Airfoil selection for a flying plank, how to achieve statical equilibrium?


Well-Known Member
Dec 31, 2009
Barcelona, Spain.

I am reading Anderson Aircraft design book. He covers everything, but there are some details that are spared. Hopefully other books will come next to study the specific details.

So far, I am working, as an exercise/divertimento, on the conceptual design of a very simple and affordable light aircraft. That is why I picked a flying plank with a vertical stabilizer as the potential layout.

I know very little of aircraft design, but I understand that, roughly speaking, you set some parameters to be able to pick the best airfoil for your requirements. Once you set a given weight, v cruise and v stall. You get a CL for your desired cruise speed. This v cruise has to be that of your best CL/CD, to get the best fuel economy. Then you have to find an airfoil that has a similar best CL/CD of the one you require for your cruise speed.

In my particular case I would like to build a tube and fabric wing. Thus I am considering flat bottom airfoils. My cruise speed would be like that of a Nano trike.

Anderson encourages to get a copy of theory of wing sections to use all the data collected of NACA airfoils. I just received a copy yesterday, I am using that book to choose my airfoil.
I guess that picking a laminar airfoil is perhaps unnecessary. The 4412 seems to be a good candidate, because the bottom is almost flat.

The 4412 has a moment of 1. OTOH the 23012 has a moment close to 0 but the bottom of the airfoil is not flat.

From what I have read on the net, most flying wings, like those used by Fauvel, use airfoils that are called auto stable airfoils; because their moment is next to zero. The auto stable airfoils I have seen, do not seem to have a flat bottom.

My doubts are those below;

Is it compulsory to use for a flying wing an airfoil with a CM that is very close to 0, or can you fix a CM that is >0 with the weight of the pilot, or with the engine weight?
When you read the CL curves You have to pick the one with closest Reynolds number to your wing chord. But what about the curve that is labelled as standard roughness, what is it for?
Given that you have to adapt the ideal CL of an infinite wing to that of a finite wing. DO you have to do the same for the CM?

Sorry if I ask something very dumb. I am just trying to digest all the info and the more I learn the more doubts I have!

Thanks for your comments, it is really helpful!