buckling is key
Buckling loads are key. That can be solved by filling the tube with the right size (tight fit) dowel rod. It's alot easier than trying to calculate critical buckling loads.
Then the bending stress will be the critical factor. Strength of material are given as stress levels in pounds per square inch (PSI). Yield stress is the stress at which the metal begins to deform. The yield stress in tension is usually the critical (lower) value. SO, step one is to look up the yield stress in tension of the type of aluminum you are using.
stress = M * y/ I
where M is the bending moment applied, I is the cross section moment of inertia(natural resistance to bending of the particular shape), and y is the distance from the neutral axis to the point being calculated. In this case y is the outside radius of the tube.
You would need to look up I in a structures book - or how to calculate I, anyway.
To get the max bending load, just rearrange the formula;
M = stress * I/y
and use the yield stress value for 6061-T6, I for a hollow circular section, and y is the radius. Getting the Moment - you can then back figure the amount of lift the wing can produce before bending the tube, neglecting the bending strength of the wood that you added for buckling. Sound like fun??