9  Plane

Adapted from Cullen, Allwood, and Borgstein (2011)

Unlike cars (Chapter 7) and trucks (Chapter 8), the mechanical drag forces on a plane are insignificant. Instead they have additional aerodynamic drag, as well as a mass which changes significantly over the flight as fuel is burned.

9.1 Characterisation

As well as the profile drag force, due to pressure drag and skin friction, planes are also subject to vortex drag, linked to the generation of lift. The design of the wing, and the way in which the plane is flown, affect how large the drag forces are. These vary with the flight speed, so each plane has an optimum flight speed for cruising with maximum efficiency.

Each plane will also have an optimum flight distance. Over short flights, the extra energy needed for taxiing and climbing is significant, leading to higher average energy use per kilometer flown. For very long flights, more fuel must be carried, which increases energy use at the start of the journey. In between, each plane design has an optimum range.

9.2 Key issues that affect efficiency

  • Reducing the weight of the plane reduces the lift force required to keep it aloft.

  • Improving the aerodynamic performance (lift-to-drag ratio) reduces energy requirements.

  • Interactions between flight speed, altitude, and flight range influence energy requirements.

9.3 Efficiency limits

Cullen, Allwood, and Borgstein (2011) estimated the current efficiency and possible efficiency limits for planes.

Specific fuel burn
Current 0.176 kg/t-km
Practical limit 0.096 kg/t-km

See section S.4.3 of the Supplementary Information from Cullen, Allwood, and Borgstein (2011) for more details.

9.4 Final Energy used for planes

This table shows the quantity of final energy \(F\) used in the UK in one year:

        Energy [PJ/year]
Fuel Sector Device EndUse  
Liquid fuel Aviation Gas Turbine Mechanical 493.3

(A small amount of energy is used in spark ignition engines for smaller planes, but we are neglecting this here).