Flat-bottomed vs symmetrical

[Ekim]

What are the advantages of havind a symmetrical wing over a flat bottomed one? Or vise-versa? :rolleyes:

[Sparky Paul]

Neither makes a good pancake, nor a spare tube for your tires, ...
What purpose do you have in mind?

[Salto]

The important thing to think of here is the mean line of the airfoil section. This is the line running in a chord wise direction (front to back) mid way between the top and bottom surfaces of the airfoil. It's sometimes called the camber line.

A symmetrical section has a straight mean line, whilst a flat bottomed section has a gentle curve to it's mean line. The maximum height of this curved mean line as a percentage of the chord is referred to as the camber of the section.

The section also has a thickness around this mean line, which can also be referred to in terms of percentage of the chord.

A section with a high camber can achieve a high maximum lift co-efficient, but will also have high drag. A section with no camber (symmetrical) can only achieve a moderate lift co-efficient.

So, if you want a slow floating model and you don't care about top speed then use a section with high camber, say 4 to 8%. But if you want a fast model then you'll have to choose a section with relatively low camber, say 0 - 3%.

Symmetrical sections also have the property that they behave the same when flying inverted, so they are popular for aerobatic models. But if you want to turn fast as with a pylon racer, then you'll need some camber in order to achieve a reasonable lift co-efficient to pull the model around the turns.

Graham.

[Ekim]

Oh I was just wondering,
And Salto, you answered it perfectly, thanks.

And I agree, balsa doesn't taste very good :wink:

oh and one other question that your answer brought up, Salto.
"would a flat bottomed wing stall sooner than a symmetrical wing inverted"

[Salto]

Ekim,
Yes, I believe it would. It would stall at both a lower angle of attack, and at a lower lift co-efficient. Others may offer a more thorough answer on this one.

Sparky,
Been thinking about your spare tube comment. EPP foam, commonly used by the enlightened in model wings, has a remarkable property of returning exactly to it's original shape even after significant deflections. This would make it highly suitable for a no-flat solid inner tube for bike tires. I'll give it a try one day - it might be a good use for all those EPP scraps that I generate.

Graham.

[Ekim]

I am doing a science fair project and during the course of research I just came up w/ these questions

[ngcebo]

Symmetrical sections also have the property that they behave the same when flying inverted, so they are popular for aerobatic models. But if you want to turn fast as with a pylon racer, then you'll need some camber in order to achieve a reasonable lift co-efficient to pull the model around the turns.

Hi. I am looking for a good wing profile to use for a small electric CAP232 (approx 800mm wing span, 210 sq in.) and do not know how to start. Can anyone guide me?

I want to be reasonably (i.e. not extreme 3D) aerobatic.

[Sparky Paul]

The NACA 2412 will be the shape you need.
Semisymmetrical, and thinnish..
In that size of plane, about all that matters is a smooth top and bottom surface.
Any quibbling about thickness and distribution and camber etc is pointless.