vasek

My Feedback: (4)

hi, i always thought that covering a speed plane with "glossy" finish (such as Monokote) would help it to go faster......

but then i came across info that a slightly rough suface is better for speed! the theory being that the rough surface will "keep" a protective "coating" of air (or water for boats; same priciple) thus reducing the actual friction = goes faster!!!!!!!!!

anyone has used this principle? i.e. using fabric (ex. Century) instead of Monokote? would the plane be faster?

would like to hear from people who know....[8D]
Vasek

Rudeboy

Depends on the plane and its application...

If you use a laminar flow airfoil, and you can build the wing precise enough... smooth is better... (with the use of an appropriatly placed turbulator)

Most people (including me) mess things up starting at the leading edge of the wing (going at it with a sanding block)... after that, it doesn't really matter anymore, because you loose laminar flow right there.

Laminar flow generally requires composite wings laid up in machined molds... you cannot get that kind of precision when hand building....

If you know what you are doing, and you use templates to sand your wing, you can keep things flowing evenly up to 40-50% of the wing chord... after that it just all goes south, no matter how handy you are with a sanding block...

Other than that, most airfoils used on models are not laminar flow anyway, so it does not really matter. In that respect you are right: a rough wing can actually improve perfomance... turbulence keeps energy in the boundary layer and prevents the forming of a seperation bubble...but you will be hard pressed to actually find any performance gain from this, unless you do some serious testing...

daven

My Feedback: (1)

My preference has always been, the smoother the better. Even then, I like to put on a fresh coat of wax to the plane prior to a race.

There is a product called "Race Glaze" that really works very well.

I have some rough surface planes, not on purpose, but my painting looks like I used a roller

Never noticed a rough surface being any faster.

SSAN

I agree when it come to boat because I have a bass boat and know about this "speed secret". However, I'm a little skeptical when it come to air because rough surfaces create turbulent and parasitic drag. In theory, the smoother the air flow, the faster air can travel, which it turn give more cleaner flow and lower drag. Just my thought on the subject!

banktoturn

This question does not have a simple answer. There are two components of drag that need to be considered: profile drag & skin friction drag. Profile drag is a result of higher pressure acting on the wing from the front than from the rear. It involves the projection, or silhouette, viewed from the front or rear, not the total area of the skin. Skin friction drag is the direct viscous friction on the surface of the wing.

The main way to minimize profile drag is to prevent or minimize separation, because the pressure behind the separation point will be lower than it would have been in unseparated flow. If a rougher surface can be used to generate turbulence, which in turn prevents separation from occurring too far forward, then it can reduce drag. Skin friction drag is significantly lower for laminar flow than turbulent flow, and is somewhat lower on a polished surface, whether the flow is laminar or turbulent. Unfortunately, a highly polished surface is not enough to guarantee laminar flow; the surface must also be free of bumps & other irregularities, and the wing must have an appropriate pressure distribution.

In minimizing overall drag, profile drag and skin friction drag both need to be considered. For example, if you build a highly polished wing, successfully maintaining laminar flow, but then get a large laminar separation bubble around 50% chord, you would probably not get a drag benefit from the polished surface. Overall, I would say that it is only worthwhile to polish the wing if you know that the airfoil is capable of laminar flow and that you can build it very accurately. If these things are all true, then you would probably also need to add a turbulator ahead of the location of the laminar separation bubble, if you know where that is.

banktoturn

vasek

My Feedback: (4)

banktoturn, your comments are interesting....

my understanding of the effect (it might be totaly wrong) was , in simple words, that the microtexture "captures" a very thin layer of air on the wing surface, thus creating mostly "air to air" contact between the "slow" air on the wing surface and the "normal" rapid flow of air just around it ... much lesser friction than any other material (even polished)...

could that be possible?[X(]

banktoturn

vasek,

The description you give is a pretty good description of what
aerodynamicists call the 'boundary layer'. It exists whether
the surface is polished or not, but there is no particular drag-
reducing effect of the boundary layer on rough surface, unless
it is preventing separation. A rough surface could do this, as
surface roughness can cause turbulence, which can have a
beneficial effect on separation. As I mentioned before, you
can't make a general statement about a rough surface and
drag; the net result depends on skin friction & profile drag,
and the relative values of those two quantities depend on
many things.

banktoturn

chris channon

Hi all, The Red Arrows display team always have the Hawks polished just before flight, they are of course seamless aircraft so polishing must be beneficial or they would not do it ?
Chris
(Englishman living in Florida ! )

combatpigg

My Feedback: (3)

18 years ago, [or so] COVERITE came out with a film / fabric that had a rough finish on one side, and smooth on the other. They recommended the rough side out if you wanted to generate lift [drag] and smooth out if you wanted less drag. I knew even less about aerodynamics back then, than I know now, but I always had the feeling that the effect wouldn't add up to much with my one horsepower, 60 mph model. Like most other things related to this model speed stuff, it's pretty hard to measure differences between one school of thought and another. I know a aerodynamicist who works for BOEING who only uses red film to cover his models, because red [according to him] is the lightest color, it takes fewer solids to create red than any other color. It works for him, I like red cuz its purty. I got to think that at close to 200 mph and up, some of these questions [arguments] about which is better, thin, round, rough, smooth, swept, blunt, etc. can start to get answered. I can settle the rough / thin controversy pretty easily with this question; At the end of the day, which finish would you rather clean?

vasek

My Feedback: (4)

banktoturn, thanx for the explanation, i learned something new[8D]

chris, i believe that the red arrows or the blue angels team polish their planes AND their shoes for show NOT for speed

combatpigg, yes you just might be right on this one! i hate cleaning the plane anyway... [X(]

now i know it's not that big of a deal as i have initially thought

Rubbie

I think this has to do with "type 3" turbulence, very similar to having dimples on a golf ball. Man Physics was a long time ago...the rough surface causes very small "ball bearings of air" to help the air move over the wing and the whole surface of the plane. Maybe the physics guys can chime in.

Mike Connor

My Feedback: (5)

Some good answers here but I thought I would ask Jeeves and came up with this short, easy to understand explanation.

There are two types of flow around an object: laminar and turbulent. Laminar flow has less drag, but it is also prone to a phenomenon called "separation." Once separation of a laminar boundary layer occurs, drag rises dramatically because of eddies that form in the gap. Turbulent flow has more drag initially but also better adhesion, and therefore is less prone to separation. Therefore, if the shape of an object is such that separation occurs easily, it is better to turbulate the boundary layer (at the slight cost of increased drag) in order to increase adhesion and reduce eddies (which means a significant reduction in drag). Dimples on golf balls turbulate the boundary layer.

Rudeboy

Mike,
That about sums it up... (in a way understandable to us mere mortals that is).

Good one!

proptop

My Feedback: (8)

How many of yaz have heard the story of the captured Zero?

Shortened version...

Early in 1942 we found a Zero that had flipped over during an emergency landing on one of the Aleutian islands.

We cleaned it up, tested it, and after awhile, decided to paint it olive drab and neutral gray (typical U.S. scheme at the time ) and it lost something like 5-7 m.p.h. (or was that knots?)

The original finish was sort of a semi-gloss I guess, and the O.D./Gray was dead flat.

Anyway...I don't know how true that story is, but I've heard/read it several times...


Another story...

We had the 416th Bomb Wing stationed here for many years until BRAC [:'(] closed Griffiss in '95...

I know a former crew chief on BUFF's and he told me that they lost speed and range when they went to the rougher textured, newer lizzard, and later gray toned paint jobs, compared to the Viet Nam schemes...