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lcfitt(at)sbcglobal.net
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 Posted: Fri Jun 12, 2009 12:24 pm Post subject: Bungee gear limit cables (was) Re: model 5 gear pictures |
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Hi Bob,
What advice are you having trouble with? I stated a fact - I do have
friends that removed the limit cables - and then made a suggestion.
For the Limit Cable fans, I did some research and made a drawing or two and
made some calculations.
First, Mil-C-5651D Load Specifications for our Type III Bungee cord call
for a load range at 100% elongation of between 90 lbs. and 150 lbs. A
minimum stretch at 140% and an ultimate breaking load of 300 lbs. For those
concerned with the 300 lbs, remember that we have six turns and 12 lengths
of bungee supporting our landing gear.
Using the drawings, it appears that a stretch of 140% would drop the height
of the fuselage at the gear, about 7" or maybe 9" at the prop in the three
point or the original 7" wheel landing. (It has been a long time since I had
a Model IV on the original bungee gear, but pictures I have of numarous
airplanes show the ground clearance in the three point to be well over half
a prop blade length.)
Given a load at the100% stretch to be at the low end - 90 lbs multiplied by
12 would be 1080 lbs. Per gear leg or close to 2 Gs if all the max gross wt.
was on the mains. This would result in a bungee cord drop of about 3 inches
or a nose drop of about five inches. (Mayabe this is where the manual
illustration measurement comes from - see below). Calculating at the high
end of 150 lbs. the 100% stretch load would be 1800 lbs. at max gross and
all weight on the mains or 3Gs. to get the same drop. It is likely that the
bungees we have - if relatively fresh would give a load somewhere between
these figures.
Most of this stuff is meaningless except regarding the length of the limit
cables. The manual - my manual - calls for a length of 4.5 to 5 inches. The
drawing shows 3 inches. The drop at the bungees at the 140" stretch would be
closer to six inches. If your limit cables are in the 4.5 to 5 inch range,
or worse yet, installed at the 3 inches in the drawing, you are cutting the
bungees effective spring action to a fraction of what it is designed to
handle and the sudden stop of "spring" movement focused on the bungee
bulkhead has in the past and likely will in the future, cause damage to the
most difficult to repair structural element.
I mentioned that some have removed the limit cables. I don't recall
recommending that as was asserted, but I did recommend lengthening them.
Frankly with a little bit of effort, I think I can now fairly safely suggest
that removing them would result in minimal risk to the prop, but can't say
much about the wing tips if someone manages to put 3gs of force on one gear
leg, but I think I would strongly recommend that they be long enough to
allow for at least six inches of downward movement of the bungee attachment
tube. If someone else wants to do the work and comes up with different
numbers, I am willing to listen.
Better landings? Definitely good advice.
Lowell Fitt
Cameron Park, CA
Model IV-1200 R-912 UL
Covering.
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akflyer
Joined: 07 May 2007
Posts: 574
Location: Soldotna AK
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| Posted: Sat Jun 13, 2009 6:21 am Post subject: Re: Bungee gear limit cables (was) Re: model 5 gear pictur |
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Lowell,
Your original post seemed to imply that you were more in favor of removing the limit cables, though you did mention that if you were to leave them on, to lengthen them. I agree that they should be lengthened but Bob was saying that leaving them off all together was not a good idea. I agree, but also added in the other thread that the safety cables should be moved in to the inside of the "triangle" formed where the rear gear legs tie into the cross tube. The failures I have see were right were the bungies wrap around the cross bar so a safety cable does NO good in the event of a gear failure at that point. I know cause I found myself on the nose in my brothers KF II (he was the one doing the crashing not me) lol.
off for a day on the river, then back to the hangar to get the new floats installed on my bird so the real fun flying can begin!
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_________________
DO NOT ARCHIVE
Leonard Perry aka SNAKE
Soldotna AK
Avid "C" / Mk IV
582 (147 hrs and counting on the rebuild)
IVO IFA
Full Lotus 1450
#1 snake oil salesman since 1-22-2009
I would rather die trying to live, than to live trying not to die.... |
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