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Old Jun 28, 2015, 08:50 PM
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Camber / Airfoil Help

Hi all! Let me start by saying that I've been flying my homemade foamies for quite a while but I never really experimented with airfoil sizes. All my wings are the same design, 7" long and a camber of 3 cm high in the highest part. The glide ratio is decent on my planes.

Then I just bought a bixler and that thing refuses to stop flying! The glide ratio is crazy. So I looked at the airfoil and it's barely half the size of my airfoils (in terms of height).

I know I'm giving very vague information but how should I choose the thickness of my airfoil? I know plane size, weight and all that definitely factor in but why does the very thin wing of the bixler fly longer then my airfoils that are twice as big? If you could give me a general idea I'd be happy! Thanks!
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Old Jun 28, 2015, 09:34 PM
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Thicker airfoils are not as "slippery" as the thinner ones, even though they can generate more lift given enough power. The higher drag will reduce the glide ratio, as you have observed.

You can use a program like Profili to examine a wide variety of airfoils to get a better idea of what to design yourself. A fairly typical camber-to-chord ratio of many practical airfoils used in modeling is around 12-15 percent. The amount of camber you described seems to be unusually high.
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Old Jun 28, 2015, 09:47 PM
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Alrighty whitecrest thank you very much for that! Seems to make sense. I'll adjust my wing and see how it works from there.

So for my 7 inch long wing I should have a camber of about 0.84 inches? Cool.
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Old Jun 28, 2015, 10:22 PM
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Yes. If you're not doing this already, you can cut your own foam wing cores using just a couple of templates and a hot-wire foam cutter. This can produce a wing section with an accurate airfoil ready for balsa sheeting or glassing. However, for most sport flying, it is not necessary to get too hung up about any particular airfoil or extreme accuracy.

I scratch build with balsa and have had good luck using the Clark Y airfoil, which is well known. It has worked well for me in models with a wingspan of about 36 inches, 210 sq.in. of wing area and about 7-8 oz/sq.ft. wing loading. These planes are easy to trim for level flight and glide very well. Other similar airfoils would most likely deliver the same good performance.

Good luck with your experimentation.
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Old Jun 28, 2015, 11:00 PM
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Firstly: "camber' is NOT the the thickness of the airfoil.
Lotsa Reading /Research into the nature of Airfoils is suggested.. before you can begin to have an articulate conversation about them
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Old Jun 28, 2015, 11:34 PM
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So Bixler has less lift but more wing area with a resultant less drag. Those planes are really engineered these days. Look at the Radian, UMX Radian, etc. They are heavily engineered, with complex manufacturing, too. Expensive molds to form those wings to spec. Even compound curves and winglets. Don't underestimate what went into that design. Thing is, there are only about 1 Million people around the world that would love to have that task - engineering model airplanes.

Bare, are there any entry level books on airfoils for model aircraft?

Probably the first thing to get into is what a Reynolds number is.

Cheers - Poughkeepsie Pete

PS I noticed these suggested threads at the bottom of the page of your post:
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Old Jun 29, 2015, 06:50 AM
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Quote:
Originally Posted by Bare View Post
Firstly: "camber' is NOT the the thickness of the airfoil.
There is a reason why I said camber/airfoil size. And while I may now know all the nittygritty breakdown of a wing I certainly know how to build them. I don't know what size to build them but I make a great airfoil from foamboard
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Old Jun 29, 2015, 06:52 AM
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Quote:
Originally Posted by PGregory View Post
So Bixler has less lift but more wing area with a resultant less drag. Those planes are really engineered these days. Look at the Radian, UMX Radian, etc. They are heavily engineered, with complex manufacturing, too. post:
I gotcha! Well thank you!
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