Re-Rigging a Fly Baby

The key dimensions here are the gap between the "ears" of the shackle--"B", and the diameter of the hole for the clevis pin "A".

Notice that "A" for both the -61 and -80 shackles is 0.375 inches, or 3/8".  This is perfectly correct for the Fly Baby, as a 3/8" clevis pin joins the shackle to the 1/4" axle plate.

But... look at "B".  This is the maximum thickness for the axle plate.  The -80 is 0.266" (which fits the plate) but the -61 is 0.203.

What the heck.  Pete specifies the -61, but it's too tight to slide over the axle plate!

From the picture, notice another difference between the two:  The -61 shackle is generally thinner (as you would expect), and leaves a bigger "hole" in the middle.  This gets important later.

I don't know how Pete handled this.  The plans (Figure 2-5) state the axle plate is 1/4" or even 3/8", but Pete is clearly showing the AN115-61 shackle here.

My airplane has the -61 shackles...but it does slide over the axle plate.  I don't know if the original builder stretched the shackle (which would be tough to do, and kind of gives me the willies thinking about what kinds of stressing this might do) or, at some point in the past, -61 shackles with a 1/4" gap had been available.

Kinda shrugged it off, at least initially.  Wanted to use the -80 shackles anyway.

Until another issue arose....

Shackle Crowding

OK, my intent was to install two 5/32" cables (to the front anchor plates) and two 1/8" cables (to the rear plates) on each wing, using an AN115-80 shackle.

Being a cautious type, I cut off some cable, and formed some eyes using the standard AN100 thimbles, using the -80 shackle.

Oh-oh.  There wasn't enough ROOM to put two AN100C-5 thimbles (for the 5/32" cable) and two AN100C-4 thimbles on the shackle...with or without the cables?

Look at the picture on the right.  The thimbles just won't fit on the shackle.  Even scarier, this *isn't* the -80 shackle... it's the -61, with a bigger hole in the middle!

One possibility here is that the shackle size may have actually physically been changed since Pete built N500F.  I've got leftover turnbuckles from N500F.   They're AN130-16S turnbuckles, but they are SMALLER than modern AN130-16S turnbuckles.  I don't mean longer, or threaded differently, but the barrel itself has a larger diameter on the modern-production units.

Maybe something similar happened to shackles in the ~60 years since the first Fly Baby was built.

Thimble or No Thimble

Ok, how to fix that.  How about leaving the thimbles off, and just looping the cable itself around the shackle?

This is controversial.  The specs for the nicopress are based on the thimble being present.  Most sources will tell you that the thimble MUST be used.

But...a number of Fly Babies loop the cables around the axle shackle WITHOUT the thimble.

Look at N500F in the Museum of Flight restoration center in Everett, Washington.  You'll see it doesn't have thimbles on its shackles.  Several other Fly Baby owners state they didn't use thimbles.  Tom Staples flew his Fly Baby quite actively for 30+ years with no thimbles.  As a caution, he replaced his flying wires every five years, but never found an issue.

The cross-section of the thimbles are broad, ensuring no points that might concentrate stress.  It's also smooth, so there's nothing to dig into the cable.  Finally, this isn't buried within the aircraft structure...it's right in the open, and one can inspect it thoroughly before each flight.

It's a decision that has to be made by every builder or owner.  I opted to not use thimbles.

STILL Not Out of Trouble

At this point, I was going to use two 5/32" cables and two 1/8" cables on the -80 shackle.

Being the cautious guy I am, I did another mockup of the shackle area.  And it STILL wasn't resolved... 

The two pictures with the red backgrounds show sample sections of the cable looped around the -61 and -80 shackle.  See....they STILL don't lie right.  Notice on both how the cable loops are riding up over each other.  This is an obviously source of stress and problems, so I wasn't willing to do it.

As of this point, I decided to go with the stock system:  four 1/8" 1x19 cables on each shackle.

The image on the right is interesting...it shows one wing's shackle and cables that I removed from the aircraft this summer.  Four 1/8" cables and thimbles DID fit on the shackle!

...but it is the -61 shackle, not the -80 shackle.  And not the arrow, showing where the stress on the cable actually bent it a bit.  Not a nominal situation.

So...the decision is made:  Rig the airplane dead stock, with 1/8" 1x19 cables.

The Anchor Plates

The decision did relieve one other aspect.

Turnbuckles to go with 1/8" cable have a fork that takes 3/16" clevis pins, those for 5/16" cable take a 1/4" clevis pins.  So, just drill the holes in my anchor plates to 1/4"?

No can do, amigo.  The centerpoint of holes must be at least twice the diameter of the hole from the edge of the metal.  And my anchor plates were drilled for 3/16" holes, with the edge margin to match.

Going to a 5/32" cable setup would have required new anchor plates, at least in the front.  I toyed with this, thinking I could upgrade the plates to 0.125" steel vs. the stock 0.090".  As it happens, my old horizontal/vertical bandsaw (bought about 40 years ago) can't handle 0.125" 4130 steel.  Finally decided to use the stock plates.

Yeah, but my plates were drilled for THREE turnbuckles.  Wouldn't it look funny leaving one spot open.

I'm the fourth owner of my Fly Baby, and the previous builder had done the three-turnbuckle conversion.  When I bought the plane, he gave me the OLD plates he had removed.  I rummaged through my drawer of old Fly Baby stuff and found the old plates.

Clean 'em off, repaint, and reuse.  While I was at it, I did the same to the aft plates as well.

Turnbuckles

Selecting turnbuckles was interesting.  Earlier this year, when I had been planning to use 5/32" cables, I needed turnbuckles to match.  This, normally, would have meant buying AN130-32S turnbuckles, rated at 3200 pounds, instead of the -16S turnbuckles on the stock bird.  However, I hate wrapping safety wire around turnbuckles.  I decided to use the MS2125X series turnbuckles, which use a small clip to safety, instead.

A small issue arose.  The stock wing-bracing cables on a Fly Baby are 1/8" 1x7, which are rated at 2100 pounds.  However, the stock turnbuckles are AN130-16S...which are rated at 1600 pounds, about 20% less.  I wanted turnbuckles to more-closely match the rating of the cables.

For AN turnbuckles, this is impossible...you've got 1600 pound units, and 3200 pound units, and nothing in between.  Besides, as I mentioned in the last section, the bigger turnbuckles would have required brand-new anchor plates.

But...the MS series has a magical alternative.  They sell a fork end, rated at 2400 pounds, that uses a 3/16" clevis pin AND is compatible with the -5 body (3200 pounds).

One drawback of the MS series turnbuckles is that there are not designation for "standard configuration" turnbuckles.  An AN130 turnbuckle has one fork end and one cable end, and there's no equivalent designation in the MS series.  You have to order the specific components to assemble your turnbuckle.  Here's the breakdown.

Forming the Cable Eyes

Now, here is where it gets fun.

Remember a few sections back, where I said it was practically impossible to form cable eyes in the 5/32" 1x7 cable?

Turns out to be very difficult to form the eyes in 1/8" 1x19 cable, too.  At least, just using your hands, you can't form the cable tight enough to wrap around a thimble.

What the Sam Hill?

The obvious alternative is to use 7x19 cable instead.  It can easily form the eyes.

Biggest problem is strength.  The 1x19 cable is rated at 2100 pounds, the 7x19 cable at 1760 pounds.  Plus, the 7x19 cable is a bit stretchier...obviously not the best solution for wing bracing.  Pete even says, "Control cable has considerable stretch to it and should not be used for wing bracing." (Plans, Chapter 9, page 9-2/9-3 depending on version).

Yet Pete blithely specifies 1x19, with NO mention of how to form the eyes.

Why?  Because I think Pete had an easier time with it, due to the availability of a specialized tool.  Pete lived most of his life in Seattle, a maritime town if there ever was one.  There was a specialized tool for ship rigging, that would grip the cable and squeeze it down to form the eye.

I was offered the loan of such a tool.  Probably would have made it easier.  But, instead, I solicited advice from various folks and used their information to form the cable eyes.

First step was to form the cable eye that would include the eye end for the turnbuckle.  These eyes would include thimbles.

The basic solution required a vice and a "push stick."  I clamped the nicopress swage in the vice, and tightened it down just enough to hold the nicopress sleeve firmly without compressing it.  Then, the cable is led through the sleeve, a big eye is formed, the thimble and turnbuckle cable eye is inserted.  Then grab the free end of the cable with one hand, pull like crazy, and push the loop end of the cable with the push stick until the thimble bottoms out on the sleeve.

Then... KEEP PUSHING on the stick, and use your other hand to tighten the swage to form the first compression of the sleeve to hold the whole assembly.  Obviously, this would work better with two people.

Here's a schematic view of the process.  You can see the recess on the end of the push stick that accepts the loop end of the cable and allows you to push.  I used a handheld swage in the vice for this.  You can use a full swage (the kind that looks like a bolt cutter) but found that it was much more convenient tightening the smaller swage with a wrench than reaching over and trying to close the arms of the big swage.

After the first (center) compression is made, loosen the swage and reposition the sleeve to compress the end closest to the loop.  Often, if the cable hadn't been fully nestled in the thimble, this compression would close it down.  After that, loosen it again and do the third compression.  Here's what the final result looks like:

Now, this was for the FIRST eye on the cable.  Dimensions aren't critical at this point; feed enough cable to be sure you have plenty to form the other end.  For which the length IS critical.

Forming the Shackle Loop

Now comes the second-to-last act:  Forming the loop that goes over the shackle.  At this point, of course, the cable has to be at the exact length (given the turnbuckle's ability to adjust for minor differences).

One thing working in my favor was my decision to not include thimbles on the shackle end.  A 1/8" cable loop on its own will slide on over the open end of the shackle, but a cable loop with a thimble is too wide.  So I didn't have to continually attach more and more loops to the shackle, and haul around the whole assembly for test fitting.  The cables could be done individually, and slipped onto the shackle at the aircraft.

So, how do you determine how long each cable has to be?  I used a pretty simple system:  I took a length of 20 gauge electrical wire, and crimped a 3/16" terminal on one end.  Used a clevis pin to hold it to the anchor plate, drew the wire to the axle, passed one end down through it, pulled it tight, then twisted the wire around itself to form the approximate loop.

You know, it worked pretty slick.  Each pair of wires is close enough to the same length, but (of course) the front ones are shorter than the back ones.  Also, on my airplane, the anchor plates were located just a bit differently.  So a cable made for the right wing was a bit too long for the left!  So do a pair of these gauges for each wing.

Forming the last, critical loop in the cable required a big jig.  I took a pine board about ten inches wide and eight long (cut it off to seven feet) and installed a sort of clamp for my swage tool.  Then I took my gauge wire, slid it through the jaws of the swage, positioned the loop just to the right of the swage jaws, then drew the other end tight.  I then parked where the 3/16" terminal hole was, and drilled a 3/16" hole into the board.

Next step was to add the rest of the turnbuckle to the eye on one of the prepared cables. I cranked the turnbuckle so that the threads were *barely* hidden (need adjustment ability both ways), ran two pieces of heat-shrink onto the cable, then ran the end of the cable through the nicopress sleeve and formed a loop.

Pulled the free end away and formed another loop using a bit of rubber tubing.  Stick a template into the loop (mine was a piece of scrap plywood about 1.25" long).  Pull on the temporary loop while feeding the cable around the loop template and through the sleeve, then tighten the nicopress swage when the loop is the right dimension.

The temporary loop does give you good power, but you do have to be careful to keep the part of the cable going to the turnbuckle from going slack.  It's going to want to, and you'll need to pick the loop with your fingers to help feed the cable to the other side.

Do the other two nicopress squeezes, and you're ready to try it on the airplane.  Interestingly, on the first try, it was a bit too short.  Was getting ready to do it again, then thought about it and tried it on the OTHER wing.  Perfect fit.

Ideally, you'll do the aft cables first...because if they're wrong, you can cut off the new loop and there's still enough cable to work on the FRONT anchors.  But I was too eager.  Ended up rebuilding two cables.

Here's a shot of the two left-wing front cables in place.  Note that the shrink wrap is in place on the upper right end (turnbuckle end), but the axle end still has extra cable and the shrink wrap isn't done.

Here's a close-up of the axle shackle, after the extra cable has been snipped off and the shrink-wrap heated.  And yes, the clevis pin at the shackle hasn't been safetied yet.  Sheesh.

Notice that the four loops are packed tightly, but are not climbing over each other.  This is right out in the open, too, so one can run their fingers over the loops during preflight to detect any cable damage.

My shrink wrap was slightly too big; it doesn't get so tight it can't be moved.  This does give me the chance to move it out of the way if I want to inspect.  Also, you can see how the shrink wrap has the bulge end when it gets to the end of the free part of the cable.  I actually pinch this during preflight, to assure myself that the nicopress hasn't slipped.

Adjusting the Rig

Before safetying the turnbuckles, I needed to get the turnbuckles adjusted so that both wings were at about the same angle.

First step was to adjust the landing wires (the ones on TOP) of the wing so that both wings had the same dihedral angle.  Simplest way of doing that, I felt, was to pick a reference point on both wingtips and adjust the landing wires so that the two reference points were the same height above the floor.

The next step was to get the wings at the same incidence angle.  Here, again, I cheated a bit.  I marked a point on the trailing edges just outboard of the notch for the ailerons, and adjusting the aft landing wires so that both wings were the same.

Now, why didn't I use a fancy level app on my phone?

Because I couldn't get repeatable results.  Every time I moved the phone it got into a slightly different position, and read slightly differently.  I was hoping to adjust things within a half-degree or so, but every time I moved the phone, the actual angle would vary in doubles of that.

So I did the measurements and figured it'd be close enough to be flyable.  I'd just tweak out any wing heaviness later.

Safetying the Turnbuckles

The MS series turnbuckles have grooves on the inside threads on the body, with matching grooves on the exterior threads of the turnbuckle ends.

When using the little safety wires, you adjust the turnbuckle so the groove on the body is matched by the groove on the turnbuckle end.  There are two grooves in the body, but only one on the ends.  The groove on the turnbuckle ends is "continued" as a polished area.

This polished area is sometimes difficult to see.  I'd recommend dabbing a Sharpie on it and making it more obvious.

It can be a bit difficult to get the body and the end lined up.  And yes, the turnbuckle can ONLY be safetied at angles of 0 and 180 degrees.  So if you're the kind that wants a quarter-turn more tension....just wrap the turnbuckle like an AN series one.

Once the pin slides all the way in, pop it into the center hole.  You'll hear a definite click when it goes in.  Tug on it, it shouldn't come out except with the aid or a screwdriver or pliers.  Each end of the turnbuckle needs a clip, and they can go in the same hole or in opposite holes.

Did I figure this all out myself? Not hardly.  Here's a Good Video showing how its done.

Test Flight

Took the plane on a brief "crow hop" after the work was done.  Back to the hangar for a thorough inspection, then take off for a single trip around the pattern.  Inspection, still no issues found.

Flew for a couple more hours, and realized the plane was slightly right-wing-heavy.  Undid the safety clips for the aft turnbuckles on the right wing, tightened the turnbuckles one and a half turns, then re-safetied them.  Perfect balance, now.