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KeysFox
Joined: 05 Jan 2009
Posts: 137
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 Posted: Fri Jun 12, 2009 9:23 pm Post subject: Bungee gear limit cables (was) Re: model 5 gear pictures |
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Lowel
I have heard about three failures of relatively NEW bungees in Kitfox
installations. In one instance the gear safety cable failed after the bungee
failed. The result was a ground loop and wing damage.
Apparently there was no structural damage caused by the safety cables in those
three cases although we have heard of it in other cases.
I don't know what caused the bungee failures but suspect fraying, installation
error or bungee end failure.
If there is a moral here, it may be that Murphy can show up in a variety
of "unexpected" ways in addition to the expected hard landing and in bungee
failures not caused by hard landings the safety cables could be valuable.
As an aside, I too installed longer cables during my last bungee replacement.
BJ
N154K N626NR
bjones(at)dmv.com
443-480-1023
Quoting Lowell Fitt <lcfitt(at)sbcglobal.net>:
[quote]
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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B Jones
N154K
443-480-1023 |
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lcfitt(at)sbcglobal.net
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| Posted: Sat Jun 13, 2009 6:47 am Post subject: Bungee gear limit cables (was) Re: model 5 gear pictures |
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BJ, thanks for the report.
Lowell
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akflyerbob
Joined: 06 Jan 2008
Posts: 30
Location: Soldotna, Alaska
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| Posted: Sat Jun 13, 2009 7:29 am Post subject: Re: Bungee gear limit cables (was) Re: model 5 gear pictures |
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Hi Lowell
I didn't mean to imply you were giving bad advice, just that
leaving off the safety cables is bad advice.
To leave them off it to subject the machine to more damage
than with them.
I once sheared a gear off a Tcraft with out any other damage,
hit a hidden object same color as the grass, wing did drag.
Is not always the cable doing the damage to the structure, often its the rearward force at or on a hard or sudden landing, such as my hidden
rock, inducing a twist. I also broke the bungees hitting s snow drift I couldn't see but flew a couple 100 miles home on the cable.
My mod 1 has a slightly twisted bungee structure from such a
force but nothing downward, previous owner incident.
Depends on how the plane is used, up here no one would leave them off.
Bob
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Bob Wolfe..Soldotna, Alaska
KF Mod I and III |
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dave
Joined: 22 Sep 2006
Posts: 1382
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| Posted: Sun Jun 14, 2009 1:47 am Post subject: Re: Bungee gear limit cables (was) Re: model 5 gear pictures |
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I make my own bungees, and use safety cables.
Not sure how hard you would have to hit the ground to stretch bungees enough to not only take cable to cables but to bend up the plane. We all make boo boos at times but man that would be tough unless the bungees were old, weak and not tight enough.
Dave
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lcfitt(at)sbcglobal.net
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| Posted: Sun Jun 14, 2009 9:40 am Post subject: Bungee gear limit cables (was) Re: model 5 gear pictures |
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A correction on my earlier post. The correct Mil Spec blungees are the type
I rather than the type III I noted. The type "I" has a double layer cotton
braid on the outer surface rather than a single layer on the type III. Also
keep in mind that their are dozens of configurations for bungee cord. If
you make your own be careful of the source and type.
Lowell
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dave
Joined: 22 Sep 2006
Posts: 1382
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