# Finding the center of gravity/centroid of an airfoil?

I'm in the process of building myself a wing frame out of balsa wood. However, as a high school student that's just started aerospace engineering, I'm not really sure how to go about it in some ways.

What I'm looking for is the center of my ribs so that I can find the best point to put my main spars in. The ribs themselves are full flat-bottoms if that helps any. Is there a go to equation for something like this?

In addition to checking out wing frame structures on the internet, why not go down to the hobby shop and get a balsa plane kit? Airplane building is a hobby you will have all your life.

Generally, in balsa, you have a top and bottom spar, a leading edge, and a trailing edge to attach the aileron (get lots of sand paper and CA glue). As mentioned, put you spars at around 25 - 33 % from leading edge, around the highest part of the rib. You may also have good luck strengthening your wing by covering it with 1/16 sheet balsa, others use heat shrink monocoat. But you must also stiffen the wing so it can not twist when you use the ailerons. So you may wish to add balsa between the ribs connecting the top and bottom spars, creating a box structure.

A lot of ways here, and you can't read too much about it. Please let us know how you are doing with it. Check out RC Groups website as well.

If you are looking for a suitable position to build a flyable model I would suggest locating them approximately 1/4 of the chord length from the leading edge. The rest of the airframe would need to be factored into computing the aircraft center of gravity and its relationship to your spar, but generally 25% LEMAC, (Leading Edge of the Mean Aerodynamic Chord) is a good starting point.

As a point of reference, the acceptable CG range of the C-130 Hercules is 15 to 30% LEMAC.

The center of pressure or lift of the airfoil will change somewhat with AOA, but I'm afraid that I can't help you with calculating it.

Structurally, depending on how many spars you want to use, there are positions based on experience. For a single spar D nose construction 25-40% is a good idea.

If you want to find the centroid, I recommend you get some CAD software and draw it, and let the software find it for you, at least that way you can print out templates straight away.

If you are calculating stability, about 25% for aerodynamic center is a good estimate