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  • June 21, 2021, 01:47:31 PM

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Author Topic: Energized Boundary Layer?  (Read 176 times)

Offline Dave Hull

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Energized Boundary Layer?
« on: June 03, 2021, 10:32:12 PM »
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

Online Ken Culbertson

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Re: Energized Boundary Layer?
« Reply #1 on: June 03, 2021, 10:49:45 PM »
. 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
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If it is not broke, don't fix it.
USAF 1968-1974 TAC

Offline Dave Hull

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Re: Energized Boundary Layer?
« Reply #2 on: June 04, 2021, 12:03:52 AM »
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


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