What is airfoil percentage?

[jcpatrick]

Well. . . what the heck is it? It appears to be height divided by chord, but I've been unable to confirm this.

Related to that, what's the (usual) effects of changing airfoil percentages.

[JetPlaneFlyer]

Yes, it's thickness divided by chord, then you multiply the answer by 100 to turn it into a %

As a rough rule of thumb thinner wings tend to give less drag and are therefore better for faster flying models whereas thicker wings can have better stall characteristics and wider operating envelope so are better for 'all rounder' type models... But this is a sweeping generalisation, the camber of the airfoil needs to be considered along with it's thickness.
Thicker wings do have the definite advantage of being able to be built structurally stronger that a thin wing.

[vintage1]

Thse that know more than I inform me that thick wings do not perform well at teh sort of Reynolds numbers we modellers encouter.

However I have been drawing up the odd few scale models from the 30's era, and here it appears that the tendency was to use a pretty thick root - up to 20% - tapering to tips of 10% or less to get a good compromise between strength and performance.

Overall 10-15% at the most seems the ideal for models . Or less for out and out speedsters.

I THINK the issue is that a thin wing at high speeds will develop plenty of lift at a very small angle of attack and reasonable drag, but as the AofA increases, its more prone to early turbulent flow separation. The thicker wing can fly slower at higher AofA.

Cambering it makes it even better, but again the stall characteristics are not so benign.

Unless you are into performance duration models, or trying to make a contest winning pylon racer, flat bottomed or semi-symetrical to about 15% seems to be the best all round section.

[Cory]

The thickness guidelines above are probably good for a powered plane, but if you are building a glider, you'll probably want a thinner airfoil. Most glider airfoils are somewhere in between 7-10%, with high speed "lead slead" slopers and some DLGs getting down to 4-6%. The mose common Dynamic Soaring airfoils are between about 8.5-9%. Not only do these have pretty low drag, but they turn well at high speeds, too. DLGs are know for being quite nimble, even with the thin airfoils. That's more a function of wingloading, however.

[BMatthews]

As a general rule if you take a NACA profile making it thicker acts to delay the stall. This was a trick that some designers used to lessen the risk of tip stalling. They would use a 2412 at the root and a 2415 at the tip for example.

This works on models up to a point since we also need to deal with the size of our wings to the "full scale" air they fly within. So for our models it's generally considered that 15% is about as thick as you want to go on anything other than the biggest jumbo models.

Along with the thickness being presented as a % value there's also the camber. The airfoil centerline is the straight line from the most forward part of the leading edge to the most pointy part of the trailing edge. Then there is the camber line which is the line that you would get if you drew it in midway between the upper and lower surfaces. The distance that curved camber line is above the centerline divided by the chord and then multiplied by 100 is the % camber.