades, experts say.
The global aviation industry is now embracing evolutionary technologies to deliver an improved environmental performance as it seeks new aircraft that contribute to the industry’s climate action goals.
Since the beginning of the jet age, lighter materials, better engine performance, and aerodynamic improvements have reduced aircraft fuel consumption per passenger-kilometre by more than 70%, according to the International Air Transport Association.
And up to 2035, it is expected that other improvements such as natural and hybrid laminar flow control, high-bypass engine architectures, electric landing gear drives, and fuel cells for onboard power generation will all deliver notable efficiency gains, IATA said in a recent analysis.
Sustainable aviation fuels, of course, will be another major component in reducing aviation’s carbon footprint.
“By applying combinations of evolutionary technologies, fuel efficiency improvements of roughly 25% to 30% compared with today’s aircraft still appear possible,” agrees Thomas Roetger, IATA’s assistant director (Aviation Environment).
The manufacturers are considering future options, forming partnerships with research establishments and start-up companies and are behind many of the new aircraft design ideas, IATA said in a recent global technology update.
The strut-braced wing, box-wing, and “double-bubble”— which are a blend of two side-by-side fuselages — are all on the drawing board.
Experts say the strut-braced wing allows for a larger wing span, reducing drag and, therefore, the thrust required. What this means is smaller, more efficient engines.
The Subsonic Ultra Green Aircraft Research (SUGAR) studies conducted by Boeing and Nasa found that a 150-seat strut-braced wing aircraft (may enter service by 2040) could be about 30% more fuel-efficient than, for example, today’s Boeing 737-800.
The most hyped design, however, seems to be the blended wing body. Essentially, this is a large flying wing with the passenger cabin or cargo storage area within its centre section. The aerodynamic shape generates greater lift, which means the benefits are most obvious in cruise flight.
The blended wing has generally been proposed as an aircraft for several hundred passengers. However, it has recently looked like a realistic possibility to optimise smaller versions of the design for the larger mid-sized market segment of around 100 passengers.
Estimated fuel efficiency gains for larger blended wing aircraft vary between 27% and 50% compared with current aircraft of similar size and range. For the smaller version, the estimate is around 30%.
The real game-changer, though, promises to be electric aircraft. Electric power generation is not emissions-free today, but it is expected that the related emissions will go down considerably until 2050, thanks to strong worldwide push towards renewable energy.
In fact, the most optimistic scenario in the International Energy Agency’s World Energy Outlook foresees over 80% of carbon dioxide-free electricity after 2040. And an all-electric aircraft would produce no emissions in its operation!
Furthermore, other aspects beyond environmental concerns improve the business case since an electric motor needs less maintenance than conventional gas turbine engines and that is a value that can be put even into dollar terms.
Battery performance will be critical to electric aircraft. At the moment, a battery-powered 100-seat aircraft is still a long way off. Simply put, jet fuel has a high energy density and getting the equivalent ability in batteries makes them too heavy for use.
“Jet fuel has been an ideal energy carrier and the alternatives have issues in terms of weight or volume,” Roetger admits.
Reports say that an Airbus, Rolls-Royce and Siemens partnership hopes to fly a hybrid-electric demonstrator aircraft, the E-Fan X — based on a BAe 146 — by 2020.
Nasa, meanwhile, is combining the blended wing design with turbo-electric propulsion — a particular form of hybrid-electric propulsion. The electric blended wing body could provide up to 70% fuel savings.
As smaller aircraft using the blended wing design become possible, the potential for the introduction of such an aircraft increases enormously as it is likely to have a huge market demand, speeding up innovation.
Conservative views predict electric aircraft in the 15-20 seat category will arrive by 2030. Larger aircraft carrying 50-100 passengers would enter into service around 2050, IATA says. 
These suggest that more time is needed for battery technology development. Optimistic views take 5-10 years off those estimates, based on interest and public funding of climate change-related technologies.
Moreover, a possible alternative to heavy batteries could be hydrogen fuel cells, especially if a worldwide hydrogen supply network becomes a reality in the next 20 years.
While the idea of greener air travel and quieter planes in our skies is not new, we are certainly closer to it than ever before, thanks to battery improvements.
That said, batteries with significantly higher specific energy, lower  CO2 intensity of electricity and lower costs are necessary for exploiting the full range of economic and environmental benefits of an all-electric aircraft.

* Pratap John is Business Editor at Gulf Times.