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Energized Boundary Layer?

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Mark wood:
This is an interesting topic and I have a number of cases in free flight where rougher is better. One case is the hand launch glider. I have done many tests with pretty nicely finished versus just sanded bare wings and in general an unfinished airplane out flies the finished and polished airplanes.  I know this will spark a few comments to the contrary but I'm not alone in this observation.

My F1C and A2 models experienced a similar transition. My F1C models had a significant performance increase when we went from very shiny purdy aluminum skinned wings to basic carbon fiber D tube covered with polyspan wings with only a mat ( a few coats of dope ) finish. Granted this transition performance increase is muddy as we also were able to increase the span and aspect ratio but the duration was measurably increased.

At low Reynolds polished and shiny can be contrary to intuition and performance. A rough ( qualified by slightly ) surface can invigorate the boundary layer naturally. I haven't done a huge amount of testing to compare surface finish with devices such as trip tape, string turbulators and other devices. I have done a lot of testing with the devices over the years including some trip testing on 4/4 airplanes which I can round up some videos of.  The real trouble with all of this is that you really have to have two very similar test articles to conduct a good A - B test to know the results and that is a tough one to do.

The biggest trouble with backing down from the nice high glossy finish is the appearance points. Judges (people) tend to like shiny and purdy which results in not so shiney not scoring well regardless of the quality of execution. I can see the weave in the silk, minus 5 points.

MikeyPratt:

The biggest trouble with backing down from the nice high glossy finish is the appearance points. Judges (people) tend to like shiny and purdy which results in not so shiney not scoring well regardless of the quality of execution. I can see the weave in the silk, minus 5 points.
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Hi Mark,
Hopefully the days for down grading for not being shinny, is in the past, the judges look for how well it was built and finished, however a Nicely built model that shines also is still hard to beat.  But things like lifting fillets, poorly done ink lines, tape leaks, trim paint edges, and other small things can hurt you, just as much as not being shinny.  But that what makes it so much fun at appearance judging.

I remember when all the Stunt flyers were looking through an open side door watching the judges line up the models to get their points, I cracked a joke that got a lot of laughs, I didn’t see my model through the door and said in my best Tim the Tool man voice, “I must be in the front row” lol.  I wasn’t to far off I was in the third row.

Later,
Mikey

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