Counter Rotating aircraft/ship propellers.
If both propellers are on the same one shaft, how are they spinning in opposite directions at the same time?
Whats the configuration of contra rotating propellers?
How did engineers do that?
And whats the purpose of contra rotating propeller?
That propeller configuration fks my brain so much.
In: 1
First, try to distinguish contra from counter rotating propellers and use them for their more narrow technical definitions to reduce confusion
[Counter rotating propellers](https://upload.wikimedia.org/wikipedia/commons/3/3c/Counter-rotating_propellers.gif) have two propellers on different shafts spinning in opposite directions, this is generally used on planes with the ones on the left spinning one way and the right spinning the other to balance roll on the plane
[Contra-rotating propellers](https://upload.wikimedia.org/wikipedia/commons/d/d6/Contra-rotating_propellers.gif) are two propellers inline with each other rotating in different directions
Generally contra-rotating propellers will have an engine that feeds a special gearbox that feeds two concentric shafts, an outer tube is connected to the nearer propeller and spins one way while an inner rod runs through the center of the first propeller to drive the second and spin it the other way. [Here’s a nice cross section of one where you can see the outer red propeller is fed by the red drive shaft that runs down the hollow green drive shaft](https://foil.zone/uploads/default/original/2X/8/8f20da8c1b58f6d2a39984a5c98c30f02ab0e443.jpg)
So why make it complicated? Efficiency!
Plane engines are fundamentally momentum based instead of your car engine which is energy based. Generally the best way to boost propeller efficiency is to make the propeller bigger, it catches a larger volume of air so it can accelerate it less and still get the same momentum boost for less energy. If you can’t make the propeller bigger because your plane isn’t high enough off the ground [like an old Spitfire XIX](https://upload.wikimedia.org/wikipedia/commons/thumb/6/6c/Supermarine_Spitfire_XIX_vr.jpg/1024px-Supermarine_Spitfire_XIX_vr.jpg) then you need to get creative.
Normally air leaving the first propeller has a twist to it. Spinning another propeller in the opposite direction cancels out that twist and converts it to a bit more forward thrust. For a given propeller size it can let you get more thrust than any single propeller would allow and doesn’t take as much extra power as you’d expect because the air leaves almost straight so there’s less wasted energy.
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