# eli5 How does Counter Rotating Propellers work?

147 views
0

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

imagine you have a straw and you put a smaller stick inside of it, they can rotate independently.

The reason for the counter-rotation is more complicated, but basically when you have something spinning it wants to also spin the thing it’s attached to, having two propellers going different directions cancels this out.

The two propellers aren’t on the same shaft.

One is on a central shaft that goes through, this is the “front” propeller. The other is mounted to a “collar” that circles the shaft, and is connected via gear the central shaft that rotates it the other way around.

The reason for it is to reduce the net torque.

Having a propeller rotating at high speeds means it’s hard to bank the aircraft “against the propeller” and easier to bank it with the propeller. Counter rotating propellers remove this problem. And can sometimes provide additional thrust.

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.

There are also mechanisms [like this.](https://youtu.be/lx01utHcFtg)
This is a wicked cool mechanisms, used in some past airplanes, but this example is all 3-d printed, and actually made in a competition to make a good computer-cooling fan.
Am I right I’m guessing you were inspired to ask this question from this video?