PA-18 lift struts on a PA-12

[autoheart]

Hello all,

New member here. I'm involved in a PA-12 restoration project and am trying to sort out the lift struts. We have the sealed PA-12 struts, but the jury struts are not in the best of shape. Our options appear to be replacing the PA-12 jury struts with new ones ($$) vs converting to the PA-18 configuration. Without sparking the debate about which is structurally better, does anybody have any direct experience with this? What does it involve? Are there any photos of a PA-12 with PA-18 struts (I can't find any)? My apologies if this has been covered previously, but my search turned up nothing. Thanks!

Joel

 

[mike mcs repair]

http://www.crosswindsstol.com

 

[stewartb]

I used PA-18 jury struts on my -12 rebuild because I preferred the way they look and they provided a better attach point for snowshoes and rifle scabbards. The tubes will require cutting to get them to true vertical and that's easily done with a cutoff wheel on a die grinder. You'll need the wings on and rigged to get them fitted correctly and you'll need PA-12 strut clamps. -18s are different. Not a big deal.

 

[flybynite]

Stewartb

Log Entry? 337?

Wayne

 

[bob turner]

I got a field approval to go from J-3 struts to PA-11. One of the few field approvals that took less than a month.

 

[Scooter7779h]

I used PA-18 jury struts on my -12 rebuild because I preferred the way they look and they provided a better attach point for snowshoes and rifle scabbards. The tubes will require cutting to get them to true vertical and that's easily done with a cutoff wheel on a die grinder. You'll need the wings on and rigged to get them fitted correctly and you'll need PA-12 strut clamps. -18s are different. Not a big deal.

Hey, that's my plane now....Cool seeing it on floats. Agree 100% with SB's advice. One more shot of it on the wheel skis.

 

[ChickenCub]

Just curious. Do pa-18 struts have the same A.D as the pa-12 ones?

 

[jrb]

How do 18 jury struts attach to 12 wing? Should it be done when wings are being rebuilt or can it be done on a covered wing? Thanks!

 

[skywagon8a]

Just curious. Do pa-18 struts have the same A.D as the pa-12 ones?

All high wing Pipers are listed on the AD.

 

[stewartb]

jrb, It should be easy to convert the jury struts on a covered, installed, and rigged wing. Fit the new tubes to keep the jury struts vertical with no bow in the lift struts. One horizontal tube, two verticals, and four screws. Personally I wouldn't bother to change out good jury struts. I had to buy new at restoration so I had an opportunity to choose.

wayne, I don't recall how the jury struts were noted after a total restoration. There were lots of details and many weren't major.

 

[Paul Heinrich]

The thread heading is misleading: it should read jury strut, not lift strut. Unless you really wanted to know about putting 18 lift struts on a 12.

 

[jrb]

I need to replace my right jury strut and i can get 2 18 jury struts for the price of 1 12 jury strut. Thanks for the info

 

[reliableflyer]

To the best of my recollection the struts are different lengths. This is due to different widths of the fuselage and because of the different angle of incidence. The jury struts are a different issue.

 

[Bill Rusk]

I am not following this line of thought. Sorry for being a pain, but sincerely asking. Are you guys changing a structural part of the airplane without an STC, 337 etc? Just a logbook entry? I am not disagreeing or saying it is unsafe, I just don't follow how you are legally doing this. Is this an experimental aircraft?


Bill

 

[pa12drvr]

"......and i can get 2 18 jury struts for the price of 1 12 jury strut......"

That paradigm shows up all over the place when looking at -12 parts! :roll:

 

[Lazy jack]

I am not following this line of thought. Sorry for being a pain, but sincerely asking. Are you guys changing a structural part of the airplane without an STC, 337 etc? Just a logbook entry? I am not disagreeing or saying it is unsafe, I just don't follow how you are legally doing this. Is this an experimental aircraft?


Bill

It is still a certified aircraft.

Under CFR 43 Appendix A (a)(viii), Alteration of the following parts when not listed in the aircraft specification including spars, ribs, fittings, shock absorbers, bracing... are airframe major alterations. To the extent that jury struts constitute 'bracing', a 337 would need to be filed by the IA signing it off. Whether the FAA then requests 'supporting data' beyond what is well proven and common to the close cousin pipers, is, I suppose, at the discretion of the FAA.

From this interesting link aopa.org/News-and-Video/All-News/2014/October/22/Aircraft-maintenance-major-vs-minor-alterations-part-2
is pasted the following:

Definitions provided in 14 CFR Part 1.1 state," Major alteration means an alteration not listed in the aircraft, aircraft engine, or propeller specifications

(1) That might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; or

(2) That is not done according to accepted practices or cannot be done by elementary operations.”
And “minor alteration means an alteration other than a major alteration.”

So under this definition, it would seem that a conversion could be done as a 'minor' alteration requiring a simple logbook entry.

Always ambiguous...

 

[autoheart]

Thanks to all for the replies, and especially for the photos. And you're absolutely right, Paul, I erred in the tille of the thread - should have said jury struts, not lift struts. While originality is always attractive, the money saved by converting to the PA-18 style is compelling, since we have to replace them on both sides. As to the 337, that will be up to our IA. Thanks again, and we'll surely be back to the well for more wisdom as we move along.

 

[skywagon8a]

I don't mean to send this thread off on a tangent, but Bill Rusk's question is very valid. Lazy jack's response also is valid. I agree that the labor involved in making this change is extremely simple and seems to be too simple to even have the question come up. However look at the differences between the two airplane's jury strut installations. The -18 jury struts support the lift struts vertically and fore and aft in relation to each other. The -12 arrangement does the same and in addition the fore and loads are directed to the wing spars. So the load paths are different. It would seem to me that at the very least a 337 with a field approval which includes an engineering approval would be required.

The -12 arrangement is structurally stronger. It may even provide some stiffness to the wing spars. Then again maybe Piper "just did it" for change purposes. We need to know why before we sign off this alteration.

Yes I expect that we will hear some folks say "O that is done all the time up here". It isn't the same as leaving the fabric off the landing gear, which in the lower 48 has produced red tags from the FAA.

 

[stewartb]

Huh? Jury struts are there to limit deflection (bowing) of the lift struts. The assertion that the original -12 type are stronger is almost certainly incorrect. The notion that they serve any purpose in fore-aft motion is a stretch. They're nothing more than an intermediate web in a truss. By limiting deflection of the lift strut that strut's capacity is improved. Given that I can pretzel knot jury strut tubing with bare hands I don't rely much on their brute strength and viewing the two styles I'd wager the more modern Supercub style is better suited for the purpose. In any event the FAA inspector didn't hesitate to sign off my -12 with Cub-style jury struts and Lord knows he had ample chance to comment.

 

[skywagon8a]

Huh? Jury struts are there to limit deflection (bowing) of the lift struts. The assertion that the original -12 type are stronger is almost certainly incorrect. The notion that they serve any purpose in fore-aft motion is a stretch. They're nothing more than an intermediate web in a truss. By limiting deflection of the lift strut that strut's capacity is improved.....

That is the type of response which I expected from Alaska. stewart, you are an intelligent person. Think about the load paths of the jury struts on the two airplanes. The -18 struts are in a pinned rectangle. The -12 struts are a pinned triangle. Which truss will collapse first? A triangle is the strongest geometric shape. They both stabilize the lift struts in the same manner except with an unusual fore and aft load. Then the -12 jury struts form a stronger assembly.

The question which Bill brought up is valid. Your FAA guy felt comfortable in signing off your alteration. In fact by signing it off, he verified that it is a major alteration. Thus your 337 is valid.

 

[Gordon Misch]

Think about the load paths of the jury struts on the two airplanes. The -18 struts are in a pinned rectangle. The -12 struts are a pinned triangle. Which truss will collapse first? A triangle is the strongest geometric shape. They both stabilize the lift struts in the same manner except with an unusual fore and aft load. Then the -12 jury struts form a stronger assembly.

Yes, but - - - How do the loads applied to and thus must be transmitted by the jury struts differ between PA-12 and PA-18? I'll offer that they don't. 18 lift struts are a little bit longer, right? If that is so, they need MORE support than 12 lift struts. So 18 jury struts on a 12 are fine. Engineering analysis complete. (until somebody shows that I'm wrong - it's happened more than a few times before!).

 

[skywagon8a]

Gordon,
The shape of the triangle stabilizes the wing spars inboard of the lift strut attach points. A wing will flap up and down pivoting at the lift strut attachments. Thus they do the same at the jury strut attachment point. Now at "high" speed the wing tip may want to deflect (twist) in pitch. The triangular -12 jury struts resist this twisting moment thus increasing the critical flutter speed. The -18 jury struts will not resist this twisting moment.

I haven't checked. Is Vne for a -12 higher than Vne for an -18? IF so this may be the reason.

 

[Gordon Misch]

Per TCDS

PA-18 -150 VNE 153
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/ab203ab0e89895af862572090071f7cd/$FILE/1A2.pdf

PA-12 VNE 148 Utility, 138 Normal http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/45b0fe3b1c17780186256a61006c7931/$FILE/a-780.pdf

 

[bob turner]

I think - I do have some structural background - that all the jury struts do is keep the wing struts from bending in the middle. The metal spar wings simply are not going to move between the root and the strut attach, and if they did, those wimpy lift struts would let them, no matter how good the jury struts.

As to an IA being able to sign a 337 for a major alteration, that is a widely held misconception. No IA can approve a major alteration without approved data, and for a change like this, there are only two sources of approved data: a Field Approval or an STC. Nothing else will do. Apparently a DER can do a Field Approval, but I have had no luck with them.

I do see major alterations signed off by IAs, but those are not worth the paper they are printed on.

 

[skywagon8a]

Per TCDS

PA-18 -150 VNE 153
http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/ab203ab0e89895af862572090071f7cd/$FILE/1A2.pdf

PA-12 VNE 148 Utility, 138 Normal http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/45b0fe3b1c17780186256a61006c7931/$FILE/a-780.pdf

Well, that blows that theory out of the water! I still say that the wing is stiffened in torsion.
And Bob is correct in that the primary purpose of the jury struts is to improve the column strength of the lift struts. However in any structural assembly one has to look at all of the components together. By forming a triangle the -12 juries change the load paths.

 

[bob turner]

But my point is there is no load. As long as the struts stay straight, they are strong enough to do their job. Bend one just a little, and it will collapse under compressive load. Keeping them straight is easy for those wimpy pieces of steel, and triangle or trapezoid will do that just fine.

 

[skywagon8a]

Bob,
You are only looking at the lit strut end of the assembly.

 

[stewartb]

skywagon, did you use old style -12 jury struts on your EX -18? With your stubborn assertions that they're superior I figure you must have.

 

[skywagon8a]

No stewart. I used the -18 jury struts. And I'm not being stubborn only trying to get folks to think a bit about how parts carry structural loads and what may be some consequences. Don't just say "That's good enough" without understanding the consequences. Look around the airport at different jury strut designs. You will see a lot of "V" or "N" arrangements. The square arrangement of a Cub is actually in the minority of designs.

I agree that the difference between the -12 and -18 is very small and very likely will not be a serious consequence by changing from one to the other. But look at the "Whole Picture" not just the fact that they stabilize the lift struts. You do understand that when there are a group of bolts holding a part with all but one bolt being loose, that it is likely that the tight one will be the one to break? Just another example of paying attention to consequences.

The thing which started this discussion was Bill's question of legality. You answered the question of "minor" or "major" alteration. I'm only pointing out that which MAY be a consequence.

 

[stewartb]

Being that I made the mod in question on my own airplane? I assure you that I thought about it more than most.

Carry on.