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PA 12 jury struts

S

Skip

Registered User
Savannah Ga
I have seen some 12`s with jury struts going from the spars down to a point between the 2 lift struts and some with a jury strut going to each individual strut. Which is best and why?
 
The vee type are standard PA-12. The others are Supercub. Some PA-12 rebuilds incorporate PA-18 jury struts. I did it because it makes a better tie-off position for rifles and backpacks.
 
I agree with Stewart. If I had to replace jury struts I'd go with the -18 style, if my IA were to find it acceptable as a minor alteration.
 
Other than personal preference and Stewart's use as a cargo rack, why did Piper change from the two verticals to the triangle? The purpose of jury struts is to stabilize the long unsupported lift struts by cutting the unsupported length in half which increases the compressive strength. The -18 method only supports the struts in a vertical direction. The -12 method supports the struts both vertically and sidewards. Did Piper have a structural reason or was it strictly as part of their overall streamlining effort? It is not uncommon to see struts bent forward from people pushing on the struts when moving the plane. Does the -12 triangle installation resist this bending? I do see a strength/load path reason for the "V" on the -12.

I would need more information before I would feel comfortable changing from a-12 to a -18 installation with a "minor alteration" entry in a log book.
 
Good point, Sky. I don't see flight loads being an issue because of the streamlined tube's much larger moment of inertia in the fore/aft bending direction. Pushing on the struts might be a different story indeed, though if the arrangement is certified on an -18, how would it not be suitable on a -12? For my plane, pushing on the struts is a no-no in any case.
 
The last PA-12 was built a couple of years before the Supercub was conceived. Why didn't the Supercub adopt the vee style?

Obviously they both work. There's a clear advantage to the Cub style for tying stuff on. And to my eye it looks better. Being that I can pretzel knot the jury strut tubing with little effort I don't think either style is worthy of being a push point. Choose one and move forward.
 
Gordon,
What about the twisting loads of the wing? I can visualize the -12 arrangement stiffening the wing and allowing a higher Vne and a better flutter resistance. By improving the twisting stiffness of the spars inboard of the strut attach point, the section of the wing outboard of the strut attach point is stiffer. The -18 arrangement does not have this twist resistance built in.
 
That is true stewart, however post #1 is asking why. Why is inviting further thought and discussion. Thought and discussion is sometimes a good thing. If it brings possible safety issues to light, it just might eliminate a problem which may not have been addressed. Your point is valid.
 
The V style has an adjustable end on the top end to position the lift struts correctly. It also seems to be stouter. I assume changing is for tying things omor when peopl install 18 wings. As I recall the V material is quite a bit heavier in construction. I'm partial to 12's anyways. A slightly different breed of cat.
 
skywagon8a said:
Gordon,
What about the twisting loads of the wing? I can visualize the -12 arrangement stiffening the wing and allowing a higher Vne and a better flutter resistance. By improving the twisting stiffness of the spars inboard of the strut attach point, the section of the wing outboard of the strut attach point is stiffer. The -18 arrangement does not have this twist resistance built in.
Click to expand...
The twisting loads are taken up by the spars transferring to the struts, and that structural path is a LOT stiffer and stronger than the jury strut path, it seems to me. If that's correct then the jury strut configuration would not have a meaningful effect. The -18's success would lend support to that hypothesis. Just an opinion - - -
 
The main roll of a jury strut is to hold the strut from bowing in the middle... to keep the load, as much as possible, a compression load on the struts when pulling negative G's so they don't buckle. The minor roll is to keep the struts from vibrating in flights.. and to strap show shoes too !
 
Gordon, This makes for an interesting Saturday morning discussion. Considering that all of the joints are flexible. Grab the wing tip at the ends of the spars. Twist the wing in the nose up/down direction. Which would be more flexible, the -18 or the -12? I think that the -18 would be more flexible because the jury struts would move vertically. The -12 jury struts would try to rotate with the wing twist against the major axis of the struts.

Yes irish you are correct, however there may be a little more to it.
 
Thinking to much make my head hurt, then I have to go lie down
 
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Sky I'd guess that any difference between 12 and 18 with the twist test you mentioned would be small enough to be unmeasurable. Reason being, that the spars and struts are very rigidly triangulated, and therefore the spars can't move at the strut attach point. Any twisting of the wing panel would have to be either outboard or inboard of the strut attach points. Twisting the wing at the ends of the spars would tend to make the wing twist in the opposite direction inboard of the struts and in that case the -12 jury struts would resist that, but I suspect the actual deflections would be miniscule.

Additionally, that would be different than aerodynamic loading. Aerodynamic twisting loads would be the same direction both inboard and outboard of the struts. So any aerodynamic twisting moment outboard of the struts would be balanced by aerodynamic twisting moment in the same direction inboard of the struts. So the jury struts would be like the Maytag repairman - nothing to do.

The problem is much more complicated than I want to tackle analytically, but my guess is that all the jury struts have to do is keep the main struts from waggling around, like Irishfield said.

Again, nothing more than opinion about an idea that's fun to bat around! Stewart, your admonition about overthinking can certainly be correct, but for those of us with thick glasses, pocket protectors, and mismatched socks this is just fun stuff to think about - nerd candy!
 
the jury struts other job is to stiffen wing... try some time throwing a wing on without leading edge skin, flex up and down at tip, with and without jury struts... scary how easy the rear spar flex....

had been taught when patching up for ferry that those little brackets jury struts attach to in wing break, and was needed to be sure they were addressed before ferry....
 
skywagon8a said:
Gordon,
What about the twisting loads of the wing? I can visualize the -12 arrangement stiffening the wing and allowing a higher Vne and a better flutter resistance. By improving the twisting stiffness of the spars inboard of the strut attach point, the section of the wing outboard of the strut attach point is stiffer. The -18 arrangement does not have this twist resistance built in.
Click to expand...

There is obviously only one way to prove or disprove this theory.Dive a 12 until the wings come off and make note of the speed at which this happens. Now repeat this test in an 18 and you'll have your answer. With Gordon's background in physics, he would be the perfect candidate.
 
Very interesting, guys. I changed from the "V" type 12/14 jury struts to the 18 type because of an event that destroyed my lift struts and very nearly destroyed a wing. I was tying down in the "corner" of my L-shaped ranch-style house thinking that I was really safe from the Wasilla winds. Well, the wind burbling over the roof had a serious downward pressure that I had not accounted for and it broke both lift struts causing the wing tip to be about 18" down toward the ground by the time I caught it. (I did not have the heavy duty lift struts yet - so it might not happen today.) Anyway, the bottom of the vertical "V" arms of the 12/14 jury strut simply became a pivot point on the horizontal cross member between the large and small lift struts and the front end failed with the front strut in an upwards direction and the back end failed with the back strut in an downward direction. As I recall, the front strut failed inboard of the jury strut cross member attach point and the back lift strut failed outboard of the jury strut cross member attach point.

The new heavy duty lift struts went on immediately, and the PA-18 style jury struts went on the next time the wings needed any work. My logic may well be wrong - but I see the direct lift-strut-to-wing attachments on the PA-18 styled of jury strut as being able to prevent that sort of pivoting action in the horizontal cross member at the bottom of the "V" in the 12/14 style jury struts that could allow inordinate flex in the lift struts. How thinkest thou?
One of you in this thread put them on for me - so maybe you ought to disclose such in the interest of avoiding any "conflict of interest." :lol:
 
This is an example of a jury strut installation which utilizes a combination of both the -12 and -18 style.
Paint022.jpg

What does this do to the spar bending loads? I would NOT use the -12 jury strut for this application.
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Gordon, A friend built a second set of wings which had more span outboard of the strut attach point for his Acy Ducy. This was for improved take off and climb performance when on floats which was accomplished. I noticed that when diving to a speed above normal cruise that the tips shook in a mild flutter mode. As I recall he had -18 style jury struts. I'm wondering that if he had -12 or "N" style jury struts would that "flutter" been dampened?
 
Sorry Gordon, I couldn't help myself. It is an interesting subject but if the truth were known I'd bet you engineering had nothing to do with it. It was done as a way of somewhat setting the 12 apart from the rest of the flock. Styling. Aftre the 12 was discontinued they had no reason to keep building this strut assy. Theoretically the V would have to be superior in reducing twist but I think the advantage would be nearly insignificant.
 
It's worthy of note that the -18 was not a successor to the -12. The "ancestor" of the -18 was the -11 and the J-3, and the -11 was in production concurrent with the -12. Using the jury struts from the J-3 and -11 on the -18 may simply have been inertia. The -12 was a "dead end" in the Piper line. Unfortunately....

MTV
 
Skip said:
I have new sealed struts but mine has the v type jury struts
Click to expand...

You'll probably need new strut clamps to fit the new struts. If you decide to use PA-18 style jury struts and buy PA-18 parts expect to trim to length. The -18 verticals will be too long. It's a simple project. Much simpler than the thread may lead you to believe.
 
PerryB said:
Sorry Gordon, I couldn't help myself. It is an interesting subject but if the truth were known I'd bet you engineering had nothing to do with it. It was done as a way of somewhat setting the 12 apart from the rest of the flock. Styling. Aftre the 12 was discontinued they had no reason to keep building this strut assy. Theoretically the V would have to be superior in reducing twist but I think the advantage would be nearly insignificant.
Click to expand...
No prob - I'll look for an opportunity to be similarly impaired!8)

I bet you're right re the jury struts. I was looking at a Tripacer today and noticed it had no jury struts at all. Shorter struts, of course, but it supported the notion that the jury struts don't have to do a whole lot - they probably function mostly to add stiffness to the main struts rather than directly strengthen anything.
 
Do the 18 struts clamp on or will I have to drill the new struts and add a threaded fitting, I don`t remember how the 18`s attach. I may just make new strut clamps and leave it as is with the new struts
 
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