The Economist - UK (2019-06-29)

78 Science & technology The EconomistJune 29th 2019

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teady improvementsin battery tech-

nology, driven along by the electrifica-

tion of road transport, are helping air taxis

and other small electric aircraft get air-

borne. But even the best lithium-ion cells

are still far from being able to power the

workhorses of civil aviation: short-haul

airliners carrying 150 or so passengers. An

electric version would not be able to rise

from the ground, because of the weight of

the batteries required to drive its engines.

Nevertheless, many aerospace experts con-

tinue to think that electric flight is the fu-

ture, at least in hybrid form.

This could be achieved by starting with

smaller hybrid airliners, such as those car-

rying 50 or so passengers on regional

routes, and then scaling the technology up.

Details of one such effort, called Project

804, illustrate how the airborne equivalent

of a hybrid Toyota Prius might work.

As the crow flies, 804 is the distance in

miles (1,294km) between a Pratt & Whitney

facility in Montreal, Quebec, and a Collins

Aerospace centre in Rockford, Illinois.

That the two firms are both parts of United

Technologies Corp (utc), which hopes to

merge with Raytheon to form America’s

second-biggest aerospace and defence

company after Boeing, suggests that the

idea is more than a flight of fantasy. Indeed,

the experimental hybrid which Project 804

plans to fly in 2022 could slash fuel costs on

regional routes.

Charged with potential

As with cars, there are different ways to

build a hybrid plane. Collins, which makes

aircraft electrical systems, and Pratt &

Whitney, which produces jet engines, have

chosen a “parallel” hybrid. That means it

will use a combustion engine augmented

by a battery-powered electric motor, as op-

posed to a “serial” hybrid in which propul-

sion is provided purely by an electric mo-

tor, but with the electricity for this motor

either drawn from a battery or produced by

a combustion engine running a generator,

depending on the circumstances. Both

sorts of hybrid limit use of the batteries,

meaning the battery packs can be smaller

and thus lighter.

For its flight tests, Project 804 is con-

verting a Bombardier Dash 8-100, a 40-seat

aircraft powered by a pair of turboprops.

These are jet turbines that turn a propeller

at the front of the engine via a gearbox.

Each turboprop produces two mega-

watts of power. Typically, the engines run

at full power during the 20 minutes of take-

off and climb, and are then throttled back

for the cruise and descent. In the conver-

sion, the jet turbine driving the propeller

on one side of the aircraft will be replaced

with a downsized version producing about

1 mw. An electric motor attached to the tur-

bine’s gearbox will provide another 1mw.

The idea, explains Paul Eremenko, utc’s

chief technology officer, is that during a

full-power take-off and climb the combi-

nation of electric motor and jet turbine

would produce the necessary 2mw. Then,

during cruise, the electric motor would be

switched off. As the aircraft descends,

which can also take around 20 minutes, the

electric motor would run in reverse to act

as a generator, turned by the windmilling

propeller. This would top up the battery for

a subsequent full-power take-off, or an

emergency “go-around” in case the landing

had to be aborted.

Project 804’s flight trials will help work

out both how such hybrid engines could re-

place turboprops on existing aircraft and

how they might be used by entirely new

models. As the downsized turbines would

be optimised for cruising, they would

themselves have better fuel economy.

Working with the electric motor, the hybrid

combination on a regional turboprop air-

liner, which typically flies routes of around

one hour’s duration, would result in fuel

savings of at least 30%, says Mr Eremenko.

Other sorts of hybrid are in develop-

ment. Earlier this month Ampaire, an elec-

tric-aircraft firm in Los Angeles, undertook

the virgin flight of a six-seat Cessna Sky-

master converted into a hybrid. Skymas-

ters have a propeller engine at the front and

another engine driving a “pusher” prop at

the rear. Ampaire replaced the rear engine

with a battery-powered electric motor. On

its own, this engine would be a series hy-

brid, except that as it works in conjunction

with the combustion engine at the front,

Ampaire calls it a parallel hybrid.

Zunum Aero, based near Seattle, is

working on a 12-seat series hybrid which it

hopes to deliver in 2022. This aircraft will

be powered by two rear-mounted 500kw

electric turbofans (which turn a fan inside

a shroud and so look a bit like jet engines).

The turbofans will be supplied with elec-

tricity by a small jet-powered generator in

the rear of the fuselage, which will also top

up batteries contained in the wings.

For larger aircraft, electric turbofans

that are vastly more powerful—perhaps up

20 mw—will be needed. Much will depend

on what Boeing and Airbus decide to do

with their future models, and how radical

their designs will be. An alternative to large

engines is lots of small ones. Giant flying

wings with many electric thrusters are one

idea. But these would require a number of

technological leaps, not just in batteries

but also in aerodynamics and electricity

distribution.

Power up

More conventional-looking hybrid aircraft

are possible. Airbus has teamed up with

Rolls-Royce, a British jet-engine manufac-

turer, and Siemens, a German electricals

giant, to electrify an example of a 100-seat-

er regional aircraft called the bae146. This

plane is powered by four conventional jet

turbofans, albeit small ones. To start with,

one of the 146’s four engines will be re-

placed with a 2mw electric turbofan pow-

ered by a combination of battery and gener-

ator. If all goes well, a second engine will be

replaced with a similar unit. Again, the idea

is that a combination of combustion en-

gines and electrical power will produce a

cleaner, more efficient aircraft. Spurred on

by environmental concerns and stricter

controls on emissions, for larger passenger

aircraft going hybrid seems to be the most

likely flight plan. 7

Airliners that mix batteries and fossil fuel could come to dominate the skies

Electrifying flight

Hybrid vigour