L.G. bungee blues


8 posts in this topic

Posted

I was in the process of replacing my bungees, and thought I'd do a stretch test with the old ones to see how bad they were, and check out the engineering behnd their use as the landing gear springs.  In my stretch test I used a fish scale and tape measure to determine the spring charactrristics, Force vs. Length.  I zip-tied one end and looped the other around the fish scale's hook, then stretched it with a boat winch.  What resulted was a sideways S-shaped curve, showing what anyone who has used a slingshot knows: that a rubber spring eventually stops stretching at some load, even as load is increased.  The Hook's law spring constant is high at the low-force end, but also becomes much higher at the high-force end, and is lower, but slowly increasing in the midrange.  Weird, but likely related to  the molecular structure of rubber.  

Knowing that a spring's stiffness is inversely proportional to length, I then applied the stretch data to the loop geometry of the Avid's landing gear.  It's convenient that in re-wrapping the bungees, the force required is totally manageable, due to the 7 wraps used.  Of course this means we are applying a preload.  At this point we are stretching the bungees to 132% of their no-load length.  Of greatest interest to me was the realization that the bungees stretch up to 172% of their no-load length.  It is here that the bungees stop stretching, even as force is increased further.  Half of the bungee-centerline wrap distance is 8.5", and with 90" no-load length and 7 wraps (14 strands) each strand's no-ĺoad length is 6.43".  Stetching stops at 172% of no-load length, so full extension should be reached at 3.4", not the 3" prescribed in the build manual!  The safety cable is only there to prevent total collapse, but if it's shorter than full extension, you risk bending &  crimping the thinwall tube under your seat; the tube the bungees wrap around, if you land too hard.  The bungees wouldn't be able to absorb the energy before the merciless cable comes into play!  That cable applies a point load to the tube, not distributed like that of the bungee.  

So those are the results of my analysis.  Am I nuts?  I'm sure others in the group have thought thru this as well.

Making up my new bungees, a special thanks is due to Fred, who showed an elegant approach!

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Posted

Oops!  In my enginerdly focus I forgot that a loop or two lie on top of others,increasing the preload and likely nulling out my 0.4 inch greater maximum deflection I got over the build manual.  It is important, however, that the cables allow full stretching of the bungees first, before coming into play.  I would consider the 3" deflection allowance as a minimum, and make sure my cable-stops were a little longer.  They were too short on my bird.

And yes, I did hard-land my bird.  It's in the weld shop getting fixed.  Uggh!

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Posted

The seat truss is a weak spot on the Avids and Kitfoxes.  At the very least, you should infill as much of it as you can with plywood.   Being as how it's in the shop, and near a welder, this may be a better approach.  I had 3" wide x .040" steel available.  That's why it looks like it does.  I wouldn't surprise me if over half of the Avids have had some reconstruction on the seat truss.  JImChuk 

New MK IV 045 (Large).jpg

New MK IV 046 (Large).jpg

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Posted

JimChuck, you will likely have to drill some holes along the tops of your seat truss plates for the seat lacing ( maybe you already have!)  I was thinking of putting some high-quality plywood in there, using zip-ties along the outer edges, letting the bungees contain it inboard.  On my bird, the original builder put in split rubber tubing as a buffer on the bottom seat truss bar, but used small hoseclamps to hold the rubber hose sections in place.  This abraded the bungees' outer fabric sheath, and strikes me as a no-no!  I suppose rubber cement is about all one can do, as even zip-ties would likely cause abrasion.

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Posted

I have the front of my sling seat tied to a piece of 3/4"  OD aluminum tube.  That tube is tied in 3 or 4  places to the seat truss where there is a space to put the rope.  This probably helps with giving more leg room, as the seat ends up further back.  JImChuk

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Posted

Turbo,

this is one of the areas where I admit poor design of the Avid Flyer... 

The good solution is to install the kind of gear Trent Palmer has but as any ideal solution it is expensive...

My recommendation is to  REMOVE the merciless safety cable to avoid having it cut the seat struss in 2 and replace it by a tight bungee between right and left leg. I know, incomprehensible as explanation so here is a picture. You can use any heavy duty bungee cord, the stiffer the better actually. In the beginning of the deflection (ie normal landing) there  will be little tention on the horisontal bungee. Only at bad landings, great deflection, will the bungee enter in action (and save both prop and seat struss). A8919615-08F8-4492-AB60-35E2E1770FF1.thu

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Posted

I ran mine without cables for 8 years for this exact reason. Interesting analysis on the bungees. 

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Posted (edited)

I've got to admit the analysis is incomplete, though.  I just got type-1 3/8"bungees from AS&S and they feel significantly stiffer.  I wonder how much tensile preload is built in to those bungees, and at what length ratio do they stop stretching.  Also, it would be reassuring to test my end loops to failure.

Edited by Turbo

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