Wing strength: beyond wing loading ?

[FlyByMike]

Reading about planes where wings snap off, I am wondering how one calculates wing strength.

I know what and how to use wing loading -airframe weight per wing area. But how does one figure out wing strength ?

I think I'm asking if the critical area for calc'g wing strength is the wing-fuse connection. Assume a 1 kg plane. Therefore the wing-fuse joint must, a minimum, be able to support(?) this 1kg. But what is the maximum case ? (A plane doing 3g's needs a wing that'll hold up to 3 kgs ? High many g's do models generate ?)

If this is correct, then what are some load bearing values for, say, balsa ? Examples please ?

[steve lewin]

Interesting question but calculating such things is way beyond me and I suspect most of us. I do know that the strength is mainly in the structure not just the materials. E.g. a 1/4" square balsa spar will not hold much weight but separate 2 of them with ribs and the load capability goes up dramatically. Add thin shear webs and it goes up a lot more.

Same with foam, a foam wing core will flap about alarmingly as will a couple of bits of thin balsa sheet. But glue the sheet to the foam and the load capability is much much greater than the sum of the parts. Just adding a carbon tow "spar" to foam helps a lot but I have no idea how to calculate how much.

You're right that the maximum loads are at the centre of the wing but I suspect models pull very high G forces (no pilots in there to black out).

Steve

[Sparky Paul]

The bending moment varies non-linearly out from the root to the tip.. max at the root, zero at the tip, as shown in this plot.
The greatest load on the structure is closest to the root.
These stresses can be calculated, but most of us go by... "The last wing built in this manner didn't break so this one won't."
The inquisitive will take parts out until it does break.. and add parts for the next one. :)
Typically a glider wing will break on winch, at the end of the dihedral brace. There shouldn't be a discontinuity in structure here.. taper the material out to the end of the joiner.

[Andy W]

A good designer keeps reducing material until the structure fails, then revert to a previous design! :)
..a

[FlyByMike]

The inquisitive will take parts out until it does break.. and add parts for the next one. A good designer keeps reducing material until the structure fails, then revert to a previous design!
Ah yes, the old "Oops, back it up a notch" method :D

[Gerald]

On full scale aircraft they used to (don't know if they still do) pile on sandbags evenly over the wing surface until something gives. The weight of the sand bags approximates the even distribution of force from airflow over the surface.

These days computer models are used to perform finite element analysis to predict loads, stress risers, and strength.

[tekochip]

Didja ever see the video on the 777 wing stress test? They had the wing in safety cage mounted to the aircraft and pulled it up until it snapped.

The analysis turned out to be right.