I've been following this thread from the beginning. Here are some FEA models showing what is being discussed. These are very simple models covering a very complex situation. I have gone thru all this before on a number of models so it is not new. I now longer have wood for a material and I've just been too busy to dig it up again. For these models I just used 6061 aluminum. It's light and in some cases probably pretty close to what some guys build in wood. I just had to put that in haha.

these show one half of a wing. the spar is 48 inches long 2 inches tall at the fixed end and 3/4 inch tall at the outer end, 1/8 inch wall thickness all around and 1/2 inch wide. I show what happens when you put the load at the tip vs a more realistic evenly loaded spar. the load is 20 pounds so it represents a fairly heavy warbird or perhaps a light 3d plane in heavy maneuvers.

Again before you young engineers take the old engineer to task remember this is a very simple representation of a very complex situation.

Shown is the spar made of 6061 aluminum weight of .94 pounds
2 pictures show end loading stress and deflection
2 pictures show even loading stress and deflection

You will note a twisting of the spar
No account is made for ribs or skin. I would design for these being adders that don't count in the design. The spar needs to be able to take all the load as the skin may have various holes that make it not a load bearing surface.

Of particular interest is that end load shows .900 deflection while the even load shows .003.

Don't even bother to get carried away with actual numbers, this is done just for clarification of what some of the posters are talking about.

You can clearly see that picking a heavy model up by the wing tips is not a very good idea and does not represent flight loads at all.

I did another version out of 1/16 material. 1/2 the weight, a little more deflection and a little more stress. Other factors begin to enter seriously that being buckling. The ribs would help here as well as some other load bearing members. As you can see, reduce the weight...increase the complexity....square it, cube it, or power it you determing how light you want. oh yeah I forgot cost....generally "black hole" defines it. haha

Now for the text book guys I'm with you all the way. If you have a Roark's, a good calculator, a nice .7mm pencil full of lead and a couple hours you should be able to check the results. Just for reference it took me only 1/2 hour to build the model and run the FEA. Don't you just love CAD. I had to do this stuff with a GD slide rule in school.