Stress testing wings

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deskpilot

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I have a couple of questions regarding the testing of wings to establish positive and negative 'g' limits. I understand that the normal thing is to load the top surface for negative limits and then turn the plane/wing over and load the top(underside) for positive limit. This seems a lot of work particularly loading and unloading and maintaining equal loads on each wing. Now I might be missing something but why not load as per this sketch

Wing strength testing.jpg then raise or lower the central cradle to stress the wing either positively or negatively.

2nd question. Test loads are normally spread along the wing span, using either a static load or a wiffle tree system. What's the difference if the load was applied only to the wing tips with the fuselage held firm?
 

topspeed100

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I have been thinkin this too. Especially if you have washout ie aerodynamical twist..it would sorta point the wing tip down and major load would actually be in the wingroot...where the strenght is at its best...how do you test that reliably ?

In negative this is vice versa unfortunately...will this affect design ?
 

Dana

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If you support the aircraft at the wing tips, the loading will not be realistic in any way and your test will prove nothing. Actually, load testing doesn't prove much anyway and is rarely used... it's impossible to make the loading accurately simulate actual flight loading. It can be useful, occasionally, to check a particular area or hard to analyze section, but it's not substitute for a proper stress analysis.

And, once a structure has been static tested, it must never be used for actual flight... it must be considered a destructive test, even if the structure doesn't actually fail.

-Dana

"If you want to build a ship, then don't drum up men to gather wood, give orders, and divide the work. Rather, teach them to yearn for the far and endless sea." - Antoine de Saint-Exupéry
 

autoreply

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Dana;91581And said:
never[/I] be used for actual flight... it must be considered a destructive test, even if the structure doesn't actually fail.
I have seen that statement quite often. Nevertheless, it looks like many people do so. In many countries static testing is required for homebuilt aircraft (don't know whether that's till limit or ultimate load) and it looks like many don't build two sets of wings. Would strain gauges, a good analysis and carefully monitoring the deflections be sufficient?
 

orion

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In general, flying a structural test assembly is a bad idea but with proper analysis and precautions it can be done. The unfortunate thing though is that most folks do not go to that level of analysis and care to do all this right so the eventual flight does present a very high level of risk. The only way to do this is to first complete a very careful analysis of the structure, including predictions of deflection. The test then will be done so as to test said deflections per given load. The key however is to keep the test short of the limit condition. But if the deflections of the structure match the analysis, it's a good indication that as long as the assembly and material selection match those used in the number crunching, the assembly will deliver the prescribed life and safety factors. But to do this right one must derive an accurate load distribution (span and chord wise) and be able to apply said load in the same manner during the static test. Rarely can one do that with cement bags (or whatever) so staying short of the limit is critical.
 
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Topaz

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I have a couple of questions regarding the testing of wings to establish positive and negative 'g' limits....
Okay, this is hanging on one word, but it's also an important point: You don't do structural testing to establish 'g' limits. You do structural testing to confirm that your loads analysis, structural design, and structural analysis are actually correct. Without that numerical work already completed and in-hand before you start building, you're reduced to trial-and-error design. That's a quick suicide unless you already know enough to actually do the calculations anyway.
 

orion

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Here are some pictures of a stress test on a vision wing, as required in SA. I thought it looked impressive!

Vision Aircraft: Stress Test: 2700kg=6G's
Yes, impressive especially since the load distribution is incorrect. Most likely the stresses seen at the root are significantly greater than what would be caused by an actual 6G load distribution, which means that this wing should have never been allowed in the air if this was to be an actual limit test.
 

flat6

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Yes, impressive especially since the load distribution is incorrect. Most likely the stresses seen at the root are significantly greater than what would be caused by an actual 6G load distribution, which means that this wing should have never been allowed in the air if this was to be an actual limit test.
whats the proper distribution? more weights toward the root?

but i dont understand why its unsafe to do that to a wing that will be actually flown. If the wing is unsafe after testing to the normal load limit then there is something wrong with the design.
 

skeeter_ca

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I think you would have the greatest load at the root and then it would get less and less as it goes outward.
 

skeeter_ca

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PTAirco

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but i dont understand why its unsafe to do that to a wing that will be actually flown. If the wing is unsafe after testing to the normal load limit then there is something wrong with the design.
If I was designing a wing to take a 9g ultimate load and a 6G limit, I would feel perfectly safe to load the wing to 6g and go fly it afterwards - provided that there was no permanent deformation after the load was removed. If there was, it's back to the drawing board and find out why. (Things are more awkward with composites or wood structures.) But if you are afraid to load test your to limit loads, whether it is 3.8, 4.4 or whatever - then I don't want to fly in that airplane. Proof loading to limit loads is often required in other countries for homebuilts.
 

flat6

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I think you would have the greatest load at the root and then it would get less and less as it goes outward.
makes sense as the lift is less outboard. it depends on the design lift distribution. a straight wing without twist will have different loading than a straight wing with twist
 

orion

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whats the proper distribution? more weights toward the root?

but i dont understand why its unsafe to do that to a wing that will be actually flown. If the wing is unsafe after testing to the normal load limit then there is something wrong with the design.
The proper test weight distribution is such that it represents the load distribution of the lift generated in flight. The loading on most wings in nearly elliptical so the distribution of the sand bags or shot bags should be so that the flight distribution is approximated as close as possible. Many of the distributions shown in the pictures seem to place too much weight outboard.

To test a wing to limit, and know that a limit load was properly achieved and that the inner structure has actually seen only limit loads, the structure would have to be instrumented throughout to make sure that no component had reached any form of distortion or yield. for instance, in composites it is highly likely that a web in a spar might deflect (wrinkle) in a manner described by semi-tension-field- beam theory. No, it's not failure at this point but if the web material was built up as a sandwich, the wrinkling would be due to delamination, which is considered a failed structure. But without it being instrumented, you don't know if it did actually debond. And overall displacement measurement would most likely not tell you whether it did or not. This type of failure is of course not immediately catastrophic but it can lead to issues down the road, even to the point of initiating more serious problems later. By flying this test article, whithout knowing it failed in this manner, would put the owner at risk. And while I use this composite example, the discussion can be as applicable to metal assemblies also.
 

Kristoffon

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The question in my mind now is whether the wing should be replaced after reaching a design 4G load during flight since again there's no way to tell what damage if any ocurred, and likewise after a given number of flights.

Of course they don't get replaced, how many aircraft even have accelerometers?
 

orion

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The question in my mind now is whether the wing should be replaced after reaching a design 4G load during flight since again there's no way to tell what damage if any ocurred, and likewise after a given number of flights.

Of course they don't get replaced, how many aircraft even have accelerometers?
Technically, yes it should. At 4 Gs it surpassed its limit load meaning it as attained a permanent set or some other part of the structure may have exceeded its functional design load. This is failure and as such, the wing should be replaced. Most folks doing annuals will check the plane's rigging so any permanent deformation would be caught at the time. Keep in mind that the operational envelope of he airframe is only to the 3.8 G limit. Anything past that is failure and the safety factor to ultimate is only just that. The airplane is not intended to be operated beyond the limit, except in the case of getting the occupants home after the excessive maneuver.
 
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