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Info on wing load testing or only Spar

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opcod

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Hi

I try to find out more and read many post, but it's not really clear about one item, in regards of the testing.

If i build a composite spar using carbon rod and web flange with carbon fiber too :

- for a static testing with sandbag, what will be the difference if testing the spar only VS testing the spar embedded into the wing with skin on. (I mean, just like a composite sailplane wing)

If I test the spar up to 8g, (according to the V-N graph, gust load of the airplane) it might be little higher load with the skin on and little less deflection might be expected. But at the end, it might gonna be the same as the spar still carry the most of the load.

But, I know the control rod cannot be tested to see if any friction or if the right deflection happen... but again..?

thanks again
 

autoreply

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The skin often stabilizes the spar against buckling (fwd/aft). The fwd load component on a wing is huge (about a quarter of the wing load) and most spars can't even withstand a fraction of that (the skin does).

If you're confident that the spar won't buckle fwd/aft, testing a "clean" spar is an easy way to verify your calculations.
 

opcod

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Hi

So if we just go with «sandbagging» the spar, if we put some guide as to make sure the spar will not move to the front or foward... we should then be ok. ? Let say by about 1/2 inch.

And about the maximum load, many told to not put the load too long on the spar. So we pile up the bag up to our max design load, measure the tip deflection.
After pile more bag up to the max load of our V-N graph at 8g, measure the tip deflection :
and right after, we unload the bag quickly.. ?

just to make sure, as the spar will be use for the aircraft...
 

autoreply

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I wouldn't test beyond limit load for a flying wing. A lone spar (if you check very careful for delaminations etc)... I wouldn't be too scared to test it to 1.5*LL

I've just tested a wing with sandbags; getting the distribution of load correct and slowly increasing is a nightmare and very physical. A hoist that's connected halfway is essential to unload the wing/spar when you're putting on more sandbags, drop one a little bit too hard and the swing will break your wing far below it's failure load.

A whiffletree (sic?) is vastly superior; make the segments the correct lengths and slowly start pulling. The time it's loaded is IMHO irrelevant. Do unload it though when people are walking around; if you only modestly bump the spar, when loaded with sandbags it'll fail.

Normally tip deflection is sufficient; for a whole wing you also want to know torsion (attach a lever or measure at LE and TE). Put a continous recording camera on the test for as much data as possible. Take pictures from the tip for the exaggerated deflection/deformation of the spar/wing.

Screenshot from 2013-08-07 15:51:50.png
 

harrisonaero

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Auto, curious, are you testing your own wing or another design?

And I'll echo what you said... sandbags don't relieve the load after failure and can be pretty hazardous. Plus they can (most likely will) cause secondary damage after failure and it's near impossible to tell what broke first.

A whiffletree pulled by hydraulics is more work to design and build but it's worth it for a serious project because as soon as something yields or fractures the load is relieved and the test article can be inspected.
 
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autoreply

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With struts, lifting the wing (and slowly loading it up), sandbags are - reasonably - safe, since the critical load is the combination of compression in the spars due to strut reaction loads, the compression due to the fwd lift components (gravity in this case) plus the bending moment. Don't stand in front of it when loading up the wing...

IMAGE_313.jpg

See the hoist, measurements of the tip's Z and torsional movement and the need for very carefully spreading out the load over the wing. The devil is often in the details and especially safely mounting the wing to a test structure can be pretty complex.
 

opcod

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Obviously it's more easier with a tree setup just like this one.
-=picture=-tree
load_tree.jpg

But with the sandbag we can go directly without the system setup or the crane at the roof. A specific sensor might the the hardest part to find out to do a home testing, at the end...

But with the graph you just joint Autoreply, is the testing facility always do several bend test up to the design load ? For sure, it's more quality and control effective than just 1 static test with the bag. Or maybe it's your test results and you have load your spar several time..?

But for torsion, doing this at home can be problematic, as if the strap or other support of the wing method move. Mine will be just like any sailplane, with pin on the root rib. But having to make a test setup for the whole wing set took longer.. But as using the main fuselage for the load setup that could be the best, but for a low wing we also need to put it higher..
 

autoreply

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But with the sandbag we can go directly without the system setup or the crane at the roof. A specific sensor might the the hardest part to find out to do a home testing, at the end...
Lever? With a fixed mass (500 pounds) and sliding it over a lever, that's attached to the cable, this isn't too hard if you have a support that can take the full force.
But with the graph you just joint Autoreply, is the testing facility always do several bend test up to the design load ? For sure, it's more quality and control effective than just 1 static test with the bag. Or maybe it's your test results and you have load your spar several time..?
Incremental loads, unload via hoist in picture, then apply some more sandbags and lower hoist again. You can apply limit/ultimate load at once. If you're very confident in your engineering data. For sure, some silly detail is going to fail, in which case you still now absolutely nothing. By doing it incremental you have hard data to connect the dots between your theory, engineering and assumptions and the actual failure load of the real-world structure.

For a conventional composite spar, I would personally not be too afraid and test it to 60, 90 and 100% of the required load. The amount of extra work is reasonable. Think through a strategy to keep the load in place though, with wing deflection that is a major issue too with sand bags.



Read this (google translate) for some more inspiration:
DG Flugzeugbau: Bruchversuch der DG-1000

2011-03-14 17:08:51.jpg
 

berridos

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Wouldnt it be more comfortable to test with water containers you progressively fill with water hanging from the wing?
 

autoreply

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The hanging does require a lot of "external ribs", load spreaders and other fixtures. Just as much work as building a whiffle tree.
Two other problems; first of all, you have to hang it very high to have sufficient clearance to the ground. Secondly; with stock 25 liter jerrycans, you might need more wing area than available to hang sufficient weight on the wing. Can't hang them behind eachother.
 

wsimpso1

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I am very doubtful that anyone can properly anchor and load just the main spar. It is so narrow and prone to failure by elastic stability. Simply put, it will roll over and buckle very easily. It hardly needs to twist off at all before it will collapse. Even clearances will hardly prevent the elastic buckling modes.

Much better to just build the entire wing and anchor the main and drag spars as in the airplane. And in most composite airplanes, the wing skins, drag spar, and any ribs will provide tremendous resistance to buckling modes while allowing you to load the wing per your pitching moment calculations, and get the pitching moment induced torsional deflection on the spar as well as the shear and bending...

Billski
 
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