## 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/e), where e 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:

transport cost = -

1 force e mass

Figure C.9. Cessna 310N: 60 kWh per 100 passenger-km. A Cessna 310 Turbo carries 6 passengers (including 1 pilot) at a speed of 370 km/h. Photograph by Adrian Pingstone.

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 e = 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