6 Technology Quarterly |Aviation The EconomistJune 1st 2019
21be both thin (saving weight overall) and long (which reduces drag).
The upshot is the eternal desideratum of better fuel economy.
Airbus, meanwhile, is working on blade(Breakthrough Lami-
nar Aircraft Demonstrator in Europe), an experimental wing de-
sign that is being test-flown this year. bladeis an attempt to create
a wing that has no irregularities to disrupt the smooth flow of air
over its surface. This, too, is intended to reduce drag. bladewings
have no joints, and therefore no rivets or fasteners, and have
smooth, glossy surfaces. They may also be fitted with flaps that de-
flect insects during take-off and landing.
The delta-wing dream will, nevertheless, not quite go away. In
theory, the optimum shape for an aircraft that relies on fixed wings
to provide its lift is such a delta, with wings and body blended to-
gether so that the whole structure provides lift. Designs for such
vehicles pop up from time to time, and might make effective
freighters. Whether passengers would like them, though, is moot.
Most seats would be a long way from a window, and evacuation in
an emergency might be hard. 7J
et airlinersare not, in fact, jet airliners. Though the first of the
breed, the de Havilland Comet, really was powered only by sleek
turbojets that fitted elegantly into its wings, it did not take engi-
neers long to work out that a turbojet works best not by itself but as
part of a bigger whole.
A turbojet takes in air through a revolving compressor, mixes
the compressed air with fuel, burns the mixture in a combustion
chamber and ejects the exhaust out of the back to provide thrust,
having first run it through a turbine which, via a shaft running
along the engine’s axis, turns the compressor. Modern engines,
however, also use the jet’s revolving shaft to spin either a propeller,
creating a turboprop, or a ducted set of blades, creating a turbofan
(see diagram overleaf ). Both of these arrangements—and particu-
larly turbofans—move more air, and thus create more thrust, than
the turbojet within is capable of generating by itself. For long-haul
flight, therefore, turbofans are preferred.
As with making large aircraft themselves, the business of
building turbofans is confined to a few big firms. According to
Market Research Future, a consultancy, General Electric (ge) and
United Technologies (trading under the Pratt & Whitney brand) in
America, Rolls-Royce in Britain, and cfmInternational (a collabo-
ration between geand Safran, of France) account collectively for
almost 93% of the turbofan market. And each is intent on upping
what is known as the bypass ratio of its engines.Top gear
A turbofan’s bypass ratio is the amount of fan-driven air it expels
from the rear divided by the amount of exhaust from the combus-
tion chamber. The bigger the bypass ratio, the more efficient the
engine. More efficiency means lower costs. It also means less envi-
ronmental impact.
The ge 9 x, as fitted to Boeing’s new 777-9x, which was rolled out
on March 13th, shortly after the second of the 737 maxcrashes, is
currently the world’s largest turbofan. It has a bypass ratio of 10:1.
The latest iteration of Rolls-Royce’s Trent, the 7000, will also have
a 10:1 ratio. And cfm, which specialises in engines for narrow-
bodied jets, and thus avoids treading on ge’s toes in the wide-
bodied-jet market, has an engine called leapwith a bypass ratio of
11:1. The daddy of the field at the moment, though, is Pratt & Whit-
ney’s pw 1000 g(branded “PurePower”). Its best-performing ver-
sion has a bypass ratio of more than 12:1.
Pratt & Whitney’s lead is the result of a gamble. Turbofans are
complicated beasts, put together from about 25,000 component
parts. The company’s engineers decided to make them more com-
plicated still, by adding a gearbox. This reduces the speed at which
the fan blades spin, relative to the shaft. The shaft needs to spin
rapidly because it also drives the compressor, but that high rate of
revolution puts a strain on the fan blades. Reducing this strain al-
lows the blades to be longer, meaning the engine can have a wider
air intake that is able to gulp in more air and thus to achieve a big-
ger bypass ratio.
Gearing is not, though, the only way to make engines more effi-
cient. Better materials also help. As with airframes, replacing met-
al alloys with composites saves weight, which saves fuel. And
composites also offer a way to deal with an engine’s intense heat.
Two composite materials are particularly pertinent to aircraft
engines: for fan blades, carbon-fibre-reinforced plastics similar toThe jet set
Old engines are improving. New ones are coming.Propulsion systems