How does the curvature of a fan blade affect its efficiency?

[poptop94]

I am having a discussion with a co-worker. He doesn't believe that the curvature of a fan blade has any effect on efficiency. That one simple has to reverse the direction off rotation to get full flow in either direction. I know this isn't the case but am having a difficult time explaining and proving it. If anyone could provide me with a quick simple explanation that would be great!

 

[Strat-O]

Well a fan blade is an airfoil and as such it's optimized for traveling in one direction. The curvature of the cross section of the blade is called camber and in aircraft it both increases the lift and also increases the drag. Typically, optimal camber in a wing is oriented such that somewhere near the center is the single high point of the cross section. This is called positive camber. Aircraft that are designed for aerobatics have symmetrical wings or nearly symmetrical so that when they are flying upside down the performance is not hindered. Aircraft with positive camber have poor inverted performance. In a fan the camber would serve to help move more air but the penalty is higher drag in the form of a higher load on the motor for a given RPM. Reversing the fan is akin to a positively cambered aircraft flying upside down. The camber is still adding a fair amount of drag to the fan motor but the camber is not serving to add any extra lift (in fact it is diminishing the lift) so less air is moved.

You can prove this experimentally by taking a piece of cardboard and forming it into a gentle camber (say a 4" wide piece with a 1" highpoint about 1/3 back from the leading edge) Sweep this cambered section through the air and note the lift it generates. Be sure to angle it about 5-10 degrees so that it has positive pitch. Now try the same thing only inverted. Again angle it so that it has positive pitch and you will see that there is no comparison. The reverse cambered airfoil hardly produces lift at all. Also harder to measure with just your hand but the drag (the resistance to being swept through the air) should be about the same whether negative or positive camber.

Marlin