# Eli5: Why is it so hard to get the same results, suchas thrust/speed, with an engine that delivers power to a propeller instead of a wheel?

79 views

So I’ve seen a couple of scenarios where somebody changes the traction from their bike/motorcyle from the wheels to a propeller and they get awful results – such as not being able to match the speed they get normally, or the “force”.

If the engine is the same, and its able to develop certain power, why does it gets so inneficient when using a propeller?

Simple example: we have a bike and by pedalling we can reach very interesting speeds (40km/h perhaps or even more). But I’ve seen videos with guys pedalling moving a propeller instead of the wheel and they are diying to get the same results.

In: Physics

If you push off against the ground you will move away, the Earth and even locally the piece of ground is a lot heavier than you, as long as you are on a tough surface like a road and not on slippery surfaces like sand. Air is 1000x less dense than water. If you push off against air you get a much smaller thrust forward for the same energy. The advantage is that the air slows you down a lot less than rubbing some wheels on the ground, so airplanes can get going a lot faster than cars ever could. The air is also thinner at higher altitudes so less drag.

The short answer? The air carries away almost all of the engine’s energy.

To move something forward, it needs to push something else backward. A car’s wheels push off of the earth, a propeller or jet engine pushes off of air.

When this happens, the two things move in opposite directions. Now, a car pushing off of the Earth won’t send the Earth flying very fast backwards. The lighter object gets more speed from the exchange and, in turn, more energy. This comes up a lot in physics.

So, if you want to put as much energy as possible into moving your bike, then you want to put as little energy as possible into moving other stuff. The Earth is so tremendously heavy that almost no energy is given to moving it when wheels push on it. The air pushed by a prop is not very heavy at all. It’s very costly to push all of that air backwards quickly.