Formation flying in the style of geese could give a 10% improvement in fuel
efficiency (because the lift-to-drag ratio of the formation is higher than that
of a single aircraft), but this trick relies, of course, on the geese wanting to
migrate to the same destination at the same time.
Optimizing the hop lengths: long-range planes (designed for a range
of say 15 000 km) are not quite as fuel-efficient as shorter-range planes,
because they have to carry extra fuel, which makes less space for cargo
and passengers. It would be more energy-efficient to fly shorter hops in
shorter-range planes. The sweet spot is when the hops are about 5000 km
long, so typical long-distance journeys would have one or two refuelling
stops (Green, 2006). Multi-stage long-distance flying might be about 15%
more fuel-efficient; but of course it would introduce other costs.
Occasionally you may hear about people making eco-friendly aeroplanes.
Earlier in this chapter, however, our cartoon made the assertion that the
transport cost of any plane is about
According to the cartoon, the only ways in which a plane could signifi-
cantly improve on this figure are to reduce air resistance (perhaps by some
new-fangled vacuum-cleaners-in-the-wings trick) or to change the geometry
of the plane (making it look more like a glider, with immensely wide
wings compared to the fuselage, or getting rid of the fuselage altogether).
So, let’s look at the latest news story about “eco-friendly aviation” and
see whether one of these planes can beat the 0.4 kWh per ton-km bench-
mark. If a plane uses less than 0.4 kWh per ton-km, we might conclude
that the cartoon is defective.
The Electra, a wood-and-fabric single-seater, flew for 48 minutes for
50 km around the southern Alps [ ]. The Electra has a 9-m wingspan
and an 18-kW electric motor powered by 48 kg of lithium-polymer batteries.
The aircraft’s take-off weight is 265 kg (134 kg of aircraft, 47 kg of
batteries, and 84 kg of human cargo). On 23rd December, 2007 it flew
a distance of 50 km. If we assume that the battery’s energy density was
130 Wh/kg, and that the flight used 90% of a full charge (5.5 kWh), the
transport cost was roughly
which exactly matches our cartoon. This electrical plane is not a lower-
energy plane than a normal fossil-sucker.
Of course, this doesn’t mean that electric planes are not interesting.
If one could replace traditional planes by alternatives with equal energy