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Ankle Saver mod.

cubpilot2

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Anchorage Alaska
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/showthread.php?54416-PA-18A-Restoration-Something-old-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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Has this mod ever been tested? I understand the reasoning but wonder where the force goes and what it does in a modified airframe.
 
thanks for the pictures, never seen one......

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

never have added that X....
 
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.
 
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.
 
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?
 
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.
 
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.


Sent from my iPhone using Tapatalk
 
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.
 
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.
 
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
 
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......
 
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....
 
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............
 
I had thought the same.

Stabilizing the existing diagonal might be more useful.

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