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?
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asked May 2, 2018 at 15:15
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17$\begingroup$ No, a wing is not being pulled up by the low pressure on the top side. NASA. Wikipedia. $\endgroup$– MichaelKMay 2, 2018 at 15:19
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12$\begingroup$ @PhilFrost Did not say that. $\endgroup$– MichaelKMay 2, 2018 at 20:44
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4$\begingroup$ I get that the equal transit theory is incorrect, I just don't see why @MichaelK is jumping to that, given what's in the question. The question doesn't seem to say much of anything about how lift is generated, just that "a wing is being pulled into the air from its upper surface in flight". Is not the pressure on the top surface lower than the pressure on the lower surface of the wing? Does this not mean there must be a net upwards aerodynamic force on the wing, "pulling it up"? How does this invoke the equal transit theory? $\endgroup$– Phil FrostMay 2, 2018 at 21:11
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4$\begingroup$ I think what @MichaelK is getting at is that if there's lower pressure above the wing, the wing is not being pulled from the top, but pushed from the bottom. $\endgroup$– StackOverthrowMay 2, 2018 at 23:11
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12$\begingroup$ @ThorstenS. MichaelK is technically correct, but hasn't clarified the misunderstanding. While there is definitely lowered air pressure on top of a wing, air doesn't "pull upward" on the wing. Fluids can't ever pull anything, they can only push. Lift is generated by the side with high pressure pushing upward. This lifting push is counteracted by pressure on the upper surface pushing downward. Thus, as Chromatix said, the bigger the difference in pressure, the greater the lift. (This all assumes both surfaces have the same area, as force is pressure times area.) $\endgroup$– drmuelrMay 3, 2018 at 8:23
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5 Answers
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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.
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@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.
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1$\begingroup$ Interestingly, this sand bag example shows how not to do the test as the wing is not inverted and I doubt the intent was to test negative Gs. The tail was tested the same way which is ok since it is intended to provide download. $\endgroup$ May 2, 2018 at 17:40
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2$\begingroup$ @Pilothead I did notice that as well but its a good picture outlining the process. Lower down on the page they are testing it upside down $\endgroup$– DaveMay 2, 2018 at 18:24
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6$\begingroup$ @Pilothead, note that you do have to test some negative Gs, as even the category with least requirement is to be tested from -1 to +2.5 G. $\endgroup$ May 2, 2018 at 21:18
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$\begingroup$ @JanHudec The photoset in question had twenty photos showing only one wing orientation, and it is almost impossible that they would only test for negative G. A different group did the test correctly. $\endgroup$ May 3, 2018 at 19:48
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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)
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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.
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$\begingroup$ This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review $\endgroup$– GerryMay 3, 2018 at 23:03
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2$\begingroup$ @garry the OP asked why they are being tested by pushing when they are pulled in flight; I explained that pressure only pushes, and that they are wrong in their belief that pressure pulls. Does anyone really need a sentence that connects "pressure actually pushes" to "test by pushing is appropirate"? $\endgroup$– YakkMay 4, 2018 at 1:16
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$\begingroup$ I'm not saying you're wrong, just that you're not really answering the question. Your statement would be a good comment adding to Dave's answer. And as the Boeing example shows, it can be tested by pulling up on the wings. Ultimately, it doesn't matter whether you push or pull or the wing is right side up or not, as long as the test applies the proper load. $\endgroup$– GerryMay 4, 2018 at 2:52
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1$\begingroup$ It seems to me that if the entire premise of a question is wrong, correcting the premise is a valuable answer. It's OK to test wings by attaching cables to them and pulling despite the fact that air lifts the aircraft by pushing on the wing, not because air pulls on the wing. I would only suggest that the answer should focus on top surface vs. bottom surface pressures on a wing, not inside vs. outside pressures on skin or the walls of containers. $\endgroup$– David KMay 4, 2018 at 11:15
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$\begingroup$ @DavidK I added that to explain why it feels like sucking and not pushing when you put your hand on a vacuum cleaner hole. The "sucking" is really just the fluid in your hand pushing out of your body and your skin being stressed by the lack of support. $\endgroup$– YakkMay 4, 2018 at 12:57
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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.
