Rough or Smooth?
 

I have a question for the experts.

What type of paint finish would be most beneficial on a racing plane? A mirror glass smooth finish, or one that is slightly rough?

There is conflicting opinions at our field, and I'm kinda on the fence...

Rough or Smooth?
 

There are a couple of things you need to consider. The idea behind the rough surface is to create turbulence and draw higher momentum fluid closer to the surface. This will delay flow separation and decrease pressure drag (normal to surface). Shear drag (tangent to surface) is increased as higher momentum fluid becomes closer to the surface. Turbulators are installed on many aircraft to do the same thing, introduce turbulence and delay flow separation.

A golf ball will fly further when dimpled due to the decrease in pressure drag. A golf ball is not streamlined like most race planes.

I believe the rough surface will create more shear drag on your streamlined plane than it will decrease pressure drag. I could be wrong.

Rough or Smooth?
 

As far as i've heard this rough surface has been tried on fins of passenger planes, its based on sharks skin, and on this scale it easy to produce.
the benifits when you come down to models are less, you cant make that surface, the closest you could get is a fabric film. getting back to golf balls, they try hundreds of diffrent patterns and you'd never be able to do this on planes.
the conclusion was:
Yes, its possible, yes, it would make a difrence under test conditions. but your racing, the advantage it would give you could be taken away just by making a turn a fraction of a second to late or even the drag off a aileron if you correct something. it wasnt worth it, also a smooth surface is much easier to acheive than the type of surface needed.

Something thats crossed my mind is steps like on boat hulls, you create a litte bit of turbelence, and it seperates the air from the fuz. i know some hi-tec design gliders have steps on the aerofoil section to do the same thing. just carried that thought on.... if you seperated the air from around the control surface when its in netural posistion, it couldnt flutter, but as soon as you moved it, it would meet the flowing air and work... something to think about.

Racing Finish
 

At low angles of attack as in the straightaway, the smooth finish should have an advantage for most airfoils. At high angles of attack in the turns, the rough finish may have a drag advantage for some airfoils. I would go with the smooth finish. In any case the difference in lap time will be very small.

Rough or Smooth?
 

Thank you for your opinions. The main reason I asked this question is because I would consider myself "a less then perfect painter". One of my planes has a pretty rough paint surface, but it is very uniform (almost as though I did it on purpose).

Some of the racers at the field thought this was terrible, but there were a couple that thought it might actually be faster than a mirror smooth finish. I have no idea to be honest, but I flew it anyways and it seemed as fast as the other planes I own. I guess I need to do a little more testing, but if anyone is in need of a rough (almost orange peeled) painted surface I guess I'm the guy to call.

This was my first attempt with the Nelson system of water based paints, and either I added too much or too little of thinner (water) or I had my pressure volume set incorrectly. In any case, I'm satisfied how it looks, but it is a bit rough to touch.

I'll let you know how it goes, this will probably be my back up plane at the Nats this year.

Airfoil
 

I don't know if it matters to the experts, but the airfoil is a Naca 66.

Rough or Smooth?
 

Daven,

I think the paint finish matters where it is on the airfoil. I personally like to have paint lines running with the airflow rather than against it. If you have lines running perpendicular to airflow, it creates turbulent air and consequently drag. I am a fan of smooth surfaces. I color sand and polish all racing wings and keep them clean. All of the stuff that I have seen indicates a clean surface is better.

If you know where you want turbulators on the wing, put your paint lines there... :)

Michael