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 *c*_{d} by 15% or 18% would improve transport cost by 7.5% and 9%

respectively.

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

0.45 *g*,

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.

### Transport efficiency in terms of bodies

Similarly, we can estimate a passenger transport-efficiency for a 747.

(C.27)

(C.28)

(C.29)

(C.30)

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.

### Key points

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 energy-per-

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.