78 Science & technology The EconomistDecember 7th 2019
2I
n the cut-throatbusiness of civil avia-
tion, every little helps. So researchers at
Airbus, Europe’s biggest aircraft manufac-
turer, have been experimenting with a
wheeze which they hope might shave up to
10% off an airliner’s fuel consumption.
This is to hitch a ride on the wake of the
plane in front.
It is a familiar idea. Evolution blun-
dered across it millions of years ago, and it
explains why skeins of geese, swans and so
on adopt a V-shaped formation when flying
in groups. Vortices of air shed from the tips
ofa bird’swingsrepresentwasted effort.
Butthateffortcanbecapturedas lift by an-
otherbirdtrailingatthecorrect distance
andangle.
Aeronautical engineers have long
dreamedofflyingplatoonsofplanes in a
similarway,withtrailingaircraft surfing
thewakesofthoseleadingtheconvoy. The
problemiscatchingthesupportive updraft
atonesideofa vortexratherthan the tur-
bulence-inducingdowndrafton the other
side—anddoingsofarenough from the
vortex’spowerfulcoretorideitsafely and
withoutspillingthepassengers’drinks.
Airbus’sresearchers,underthe aegis of
Sandra Bour Schaeffer, head of Airbus
GroupDemonstrators,havebeen trying to
workoutthedetailsbyflyinga series of
testsinwhichana350-900follows in the
wakeofana380,bothhavingbeen loaded
withballasttosimulatetheweight of pas-
sengersandcargo.Todothis,the test pilots
neededa waytoseethevortices’ cores, in
ordertoavoidsteeringintothem—whichwould risk crashing the plane. Early experi-
ments used smoke to make vortices visible.
This approach was then replaced with li-
dars (the optical equivalent of radars). Us-
ing these, Airbus’s researchers were able to
measure the shape of a vortex at different
distances behind the leading aircraft.
The trailing pilots then proceeded, in
careful stages, to approach closer and clos-
er to the outer portion of the wake, while
engineers in the back of the plane
crunched data such as fuel consumption
and the speed and accelerations of the two
aircraft. Early in the tests, in 2016, over
southern France, the team observed that by
positioning the trailing aircraft at a partic-
ular distance—a “sweet spot”—the ride
would be especially smooth, with the fuel-
burn reduced by more than 10%.
This sweet spot, they found, is between
one and a half and three kilometres behind
the leader, and slightly to its side. Since the
vortex shape and position change with alti-
tude and temperature, as well as the veloci-
ty and weight of the leading aeroplane, so
does the location of the sweet spot.
Working out how to incorporate all this
into an aircraft’s autopilot will take a while.
Ms Bour Schaeffer hopes to run further
tests next year and then, in 2021, to extend
these to involve a pair of commercial air-
lines. The biggest obstacle, if those tests
prove satisfactory, will be gaining the ap-
proval of air-traffic controllers and regula-
tors. At a typical cruising speed a distance
of two or three kilometres takes only a few
seconds for a plane to cover, and the idea of
flying that close for long distances has
raised eyebrows among both pilots and en-
gineers. Flight-control and precision-navi-
gation technology are, though, getting bet-
ter and better. And regulators may also
wish to take into account the disfavour the
air-travel industry is experiencing as a re-
sult of the carbon dioxide it is adding to the
atmosphere. Saving fuel not only saves
money, it also saves CO 2. 7If aircraft can copy the way geese fly,
everyone will benefitAviationTrail blazers
Geese do it. Why not planes?centipedefoundinto a solution of ethanol,
to kill and preserve it. Also, before them-
selves descending the trees, the collectors
put data loggers into some of the ferns they
had left in place, to measure the tempera-
ture within and outside the plants.
It quickly became apparent, when the
researchers began pulling the ferns apart,
that some of them contained centipede
nurseries. Deep inside they discovered
special chambers that the creatures had
made by chewing through the fern’s inner
roots. Here, mother centipedes were curled
protectively around clutches of eggs or ju-
veniles. The team found ten such nests.
And three of them were shared by females
of different species.
Maternal behaviour by tropical centi-
pedes is not unknown. In particular, fe-
males will hang around to keep eggs and
newly hatched larvae clean, to stop fungal
infestations developing on them. They also
bring prey for the youngsters to feed on.
Tolerating nest mates, though, is a differ-
ent matter. Dr Ellwood and Ms Phillips
reckon that this curious behaviour is dri-
ven by matters climatic.
The climate in question is not, however,
that of the rainforest as a whole. Rather, it is
the microclimate inside a fern itself. The
data loggers left behind by the fern collec-
tors showed that during the hottest part of
the day the temperature inside a fern is as
much as 6°C lower than that outside. Dr Ell-
wood suspects this heat-shielding makes
ferns attractive places for centipedes to
raise their heat-sensitive young—and that
the limited space available inside a fern has
caused natural selection to put mother
centipedes’ aggressive instincts on hold
and make them considerably more tolerant
of one another’s company when nesting
than might otherwise be the case. 7Come to Mummy