Why are wings load tested by putting weights on the underside of the wing (placed upside down)? A wing is being pulled into the air from its upper surface in flight, so shouldn't it be load tested in the other direction?
A wing can be tested in any orientation as long as the load is applied correctly. The classic wing test photo is the 787 in a fixture showing its extremely flexible wings.
I thought it might be fun to add an ultimate load test, so here is the 777 tested to failure. Sorry for the early 1990s video quality.
@PilotHead is correct, but to elaborate a bit on why traditionally weight was put on the bottom is largely because its just easier. If you are Boeing you can afford to build a rig large enough to hold a plane down while you pull the wing up. If you are building a home build in your garage its far simpler to put some sand bags on it and let gravity do the rest.
Think about the forces on an airplane (free body diagram). Where does most of the lift come from (wings)? Where is most of the weight(fuselage)? That means that the wings are pulling the fuselage up against gravity (or the classical way of thinking about it, the fuselage is pulling the wings down)
Pressure always pushes.
Low pressure doesn't pull. Low pressure just pushes less than high pressure.
When you have a stretchy fluid-filled pressurized sack (like a human), its internal pressure causes it to really want exterior pressure on it. When you don't, you can have fluid leaking or the stack tearing itself apart.
Similar problems can happen with metal holding back pressure. But even there, the force comes from the air inside the metal object pushing more than the air in the low pressure zone.
Vacuums don't suck things. It is the air pressure surrounding is that is violently shoving things into the lower pressure zone they create.
It is true that you don't get the same forces from upside down weights that you do from a wing in flight; but there is always going to be a mismatch between your model and your test.
As far as I know, all aircraft have structural limits for both positive G and negative G (typically a lesser value) that need to be proven. A small aircraft may be designed for loads of positive 5G (wings pushed up relative to fuselage), but only negative 3G (wings pushed down). The well-known image of fighter aircraft quickly descending by steep banking first is an illustration of this difference in structural performance (as well as visibilty concerns). The aircraft can withstand quite high loads when suddenly pulling out of a dive, but usually much lower loads when suddenly departing level flight into a dive.