While today’s aircraft offer one of the most efficient ways of travelling long distances for individuals, rising oil prices and a concern for the amount of CO2 emitted mean future aircraft will need to be more environmentally friendly—emitting fewer greenhouse gases while offering the same or better range, speed and comfort we are used to. In order to accomplish this challenge, research institutions and aircraft manufacturers alike are investigating the feasibility and use of electric and hybrid electric propulsion in aircraft.
An integral idea of hybrid electric aircraft is to use a generator; optimised for cruise flight, and to use batteries to supply additional power to the electric engines during portions of flight when peak power usage occurs, such as takeoff and climbing to cruise altitude.
Using a generator optimised for cruise reduces power requirements and weight, due to reduced drag at higher altitudes. This brings the advantage of fuel savings, further reducing weight and emissions while saving on costs.
As electric motors are much quieter than conventional piston or turbine engines, noise emissions are greatly reduced and increased night flights become possible. Although using a generator means an increase in the noise level over that of entirely electric aircraft, hybrid electric aircraft will still be quieter than conventional aircraft, due to the lower velocity of exhaust gasses exiting the generator turbine and its lower power and therefore also noise production. To further reduce noise emissions, it would be possible to use fuel cells to generate power; however, they currently lack the power to weight ratio in order to be competitive with fossil power generators.
The use of electric propulsion allows for some unique design possibilities, which are not feasible for aircraft using fossil fuel propulsion. NASAs LEAPTech project aims to demonstrate electric propulsion technologies in the X57 Maxwell X-plane and to compare them to the conventional counterpart. Due to the low energy density of current batteries in comparison to kerosene, one of the design goals of the X57 is to be as aerodynamically efficient as possible. This is accomplished by having counterrotating wingtip mounted propellers, which reduce vortex drag, and by having 12 additional propellers installed along the leading edge, which blow air over the wing during takeoff and the approach for landing, dramatically reducing the required surface area of the wing. The low chord length of the wing, in combination with its high aspect ratio, reduces the tip vortex strength, resulting in lower induced drag. Furthermore, as visible when looking at the expanded drag formula
the reduced surface area S of the wing allows for a higher speed V at the same drag, facilitating faster travel without losses in efficiency.
The largest disadvantage and reason why currently almost no aircraft use batteries and generators, is the inferior energy and power to weight ratio respectively of these in comparison to turboprop and turbofan–jet engines. While the energy to weight ratio of batteries is continuously improving, it still severely limits the range of all–electric aircraft.
Another problem is the upfront cost of building an electric or hybrid electric aircraft; because they are unproven technologies in aviation, the first production models will come at a high price. Although the weighted cost and maintenance costs are likely to be lower, airlines might be reluctant to introduce these aircraft into their fleets, at least until they know more about their long–term performance, safety and cost margins.
Currently, a number of aircraft manufacturers and airlines are investing in electric propulsion. Zunum, backed by Boeing, is building the Zunum Aero, a hybrid–electric short–haul passenger aircraft, for JetBlue, with plans to use the airplane as early as 2022. According to Zunum, these aircraft are expected to save on 40 to 80% in operating costs. Airbus, Siemens and Rolls–Royce are working on the E–Fan X, a testbed for further development of hybrid–electric aircraft, which is supposed to fly in 2020.
While much development still is necessary before large hybrid–electric passenger aircraft take to the skies, important work is being done to get there. If advances in battery technology continue to be made at the current rate, and more innovative companies introduce new concepts, it is not improbable that the first hybrid electric or all electric passenger flight will take place before 2025.