Bungee questions

29 posts in this topic

Posted

 

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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. 

It looks like you have 7 wraps of cord on the pilot's side gear and only 6 wraps on the passenger side. Am I seeing that incorrectly or maybe there is a reason?

I am ordering the shock cord right now to replace my bungees because I found a couple of spots that were chafed. The bungees I am removing have only a single black fleck so I am assuming they are the marine or some other non-aircraft part. Now I wonder how much it will change the handling with the new bungees when I get them on. I am also seriously considering leaving the cables off after reading all the discussion on them. I have flown around 200 hours on my Avid with no issues but why risk that kind of damage if I happen to have a hard landing.

Thanks,

Dan

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Posted

Hi Dan,

no, there 7 wraps on each side, the wraps are slightly overlapping on one side.  What you want is the part number 06-12400 at Aircraft Spruce 3/8 " shock cord. "MIL-C-5651D TYPE I SHOCK CORD: This cord has a double cover of glazed cotton over natural cotton. It has 3 markers: 2 for the year and 1 for the quarter. " so yes, yours is probably marine cord (like the one between the legs on my photo).
The change all depend on the characteristic and condition of the current bungees. When you change to the same type the difference is better ground handling but be prepared for bouncing at landing if the old once were tired...

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Posted

Fred, I also have another question if you don't mind. I have been thinking about this since I will probably be eliminating the safety cables from my gear. Did you test your secondary bungee with the main bungee off to see how far the gear moves before the bungee holds it? I thought that the cables were a safety in case one of the main bungees actually break which would allow the gear to collapse without the cables. I was wondering if a slightly stretchy nylon line of the appropriate length rigged like you have your second bungee would be adequate as a safety without causing damage if a main bungee should separate. It doesn't look like the bungee mounts on the gear will move very far apart from each other until the gear has traveled up quite a bit.

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Posted

Fred, I also have another question if you don't mind. I have been thinking about this since I will probably be eliminating the safety cables from my gear. Did you test your secondary bungee with the main bungee off to see how far the gear moves before the bungee holds it? I thought that the cables were a safety in case one of the main bungees actually break which would allow the gear to collapse without the cables. I was wondering if a slightly stretchy nylon line of the appropriate length rigged like you have your second bungee would be adequate as a safety without causing damage if a main bungee should separate. It doesn't look like the bungee mounts on the gear will move very far apart from each other until the gear has traveled up quite a bit.

Hi Dan,

no I have not as what your question implies is not the way the horisontal bungee is expected to work. The problem with the safety wire is that it comes to a hard stop, and hard stops hurt. The idea with the horisontal bungee is to provide a progressive resistance when the gears travel too far apart. As you point out, there is initially next to no tension on the horisontal bunge, it progressively sets in when the gears travel further apart.
I have never heard about the main bungees braking (they should be replaced every 5 years) but we know just to well about hard landings maxing out the main bungees and stopping hard on the "safety" wire and breaking the seat struss...
So no, I don't think the horisontal bungee would hold the plane if one of the main bungees breaks. And yes, if you land hard enough you can still get a prop strike or worse. But my experience is that the horisontal bungee really softens hard landings.

The downside of bungees is that there is nothing to dampen the recoil...  The harder you land the higher you jump! 
I'm trying to find a creative way of adding some kind of damper but I'm not there yet.

Note that breaking the seat struss and potentiall warping the fuselage might be preferable to a prop strike on a direct drive engine...   On a reduced engine with built in rubber damper and a low price, high efficiency prop from Ukraine the Outcome of the risk calculation is different.

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