I have had two bungee cord fail on a 750 in the last few hours

not this is on a 750 with a zenith Rotax FWF kit

 

 Just the way that they failed made me think that it might be a possable heat issue

I know that I had not been hard on the bungee

 so i put a remote temp sensor on the area of the bungee that had failed

also I did some measureing on the area of the lower cut out

the round radius is 7"

and the rear exit area is 1" wide by about 17.5" long

 all you math wizzes can figure out the area !!

 

well, with that size exit I run about 234 cht and 192 on oil temps

and I took  the temp on the bungee ,,,160 degress !!!  all the time

and if you didnt open the oil inspection cover the ambient gets even higher just sitting there

i would think that at that temp it wouldnt take long to reallly couse problems with the rubber bands inside

 

well the first thing I tried today was make the radius hole on the exit 2" wider in radius

ten minute flight and the bungee temp was the same , I lowered the cht and oil   227 and 187

 

next I duct taped on a 2" lip right ahead of the radius , not it wasnt really solid and i think it flexed back a little  in flight

but it did drop the bungee temp down to 145 degress

I did not notice a big differance on the cht and oil

 

so I put a SCAT tube on the bungee area with the front of the tube up by the carb holes on the upper cowling

flew it for ten minutes and never got it over 117 degress!!!

 

big improvement

 

and for all the testing the ambient was 84 degress

 

I know there is alot of 701 running the Rotax setup that have not had any problems , but i think the 750 might need a little help on that area.

Guys should keep an eye on thier bungees .

 The lower strap welded on the gear leg saved lots of headaches ,hats off to Zenith on that !!

hope  this helps someone

 

Im thinking im going to make a cover over the bungee nose gear tube and run a SCAT tube  to the top , it will direct cool air and act as a heat shield at the same time , ill get some pics posted of that soon and get some temp results

also I will make a permanet lip on the  lower cowl exit

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Comment by Tracy Buttles on September 19, 2012 at 4:11pm

Pat

The bunge cord works great , I beleive the thought on that design is simple ,light and cheap .

it has worked great on Piper products for 75 years

 

and Milan

im not sure on your situatuion but next time you fly just fell around  the bunge area and get an idea on what kind of heat is on it , but you also have to look for anything that might have rubbed it some how and damaged it

Im not saying heat is the reason for every bungee fail

Comment by Pat Kelly on September 13, 2012 at 3:45pm

just curious, why a bungee cord and not maybe a coil spring or something like that?

Comment by Tracy Buttles on September 11, 2012 at 3:47pm

Im waiting for an answer on that

 

Comment by Roger Lambert on September 10, 2012 at 8:46am

What temperature range is the bungee cord designed to operate within?

Comment by Geoff Klestadt on September 7, 2012 at 3:09pm
Well done! I think you have demonstrated the cause of the problem.
Comment by Bob McDonald on September 4, 2012 at 4:32pm

Interesting stuff.... I have not had a bungee fail on my CH750 yet !  I have had a nose gear bungee almost fail (walk around inspection caught it) on the CH601HD nose gear. This bungee notice will have me and others looking closer on the inspections.

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