# How does the relative wind on a propeller, strike the blade from behind (the non camber side)?

This may be a stupid or simple question, although any help is appreciated! :)

If possible, please answer without mathematical terminology. So I understand the concept of the blade increasing air momentum at the front, thus creating lower pressure, and I also get that the blade acts as an airfoil, lower pressure at the top, and higher at the bottom thus pulling it forward, and producing thrust.

My main issue is that I can't visualize how the relative wind physically travels behind the camber side, and hit the back side of the airfoil like in the diagram? Wouldn't it hit directly on the camber side if it's moving forward, not travel below, and strike it from the back? If someone can explain, that would be amazing!

Plane moving forward plus blade moving down gives us a combined velocity vector, shown above in red.

Relative air(speed) is still coming from ahead—but this now no longer concerns the propeller as it has its own velocity vector.

Tilt your head (or check right image to avoid neck injury) and you'll see the blade's angle-of-attack. Remember, air (shown above in light blue) comes in from the direction opposite to the velocity vector.

Increase forward speed, velocity vector changes, angle-of-attack is now smaller, but the airfoil is also faster.

• So you could say that individual velocity create a combined velocity, and relative wind opposes the direction of velocity, which is why it strikes it from the back? – ApolloMission4 Dec 2 '16 at 17:44
• Yep, much appreciated! :) – ApolloMission4 Dec 3 '16 at 18:33

I assume you understand that that the propeller is twisted (the black bit of your picture). You might expect the blade to be flat like a ceiling fan, but instead it is angled towards the front (this angle does reduce at the propeller tips though).

The key is that the propeller is moving downwards - very fast. Consider when the aircraft is stationary - the relative airflow is coming from directly underneath. Because of the angle mentioned above, you can see that there is a high angle of attack.

When the aircraft is moving, there is also some airflow coming from the front. The combined effect is that the propeller thinks the airflow is coming at an angle, in your diagram labelled relative wind.

Just Imagine the blade to be a rotating wing. This happens in wing also right; the relative wind velocity is below the chord and this is the AOA of the wing. However, due to the high velocity of the rotation of the blades, the rotational velocity becomes significant over the froward velocity and therefore the relative wind hits behind. But if the blades are in the feather position they are are approx straight to the forward wind and hence produce no thrust.