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Supercub "crashworthiness" improvements

cubpilot2

cubpilot2

SPONSOR
Anchorage Alaska
I'm about finished with all of the repairs / changes to my 18A fuselage and started thinking thru the (what have I forgotten) checklist and the thoughts of "crashworthiness" came to mind.
I have all of the standard stuff with the X bracing, harnesses, seat belt attachment improvements etc. but......

Over the years I've seen many cubs that were involved in relatively minor crashes and have noted that the "front bulkhead assembly" (doorposts forward) is fairly weak, as the lower longerons will often "buckel" upward or fold towards the instrument panel.

One cub that I retrieved experienced this when heavily loaded; lost an engine and was forced down in a pasture. The gear failed (no cables) and when the prop and engine hit they tucked under the boot cowl. The floor boards were buckeled upward to within about 8 inches of the instrument panel. Pilot got out ok but easily could have been trapped. All he got was splinters from the floorboards breaking.

My thoughts are these: Has anyone ever installed an additional diagonal 7/8 in. tube on each side of the bulkhead assembly; routed from the lower front corners and extending to the upper longeron clusters near the panel?
This tube would just clear the "opposite diagonal" tube and should not interfere with any other components (except for perhaps a map pocket)

These tubes would serve as "compression members only" to hold the bulkhead in place in the event of an accident. They would not alter any other structural characteristics of the airframe so they should be "approvable".

What do you think?? Take some shots at it. :9mm
 
Might have some merit Ed..........an old friend of mine that used to spray with 9 cubs told me that the accidents he has had with his pilots over the years involved just what you are talking about. He also said that in most cases it broke either one or both of the pilots legs.
 
crash ?

Cubpilot2,
I have done that very thing on a few cubs that wanted more safety and also for
larger engs. Also a cross tube at the bottom of the dash keeps the cabin from opening
up when the eng. is pushed back. J3 has that. I did a lot of wreck hall outs when in AK
and really looked into what gave up and hurt the pilots in minor wrecks. Wayne
 
Wag Aero shows this mod on their Sport Trainer drawings for engines over 125HP, they use a 3/4 X .035 tube. This was one of the first additions I decided was a MUST HAVE on my Experimental. Just looked like to much of a ankle and leg saver to pass on.

Frank
 
I am just welding up my experimental fuselage now, has anyone got a photo or drawing for this? Sounds like something I would like to do. Joe
 
Joe:
Whats your e-mail address?
I will send you a marked up drawing.

Anyone else while I'm at it?
 
Can you add atlee strut spar beef-ups, tie down straps and top deck X-braces without waiting until rebuild time. I assume you can add the tail brace easily if you have a metal belly.
 
Yes to all.
Minor surgery required for the struts and tiedown brackets.

Moderate dissassembly required for the X brace welding, Windshield, skylite, upper trim panels, headliner etc.

Atlee has a clamp in tail box brace. Easy with metal belly access. Minor fabric work otherwise.

All are very much worthwhile, especially X bracing!
 
X-Bracing

I'm seeing lots of references to X-bracing, but I don't know the lowdown on why. I'm rebuilding my 12, and am interested, but don't know what it would get me.
Gordon
 
Right now you have a tube running from the rt. front wing attach fitting to the left rear wing attach fitting. The X brace adds a tube from the left front wing attach fitting to the right rear wing attach fitting. This beefs up the upper deck in the case of an accident. I was involved in a midair collision in my Piper Clipper which has the same upper deck structure without the X brace. When the rt. wing hit the ground it buckled the rear spar carrythru tube right behind my head and allowed the rt. side of the fuselage to cave into the cabin. I am afraid that if I had a passenger they would have been seriously injured or killed. I am lucky that I went forward and didn't get hit in the head by te rear spar carrythru. I am presently rebuilding another Clipper for myself along with my Dad's fuselage along with a customer's. I have added the X brace under the field approval process. A fellow Clipper owner was unable to get the Feds to approve the modification to his fuselage. I sent him pictures of my damaged airplane to show the FAA inspector. Maybe that will wake him up.

I believe Piper made changes thru the line of aircraft as they gained experience. The later model Pacer fuselages have a V brace in the upper deck as does the Husky. I believe this makes a better structure especially when attaching a shoulder harness.
 
Why hasn't someone made a clamp in X-brace. It does not seem to be a difficult problem to solve. Has anyone ever thought thru this problem? So far, the X-brace has been defended as a necessity very clearly to me. The tail-brace is very questionable to me, since I fly mainly floats and hire a pro when I need off-airport wheels (I gladly pay someone to take the risks with their airplane). Is it only cost effective in heavy tailwheel abuse situations. The wing mods seem like a good situation, but I haven't heard enough horror stories of problems when people didn't have them. Can anyone sell me on the ides other than the X-brace?
 
Ground loop:
Stop by Atlees and look at some of the wing attach fittings that he has on display.
These were damaged due to high winds coupled with tie downs being used at the attach points. This damage may not be detected during your preflight.
Many people will tie their wings while parked on floats. If the wind and big waves hit and the ropes are too tight the stress will start to rip the fittings from the wings.
I was told of a cub on Cambell lake (several years ago) that departed after a bad wind and a wing came off. Fitting was damaged from this very problem.

You sound like a cautious pilot; hiring someone to do the flying in tough situations. It is always best to err on the side of safety in all aspects, while flying and also by trying to eliminating the weak points in the aircraft design. I think Atlee has a genuine interest in keeping people safe, and didn't just create these things to make money. Why else would he offer the X brace STC for free. Anyone else would want a couple hundred bucks.

Just do it..... you'll be glad you did.
 
Field-approval process for the cabin X-brace in the -14, anyway. I got it for the -14 years ago. With the new FA process, not sure how it'd go, but I believe a good IA with decent rapport with his fed could get it easily even with todays field-approval process. I think a clamp-on x-brace would be a cheezy knock-off of the welded X-brace. The welded installation can be handled easily with a little bit of disassembly, and if you have a -12 without a skylight, now's the time to add it and get out of that dungeon.

Definitely go look at Atlees collection of wasted wing fittings. I'm a true believer in both the fitting beef-up plate and the auxiliary tie-down bracket. You'll understand how each works when you see the ruined stock pieces without these items.

I like the additional compression member in the forward fuselage area as discussed by Cubpilot2.

Have you guys recieved an FA for this? Lately??

I think guys don't use their gust-locks enough, especially float guys. I've seen so many cracked flap nose-ribs from flapping flaps in the wind, and a guy won't even know about it. Same goes for rudders. Also, I've been doing a flap-belcrank area inspection opening with a real :D aluminum doubler supported by the flap cove skin and the rear spar. It's nutplated so you never lose your inspection panel screws. It eliminates those pesky walking-round inspection covers in the propwasharea, and also those cracked-out rectangular fabric reinforcement plastics for you rectangular flap belcrank inspection opening guys. Also, if you've seen the new Dakota flap belcrank return springs, I'm sure you'll be installing these from the get-go on new rebuilds.

Also, have been seeing a lot of cracked out butt ribs. The mount tab/angles have been cracked away from the ribs and/or #4 PK screws sheared. Overspeeding the flaps? Flapping flaps in the wind? Passengers using the inboard flap hinge arm for a boarding handle? :)

Something like the rib beef-up part of Wiplines up-gross kit would be a good way to solve this from the get-go.

DAVE
 
wayne and ed; in my wag-aero plans (sport trainer) it shows two tubes that come up from the center in between the bottom engine mounts and go out to the top longerons. then theres a tube accross from one longeron to the other at the bottom of the insturment panel like wayne says. the only thing in my plans it shows these two tubes removable, sortve like if you wanted to crash , you have them in and if you didnt, leave em out, it shows welding sockets at the top and bottom, to make them removable. could it be something for the tank maybe? to take it out? all the tubes are out of the way for the rudder and brake pedals. and adding eds idea it would sure make the front end rigid. in the event. doug
 
cubpilot2 said:
Drawing on the way.
Click to expand...
Ed,
I'm rebuilding a supercub for a fella and he was interested in the modification you were talking about, by adding the diagonal braces to beef up the fwd lower longerons. Could you send me some drawings if possible. My email is danb@acsnet.com. Thanks
Dan
 
Dan:
Drawing on the way. Hope it helps.
I have submitted this for field approval consideration today. Will post results when I know something.
 
OK guys:
Just got word from the Anchorage FSDO today on my application for a field approving the two added "compression members" to the front bulkhead.
In a nutshell they (more then one inspector) reviewed the paperwork and came to the consensus that this could be done as a "minor change".
Basically since the tubes do not intersect the existing tubing; it does not "appreciably" effect the structural characteristics.
They are to send me a written response of their evaluation which will be included as part of the aircraft records. (and mine, for future reference).
Can't get any easier then that!!!
I am all smiles on this; as many can maybe be "a bit safer" without all of the paperwork worries. :cheers

Please be advised that this modification has not been functionally tested nor do I ever intend to! Therefore the exact benefits may be unknown. I advise it only because it looks like the "right thing" to do. We can only hope for the best. Kind of like insurance and ELTs. You have it but never plan to use it.
 
If the brace you are talking about looks like the "seaplane" brace in the drawing on this page

http://www.cubparts.com/pages/fuselage.htm

then yes, I have them field approved in my PA-12.

They are a bit of a pain in that they change the interior shape slightly and make the floorboard fit tighter and access to the boot cowl a bit more difficult.

I read about a very similar install done in PA-25's based on Fred Weick's work at Texas A&M and decided it might be worth the effort.
 
The tubing in the front bulkhead area of a PA-12 and 18 differ. The original diagonals are installed opposite to each other. The diagonal that is added to the 18 is essentially in the same position as the original on the 12, running from the lower engine mount attachments to the upper longeron cluster at the instrument panel. When installed on the 18 and viewed from the side it forms an X bracing, altho they do not intersect each other. There is about 1/8 to 3/16 inch clearance between them.
 
I looked at these tubes on my Dakota 12 last night. Forward of the cluster, the diagonal tubes converge toward the centerline, and are on a different plane than the fuselage sides, when viewed from the top. I'd think the tubes would splay out (where the planes intersect) at impact. You could add a dozen tubes and there would still be a hinge point. I'm referring specifically to a 12.
SB
 
SB
You are definitely correct about it not being a plane from from door post to firewall. Since tubes were required to be bolted into place on some SN PA-12's through that area, I figured it could stand the beefup and welded them into both sides. The tube from instrument cluster to front gear attach will now be loaded with any compression forces applied to the lower engine mount via the new tube, and pass some portion on to the door post, though the new continuation tube.
KL
 
A different take on safety: stronger not always better?

The X-brace is a commonly recommended cub safety modification. In thinking about crashes, something has to give to absorb the energy of a crash. My PA-12 still has only a 3/4 wrap leading edge. I'm not sure when Piper starting making the full wrap or "hard" leading edges. I've seen the installation and they clearly make the wing more rigid and stronger. I've had some strong recommendations to install them during my rebuild. Initially I thought stronger must be better, but now I'm thinking that may not be true in a crash. Increasing the strength of the wing will mean that any crash impact to the wing will transmit more stress to fuselage top deck next to my head. Could this be part of the reason why some many people see the top deck folded down into the cabin area in crashes. Could it be better to leave my weaker 3/4 wrap leading edge inplace to allow the wing to absorb" more of a crash impact and transmit less force to the cabin area.

Is there any consenus out there about whether or not a "hard" leading edge is considered an important safety mod. I know it is required for the Dodge 30.5 gal tank installation, but I'm guessing that is to reduce the wing flexing which might lead to the tanks leak and not for any major structural reason. Is there anything wrong with having a more flexible wing?
 
I was at a crash survival seminar. The speakers suggestion was that the best chance you had of surviving a crash was by hitting the tip of the wing first, which would rotate the airplane around in a circle totally destroying the airplane from the outside in and also absorbing all the energy of the crash. The idea is that the only thing left intact was the pilot. Floats are also known to absorb energy in a crash. BUT, I can't see any logic for not having the strongest possible cage surronding me in the cockpit!
 
The leading edge won't make the difference between caving in the cockpit or not in my opinion. The spars tied together with compression struts and drag and anti-drag wires feels pretty strong. The leading edges do make a big difference when it comes to twisting. Like ground loop stated, I want the strongest structure around me.
 
Well, this ought to provide an opportunity for Steve to throw this thread into the Rant and Rave section:

If you want a more crashworthy airplane, get a Husky. Folks, everything you describe here is a great idea, but it adds weight to the Cub. A little at a time. Personally, I'd add most of this stuff, but....

For those who think the Husky is heavy, this is the primary reason why: The Husky was designed to be a much more structurally stiff and strong airframe, and to protect the occupants in the event of an, er, minor error in judgement by the pilot.

Now for criminy pete's sake, DON'T quote me as saying that therefore the Husky is a much better airplane!!!

Your Cubs get heavier and heavier, just like Frank Christiansen's did, when he created the Husky.

That's not all bad, by the way.

I've seen at least one stall/spin accident involving a Husky, where the rear seat passenger survived, and it was hard for me to believe that he did. There's no way to tell for sure, of course, but I don't think he would have survived had he been in the back of a Cub.

Okay, go ahead, fire away, I've donned my helmet and body armor!!

MTV
 
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