Flight
Propulsion & Thrust
14 min · 2 worked examplesFree
Thrust is momentum thrown backwards. Why big slow propellers beat small fast ones, and what a rocket does differently.
After this lesson you can
- Explain thrust as a change of momentum
- Say why a bigger propeller is more efficient
- Tell a jet from a rocket, and say why only one works in vacuum
Thrust is momentum
Every propulsion system works the same way: accelerate mass backwards, and the reaction pushes you forwards. Thrust equals mass flow rate times the change in velocity of that mass.
That leaves a choice. Move a lot of air slowly, or a little air very fast, for the same thrust.
Efficiency strongly prefers the first, because the energy you must supply goes as the square of the velocity change while the thrust goes only linearly. Accelerating a large mass gently is much cheaper than accelerating a small mass violently.
That is why a large propeller is more efficient than a small one, why a high-bypass turbofan replaced the turbojet on airliners, and why a helicopter has such a large rotor.
In the full lesson
- Thrust is momentum
- Propellers
- Jets and rockets
Free with an account. Read it, work the examples, answer the questions, then build it in the simulator.