Designing a 50" Pitts S-1S - Reading airfoil polars and airfoil selection - RC Groups
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Feb 06, 2003, 08:06 PM
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Designing a 50" Pitts S-1S - Reading airfoil polars and airfoil selection


So I'm in the process of designing a Pitts S-1S. Fuselage will be molded of Kevlar, and will be roughly 1/4 scale having a 50" wingspan.

I'm finding myself scratching my head now that I've come to the part about selecting an airfoil.

I want a relatively smooth handling plane, but a nice abrupt stall helps for entering spinning maneuevres. I've been considering something like an Eppler 474 or 475, but don't have the knowledge needed to really evaluate the airfoil.

I can plot the polars in Profili, but am left wondering if there's an online resource that can teach me how to read them.

Please help, anyone !!!
Dan


Dan Kitching
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Feb 06, 2003, 08:52 PM
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See:
http://www.mh-aerotools.de/airfoils/hdipolar.htm

Reading a polar diagram is one thing, knowing what is relevant to a particular type of aircraft is another.

One thing you are looking for is the stall characteristic. For that you want data that comes from a windtunnel rather than a simulation program such as the Eppler program or X-foil. The coefficient of lift versus angle of attack plot gives the stall characteristic. You want an airfoil that stalls abruptly. Its coefficient of lift increases with angle of attack up to a maximum and then you want an airfoil where the coefficient of lift drops suddenly with little or no further increase in angle of attack.

You don't have to concern yourself much with the drag polar because you will have enough thrust to over come drag over a wide enough speed range assuming that you are powering the model for strong vertical performance.

The maximum coefficeient of lift and the wing loading will determine the stalling speed and, with allowance for stall margin, determine the minimum landing and take off speed.
Feb 06, 2003, 09:40 PM
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Thanks Ollie,

I feel like I'm learning a new language, but I also get this feeling that there is a whole new fascinating side of the hobby just waiting to be explored.

Should I think differently for a biplane when it comes to choosing an airfoil? Or are the criteria all pretty much the same?

Thanks
Dan


Dan Kitching
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Feb 06, 2003, 10:50 PM
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Off hand, I can't think of anything about an airfoil that makes it more suitable for a biplane than for a monoplane.