Smooth vs Shiny

[Ken Culbertson]

Quote from: Dennis Toth on January 08, 2022, 05:43:03 PM
should be very strong, smooth and round with the 8 braids. I was wondering if the form of the braid has a significant impact on the line drag through the air. What I mean is the outer bumpiness of the number of braids. It seems that not only does the thickness matter, but the smoothness of the round shape.

Interesting point although a non-smooth surface can be often less draggy than a smooth one! Compare a dimpled golf ball against a smooth one - the former flies further with a better flight profile compared to the latter! Think of full-on, glossy paint against the semi matt versions seen on recent F1 race cars - it was found the latter experience significantly less drag when compared. It goes against 'common sense' but boundary effect on un-smooth surfaces can be a thing to consider as maybe a benefit not a hindrance?

I copied this from the "Adventures in Spectra" thread because it is one of my pet "why is this" things that I would like to understand better.  All of my PA planes seem to fly better with the satin finish (i use satin spray, shows flaws better and more colors) than after making them shiny later.  When I comment on this the answer is always, "but the judges like shiny".  Since for the most part, WE are the judges, why is this?

Ken

[John Park]

I don't have any difficulty believing this.  Ever since film covering came into common use, many free-flight modellers have reported superior duration from a wing covered in doped tissue, as compared with film.  This seems to be especially noticeable with big towline gliders, which would traditionally have been covered in heavyweight English 'Modelspan', or even silk, and not given more than three or four coats of clear dope to keep the weight down.  The surface of such wings would be regarded by a C/L modeller as distinctly rough!

[Dennis Adamisin]

Ken:
I share your pain, have tried the same argument - with no more success than you, even on semi-scale beasts!

I have flown airplanes before rubbing out, waiting for the dope to gas off.  After polishing, the airplanes flew different and not necessarily better.

Like John Park mentioned, I understand that the hot tip in FAI FF events is now a mylar covered LE with doped polyspan or Icarex covering aft of the highpoint.  Observation is improved dead-air glide time.

[Brent Williams]

The model I learned the pattern with used a big Corehouse 54" Gotcha foam wing covered in clear film. The airfoil acquired many, um., errr, uh...spanwise flow enhancements, turbulator ridges, and boundary layer excitement experiments during its lifetime.  It must have been the most aerodynamically slippery plane out the field with its myriad of CA, Gorilla glue and packing tape repair scars!...  I broke and repaired the wing on that plane at least 15 times.  The fuselage, 5 or 6 times.  The rudder 7 or 8 times.  It bears the scars of a true stunt warrior!   

[bill bischoff]

There was a stunt flyer in the Dallas club a while back that found similar results with props. He was absolutely convinced that slick, shiny props performed better after they were sanded with 400-600 sandpaper. Anybody else experiment with this?

[Ken Culbertson]

There was a stunt flyer in the Dallas club a while back that found similar results with props. He was absolutely convinced that slick, shiny props performed better after they were sanded with 400-600 sandpaper. Anybody else experiment with this?

Bill, I was not that flier, but I have always sanded my wood props.  Not to remove the shine, but to get rid of all the dust bumps trapped in the finish.

Ken

[Mark wood]

The CLPA model operates in a Reynolds number range that is on fringe of exiting the zone of laminar separation likelihood. I've said it before on here that I don't work to a fine gloss finish, mainly because I'm lazy, and have experience that some roughness is not bad. The big fat airfoils used in CLPA, especially the ones with the forward thickness are prone to creating Laminar Separation Bubbles around the turning point of the airfoil. The trip strips, when placed properly, invigorate the boundary layer. The same thing applies for roughness. A slightly rough surface also invigorates the boundary layer. I have many examples flying flight of measurable performance improvements with both treatments. The transition from aluminum skinned balsa to lightly doped polyspan  made a significant improvement in performance. Some of that was the increased aspect ratio, some from cleaner running airfoils.

The formation of a LSB creates an increase in drag and a decrease in lift. What this means is that our intuition is that shiny is good because it is smoother. The trouble is that contact layer becomes tightly fixed and the velocity gradient is not great enough for the freestream to mix well and allows the circulation to feed back forward. When this happens the bubble is formed. Adding roughness causes some localized thin circulation, turbulence, to occur and keeps the larger shear from feeding backwards. There's some pretty good articles on this these days. You could think of trip strips or surface roughness as kind of creating a layer of marbles.

[Scott Richlen]

  There's some pretty good articles on this these days. 

Could you recommend some?  Are there any that those of us who are not aerodynamicists could understand?

[BillP]

Shiny finish is seen on the vast majority of general aviation planes. If a dull finish gave better performance it seems they would be dull too.

[Lauri Malila]

Shiny finish is seen on the vast majority of general aviation planes. If a dull finish gave better performance it seems they would be dull too.

I think we are talking about completely different Re-numbers, airfoils and a different way airplanes fly, so you are kind of wrong. L

[Mark wood]

I think we are talking about completely different Re-numbers, airfoils and a different way airplanes fly, so you are kind of wrong. L

Exactly. Above a Reynolds number of about 500,000 laminar separation stops being an issue as there is enough energy in the boundary layer to prevent the occurrence. Since most full size aircraft operate with Rn's of 3 million and more this isn't a concern for them. For the low speed aircraft, doing things like inducing turbulence early is actually preferred whereas in the full size arena is isn't as the turbulent drag is greater than the laminar drag. In the low speed arena, the highest drag mode is the laminar separation bubble, about 4 times higher than the turbulent drag or even greater. Additionally the laminar separation can cause significant loss of lift. So triggering the boundary layer to go turbulent has two benefits in recued drag and increased lift. See the work of Theodor Von Karman, Michael Selig and Mark Dreyla for a better understanding.

[BillP]

Thanks for the correction. So does oil accumulating on a control line wing affect the RN calculation?

[Mark wood]

Thanks for the correction. So does oil accumulating on a control line wing affect the RN calculation?

No , it doesn't. It does effect the shape and smoothness. Reynolds number is a function of the fluid density, chord, velocity and kinematic viscosity of the fluid. If you have look at this link on Rn, the boundary layer is the region where shear occurs. If you looked at a line drawn through the vectors within the boundary layer you would notice that the line would tend to rotate. We call that circulation. For low speeds that circulation is low which allows higher pressure air from behind to feed back in to the main stream. This is what causes the main flow to separate. Tripping the boundary layer causes the rotation to increase which adds energy to the boundary layer which in turn prevents the back flow of the higher pressure air.

https://www.grc.nasa.gov/www/BGH/reynolds.html