Recommended bungee for a fat plane?

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Posted

Hey everyone 

as some of you may know. My Subaru converted mk4 is sitting at 716lbs empty weight and I'm noticing that my new bungees 3/8", 90" long and 8 wraps is flexing quite a bit. The heats stance is pretty wide at empty weight and the tires running on the inside edges. 

Any suggestions or advice whether or not this is ok and if not what would be a better bungee setup to go with? 1/2" maybe? More wraps?

thanks everyone 

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Posted (edited)

Are you really using shock cord?  Not the bungee stuff you can find in some hardware stores.  This is the stuff you should be using.  http://www.aircraftspruce.com/catalog/lgpages/shockcord.php   JImChuk

hey Jim

looking at my reciepts I had purchased 23' of the 10mm bungee cord from AS. Here's the link http://m.aircraftspruce.com/catalog/lgpages/bungee10mm.php?clickkey=8478

should I have purchased the shock cord? Also now I'm trying to remember why I bought so much. I'll dig up the old cords and see what their length was because I think I just duplicated them. 

UPDATE- measured the old ones and they're 124 inches tip to tip and counting the wear/paint impressions on them there 8 total wraps starting from connection to connection. This cord is also 1/2" and I think I was able to get much more out of the 3/8 and did 10 wraps. Here's pics of the old stuff and also my gear how it's sitting unloaded. Note, the safety cable slack is  pulled up into the cabin, they aren't even close to that little amount of allowable travel.

 

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Edited by fusionfab
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Posted

Using the mil spec shock cord you will be much happier.  It made a big difference on mine when I was running bungee gear.  Also take a look at the attached drawing. 

Trust me, when the gear leg breaks as it has done to many people, your day will get really shitty.  With the safety cable where you have it there is nothing to keep that gear leg under you.  It folds up and smacks the bottom of the wing and makes one hell of a racket when the plane flips over so damn fast it will make your head spin.  Yes, I am talking from experience.

gear leg.pdf

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Posted

Ok thanks I'll get new ones put on today. What's your thoughts on the length of bungee and number of wraps I should be using weighing 716lbs?

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Posted

On our Avid plus aircraft, with Subaru engines, the length of the bungee was around 104". Eight wraps times the distance around the truss and gear leg.  The last couple of wraps were tough to get on , but with two people doing the install it was possible. I seem to remember that the gear was supposed to be snug up against the bottom of the fuselage until gross weight was reached. Another thing we did was to move the safety cables out to the inside of the cluster rather than where they are normally wrapped around the gear leg. There have been instances where the portion of the leg that has the bungee wrapped around it, broke off if you had a really rough landing.  Just to be on the safe side we also slipped a piece of 4130 inside the gear leg about 12" to reinforce the leg at the cluster outboard of where  the bungee wraps. This is on the wide Avid gear. 

Jack 

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Posted

On our Avid plus aircraft, with Subaru engines, the length of the bungee was around 104". Eight wraps times the distance around the truss and gear leg.  The last couple of wraps were tough to get on , but with two people doing the install it was possible. I seem to remember that the gear was supposed to be snug up against the bottom of the fuselage until gross weight was reached. Another thing we did was to move the safety cables out to the inside of the cluster rather than where they are normally wrapped around the gear leg. There have been instances where the portion of the leg that has the bungee wrapped around it, broke off if you had a really rough landing.  Just to be on the safe side we also slipped a piece of 4130 inside the gear leg about 12" to reinforce the leg at the cluster outboard of where  the bungee wraps. This is on the wide Avid gear. 

Jack 

ok thank you Jack, was that 1/2" bungee? 

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Posted (edited)

I would use the 3/8" shock cord that I linked to on the Aircraft Spruce site.  Here is a link to Steve Winder's explanation on how to install the bungees.  It works good, although, I also put marks halfway between the 13" marks.  That way both people (top and bottom) have a mark to pull to.  I've flown my Avid MK IV lots of times at the gross weight of 1150 with the 7 wraps of 3/8" shock cord.  Obviously, the black cord you used isn't near as strong as the shock cord.  JImChuk

 

Bungee installation (1) Steve Winder's how to.doc

PS  incase my link doesn't work, here is a bunch of posts on bungees with the same link in it.

Edited by 1avidflyer

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Posted

Roll your plane forward and back as this will give an indication of alignment.My 3 has good bungees and sits with the gear up snug to the belly,but push it backwards into the hanger the legs pull away at different rates and looks like yours.

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Posted

Dusty,  if your gear legs get wider when you back up the plane, that sounds like you have a toed in condition.  Most seem to think neutral or slightly toed out is preferable to toed in.  Easier ground handling.... JImChuk

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Posted

Mine had widened when pushing forward into the barn....but very very slightly. But the gear wouldn't return snug up against the belly. 

I just got back from my flying club though, there was someone there who had a small roll of 1/2" shockcord leftover from his bungee setup. He was even nice enough to have them cut and ends made for me when I got there. Thanks Ernie. 

Hoping to get the bungees changed out now but damn this shockcord is a heck of a lot harder to stretch than the black 3/8 bungee. 

Thanks for all your help everyone it is very much appreciated. 

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Posted

Yes we used the 1/2", but our target gross is 1320 lb.s.

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Posted

My gear is slightly toed in but hasn't been a problem but she is a bit squirrelly,I intend to put a grove setup on in the next few weeks but i don't want to make it too easy for the instructors or students;)

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Posted

Regular hardware store bungee cord is not advised as it is simply too flexible. Bungee's ie. shock cords comes in all different patches, sizes, flex, you name it and sourcing it consistently is at time a bit of a task. I fully agree with using Aircraft Spruce as a trusted supplier or you can sweet talk the good folks at Kitfox and order a set for your Kitfox4-1200  ;-) or 5 project. Just to show you that this is not an uncommon issue here is a somewhat dated PDF. I'm sure Avid in its days went through similar problems. And these problems still exist today but as "Red Green" would say, ..remember, we're all in this together! 

http://www.kitfoxaircraft.com/images/serviceletters/SL-20.pdf

 

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Posted

Regular hardware store bungee cord is not advised as it is simply too flexible. Bungee's ie. shock cords comes in all different patches, sizes, flex, you name it and sourcing it consistently is at time a bit of a task. I fully agree with using Aircraft Spruce as a trusted supplier or you can sweet talk the good folks at Kitfox and order a set for your Kitfox4-1200  ;-) or 5 project. Just to show you that this is not an uncommon issue here is a somewhat dated PDF. I'm sure Avid in its days went through similar problems. And these problems still exist today but as "Red Green" would say, ..remember, we're all in this together! 

http://www.kitfoxaircraft.com/images/serviceletters/SL-20.pdf

 

yeah the bungees I installed were from aircraftspruxe and labeled as suspension bungee. But I think it's just too light for my weight so I went to 1/2" yesterday and tested on the runway this morning....MUCH BETTER. 

 

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