Have you ever seen dimples (turbulators) used in RC?

A few weeks ago I read in a magazine (I don't remember the name, but it was for home built full scale aircraft) about dimples being used in the wing and propeller. They compared this to the dimples on a golf ball. There was a thin strip of these tiny dimples placed on the high point of the wing/propeller airfoil. Several miles per hour increase in speed were claimed to have been achieved. My question is: Have you ever seen this done or used this technique in R/C?

From : Don Stackhouse

Brian, it's very common on R/C models, especially older designs, but not like you might think.

The dimples on a golf ball are nothing more than turbulators. There's a lot of "smoke-and-mirrors" in the golf ball advertisements about different sizes, shapes, numbers and arrangements of dimples, but in the end it boils down to turbulators, nothing more. By turbulating the boundary layer on the golf ball, they help the boundary layer stay attached to the ball longer, which reduces the size of the ball's wake and therefore its drag. Dimples work well for a golf ball because we have no way of knowing how the ball will be oriented in flight, and dimples work about the same in almost any position.

Some airfoils that are prone to separation of their laminar boundary layer may benefit from turbulators. A turbulent boundary layer is always stirring and mixing itself up, which tends to increase the "scrubbing" of the air against the skin and increases skin friction, but it also pumps fresh doses of kinetic energy into the bottom of the boundary layer. In cases where a laminar ("smooth") boundary layer might tend to separate, this extra energy may help a turbulent boundary layer to stay attached. The overall drag will go down if the reduction in separation drag exceeds the increase in skin friction. This is often the case with some of the older airfoils.

If the airfoil doesn't have any separation problems, then turbulating it will only increase the skin friction and performance will get worse. This is often the case with some of the more modern airfoils.

The same airfoil may benefit from turbulation at one size and speed (i.e.: Reynolds number) and lose performance with a turbulator at a different size and speed. You have to look at each individual case to see if it makes sense there.

As far as dimples vs. other types of turbulators, it's not usually that significant. The turbulator needs to stick up high enough to trigger the transition to turbulent flow, but not so high as to create excessive drag of its own. Other than that the shape doesn't make much difference in most cases. As I said, they use dimples on golf balls because of the orientation issue, but on model wings where we know already which direction the airflow will be going, the shape of the turbulator doesn't make a lot of difference. Straight tape, zig-zags, dimples, whatever, they all get the job done.

Don Stackhouse @ DJ Aerotech
djarotec@bright.net