To most people, the suggestion
that it takes less fuel to overcome more drag is, on the face of it, perverse.
The problem with the statement above is that is based on the assumption that the amount of fuel we burn must be determined by the amount of thrust we are producing. This is a false assumption.
I weigh about 210 lbs (yes I am a bit of a fat ……). If I sit or lie on the ground I exert a force of 210 lbs on the ground. Exerting this force uses up no energy. But if I were to repeatedly lift a 100 lbs weight above my head and lower it back to the ground, I would expend a great deal of energy. Isn’t that perverse?
So why does lifting the 100 lbs weight expend more energy than exerting a 210 lbs force on the ground? The amount of energy that my body expends in doing this type of thing is determined by the amount of mechanical work that I am doing. Exerting the 210 lbs force on the ground does not require me to do any mechanical work, so no energy is required (I don’t even need to stay awake). But repeatedly lifting the 100 lbs weight involves doing 100 ft lbs of work for every foot that I lift the weight above the ground.
The key fact in all of this is that the energy used is not determined by the force that I apply, but is determined by the amount of work that I am doing. Power is the rate of doing work, so the energy (fuel) consumption of my body is determined by the power required to do the work. Exactly the same situation applies to propeller aircraft. Fuel consumption is not proportional to the thrust produced, but is proportional to power output.
There are a great many aspects of aerodynamics and physics which appear to be perverse.
For example:
1. Fuel flow in a jet is proportional to thrust, but fuel flow in a piston/prop is proportional to power?
2. At speeds below Vmd, the faster we fly the less drag we have.
3. At speeds below Vmp, the faster we fly the less power we need.
4. For jet aircraft at speeds below Vmd, the faster we go the less fuel we burn.
5. For piston/propeller aircraft at speeds below Vmp, the faster we go the less fuel we burn.
Looked at individually each of these statements may appear to be perverse.
But if we rearrange the order of the statements slightly the situation becomes a little bit clearer.
6. Fuel flow in a jet is proportional to thrust.
7. At speeds below Vmd, the faster we fly the less drag we have.
8. For jet aircraft at speeds below Vmd, the faster we go the less fuel we burn.
Statement 8 is true because statements 6 and 7 are true. As we accelerate towards Vmd, the reducing drag requires us to produce less thrust, and because fuel flow is proportional to thrust, producing this reduced thrust reduces our fuel flow.
9. Fuel flow in a piston/prop is proportional to power.
10. At speeds below Vmp, the faster we go the less power we need.
11. For piston/propeller aircraft at speeds below Vmp, the faster we go the less fuel we burn.
Statement 11 is true because statements 9 and 10 are true. As we accelerate towards Vmp, the reducing power required enables us to produce less power, and because fuel flow is proportional to power, producing this reduced power reduces our fuel flow.
The real conundrums are why are statements 6, 7, 9 and 10 true.