The Economist - UK (2019-06-01)

The EconomistJune 1st 2019 Technology Quarterly |Aviation 7

2 those used in airframes; and for components that
need to be heat-resistant, ceramic-matrix compos-
ites, in which both fibres and matrix are made of a
material such as silicon carbide. At the moment, ge
leads the way in the use of these materials in en-
gines. It has been making carbon-fibre blades for its
turbofans since the 1990s. In the case of leapthis
halves the number of blades required, compared
with its predecessor, the cfm56, from 36 to 18. ge
also makes extensive use of additive manufacturing, commonly
known as 3dprinting, which permits the manufacture of shapes—
including weight-saving voids—that conventional manufacturing
can manage only with difficulty, if at all.
Rolls-Royce, meanwhile, is planning to go the whole hog, using
a gearbox, carbon-fibre blades and a range of ceramic-matrix com-
posites in what it calls its “ultrafan” technology, which it hopes
will permit bypass ratios above 15. Ultrafan will also make use of
additive manufacturing. And, naturally, every component in an
ultrafan engine will have its digital twin in Rolls-Royce’s comput-
ers, keeping track of what is happening to it and flagging up any
maintenance-related issues.

Our friends electric

In the end, though, all these improvements to turbofans lead sim-

ply to better turbofans. The propulsion systems of large passenger

aircraft seem unlikely to change radically from this arrangement

in the foreseeable future. But that is not the case for small passen-

ger aircraft. There is now a real possibility that many of these will

be converted to electric propulsion.

The aircraft in question are those driven by propellers. Replac-

ing piston engines, or even turboprops, with electric motors

would have several advantages. One is that such motors are easier

and cheaper to maintain. Another is that they are quieter, which

would make planes using them popular at urban airports with a lot

of neighbours. A third is that, depending on how the electricity is

generated, electric propulsion sometimes offers a saving in green-

house-gas emissions.

As is also true of electric road vehicles, the terminology of elec-

tric aircraft has become confused. An electric motor is an electric

motor regardless of where the electricity comes from. There is a

tendency, however, to refer to motors supplied only by batteries as

“pure” electric systems, whereas those that employ

both batteries and an on-board generator are

known as hybrids.

In an aerial context, where weight is everything,

a large or long-distance electrically propelled air-

craft will, for the moment, have to be a hybrid. Ex-

isting batteries cannot store enough juice per kilo-

gram to allow otherwise—though, as Grazia

Vittadini of Airbus observes, lithium-ion batteries

are improving by one or two per cent a year. And, like compound

interest, such incremental change adds up.

For small, short-haul planes, however, today’s batteries will

suffice. One airline running such services has already announced

that it is going down the electric route. On March 26th Harbour Air,

a firm in British Columbia, Canada, made public a deal with mag-

niX, an aspiring manufacturer of electric motors for aircraft. Har-

bour Air will, if all goes according to plan, fit its fleet of around 40

seaplanes with magniX’s motors, using lithium-ion batteries as

the power pack. The first in line for conversion is a six-seater, but

the largest plane in the fleet has three times that capacity.

Harbour Air may not be alone for long. Ampaire, a firm in Haw-

thorne, California, hopes, later this year, to do something similar

in collaboration with Mokulele Airlines, a Hawaiian company. Mo-

kulele’s planes, unlike Harbour’s, touch down on runways.

Fitting existing airframes with electric engines is clearly the

quickest way of getting electricity-driven aviation airborne. But it

may not, in the longer run, be the best. Most of the firms that plan

to launch electrically powered regional aircraft are starting from

scratch, using airframes made from carbon-fibre-reinforced plas-

tics as well as specially designed motors.

Alice goes to Paris

The most advanced of these startups is Eviation, an Israeli firm. Its

proposal, called Alice, is a pure-battery system intended to carry

nine people 1,000km. Alice has three engines, supplied either by

magniX or by Siemens, a German engineering firm, at the custom-

er’s choice. These are mounted, one on each wing tip (where they

also serve to reduce drag) and one at the stern, with the propellers

facing backwards to push the plane through the air. A prototype,

built in north-west France, awaits certification. Eviation hopes to

fly it thence to the country’s capital and show it off at the Paris air

show in June.

Several other firms, mostly American, are following in Evia-

tion’s wake with proposals to build aircraft of similar range and ca-

pacity. Most of their designs, however, differ in one important re-

spect from Eviation’s. They use ducted fans rather than propellers.

These are more efficient at producing thrust, and also quieter.

Ampaire, for example, has a proposal it calls “Tailwind” that

would be pushed along by a single such fan at the stern. Zunum

Aero, in Seattle, proposes two ducted fans mounted on the sides of

the fuselage, near the rear. And Wright Electric, a firm in Los Ange-

les, backed by Larry Page, a co-founder of Google and now boss of

Google’s parent company, Alphabet, has teamed up with easyJet, a

British airline, with the aspiration of producing a nine-seater with

enough range to be able to fly from London to Amsterdam.

Just how far electrification can be pushed is unclear. Airbus

seems to think it could be quite a long way. It has teamed up with

Rolls-Royce and Siemens to electrify an example of a 100-seater re-

gional aircraft called the bae146. This project is intriguing, not

least because the plane is not even a propeller aircraft to start with.

It is powered by turbofans.

The consortium is proceeding cautiously, and will replace only

one of the 146’s four engines on the test runs planned for next year.

If those go well, a second will be swapped, too. It is hard to imagine

an electric version of a plane like a 146 being purely battery-pow-

ered. But even a hybrid version of a plane that large would demon-

strate that electric engines could have a big future in aviation. 7

Airflow

Thrust

Gearbox

Fan

Combustion

chamber

Compressor Turbine

Geared turbofan

A gearbox allows a bigger fan to rotate more slowly than the rest of

the revolving components, to push an even larger volume of air around

thejet’score

Source: The Economist

Replacing piston

engines with electric

motors would have

several advantages