Energized Boundary Layer?

[Dave Hull]

Was out at the field flying last weekend, and noticed something kind of interesting. While I was assisting a guy with his plane (ok, I was flipping and stooging), I could see the covering (MonoKote) had a very high-frequency displacement. Let me try to describe it:

--The sun was nearly overhead, and the Monokote was still fresh and shiny. It was chrome yellow, which might have made things much more observable;
--Between any pair of ribs, the reflection of the sun was broken up into very small "patches" of specular light. The patches were surprisingly small, perhaps only 3/32" or so;
--The patches behaved independently, with each shimmering in a random pattern. Not a checkerboard, but that at least conveys the idea that there were lots of "segments" out of phase;
--Since this was being optically measured, you could make an estimate of the angular displacement of each "patch" as the light would go off-axis to your eye and eliminate the glint;
--No accurate idea about the angular frequency of the "patches" other than to guess maybe 10 Hz or so.
--There was no easily visible low frequency "drumming" where the entire skin between ribs was oscillating. I'm sure that it was, but this high frequency displacement was superimposed on top and dominated the eye;
--I saw it on the covering in the bays nearest the fuselage, which implies that the prop blast, and not just structure-born vibration was at work. I didn't check the outer bays, since I was supposed to be starting and launching the plane, not running a physics experiment....
--I have read about skin drag experiments in Horner where they discussed what seems to be a similar effect. I believe that was in an arcane section that was discussing changes to aerodynamics in engine-out conditions--if memory serves.


For those who might want to know if it is happening on their planes--look for it and let us know. If you have to, wax up your plane for a better view. It's a good excuse, anyway. For those who say it is likely to be an oddity of construction, know that this was a wing modified from a Ringmaster S-1, so about as mundane as these things go. Silkspan and dope may have more damping and not be prone to this? Foam and balsa have too much rigidity to be influenced by some minor delta-P from turbulence, I'd assume.

Dave

[Ken Culbertson]

. If you have to, wax up your plane for a better view...

For as far back as I can remember (and at my age that is probably about 11:30 this morning) I have wondered why my planes always flew better before I rubbed them out an waxed them.  Is it possible that the slight roughness of an un sanded/rubbed finish is actually aerodynamically better than a smooth shiny one? 

Ken

[Dave Hull]

Depends on the scale.

When I worked on elements of wind tunnel models of the F/A-18, these were "tuned" by altering the surface of the item to get it to match a dynamically scaled result. Once the model is old, and known, and correlated to full scale, they can fiddle with the model until they get a more accurate result in terms of drag counts. Lots of stories about how modelers (in the hobby) and model makers of tunnel articles roughen surfaces. I have some doubt about the difference of any wax buildup changing the surface roughness at our Reynolds numbers....

Perhaps all the effort put into rubbing tired out your arm before flying? Or, perhaps after the investment of time you were more cautious how you flew?

Dave

[Scientifiction .]

For as far back as I can remember (and at my age that is probably about 11:30 this morning) I have wondered why my planes always flew better before I rubbed them out an waxed them.  Is it possible that the slight roughness of an un sanded/rubbed finish is actually aerodynamically better than a smooth shiny one?

Yep , this is ADHERANCE . Like wot if it were say ' in water ' , LIQUID . The more interaction with the atmosphere , the more they work . Liftwise . Be better supersonic, as it'd be as liquid, the air .
But a bit hard to fly the pattern at that speed. And theres a small matter of inertia .

DRAG reduction is another matter , almost irrelevant - power . But Constant Drag . Or Minimalilist variation in drag , gives a better velocity constant ( more steady speed ).

Out of town driving , the road filth & speed , got patterns on the auto from the windflow . Be another idea to spray on thin ' mud ' to trace airflow on the surfaces .



Cept it was RED . 4 door .

[PJ Rowland]

There is so much " i often wondered" about this or that..

I no longer say things like that. Do the testing, you will have answers.

[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.
[/quote]

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