Wing spar damage

[kiwicubber]

We have a rear spar with some damage to the cap, about 1 foot from the root, less than 1/32” deep and about 3/16” diameter. Looks like a screw from the fuel tank panel was too long and wore away some of the cap.
Can anyone tell me what the limit is for blending out such damage.

Also the damage is on the left rear spar, and we have a right rear spar on hand. Are the spars really handed?

 

[skywagon8a]

What is that little kink to the right of the pen point upper edge? Just a distortion in the photo?

 

[Steve Pierce]

We found the same thing on a friend's Super Cub. I told him it had been flying that way for a long time but what was it worth when he was at gross weight or above in turbulence over Idaho. He decided to replace. $300 or so for the spar and $100 in shipping. I bet the shipping would be a lot more for you. I don't know of anything giving the criteria for blending this area. It is in compression. The only thing that I can think of that makes the rear spar left and right are the holes drilled in the cap to retain ribs and false spar in the tank bay area.

 

[mike mcs repair]

What is that little kink to the right of the pen point upper edge? Just a distortion in the photo?

that caught my eye too... have seen bent rear spars under metal behind tank(flying plane just in for recover, old crash damage!! looks to be within INCHES of same spot on your spar..)the ding damage is on top of spar, and close to root, it's fine, the forces are not on the top of spar there...

 

[bob turner]

Mike has a great point. Very close to the root the forces concentrate on the web and attach fittings. And don't forget the caps have a gazillion holes in them for sheet metal screws holding the leading edge sheet metal.

 

[kiwicubber]

Good spotting Skywagon8a. There is an area of corrosion that has previously been treated there, which would meet the same criteria for blending limits and would be in our consideration for replacing the spar. Starting to add up to replacement.

 

[kiwicubber]

Thanks Steve, yup our shipping more than doubles the price, but still not much compared to having a wing fail. Or worse having our engineer reject it when we have finished. We can economise by shipping in 3 spars in the same box.

 

[kiwicubber]

Thanks Mike and Bob, I guess the web takes the vertical forces and the cap takes the horizontal forces, all supported by the ribs and bracing wires etc stiffening it all up.

 

[skywagon8a]

kiwi,
Do you have access to a structures engineer? I am not one, however in looking at the suspect spot in detail, it is centered on one leg of the "T" between the web and bulb on the cap. The spar is made up of several sections (in one extrusion) forming an "I" or "H" beam. Each section is made up of a web and cap. Generally the center point of a web carries no load. This abraded or corroded spot is in the center of it"s web. I think that it would be worth your while to have someone who knows look at this before you commit to scrapping the spar.

 

[Gordon Misch]

Generally the center point of a web carries no load.

Sorta, but not quite - - A beam, which the spar is, when subjected to a pure moment (bending) will experience no tensile stresses at its center (neutral axis) as a consequence of the applied moment, however it does experience shear stress. If the beam is subjected to axial forces then there will be tensile stress at the neutral axis as well as everywhere else in the cross section. If both moment and axial forces are present, then the stress at any given location will be a vector sum of the component stresses. Google "shear moment diagrams", "combined stress" and "Mohr's circle" if you're interested in details.

This abraded or corroded spot is in the center of it"s web.

This is only useful considering lateral (fore and aft) stresses on the spar. In this application I'd think those would be minimal or near-zero. However in the vertical plane, the spar will be subject to both bending and axial loads. In that case, the corrosion is at the so-called extreme fiber for the beam, a region subjected to shear, axial, and moment induced tensile stresses.

I don't have enough knowledge to truly analyze those stresses, only enough to know that it may not be real simple. I'm perhaps somewhere slightly to the right of the bottom of the Dunning Kruger curve:roll:. I suspect, but don't know, that in close to the hinge point the damage might not matter much. I'm also pretty sure, but don't absolutely know, in reference to the rib screw holes, that the vertical part of the spar cap is to provide transverse (horizontal plane) stiffness which would probably be most important in handling the spars, though would also contribute to stiffness in the vertical plane. Obviously all those little screw hole stress risers aren't detrimental and most likely the minor damage to the spar cap isn't either.

I agree with your assessment that the opinion of somebody well versed in the analysis of this particular application would be very valuable. So is the opinion of those with lots of experience with flaws of this nature. Mike MCS's and Steve Pierce's comments probably apply.

 

[txpacer]

This is why I changed my spar

 

[kiwicubber]

Ouch, good to find before it got bigger

 

[46 Cub]

It looks like the area just below the pen is swelling from corrosion. The part pointed out by the pen looks like it would go deeper if you went for the bottom of it too. If it were only a small indentation from wear it would be nothing. If it is the open part of deeper corrosion you might have a bigger problem. I think it would be worth looking at all of that spar and grinding at any swelling, splits, or open spots like you point out. That corrosion might be prevalent throughout your spar.

 

[SpainCub]

Hum... I would go with a spar change. Piper's design of extruded spars is great and can take a lot more than most people care to consider, but there are limits.

Here is a simulation I ran with the origina Spar design, 0.075" thin web...





Here is he maximum and min of the spar... I's done it with a safety factor of 2x for loads exerted are an as hand calculations and physical test from the FE SW are off " a little " (I should day that it's mostly my laziness to run through a longer workflow to derive a better mesh.)



If you understand what I'm doing, you can clearly see some examples of where lightning holes could be added. 8)

Does anyone have the weight of a Spar Blank at hand? This was calculated a 12lb 4 oz. Adding the steel attachments with it up to 14lbs all together. Note I made a slight change to the strut attach point... hence the extra weight.

Bottom line is, I would change the spar, considering that you like to fly say for a long time and not feel like jelly´knees if you run into some turbulence... BTW, picture is exaggerated and displacement is something like 3 mm at gross weight. Easy to get to 3200 lbs out of these spars, maybe even more with just something like 4 lbs of extra hardware. This simple study leaves the door open to a single strut design ala Luscombe and maybe even folding wings.

 

[Steve Pierce]

This is why the Piper clones with lightening holes place them outboard of the struts.

 

[Gordon Misch]

Spain, could you please identify your input parameters? Are the colors tensile, shear, combined? Thanks for sharing the simulation!

 

[mike mcs repair]

if you want to see the forces, just put a wing on with no leading edge and trailing edge, and no jury struts, then lift tip, quite impressive how much they bow down at the just strut location on rear spar.... those jury struts are important...

 

[SpainCub]

I was testing for 3500 lbs + 2.0 safety (Gross on the plane ) , I still get 0.98 safety factor off the design. I was been lazy with the mesh, the better mesh to work with x1.5 safety factor is too much work to the workflow. In the end, I'm with in 99% of excel calculation. mostly because of the centroid, integration of all these parts are, well, lots of work. The Spars and the attach points are key and my end goal was not to answer the OP, just had is sitting o CATIA for my single strut design ala Luscombe, adding what they reference as a Jack Strut. I believe that the GlassStar Sportsman 2+2 uses the same to tie forces to booth spars.

 

[SpainCub]

This is why the Piper clones with lightening holes place them outboard of the struts.

Agreed Steve, Some of them, unless they are adding extra HW, they seem to be borderline for my personal sanity. I have another simulation that provides a better means to reduce the material while maintaining the original strength that is you trust the math, is just as strong but as light as you can get with in the parameters adjusted for. To me, extruded spars give you the best bang for the bucks for a production part, lower manual labor, they might be a little more expensive for a low production run.