7.2.5 Gurney Flaps


A Gurney flap is a small strip mounted perpendicularly on the trailing edge of a wing. It's also called wickerbill, or Gurney lip. It was named after Dan Gurney, if you want to read the entire story behind it, visit http://www.allamericanracers.com/gurney_flap.html.

"A Gurney flap reduces trailing edge flow separation on the low-pressure side, hereby reducing drag and increasing downforce."
Right. Allow me to explain.

Let's return to the previous example of the inefficient wing beause of flow separation. Suppose we mount a Gurney flap.
Gurney flap at work Detail

First thing that will happen: drag will increase. Naturally, we've just imposed a barrier for the compressed air, which is travelling at high speed, this is not a very bright idea as such.
But, the very interesting side effect is that behind the Gurney flap, we've created a region of very low pressure. This is much the same effect as the 'rear vacuum' which was discussed in the part about drag.
The extra low pressure at the trailing edge reduces flow separation on the underside of the wing, because the low pressure 'sucks' the air upwards, effectively _forcing_ the air to follow the contour of the wing.
If there is not flow separation, the region of low pressure we've just created also sucks the air passing underneath the trailing edge upwards, which creates just a little extra downforce.

So, we've found an effective means of combatting flow separation at high speeds: the Gurney flap. You don't get something for nothing though: you'll have to carry the burden of more drag at speeds lower than the speed were you'd get flow separation had you not mounted the Gurney flap.... if you're still with me ;-) I just want to say there's a slight penalty in drag.
Try to remember this when you're trying to figure out how to make a small wing more effective at high speeds.

I already mentioned that I think the rules are a little tight when it comes to wing dimensions. Our relatively small wings can use a little help at high speeds, that's why Gurney flaps are often used; they increase the effectiveness of rear wings at high speeds.
The reason realistic wings don't work in R/C is that fluids don't scale: you can scale down cars, boats and planes, but you can not scale down water and air.

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