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Old Feb 19, 2013, 11:37 PM
ian murdock is offline
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Question

laminar flow airfoils


Hi guys, I'm starting to get into designing my own dynamic soaring gliders, if you don't know what this is look it up on youtube. My question is how to pick and airfoil that will allow a glider to reach 500mph!!!! or try and hit 400mph. Laminar flow seems to be the way to go for very high speeds. If you can point me in a direction that will help in foil choice or a straight up answer of some airfoils to try out that's great to.
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Old Feb 20, 2013, 04:05 AM
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There's far more to it than simply laminar flow style airfoils. The key is to figure out the reynolds number for that sort of speed range then to look at the G loads that the models encounter during their angled horizontal looping style flight pattern during the speed runs and to find airfoils with low drag values that occur at the resulting lift coefficients that are experienced during this style of flying. Laminar or not is the last thing you need to look at and only in comparison to the airfoils that are known to work well at the expected Reynolds numbers and lift coefficients encountered during this style of flying.
Old Feb 20, 2013, 08:24 AM
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Thanks man, i really should of gone into engineering and not pre-med.
Old Feb 20, 2013, 09:57 AM
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Deniable plausibility
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Quote:
Originally Posted by ian murdock View Post
Thanks man, i really should of gone into engineering and not pre-med.
Now that's funny because I went into engineering and now realise I should've gone into medicine!

But back on topic, IIRC (and that's a fair old caveat..) the airfoil used on the little Dynamics is called 'RK40' - maybe a google will turn up coordinates?
Old Feb 20, 2013, 11:02 AM
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Thank you for the help, I tried to look up the foil in Compufoil and not there but Googled it and found this website. http://www.dynamic-soaring.de/profile.htm
Old Feb 20, 2013, 11:16 AM
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Here are a couple of other well-known airfoil designers with DS profiles:

http://translate.googleusercontent.c...aKYYGYIQbL13iQ

http://www.habebert.com/index.php?op...mid=18&lang=en

Kevin
Old Feb 20, 2013, 01:54 PM
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Looking at 2 second laps and 400 mph, the g-loading comes out very high!
Old Feb 20, 2013, 04:18 PM
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18G's isn't very high for an RC airplane. I suspect DS airplanes see far higher numbers than that due to turbulence, probably 60G or higher.

DLG launches are over 50G, and maybe as high as 70G, but laterally, not in pitch.

39G measured for a zoom on thermal duration glider:

https://www.rcgroups.com/forums/show...&postcount=160

Kevin
Old Feb 20, 2013, 06:22 PM
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With the G load more or less in the 20 to 25G range you can begin looking at what the lift coeficient will need to be on your wing and with that to begin picking an airfoil which has lower drag values for this sort of lift range.

Just for giggles a 2 meter span wing with 650 sq inches, or 4.5 sq feet, of area and 3 lbs weight will need to generate 60 to 75 lbs of lift during the circuit shown by Sparky.

Cheating a little and being lazy I tend to use NASA's Foilsim for quick answers and get a feel for such things.

http://www.grc.nasa.gov/WWW/k-12/airplane/foil3.html

So our 4.5 sq foot wing at 60lbs of lift and at 400 mph only needs to work at a Cl of a mere 0.087. And at 75 lbs of lift the Cl rises to 0.106. The airfoil is very limited in what you can do with camber and thickness. I set the camber to 1.2% and the thickness to just a hair under 10% for this example.

Even building heavy and ballasting to 7 lbs at 25G's the required Cl only rises to 0.247. Still fairly low overall. But enough of a rise away from the very low values that it would be worth using an airfoil with enough camber that the low drag range of Cl's is centered at around that value.

So whatever else you do the wing will obviously be working at a very low lift coefficient due mostly to the speed.
Old Feb 28, 2013, 10:03 AM
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I think there might be an error in Sparky's calcs...???

I get
a=v^2/r
v=400mph or 586.7ft/s
r=186.5ft
so
a=1,845ft/s^2

and in G's, 1,845ft/s^2 / 32.2ft/s^2 = 57G of acceleration

I wish it were only 18G! I could save alot of carbon!!! ;o)
We've measured acceleration with onboard accelerometers and pegged an accel that was only good to 90g! The calculation above only offers an average value based on circular orbit and like Kevin mentioned above doesn't account for non circular orbits and the impact of turbulence and hard corrections. A scary way to look at it is to consider that I know my plane can pull Cl of at least 0.9 without stalling. I also know that I've given full up elevator commands at 400mph to avoid piling it into the hill. That boils down to 150G!!! Never measured that much, just sayin...
Last edited by sll914; Feb 28, 2013 at 04:51 PM.
Old Feb 28, 2013, 12:11 PM
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Yup! Oh, gosh darn! I made a boo-boo! And I poste...
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Last edited by Sparky Paul; Feb 28, 2013 at 02:11 PM.
Old Feb 28, 2013, 02:50 PM
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Wooooooooo...... That changes things.

Oh, I don't know what I did in my last example but I see now that Foilsim only goes up to 247mph. So it looks like I'm in the same boat as Sparky by posting flawed information. Those Cl values I posted were for a "mere" 247mph.


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