the thrust of a cruising 747 is 200 kN, just 50% more than our cartoon

suggested. Our cartoon is a little bit off because our estimate of the drag-

to-lift ratio was a little bit low.

This thrust can be used directly to deduce the transport efficiency

achieved by any plane. We can work out two sorts of transport efficiency:

the energy cost of moving *weight* around, measured in kWh per

ton-kilometre; and the energy cost of moving people, measured in kWh

per 100 passenger-kilometres.

### Efficiency in weight terms

Thrust is a force, and a force is an energy per unit distance. The total

energy used per unit distance is bigger by a factor (1/*ε*), where *ε* is the

efficiency of the engine, which we’ll take to be 1/3.

Here’s the gross transport cost, defined to be the energy per unit weight

(of the entire craft) per unit distance:

(C.24)

(C.25)

(C.26)

So the transport cost is just a dimensionless quantity (related to a plane’s

shape and its engine’s efficiency), multiplied by *g*, the acceleration due

to gravity. Notice that this gross transport cost applies to all planes, but

depends only on three simple properties of the plane: its drag coefficient,

the shape of the plane, and its engine efficiency. It doesn’t depend on the

size of the plane, nor on its weight, nor on the density of air. If we plug in

*ε* = 1/3 and assume a lift-to-drag ratio of 20 we find the gross transport

cost of *any* plane, according to our cartoon, is

0.15 *g*

or

0.4 kWh/ton-km.

### Can planes be improved?

If engine efficiency can be boosted only a tiny bit by technological progress,

and if the shape of the plane has already been essentially perfected, then

there is little that can be done about the dimensionless quantity. The trans-

port efficiency is close to its physical limit. The aerodynamics community

say that the shape of planes could be improved a little by a switch

to blended-wing bodies, and that the drag coefficient could be reduced a