little by laminar flow control, a technology that reduces the growth of tur-
bulence over a wing by sucking a little air through small perforations in
the surface (Braslow, 1999). Adding laminar flow control to existing planes
would deliver a 15% improvement in drag coefficient, and the change of
shape to blended-wing bodies is predicted to improve the drag coefficient
by about 18% (Green, 2006). And equation (C.26) says that the transport
cost is proportional to the square root of the drag coefficient, so improve-
ments of cd by 15% or 18% would improve transport cost by 7.5% and 9%
This gross transport cost is the energy cost of moving weight around,
including the weight of the plane itself. To estimate the energy required to
move freight by plane, per unit weight of freight, we need to divide by
the fraction that is cargo. For example, if a full 747 freighter is about 1/3
cargo, then its transport cost is
or roughly 1.2 kWh/ton-km. This is just a little bigger than the transport
cost of a truck, which is 1 kWh/ton-km.
Similarly, we can estimate a passenger transport-efficiency for a 747.
This is a bit more efficient than a typical single-occupant car (12 km per
litre). So travelling by plane is more energy-efficient than car if there are
only one or two people in the car; and cars are more efficient if there are
three or more passengers in the vehicle.
We’ve covered quite a lot of ground! Let’s recap the key ideas. Half of the
work done by a plane goes into staying up; the other half goes into keeping
going. The fuel efficiency at the optimal speed, expressed as an energyper-
distance-travelled, was found in the force (C.22), and it was simply
proportional to the weight of the plane; the constant of proportionality
is the drag-to-lift ratio, which is determined by the shape of the plane.