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Ankle Saver mod.

Pete, Supercub MD is correct. Buckling theory is all about exploiting offsets from straight. Here's more than you'd ever want to know about that! https://www.continuummechanics.org/columnbuckling.html

Folks sometimes make the mistake of thinking that a single reinforcing tube intersecting a column mid-span effectively shortens the length of the column. It can a little bit, based on it's own resistance to cantilevered bending, however the assumption ignores the potential for buckling perpendicular to the plane formed by the original tube and the reinforcing tube.

Bottom line, it's not as simple as it may first appear.
 
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stabilizing the “column” increases its buckling/compression strength. Notice I placed a period behind that statement.

Imho as a “farmboy”. ....and also a hack bicycle mechanic.
 
Right here
A hat section is generally a piece of flat stock formed like the shape of a top hat with a brim. Tubing does not form a hat section.

Gordon, The two diagonal tubes as described in cubpilot2's pass offset from each other. If they were cut where they cross and then joined in an X cluster. The offset portions of each tube would join in the center balancing each others offset stresses. As I said, I am not qualified to do the actual stress analysis. You know better than I.
 
Let me restate the question I sent to Pete.

The original post talked about mitigating floor and tube buckling to stop trapping the pilots legs due to the original design. If cubpilot2's diagonal is added, what effect will this have on floor buckling? Will these effects change if the new tube is connected to the original diagonal or not? The energy developed during the crash will not 'go away', so where does it get transferred to and how does this path change with the added tube?

These mods always bring me back to old guys telling me about beefing up Cub gear legs. The old school small tube legs would buckle during a crash landing. Sometimes they could be straightened and braced for a flight home. Usually they could be be replaced, even in the field. But with the new heavy duty legs, instead of buckling, they get swept off with the gear fittings. This means welding on new fittings. So the energy that used to buckle the gear legs now pulls the fittings off the fuselage tubes.

I'll stand toe to toe with anyone arguing electrical design but I'm no structures guy, so explain these questions in basic English.

Web
 
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I owned a Cub that had experienced the buckling during a fatal crash. I was told (but don't recall) that there were tubing replacements and strengthening done during the rebuild. Most side work was recovered by fabric and interior metal paneling. The only residuals were the slightly twisted rudder bars that I never had straightened.

Gary
 
Well, a couple things here.

First, I won't pretend to offer a valid analysis in a crash situation. Who can predict just what the geometry and energy of an impact will be?

Second, Yes Dave and Pete, any ol' support will help stabilize a column. My point though, was that it's not valid to just assume the effective column length is reduced by half with any ol' support.

Third, Slenderness of the column matters for the mechanism of failure, i.e. buckling / crushing / combination.

Fourth, the critical buckling load for slender columns varies inversely as the length squared. i.e. double the length and the critical load is 1/4.

Fifth, the critical buckling load varies directly as the fourth power of the diameter of a thin tube. i.e. double the diameter and the critical load is 16 times as great.

So the easiest way to increase buckling resistance is to increase tube size. And keep the column straight. On the PA-12, the firewall brace tube runs from the bottom of the firewall upward and aft. I may be mistaken, but I think on the PA-18 it runs from the top of the firewall downward and aft. Personally, I think the PA-12 arrangement is preferable for crashworthiness.

Finally, yes I've experienced the firewall folding back over my foot. Happily, just a dislocated big toe. :smile:

Edit: If you feel like playing with numbers while you're stuck at home, the basic formula is here. https://en.wikipedia.org/wiki/Euler's_critical_load And the formula for "I", the transverse moment of inertia, is in the middle of the page (Hollow cylindrical cross-section) here. https://www.engineeringtoolbox.com/area-moment-inertia-d_1328.html
 
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Why not weld two half sections of the next diam up 4130, into an X with a square stock of the proper dimensions between, to provide a clamp on brace? You would not be welding the tubing together, just the X brace, spaced to where you would clamp on to the diagonal tubes. This would work for certified aircraft...
 
If it means anything, this probably shows the FAA has already approved the addition of the new tube and the stub that ties the two together though maybe in a roundabout way. They may not have even known it was an add-on over a stocker but regardless, it made it through the certification process. Does this compare apples to oranges? Maybe but not really. It is an -18 with many of the same size tubes as a stock -18. Evidently Airframes thinks enough of the mod to add it to this 4 place. Do they do this to their standard or bush mod Cub Airframes? Apparently not as a regular item as shown in the lower photo. That airframe delivered late 2019 or early 2020. The green 4 place shows the two tubes joined together where they cross just below the “Certified” sign. A little hard to see but it’s there. The other two pics ( upper and second) are PA-12, one stock, one with the other tubes added, for comparisons sake.

Maybe this raises more questions than it answers but it shows there’s some activity going on that’s somewhat “new”, fwiw.

Oz
 

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OzAK, The FAA may have approved it for this particular application. That does not mean you can copy it on your airplane and consider it approved.
 
Sky,
Roger that, I absolutely understand that fact! Merely pointing out some examples of the two tubes having been tied together at the cross as was being discussed earlier this year. Like I said, evidently someone thinks it has merit and there seemed to be some speculation as to if anyone had done it... as always, for what it’s worth.

Thanks!
Oz
 
wide body.jpgnarrow.jpegI built a Javron wide body (experimental) and it had the extra tube. I also built a Javron narrow body (experimental) just recently and it did not.

Greg
 

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