Light wing build

Broken down and rebuilt a lot of wings from the mid 1940s and up and have never seen corrosion issues with the stainless fittings. The fittings that attach the drag/anti-drag wires to the spar are pretty malleable and can be adjusted if you get the wrong one in the outboard bay where the wires are longer. 4130 is harder and I guess my concern would be vibration and cracking at the corners. I am not an engineer and respect what Piper did, the robustness and longevity of the design. I would do some serious calculating before I changed a major structural component.

Its experimental

Steve Pierce said:

Broken down and rebuilt a lot of wings from the mid 1940s and up and have never seen corrosion issues with the stainless fittings. The fittings that attach the drag/anti-drag wires to the spar are pretty malleable and can be adjusted if you get the wrong one in the outboard bay where the wires are longer. 4130 is harder and I guess my concern would be vibration and cracking at the corners. I am not an engineer and respect what Piper did, the robustness and longevity of the design. I would do some serious calculating before I changed a major structural component.

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These guys are having trouble getting the stainless Steve, maybe they should come to New Zealand to get some where between the dairy and marine industries it is available in buckets. :lol:

These are experimental aeroplanes; I would not like to see these guys discouraged from anything, provided they've gone through a process to get there. Gee, one guy that frequents this forum even built his Super Cub without any fabric on the back of the fuselage!

With the greatest respect, I would counter that 4130 is a very tough material and vibration should not be a problem. That is why we build engine mounts out of the stuff. Polish the edges, like I said in an earlier post. The Pitts wings I have did over 1100 hours of very punishing aerobatics. All the fittings (including the drag/anti-drag wires) are made out of 4130. No cracks.

Cheers,
Andrew.

I agree but take everything into account. Pitts wings are a wood structure where a Piper wing is aluminum. Different things flex in different ways. I am all for experimenting, I am just saying don't decide more is better just because. Think long and hard about everything in the system. When you modify one thing it usually effects several more. There were some after market Luscombe struts made several years ago. Luscombe used mild steel and these were built from 4130. The fittings started to crack because 4130 couldn't be formed the way the mild steel was. Just an example.

Gentlemen thank you for discussing this trivia. This is exactly why I started this thread. I wondered why Piper built these out of stainless. Price and availibiltiy are secondary at this point. The finished fittings are less than $150 so not really a consideration. Safety, longevity and ability to claim these parts as "home built" are my primary concerns here. I hope to include notes from this discussion in my blog for others in the future to examine when searching drag wire pulls or wing builds. Steve has me re-thinking again the stainless issue. I too have not seen problems in the boat world with stainless against aluminum as compared to steel against aluminum. Stainless gets "crevice corrosion" if it is not exposed to oxygen so it's still a concern wherever it is used. Stainless wire crimped in a "standing rigging" fitting will "rust" just like any other metal inside but it becomes invisible since the exposed surface looks fine. Prop shafts left sitting and not "dripping" in a stuffing box where water with it's oxygen cannot get to them will develop corrosion inside the box (the standing trapped water will give up it's oxygen to the metal and then the corrosion starts). Apparently you have not run into that Steve under the fittings against the spar web. Stainless is a "cheesy" material to work and my least favorite metal to machine so if I finally decide to use the original material as spec'd by Piper I'll just buy them already made. I know that bending it into position is easier than regular steel but the stuff really work hardens quickly when you machine it. This makes me think corrosion must be the real reason Piper used it because vibration should trash this stuff quickly. If you interupt a cut while threading you are likely to brake a tap or lathe cutting tool when you try to start again. Nasty stuff IMHO.

Acording to the Brutons round tie rod data in Aircraft Spruce a #6 rod has 1,000 lbs minimum breaking strength. I put my barnyard engineering to use and am using an AN-170 cable eye end with a ms21042 nut and 1/8 cable for my drag/antidrag wires. Lightweight, not too expensive and easy to make to any length. The breaking strength of the cable eye is supposed to be 1600 lbs.

Be careful here....Wire rope is less then half as STIFF as a solid tie rod.....It's Probably not a good idea to use cable for anti-drag wires......
DaveG

1/8 stainless cable has a breaking strength of 1760 lbs. Since it is only loaded in tension it should be really strong. If I remember right aircraft cable has to flexed and subjected to many thousands of load cycles and then it is tested for breaking strength. I know some of the older Rans S-7s used 3/32 cable for thier tail brace wires. They really take a beating back in the prop blast, never heard of any failures. Anyway I appreciate your thoughts on this.

Too heavy for me when you figure in cable, eyelets and turn buckles.

Cable

Steve Pierce said:

Too heavy for me when you figure in cable, eyelets and turn buckles.

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And too expensive!

DaveG said:

Be careful here....Wire rope is less then half as STIFF as a solid tie rod.....It's Probably not a good idea to use cable for anti-drag wires......
DaveG

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Dave I bought the rod but aren't these wires always in tension? If it's "give" you are concerned with isn't that a good thing? Steve once the wire lengths are set using turnbuckles, the wires can be pre-fabricated to the proper length (make a jig) . (The following is conjecture on my part) You would install the wires first and then "force" everything else (compression struts) into position . Wires give a little adjustment with a half twist either way but they do have to be "set" before final rigging. It's my opinion they would be a superior way to do the drag/anti drag wires potentially stronger, tougher ( a little flexible), and less shock to surrounding parts during turbulance. They are naturally spring loaded... BUT I am going with rolled thread PMA'd rods. Go figure...

It's not a strength issue. 1/8th cable is plenty strong. It is a stiffness issue. Using cable will allow the wing to deflect inplane (rack/paralellagram) more easily. This will put added load on everything from the wing ribs to the covering.
DaveG

I put my barnyard engineering to use and am using an AN-170 cable eye end with a ms21042 nut and 1/8 cable for my drag/antidrag wires.

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I did the same thing. Probably about 2 lbs heavier than stock and there's no way the rod gives any compressive strength especially with the way it's attached.

Wow, this is a great thread, and those are some beautiful wings! Are there any plans available to build a SC wood wing with flaps? I was thinking of building me a set... sort of a long term project... I was thinking of building a light cub with 100-120 hp.

Cables.

qsmx440 said:

Dave I bought the rod but aren't these wires always in tension? If it's "give" you are concerned with isn't that a good thing? Steve once the wire lengths are set using turnbuckles, the wires can be pre-fabricated to the proper length (make a jig) . (The following is conjecture on my part) You would install the wires first and then "force" everything else (compression struts) into position . Wires give a little adjustment with a half twist either way but they do have to be "set" before final rigging. It's my opinion they would be a superior way to do the drag/anti drag wires potentially stronger, tougher ( a little flexible), and less shock to surrounding parts during turbulance. They are naturally spring loaded... BUT I am going with rolled thread PMA'd rods. Go figure...

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I don't think I'd go about it this way....... you will want those drag/anti-drag wires adjustable. The accepted way of assembling a wing is to put it all together and then square it up using the adjustment in the drag/anti-drag wires. I think you'd be hard pressed to make thimble end cables that accurate. The thimbles tend to elongate as you proof load test them as well.

People are using them for drag/anti-drag wires as shown in this link: http://www.homebuiltairplanes.com/forums/tube-fabric/8339-j6-karatoo.html

Cables are widely used as tailbrace wires, at least on lighter aircraft. The topic has come up on the Bearhawk forum and even Bob Barrows, the Bearhawk designer, who they say is a real scrooge and an advocate of lightness and simplicity advises that it is satisfactory. I used to fly an Issacs Fury that used cable for the wing flying and landing wires; that aeroplane is nearly 40 years old now and is still flying around. (I guess no one told the aeroplane it can't be done!)

The usual objection raised to using cable to replace tie-rods is that the cable with stretch. Engineers involved with cable manufacture have told me this is a myth. I'm not going to wade into this debate myself........

In this application, I would consider using swage end cables with the plain threaded terminals. Then you have basically replicated a tie-rod functionality wise. You could then use the Piper nipples, clevises and square washers to connect to the wire pulls and have the required adjustment. You may want to use Tri-Pacer ones which are 10-32 'cos I'm not sure there is 6-32 cable hardware. Of course if you were going down this path, you may want to dispense with some of that Piper stuff, there will be other ways of doing this. But the point is that structurally, cable would be an acceptable substitution.

Go for it!
Andrew.

You have to do quite a bit of adjusting at each bay to square. I had to go 7 turns on the wings I jsut assembled. The Legend and Back Country Cubs both use compression struts like Piper used in the tank bay. You just have to be real precise.

I'm attempting a 1320# gross weight super cub

"Ron Normark" owns and has rebuilt his three Super Cubs, all of them are light.

One, a 1950 PA-18 with flaps, balanced tail, non-electric, one wing tank, Lycoming
O-235 ..... is 850 lbs. empty.

He says it's his best flying cub, and it would be a nice light sport pilot SC.

Maybe we could get Ron to respond to your thread and give his rebuilding details.

I imagine an O-200 would be even lighter.

I would like to hear more about his builds. Does anyone know what the modern equivalent of a 53S-W rivet is? It is used to hold the "plug" in the compression struts. I don't want to use bolts.. Shear would be the defining parameter.

I weighed a wood spar this afternoon, standard Super Cub dimensions; length set up for wood tip bow. I only had a rear spar and it came to 9.5 lbs. Calculating percentages, a front spar from same spruce should be about 13-1/4 lbs. Any idea how that compares to the aluminum spar? Wag lists the gross weight of the 2+2 with this wing at 2200 lbs.
Marty57

This thread is just another example of the knowledge in this group. Really interesting. As far as cable being used in "squaring" aircraft rigging, remember that most aircraft up into the thirties were braced with cable. The Jenny had flying and landing wires that were cable back woven and soldered. Once rigged, you usually don't have to touch it from then on. The trammeling cross braces on my Travel Air are made from rolls of "Hard wire", hence the term hard wire bracing. The wire is simply loped through a turnbuckle eye, bent back over and run through a wound ferule and bent back. The compression members are just ribs made out of 1/4" solid spruce rather than the normal 3/16" spruce. They also glue a diagonal piece of 1/2 or 3/4 (I can't remember) square spruce lengthwise to strengthen the rib and that's it. I guess what I'm trying to say is there are a lot of ways to skin a cat when it comes to maintaining wing squareness.

qsmx440 said:

It is used to hold the "plug" in the compression struts....Shear would be the defining parameter.

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The compression members are only there to keep a preload off of the ribs. In other words, the two spars are two sides of a box. The trammel wires are there to square the box but in order to square the box you have to draw the wires in an appropriate way to produce that squareness. However, when you draw the wires in you put in inward pressure on the spars. The compression members are there to hold the spars apart, creating the final two sides of the box. Properly done, there should be no preload or compression of the ribs. Also, there really shouldn't be any "Shear" on the plug in the square aluminum Piper compression member. The bolt which goes through the spar is basically just a pin. Some Pipers didn't even use steel bolts. The plug is just there to hold the bolt (pin) in. Once the square tube is pressed against the spar web, there shouldn't be any shear load between the tube and the plug. The Stearman, for all its load carrying and G pulling capability has aluminum bolts holding the compression members in. A Pitts (I believe) has wood compression members and drills holes through the spars to run the trammel wires through and tightens the nut on the other side of the spar against an angled wood block. The Travel Air even has hard wire bracing in the fuselage bays as does a Jenny. Wires or cables or anything "crossed" in a trammel bay is not designed for compression in any way, only tension. The cross braces like Piper's tank bays are both compression and tension. You could build the entire wing with either. And if you have a stressed skin wing, wood or metal, you don't need internal trammeling or Vee wing struts at all (The Maule is a long story). Chris and I could build our wood SC wings (actually Wag 2+2) with wood compression members if we wanted.
Many different ways to do the same thing and they all work. Good discussion. Sharp group!
D.A.

Compression members also help keep spars from twisting. Serious Clipped Wing Cub guys used to double the ribs for the same purpose. They figured out a compression strut at the top and bottom of the spar worked better to keep from twisting during aerobatics.

Steve Pierce said:

Compression members also help keep spars from twisting. Serious Clipped Wing Cub guys used to double the ribs for the same purpose. They figured out a compression strut at the top and bottom of the spar worked better to keep from twisting during aerobatics.

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I did not know that! You're a wealth of information Steve. I'm glad I tuned in tonight. Thank you sir!

Too many hanger discussions with the old guys. They should write a book because this stuff gets lost when they go and we all have to learn it all over again.

Marty57 said:

D.A.
I am building a wood wing for my 2+2/PA14, same as Rogers but with flaps.Marty57

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Roger Peterson said:

I think wood spars and ribs are about as strong as you can get and yes I would go wood on a experimental SC.

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Marty/Roger, do you guys remember how much plywood it took to make your wing ribs? Chris will be driving over next month and I'd like to have the jig built and material on hand to give to him. I've got the cap strip figured out but I'm not smart enough or rested enough or some other excuse I haven't thought of yet to figure out the plywood. I'll only need aileron bay ribs except for the tip rib which will be full length. You guys remember?
Thanks in advance.
D.A.

Steve Pierce said:

Too many hanger discussions with the old guys. They should write a book because this stuff gets lost when they go and we all have to learn it all over again.

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We are writing a book Steve. The reason I believe the rivet through the plug is in shear is that besides holding the plug in position it does double duty holding the triangle shaped bracket that resists twisting of the spar therby transfering the twist load to the compression strut through that rivet (that duty is shared with the aluminum re-inforcing angle and bolt through the far side of the spar web. My understanding is that "twist" is the failure mode of the spars.
I need to ask again. Piper used an aluminum rivet for the plug. It's call out number is 53S-W. Several hours of searching the net for the last couple of days did not turn up any reference to that number. I did finally find a AN470AD6-18 which is the 3/16 X 1 1/8" correct size but don't know if it would have the required strength. I am aware everyone else is using bolts and nuts for this application and if I have to I'll do the same but I would like to use rivets..

D.A.
I'm not sure how much ply exactly. One of the reasons is that I screwed up on my spar thickness and had to remove the ply and and glue on another piece. I remember having to order a couple of times from ACS; maybe 4 sheets total???. I started out with two or three pieces of there biggest shippable (UPS) ply; 2' x 4' maybe? For the nose pieces (the big "D"'s) I actually cut out paper patterns for all what, 50 or so pieces and laid them out to figure out best material usage. So, sorry about not remembering exact amounts. Remember, you have to make a lot of those little gussets. I can help you out with your wood spars though. I got my Spruce just around the corner from you, found it in Forks Washington at a little mill; might still be there. I milled my own spars from their stock and some of the cap strips. I found out that the cap strips from ACS are a deal, good price and saves a lot of time. Here's the mill in Forks.
Marty57


Marty57

Rivets

qsmx440 said:

We are writing a book Steve. The reason I believe the rivet through the plug is in shear is that besides holding the plug in position it does double duty holding the triangle shaped bracket that resists twisting of the spar therby transfering the twist load to the compression strut through that rivet (that duty is shared with the aluminum re-inforcing angle and bolt through the far side of the spar web. My understanding is that "twist" is the failure mode of the spars.
I need to ask again. Piper used an aluminum rivet for the plug. It's call out number is 53S-W. Several hours of searching the net for the last couple of days did not turn up any reference to that number. I did finally find a AN470AD6-18 which is the 3/16 X 1 1/8" correct size but don't know if it would have the required strength. I am aware everyone else is using bolts and nuts for this application and if I have to I'll do the same but I would like to use rivets..

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Just so you get an answer!

Don't use an AD rivet, the force required to drive it (at least if you've got an original Piper strut and end fitting or plug) will surely ruin the parts! The piper rivet is a very soft one. You could use an A rivet, which are the soft ones, if you could find them in that size. Or find a commercial one somewhere.......

I've done mix and match on those plugs or inserts or end fittings whatever we want to call them and drag struts and secured them in place with Cherry rivets. Two each in from each side (i.e. four total). I can be sure that is way more stronger than the original soft rivet. I have a recollection that a PMA drag strut I bought from Dakota Cub was done the same way. I think they look kinda neat and tidy done that way.

Now, I betcha Steve Pierce will leap in now and comment, one way or the other!

Cheers,
Andrew.

I've got some Univair compression struts that came out of a Husky wings. I will look and see what they use.

I have riveted them before.Someone here suggested using a plumbers torch to soften the rivet prior to shooting it. I don't agree. I used a big rivet gun and bucking bar. Don't remember the rivets though but I was thinking they were AN470AD rivets.

Resaerching 43.13 section 4-58-a it talks about using rivets to splice round tube (should equal square tube for proplems) and they give an example of "buckling" of the rivet in the unsupported center of the rivet (fig 4-12) so that is a real possibility (as mentioned in the post above) . They say to not form the normal round head but "only hammered enough to form a small head" whatever that means and the rivet has to be 1/8 of tube diameter (this square tube is 7/8 so rivet min is 1/8 but speced is 3/16 so meets the requirment) . I will get extras to practice on. I am making the plugs from 6061 rod so it would be possible to leave some material to support the rivet but then that defeats the "rivet vs weight of the bolt and nut" thing. Steve thanks for checking on the rivet type. I'm working from those early 50's drawings of the L-21 and don't have the later drawings. It's possible they spec a modern rivet in those. OR Piper joins the dark side and uses bolts and nuts in the later builds. Are you having a seminar this year?

It's been awhile since I've looked at the Piper compression strut but if memory serves me well (sometimes it doesn't) the plug in the end filled (virtually) the square aluminum compression strut and was flush with the end. So once you've got the square tube/plug against the spar web, there shouldn't be any shear between the square tube and the plug because they're both the same length and both bottomed out against the spar web. Way back when I looked at those compression members and thought about how I'd rivet the plug in, I figured I'd put a solid plug in, shoot it with whatever rivet happened to be closest to the bench and then drill and tap the plug afterwards. The plug just holds the square tube in place until you trammel the wing. At that point it couldn't go anywhere if it wanted to.