Hi I'm curious about the difference in drag between various surface finishes and the effect on speed.
For example would there be a "noticeable" difference in performance between a molded glider and the exact same model covered in Solartex ? and Solartex after a bit of scuffing from a few landings?
Is there a "friction coefficient" for different surface finishes?
I've tried Googling but most references are to technical for me or don't seem to apply at our Re numbers.

actually ,there is a difference but you'll need to get some stick time on both types of aircraft. I had flown the slick fiber fuse and composite wing and there are differences in wind penertration and total airfoil efficiency verses the covered aircraft. of course the drag is more noticable when the covering comes loose. but on the other end of the spectrum, a film covered thermal plane flies really well at slow speeds verses the other for speed. in the end though it just depends on what you're looking for...

In the sizes we fly - at the speeds we fly the differences will be really small .
Th new molded stuf is awfully good

I have just recovered an old beaten up Widthing in Lam film and I think the difference in speed is quite noticeable. I looking at some of my other gliders and wondering whether to recover them with Laminating film. I do have a Miraj and Eraser so I am used to hoe mouldies fly but it would be interesting to hear if anybody has done a direct comparison.

The trick is to do an exact comparison
the difference in structure changes the way they fly - My injected molded gliders flop around
in wind-but that seems to be only real difference.l
built up or carbon flyers are less prone to this

Years ago, Don Edberg while at Stanford did a study and I beleive he had access to the wind tunel at Nasa Ames and he found that a Monokote covered panel was less drag then a primed/panted panel. At the time Don was very competitive sailplane pilot and if I'm not mistaken was on a Worlds team. Obviously a moulded airframe will have less drag overall but I think this has more to do with more accurate airfoil sections and skin hinging that the technique provides.


On the other end of the theory, lots of boat guys run a rough bottom. The theory is that at speed, water adheres to the rough surfave and the boat glides with water on water. I know when I go pylon racing, on of the fastest guys out there has a finish close to 150 grit sandpaper. Personally, I also think that at our model speeds it dosent make alot of difference.

My take is that unless you really do your homework, smoother is better. There has been evidence to suggest that some intentional roughness can offer reduced skin friction (try Googling: friction riblets), but in order to see the small benefit, the orientation and geometry of the roughness needs to be precise. My intuition is that for most Reynolds Numbers in play for RC, if your finger can't tell the difference in roughness, the air probably won't either (I suspect there will be disagreement on this). I agree with rmh that it's tough to isolate the effects of roughness for different construction techniques because other factors (like stiffness) will come into play.

Nicholdime,

Are you sure that no other aerodynamic improvements were made to the old glider that you recovered.

If not, then you have provided the response to your question.

If so, then the form drag may be more influential than the skin drag (as I suspect).

http://en.wikipedia.org/wiki/Parasitic_drag

http://www.grc.nasa.gov/WWW/K-12/airplane/shaped.html

http://en.wikipedia.org/wiki/Friction_coefficient

Of course, the final performance results by improving the friction coefficient of the skin depend on several factors (amount of wet surface, coefficient and condition of old skin, velocity of flight, etc.) and it should be different for each case.

You can determine the difference in friction coefficient of each skin by testing a sample.
The material should be used as the surface of an inclined plane.
The coefficient (between the materials of the skin and testing block) is always the tangent of the angle at which the block starts moving down the plane.

http://www.ejsong.com/mdme/memmods/M.../Friction.html

Thanks for all the info .I was looking for objective conformation rather than my subjective "I've put a lot of work into the model so it must be better" and the lift is beeter today than when I last flew it in its rough state.
I wonder whether it would be wort while setting up a crude wind tunnel out of some boxes and a fan and doing some simple experiments ?

Hello nicholdime,

I think the Universal Answer applies here: "It depends." One of the primary effects
of surface roughness is that it affects the transition from laminar to turbulent flow. Less
roughness will tend to delay the transition. The effect of the transition location is a bit
complicated to assess. Laminar flow can make one component of drag smaller, but
turbulence can prevent a laminar separation bubble or delay trailing edge separation,
either of which will probably reduce drag.

banktoturn

Given the work required and the size of tunnel needed to confirm what you are trying to measure, probably not. That said, if you care to go back through your 1940's~50's 'Aeromodellers' and 'Flying models' and 'MAN's you will probably find much 'theoretical' analysis of model wings, turbulators and surface turbulation relating to Reynolds numbers typical of what you seek. Much of this stuff is grounded in thin air, but the problems, and solutions, of the airfoils used and speeds where the problems manifest, is well covered. It does make interesting reading, if nothing else.
Evan, WB #12.

There is a lot of complicated papers and theory on the subject. Enough hot air to produce a good sized thermal.But nothing directly relating to model gliding. I had hoped to find that somebody had done some simple tests , like taty old tape covered wing produced 8oz of drag in a 20mph air stream (top of a truck ?) .Wing recoverd in smooth shiny stuff produced 6oz of drag

Airfiol design is a big factor. L/E radius and trailing edge thickness would be were i would find less drag.. You can use, water, as a easy and fairly accuate method to test drag. Glass or sand paper will let you see what is related to, surface drag only. By running water down an incline, with test material ,on the incline you can see it live!! Turbulance is the real problem. "Trailing edge is a killer".. Drag it through water,behind a canoe..or build a test rig,run water over it and read drag ..Then ,after all that work,you can tell us ,for free, what you foud out!!!! lol [X(]

Nicholdime, why not do your own testing? As you see here there really isn't any figures for such things.

An arm extending out from a car window would not be a bad way to test such things. First off you'll want to make up an airfoil shaped arm of wood that is about 4 to 6 feet long. On the arm at a few points on the last foot or two you'll attache some wool tufts. At some speed, such as 20 and 30 mph, you'll extend the arm out from the car while watching the tufts. When you see the last couple of tufts smoothen out and flow straight back you know you're out beyond the turbulence from the car's body. So your test models can be placed at or out further than that distance and get some decent readings.

When you have some numbers get back to us with the results, OK?

From personal (literally) experience, I skydived for many years and had several suits for different disciplines. I had a fast suit (belly to earth) for when I was skinny that was made of nylon and was fairly tight. A cotton and cordura suit that was fairly tight also for slower fall rates (had to buy this one for when I became heavier and more bullet shaped). I had a loose cotton suit for head down, sit, and standing (freeflying). I had a "Pro ditter" that would record fall rates and belly with skinny suit i fell as slow as 125 and as fast as 170. 170 was flying with freeflyers, belly with elbows in and fists in chin with heels in my ears. My slow belly suit I could slow to sometimes slower than 110 and as fast as 140. Freeflying is a whole other story, have gone 225 in a stand. So, at those speeds the fabric made a huge difference in speed. By the way, in a track, we could go close to 100 mph horizontal and the slowest vertical in a track I went was 94. Had to really make your body into a wing.

Brooks