MyWay

Has anyone out there found the best surface finish on planes going over 150mph? Is a waxed surface better than a non waxed surface? Is a rough surface better than a smooth suface. Does it even matter? What have been your experiences.

rockhouse

My Feedback: (5)

The military is fond of aluminum.

daven

My Feedback: (1)

Many of the top racers use highly glossed painted fiberglass planes and wax them prior to the races. After every heat they carefully clean all of the goo left on the finish after the flight including the propeller.

As to your basic fast sport plane, I doubt you would ever see a difference.

banktoturn

MyWay,

The answer to that question can be a bit complicated. I will say that it is generatlly better to have as smooth a surface as possible. Under some circumstances, you actually want to intentionally cause turbulence, to prevent separation. A rougher surface, at least on part of the wing, can be beneficial in this case.

banktoturn

Kaos1

The smoothest surface will provide the least resistance when passing through the air. Parasitic drag is what you want to avoid and a rough surface creates plenty of drag.

Being a pilot, one is taught that you don't even attempt to take-off with the slightest haze of frost on your plane. You want laminar flow!!! NASA even experimented with perforating the upper surface of a wing to draw the micro turbulent flow into the wing to provide better flow for increased lift. http://www.dfrc.nasa.gov/History/Pub...DF/Laminar.pdf This worked but they had trouble keeping the holes and all of the pneumatic tubing clean.

An example of turbulent air flow is seen in the question, Why does a plane stall? Ultimately, it is because the flow of air has separated from the wing and this creates turbulence on a grand scale, lift is lost, and the control of the plane too. This is often caused by exceeding the max angle of atack for any given surface.

Given all other factors being equal, the slipperiest surface would be the fastest.

MyWay

Has anybody tried different surfaces and know what works best for them? I want to compare your results with my results.

Rudeboy

It depends on the design of your plane, the airfoil used, the precision of construction...

Some airfoils benefit from a (partly) rough surface... sometimes turbulence is even artificially induced with turbulators...
Other airfoils benefit from a perfectly smooth surface.

It will be very hard to measure any difference on a "common" speed plane...

I say go for smooth... it looks way better...

der_steuermann

My Feedback: (2)

Yes, go as smooth as possible[8D]

thunderjet

My Feedback: (1)

I remember reading something several years back. About if you have a rough surface. That air gets trapped in the lowspots and creates a boundary layer. That in simple terms means that air is rubbing air. Not air rubbing surface. I guess its like wet rubbing agaianst wet ice. They said that this reduced drag. Most millitary aircraft have a rough primmer dull surface. I know that a lot is for low vis. But is this a secondary reason..

I also remember an f-14 pilot told me that his airplane flew better dirty than clean. And that if it werrent for corrosion prevention they would leave them dirty..

But millitary aircraft are tool of war.

The Reno racers are the ones we should ask.

My free advice may be worth every penny.
Thunderjet

MyWay

So far all the thoughts put forward are helpful but somewhat confusing. Thanks for your comments.

daven

My Feedback: (1)

Why don't you radar test a plane as is, and then spend half an hour cleaning and waxing the plane as best you can and repeat the speed test.

See if the difference is even noticeable.

banktoturn

MyWay,

You're right, it is confusing, because it's a fairly complex situation. Running the risk of complicating it further, I'll make a couple comments.

There are several different components, or types, of drag. One is often called 'pressure drag', or 'profile drag'. This is a result of the pressure on the front of the wing being higher than the pressure on the back of the wing, which exerts a net force to the rear of the wing. Another component is 'skin friction drag'. This is a direct result of the viscosity of the air. Imagine putting your finger in a blob of honey on a table, and trying to slide it along the table. Because honey is highly viscous, it takes a lot of force to slide your finger. This is just like skin friction drag.

Now, to the smoothness issue. A smoother surface always reduces skin friction. Skin friction is also lower for laminar flow than for turbulent flow, but in either case, the skin friction is lower when the surface is smoother. It turns out that a very smooth surface is a double win, because it tends to preserve laminar flow, which reduces skin friction, and also lowers the skin friction after the flow turns turbulent. Now comes the complication. If the flow remains laminar all the way to the trailing edge of the wing, there will almost always be a thing called a 'laminar separation bubble', especially for model planes. The laminar separation bubble increases pressure drag. This is a fundamental tradeoff. If we make the surface smooth enough to preserve laminar flow, we will reduce skin friction, at the penalty of increasing pressure drag because of the laminar separation bubble. To try to get the best of both worlds, we can preserve laminar flow part way back, and then intentionally cause turbulence, using a turbulator. The turbulent flow prevents the laminar separation bubble. Behind the turbulator, skin friction will be higher than it would have been for laminar flow, but the drag penalty of the separation bubble is avoided. To get this benefit, the turbulator needs to be placed a little bit ahead of where the laminar separation bubble would have occurred. It is worth noting that in order to preserve laminar flow, you need to do more than just have a very smooth surface. There are certain airfoil sections that are better for preserving laminiar flow than others, and you need to avoid bumps and dips in the surface as well.

So, the answer to your question might be: "A smooth surface is best, as long as it doesn't result in a laminar separation bubble". If you build a wing with a beautifully smooth surface, and then let a bunch of bugs and fuel spatters accumulate on it, then you lose the benefit, as these little particles will probably cause turbulence.

banktoturn

MyWay

I have tested but without radar. I looking for someone that has done testing with radar and can give me there results.

MyWay

banktoturn. Thanks for all the info. Makes it a little easier to digest.

daven

My Feedback: (1)

Honestly, I doubt you will see enough difference in your typical fast sport plane to make a difference.

I have a very tough time distinguishing from 165 mph and 170 mph in the sky. However, after a 10 lap race that speed difference correlates into slightly over 2 seconds of time. 2 seconds on a standard pylon course is about 1/3 of a lap down.

MyWay

Wouldn't that 2 seconds of time be an advantage if the surface finish with the least amount of drag allowed the extra 5mph?

Kaos1

You don't need radar, you need a wind tunnel.

MyWay

A wind tunnel would be nice. Thought I could here from someone that has gone to the trouble of working on this surface finish question.

Rudeboy

People have gone through the trouble of working on this surface finish question basically since this thing called "drag" became apparent when building and operating flying machines...

The bottom line is: there is no simple yes or no answer to this question!

If you really are interested in the matter, I suggest you buy the books, and start studying all the material of people who have gone through the trouble...
Be prepared to invest a fair amount of time though (and brush up on your math )...

Other than that, all you can do is build the best possible airframe you can... You won't believe how hard it is to build an accurate wing until you have truly tried it...
Accuracy is far more important than surface finish... That is, even if you manage to pick or develop the right airfoil for a given application... Which is why you have to buy and read all these books anyway...

"Theory of wing sections" by Abbott and Von Doenhoff is a good place to start for the basics...

MyWay

Rudeboy. I will do some reading on the subject. In the meantime try this. Take your best flying plane and wax it with any silicon based wax. Fly it and see how it flys. Get back to me so we can compare notes. There is a difference and not neccesarily for the better.

daven

My Feedback: (1)

What is silicone wax, and how does it compare to your standard Car Wax?