Surface Roughness?

In high speed full scale aircraft, like say the P-51, surface roughness is an issue. In some cases great care is taken to fill in and present an absolutely smooth surface to the air. You hear stories of bugs on the leading edge destroying laminar flow and thus reducing performance. Miss America gained 15 MPH or so at the top end when they rebuilt the wing to very high tolerance a few years back.

Meanwhile the time and effort to put a fine finish on a stunt model (ignoring monokote, I tend to fabric and dope) rises, it seems, in a non-linear arc the more you try to hide the grain and those odd ripples you can get on a brush painted dope job.

While intuition and esthetics tell you that the smoother the final surface of a stunt model the better, I'm wondering if anyone has done real research on the effects of surface roughness at the low reynolds numbers we operate at.

Which is to ask: How smooth is practically smooth enough?

Assuming you have the basic surface properly contoured and sanded smooth, is a light finish that shows a little grain or dope roughness good enough aerodynamically?

Or is there a real performance value to bringing the finish up to mirror smoothness?

Cy.

Rutan has done alot of work in this area along with John Ronsz (sp?) for the Voyager round the world plane. (bugs were a SERIOUS concern)

Earlier, on the Eze's, the canard exhibited startling changes when flown into the rain. Surface finsh and flow were all looked at.

Sailplane guys are also really involved in this.

If I recall, it seams for our models, if the surface is too smooth, the wings tend to remain in laminar flow longer but break and stall un-predictably. A little surface roughness can trip the flow in a more predictable fashion. THis is the reason for the turbulators experimented with by Ted Fancher and others.

consistent and predictable stall characteristics tend to be more important than a flakey extension of the operating envelope. (except in bleeding edge applications of course )

Hopefully those more knowledgeable will chime in as well.

Just my understanding