Bungee questions

[Columbus 30]

Morning-

so, I am about to swap the bungees on my MK IV with the wide stance gear. I’ve done my research and homework yet still have two issues.

i ordered the shock cord from AS and what came in is ALL black, not flecked. It does seem relatively stiff (as compared to Ace hardware stuff) but I’ve nothing to compare it to. AS says, both in the catalog and when Inspoke to the rep on the phone, that it’s for landing gear...thoughts?

Secondly, with the slick outer casing, should I up the number of wire wraps? Would too many be detrimental and possibly cause premature failure by cutting into the cord? 

Suggestions and advice GREATLY appreciated

[ChrisBolkan 130]

I think you bought the wrong stuff. You want P/N 06-12400. 

http://www.aircraftspruce.com/catalog/lgpages/shockcord.php

[Columbus 30]

Thanks! This is what I got when I ordered over the phone

http://m.aircraftspruce.com/catalog/lgpages/bungee10mm.php?clickkey=6282

when it showed up, I called and she read to me over the phone and I looked it up in the catalog - I didn’t think it was correct!

appreciate the help

[ChrisBolkan 130]

As for actually terminating the ends and winding the bungees there is LOTS on this site to make the job easy, but you have to spend some time searching the various threads. Very well worth the time though. Saves you having to re-invent the wheel! :-)

[chopndrag 226]

How many feet does it take to do the landing gear ? I forgot how much I ordered a couple years ago amd want to change mine soon . 

[MnAvidFlyer 34]

Just finished redoing my bungees. The Avid Flyer gear requires 90" between the loop holes so add 6" to each end to make the loops.

If you order 18' you will have about a foot left over. If you do it right there will be 6 loops on the top and 7 loops on the bottom.

I pre-made my loops before installing and did the 0.040" safety wire wrap to secure the loops  with shrink tube over the wire. There is no way I could have wrapped the safety wire in the plane.

I came up short on both legs so I used a alignment bar to roll the bungee over the gear leg and over the fuselage tubes.  Postition the bar in the middle of the bungee and once it is through roll the bungee over the bar or leg. Starting at the beginning and working bottom to top, bottom to top etc...Got enough slack to attach the bungee! Whew!

It's not easy and you need a helper. I worked from the top and my son fed the bungee up from the bottom.

My seat truss is modified so it looks different than most.

Good luck, Brooks

[Avid Bart 39]

You're making progress! I remember you saying someone reinforced the seat truss. Did you have to add the bolts to hold the bungee ends?

[MnAvidFlyer 34]

A previous owner added the bungee bolts to the seat truss. They cut the ends off the bungee hooks and wrapped a steel plate over the truss tubes. Then drilled a hole through the plate and installed the bolts.

It seems to be strong and I got the wraps to layout nicely with some planning.

I kept the loops shorter because the first and last cord has to stretch as much or more than the middle cords. I didn't want to over stretch those 2 cords.

Pic is of the previous owners installation  the bungees. Only 54" long 1/2" cord and hose clamped together. The cord wasn't mil spec either. How it did not break, I don't know.

The stretching of the old cord was very inconsistent.

Brooks

[MnAvidFlyer 34]

In the old pic you can also see the metal seam and the rough welds of the 2 piece truss plate. I didn't want the new cord cut by them so I blended the seam and the welds.

I also spread a layer of epoxy over the top of the truss and sanded it smooth. It provides a smooth transition for the bungees to ride on the top of truss plates.

Brooks

[Avid Bart 39]

I see the original builder left one thread showing. Inside the lock nut. Nice touch.

What is mounted behind the seat truss with the tubing fittings? Fuel pump?

[MnAvidFlyer 34]

Those are the hydraulic brake master cylinders! Another issue I need to deal with. Working on coverting it to toe brakes. Looking for some compact master cylinders ~6" long. I may fabricate the Kitfox style rudder/brake pedals.

I don't know how to manage the stick, throttle, and brakes with only two hands. Plus dealing with differential brake pressures with two levers in one hand.

This setup may work with runup and taxi but not 15 knot crosswind landings.

Brooks

[TJay 1041]

Easy just take someone with you. You can be like those putt putt race cars where one guy steers and the other guy runs the throttle 

[Avid Bart 39]

Just remember, the guy on the throttle and brakes needs to be blindfolded 

 

Brooks, let me know if you need to take a look at how mine are set up. They are the larger diameter matco master cylinders on the toe brakes. Sometimes it helps to see it in person before you start fabricating.

[Columbus 30]

My first loop. Still have to shrink wrap it but I wanted the forum’s opinion if it looks ok? Too tight or too loose- thanks. I’ll wait to see what the consensus is before I move forward 

[NorthIdahoAvidflyer 374]

I searched for instructions on how to wire tie these ends. Can anyone lead me to an article or video on how to properly do this, All I found was some goofy bungee jumper tying up eyes with cloth thread. 

[Columbus 30]

Search on this thread and you’ll find a TON.

[1avidflyer 1983]

I do a double wrap, grab it tight in with plyers, pull it tight and give it a full twist or so.  That locks the first two wraps in place.  Then go two more wraps and pull it tight and twist it like you would a safty wire.  Roll the wire over so it can't poke anything and then tape the wires good with electrical tape.  It really isn't going to take a real lot to hold the loose end in the loop if it's larger then the wire wraps that it would have to go through to get loose.  JImChuk

[Allen Sutphin 705]

Just a thought, has anyone tried or measured the standard GA shock cords as used on Pipers and T-crafts and such. They come in several different diameters and lengths and different strengths.  I've installed dozens of them and have yet to see one break on a hard landing. Never installed any on Kitfoxes, though.

[1avidflyer 1983]

I don't think any bungees break on the Avids or Kitfoxes, but in a hard enough landing, I think the extra shock provided by reaching the end of the slack in the safety cable buckles the seat truss.  Also, the horns on the seat truss that the bungees hook onto probably wont fit on anything larger than 3/8" bungees.   JImChuk

[FredStork 453]

I don't think any bungees break on the Avids or Kitfoxes, but in a hard enough landing, I think the extra shock provided by reaching the end of the slack in the safety cable buckles the seat truss.  Also, the horns on the seat truss that the bungees hook onto probably wont fit on anything larger than 3/8" bungees.   JImChuk

"I think the extra shock provided by reaching the end of the slack in the safety cable buckles the seat truss." I can confirm this is true... Been there, done that (even documented in a film - "I had a dream").
If the safety cable is there to protect the prop, by breaking the seat struss instead, it is a bad calculation... particularly now when we have found the magic and inexpensive NR-props...
I have removed the struss breaking safety cable and installed a 2 wrap horisontal rater stiff bungee between the legs:

It only comes into play at any greater deflection and hardly at all at normal landings. I have had at least one landing that could compare to when I (the safety cable) broke the seat struss and had no issues, no prop strike or other damage. 

[FredStork 453]

Those are the hydraulic brake master cylinders! Another issue I need to deal with. Working on coverting it to toe brakes. Looking for some compact master cylinders ~6" long. I may fabricate the Kitfox style rudder/brake pedals.

I don't know how to manage the stick, throttle, and brakes with only two hands. Plus dealing with differential brake pressures with two levers in one hand.

This setup may work with runup and taxi but not 15 knot crosswind landings.

Brooks

 

Brooks, this is actually an interesting brake setup... do try a toe brake conversion but don't throw away this stuff... you might want to give it a second thought one day!
Normally when braking your engine will be on idle so no throttle to manage and the throttle hand will be free for braking. I think you will find it easier to brake evenly with the two levers in one hand than with the feets that at the same time will control the rudder (or tail wheel depending on speed). 
And yes, the KitFox pedals are better than the original Avid pedals, actually any pedals are better...

[MnAvidFlyer 34]

I have removed the strut breaking safety cable and installed a 2 wrap horizontal rather stiff bungee between the legs.  It only comes into play at any greater deflection and hardly at all at normal landings. I have had at least one landing that could compare to when I (the safety cable) broke the seat strut and had no issues, no prop strike or other damage.

I really like the secondary bungee setup. I was thinking of doing something similar to lessen the load on the seat truss in a hard landing.

The off road race trucks use a hydraulic damper on their bump stops to gradually slow the suspension travel rate when bottoming out. It reduces broken parts. Unlike our limit cables which try to immediately stop suspension travel at full travel.

Thanks for sharing!

Brooks

[FredStork 453]

I have removed the strut breaking safety cable and installed a 2 wrap horizontal rather stiff bungee between the legs.  It only comes into play at any greater deflection and hardly at all at normal landings. I have had at least one landing that could compare to when I (the safety cable) broke the seat strut and had no issues, no prop strike or other damage.

I really like the secondary bungee setup. I was thinking of doing something similar to lessen the load on the seat truss in a hard landing.

The off road race trucks use a hydraulic damper on their bump stops to gradually slow the suspension travel rate when bottoming out. It reduces broken parts. Unlike our limit cables which try to immediately stop suspension travel at full travel.

Thanks for sharing!

Brooks

...and I’m still thinking about a way to add dampers to the original Avid landing gear... Maybe adding a cabane « V » to support 2 dampers...

[Turbo 162]

Your wire wrap looks great, but may be twice as long as needed.  Note that this type of bungee cord can stretch fully 100% of its original length.  Slippage of the shock cord inside the wire wrap will occur if the cord is not stretched sufficiently when wrapped, since the long end will shrink in diameter when stretched.  If stretched beyond the condition at which it was wrapped, it will slip, shrinking the eye.   

Fred double-wraps his ends; I think this is a good idea.  I built a wooden gantry with 6:1 purchase using pullies, which allowed me to re-rig the bungees without help.  I shied away from heat-shrink, and just used electrical tape.

[FredStork 453]

Your wire wrap looks great, but may be twice as long as needed.  Note that this type of bungee cord can stretch fully 100% of its original length.  Slippage of the shock cord inside the wire wrap will occur if the cord is not stretched sufficiently when wrapped, since the long end will shrink in diameter when stretched.  If stretched beyond the condition at which it was wrapped, it will slip, shrinking the eye.   

Fred double-wraps his ends; I think this is a good idea.  I built a wooden gantry with 6:1 purchase using pullies, which allowed me to re-rig the bungees without help.  I shied away from heat-shrink, and just used electrical tape.

I did a 50% pre stretch (starting with 20 cm pulling it to 30 cm) before applying the wrapping and yes I did a double wrapping. I don't think it is needed to prevent slipping but if one wire breaks hopefully the other still holds.Have been using them for over 3 years and done some pretty bad landings without seeing any degradation in the wrappings or tendency to slippage.

This little tool makes it really easy to make the wrapping. I would say say the time to make it is almost gained back when making the second set.

A little neoprene glue add to the friction between the cords. 

A simple cotton band glued with textile glue protects the wrappings. And a shrink rapping gives a nice final touch as well as protection.

...and I just made 5 new sets for Avid pilot friends here in France...

I have also made a new set of "horisontal-between-the-landing-legs-bungees-to-replace-the-seat-struss-breaking-safety-wire" using the same wrapping method. I think wire wrapping is less agressive to the bungees than the metal clamps.