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Thread: Ankle Saver mod.

  1. #1
    cubpilot2's Avatar
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    Ankle Saver mod.

    Ran across a couple of old photos of one example of why I started trying to promote this mod over the last several years.
    This was a wreck of a heavy loaded sprayer in 1974 which lost an engine on takeoff.

    See the following thread post #13 to see what it looks like.
    https://www.supercub.org/forum/showt...-something-new



    It is intended to reduce the collapse of the front bulkhead which can trap you inside. The consequences are obvious, especially if there is a fire.
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    Ed
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  2. #2

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    Has this mod ever been tested? I understand the reasoning but wonder where the force goes and what it does in a modified airframe.

  3. #3
    Cubus Maximus's Avatar
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    I think Jay at Javron adds this automatically to all his frames, it's not an option.

  4. #4
    www.SkupTech.com mike mcs repair's Avatar
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    thanks for the pictures, never seen one......

    you gotta crash just right to do this.... edge case...

    never have added that X....

  5. #5
    cubpilot2's Avatar
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    When I first brought this up about 15 years ago, several people contacted me or responded with their experiences.
    TJ (on this site, who later deleted all his activity) as I recall said that he had two separate friends killed in cubs by being trapped inside and fires started.

    One individual contacted me directly who was recovering from a bad wreck on a beach in SE Alaska. He had to be extracted by rescuers. He had a broken leg and hip. He was certain that this would have lessen the severity of his injury.

    Can it be tested? Don’t think so; but it is definitely in the direction of goodness!
    In my mind it is sort of like adding the box or X brace in the tail. Hard to say what tests would show; but we do know that longerons don’t bend like they did without it.

    I won’t rebuild one without it. Best part is that the Feds said that it is a minor change as it does not intersect the original tubing; therefore altering the original structure.
    Ed
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  6. #6
    skywagon8a's Avatar
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    Quote Originally Posted by cubpilot2 View Post
    Best part is that the Feds said that it is a minor change as it does not intersect the original tubing; therefore altering the original structure.
    Key point: Do not weld or add a small joining tube at the X as this will change the characteristics of the original tubing.
    N1PA
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  7. #7
    nanook's Avatar
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    Seems it would be stronger if it did intersect and form an ‘X’ brace, instead of a straight tube. So we design things to avoid involving the FAA’s scrutiny, instead of the best design. What size 4130 are you using on this diagonal tube?

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    skywagon8a's Avatar
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    Quote Originally Posted by nanook View Post
    Seems it would be stronger if it did intersect and form an ‘X’ brace, instead of a straight tube. So we design things to avoid involving the FAA’s scrutiny, instead of the best design. What size 4130 are you using on this diagonal tube?
    I tend to agree with you. However just imagine all the hoops you would have to go through in order to get an STC or other type of FAA approval to connect the two tubes. It likely would be many more $$$$$ than would be worth your while.
    N1PA
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    FWIW, this is what Backcountry does.

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    Quote Originally Posted by skywagon8a View Post
    I tend to agree with you. However just imagine all the hoops you would have to go through in order to get an STC or other type of FAA approval to connect the two tubes. It likely would be many more $$$$$ than would be worth your while.
    Looks like a pretty easy DER approval. A couple hours writing it up tops.


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  11. #11
    skywagon8a's Avatar
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    Quote Originally Posted by stewartb View Post
    FWIW, this is what Backcountry does.
    On a noncertified experimental airplane. This is not applicable to the OP's airplane.
    N1PA

  12. #12
    cubpilot2's Avatar
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    Quote Originally Posted by nanook View Post
    Seems it would be stronger if it did intersect and form an ‘X’ brace, instead of a straight tube. So we design things to avoid involving the FAA’s scrutiny, instead of the best design. What size 4130 are you using on this diagonal tube?
    i used 7/8” x .035
    Ive considered the X bracing on this but looking at the geometry to bring them together with the existing diagonal puts about 1 1/2 in deflection into it. This tube is to work in direct compression so any offset would weaken it.

    Ive also thought that a clamping type device to the opposite diagonal would add stiffness. (Lessen deflection) This could be as simple as a hose clamp with a spacer between them.
    Ed
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  13. #13
    RVBottomly's Avatar
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    Quote Originally Posted by cubpilot2 View Post

    Ive also thought that a clamping type device to the opposite diagonal would add stiffness. (Lessen deflection) This could be as simple as a hose clamp with a spacer between them.
    I was going to suggest a heavy duty hose clamp and spacer, but didn't want sound foolish. It does sound like a good idea.

  14. #14
    wireweinie's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Key point: Do not weld or add a small joining tube at the X as this will change the characteristics of the original tubing.
    I agree that this could be a minor. But it shows the stupidity of semantics. We are in fact trying to change the characteristics of the original tubing; we don't want them to buckle where they do now.

    The games we play to keep 'them' happy.

    Web
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  15. #15
    cubpilot2's Avatar
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    I wish one of you creative types would figure out some sort of bolt or clamp in fixture for this. You could probably sell a bunch.

    Otherwise it will only get put in during a rebuild......
    Ed

  16. #16
    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by cubpilot2 View Post
    I wish one of you creative types would figure out some sort of bolt or clamp in fixture for this. You could probably sell a bunch.Otherwise it will only get put in during a rebuild......
    very simple... but not worth the weight....but I'll take your money if you wish to hire me....

  17. #17
    www.SkupTech.com mike mcs repair's Avatar
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    if you actually look at the damage, the brace being installed is not as useful as a smaller lighter one in middle of lower side tubes to front top............

  18. #18
    Dave Calkins's Avatar
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    I had thought the same.

    Stabilizing the existing diagonal might be more useful.

    Eds picture above is worth a thousand words. Old Chinese saying.

  19. #19
    SuperCub MD's Avatar
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    The last time I had my boot cowl off I looked real hard at adding that tube. I figured I would x it through the original cross tube, but realized with the offset it may not do much good. I thought about adding it straight with a stub tube connecting the cross, but in a crash the stub tube would probably just break and it would collapse anyway. The only way I thought it would be worth while would be to combine the two ideas. Add the cross tube, and the stub tube at the cross. Then scarf two tubes on to the ends of the added tube that intersect the original tube.

    This would create a truss, not just a added tube which might stand a chance of holding the floor down in a a crash.

    After all this thinking, and looking at the added weight and work involved, I decided that for myself it would just be alot easier to just not land prop first. So far so good.

    Anything you add in that area will add strength, and not a bad idea, I just hope nobody ever puts it to the test
    Last edited by SuperCub MD; 03-28-2020 at 04:47 PM.
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  20. #20
    Randy's Avatar
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    I thought I saw a photo of the tubes joined together here on the site, Bill Rusk's Javron Cub Build?

    EDIT:
    The photos are here, several down from the start/top of thread
    #154 - Modifications with Pictures and explanations - http://www.supercub.org/forum/showth...l=1#post525332
    Last edited by Randy; 03-28-2020 at 07:54 PM. Reason: found pictures
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  21. #21
    skywagon8a's Avatar
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    Quote Originally Posted by Randy View Post
    I thought I saw a photo of the tubes joined together here on the site, Bill Rusk's Javron Cub Build?

    EDIT:
    The photos are here, several down from the start/top of thread
    #154 - Modifications with Pictures and explanations - http://www.supercub.org/forum/showth...l=1#post525332
    Randy, Bill's Cub is "Amateur-Built" and cubpilot2's is not. Bill can do it if he wishes, yet cubpilot2 needs official permission from the FAA. Be careful that this can be misleading some to just modify their Type Certificated airplanes without FAA approval. If this is not made clear some will just modify their Type Certificated airplane because "joe or Bill" did it and it looks good. It may be good. However if the FAA doesn't say so, it is not.

    In my opinion the X may be stronger and thus better if the original Piper diagonal was removed and four lengths of tubing were joined where these two cross into a permanent cluster. Yes the original would no longer be straight. Yet all four joined in one place would increase the compression column strength of each leg. Each leg would stabilize the now offset X joint thus in total increasing the strength in all directions of that bay.
    Last edited by skywagon8a; 03-29-2020 at 04:43 AM.
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    Who’d know if a guy did it? Lots of things are done under the radar.
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  23. #23
    skywagon8a's Avatar
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    Quote Originally Posted by stewartb View Post
    Who’d know if a guy did it? Lots of things are done under the radar.
    Stewart, While we all accept that is so, it sends the wrong message towards promoting aviation safety and legality. It may also create problems for a subsequent owner for some unforeseen issue. Please, let's keep the amateur-built experimental separate from the Type Certificated requirements. They are many uninformed people who do not understand the difference.
    N1PA

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    Everyone understands the difference. If a guy truly believes a tube will improve his safety and the FAA won’t give approval? Don’t ask for permission.
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  25. #25
    skywagon8a's Avatar
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    You sir are not understanding the difference.
    N1PA

  26. #26
    wireweinie's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Randy, Bill's Cub is "Amateur-Built" and cubpilot2's is not. Bill can do it if he wishes, yet cubpilot2 needs official permission from the FAA. Be careful that this can be misleading some to just modify their Type Certificated airplanes without FAA approval. If this is not made clear some will just modify their Type Certificated airplane because "joe or Bill" did it and it looks good. It may be good. However if the FAA doesn't say so, it is not.

    In my opinion the X may be stronger and thus better if the original Piper diagonal was removed and four lengths of tubing were joined where these two cross into a permanent cluster. Yes the original would no longer be straight. Yet all four joined in one place would increase the compression column strength of each leg. Each leg would stabilize the now offset X joint thus in total increasing the strength in all directions of that bay.
    I agree that the X style tubes would be stronger. But the original post was about preventing the floor/lower tubes from buckling and trapping the pilots legs. Could you explain further how the x bracing would prevent this? The energy of the crash would still be there so where would it be transferred to?

    Web
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  27. #27
    skywagon8a's Avatar
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    Quote Originally Posted by wireweinie View Post
    I agree that the X style tubes would be stronger. But the original post was about preventing the floor/lower tubes from buckling and trapping the pilots legs. Could you explain further how the x bracing would prevent this? The energy of the crash would still be there so where would it be transferred to?

    Web
    The two tubes would still be in place doing their jobs. However when solidly joined at a midpoint, each leg's compression column strength would be increased. The external dimensions of each triangle in the truss being less with the same sized tubing would create a stronger section overall. So instead of there being two large triangles forming that load path as on the original, there are now four. I'm sure that a structural engineer could resize the tubing to reduce weight. I'm not that person.
    N1PA
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    SuperCub MD's Avatar
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    Quote Originally Posted by skywagon8a View Post
    The two tubes would still be in place doing their jobs. However when solidly joined at a midpoint, each leg's compression column strength would be increased. The external dimensions of each triangle in the truss being less with the same sized tubing would create a stronger section overall. So instead of there being two large triangles forming that load path as on the original, there are now four. I'm sure that a structural engineer could resize the tubing to reduce weight. I'm not that person.
    I would question if you would gain any strength replacing a straight diagonal tube with a crooked hat section. And the hat sections would point into the cockpit so when they fail, that is where they are going to go. I don't think I would put my name on that approval. You are talking about replacing existing structure with something completely different, not just reinforcing what is already there.

  29. #29
    skywagon8a's Avatar
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    Where did the hat section come from?
    N1PA

  30. #30
    SuperCub MD's Avatar
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    Quote Originally Posted by skywagon8a View Post
    Randy, Bill's Cub is "Amateur-Built" and cubpilot2's is not. Bill can do it if he wishes, yet cubpilot2 needs official permission from the FAA. Be careful that this can be misleading some to just modify their Type Certificated airplanes without FAA approval. If this is not made clear some will just modify their Type Certificated airplane because "joe or Bill" did it and it looks good. It may be good. However if the FAA doesn't say so, it is not.

    In my opinion the X may be stronger and thus better if the original Piper diagonal was removed and four lengths of tubing were joined where these two cross into a permanent cluster. Yes the original would no longer be straight. Yet all four joined in one place would increase the compression column strength of each leg. Each leg would stabilize the now offset X joint thus in total increasing the strength in all directions of that bay.
    Right here

  31. #31
    Gordon Misch's Avatar
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    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.
    Last edited by Gordon Misch; 03-29-2020 at 01:50 PM.
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  32. #32
    BC12D-4-85's Avatar
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    Could adding a 90* offset bridge over the X reduce the potential for buckling?

    Gary

  33. #33
    Dave Calkins's Avatar
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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.
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  34. #34
    skywagon8a's Avatar
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    Quote Originally Posted by SuperCub MD View Post
    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.
    N1PA

  35. #35
    wireweinie's Avatar
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    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
    Last edited by wireweinie; 03-29-2020 at 11:37 PM. Reason: bad speller
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  36. #36
    BC12D-4-85's Avatar
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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

  37. #37
    Gordon Misch's Avatar
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    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.

    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%27s_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/a...ia-d_1328.html
    Last edited by Gordon Misch; 03-29-2020 at 05:46 PM.
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  38. #38
    nanook's Avatar
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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...
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