conventional gear

[klehman]

Hi
Further to Geert's message of last August on the conventional landing gear, I
notice that the 1998 manual now says to size the safety cable to allow 3" of
movement. The previous 1995 bulletin said 2.375". My LG-5 slides 5.75" into
LG-43 so 3" of movement seems reasonable.
Ken

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[Wayne G. O'Shea]

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Your following message has been delivered to the 155 members of
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Ken and ALL, If you are going to stick with the stock MAM aluminum bungee
struts, do yourself a big favour and put external 5/64th or 3/16 safety
cables on them (similar to an "Alaskan" Piper Super cub). Dave Bangle (and
many others) can tell you how good the internal 1/8th ones will work if you
are unfortunate enough to need them! I would suggest that when you install
them to hook the bottom of the cables to the inner axle gusset bolts (not
the strut attach bolt) and the top of the cables either hook to the opposite
sides top strut bolt, or make a steel bracket that joins both top strut
bolts. This bracket would "drop down" and have 2 more holes to install the
top of the safety cables. This attachment procedure will allow the cable to
stay attached (and hold the aircraft up) even if a struts attach bolt rips
out of the fittings. Any other way and when the strut (or it's attachment)
fails, the cable will also! When I designed and built my compression spring
struts, I figured out all the loads in the struts to determine spring
strength/ travel etc.. I don't have the numbers in front of me, but static
(1g) each strut has over 1200 lbs of tension on it at gross weight of
1650lbs (if you remove the friction influence of the tire). A "lightly hard"
2 G landing would produce over 2400 lbs of tension. This is over the
breaking strength of 1/8 S.S. cable. Also it is not so much the pull amount,
as it is the sudden snap when the cable hits the end of it's travel. Also
with the cable inside the strut, you can not see how quickly it is wearing
and it will wear quick when jammed inside the tube and twisted up to get it
in there. It's not easy trying to pack 3 extra inches of cable inside that
strut! Take note that Dave B. was pretty much sitting still/taxing when his
bungee and safety cable failed at Oshkosh '95.

As always just my 2 cents,
Wayne G. O'Shea
www.irishfield.on.ca

-----Original Message-----
From: klehman@albedo.net <klehman@albedo.net>
To: Murphy Rebel <murphy-rebel@dcsol.com>
Date: Tuesday, April 18, 2000 10:15 PM
Subject: conventional gear

Hi
Further to Geert's message of last August on the conventional landing gear,

I

notice that the 1998 manual now says to size the safety cable to allow 3"

of

movement. The previous 1995 bulletin said 2.375". My LG-5 slides 5.75"

into

LG-43 so 3" of movement seems reasonable.
Ken

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[klehman]

Wayne

Your analysis seems to be in the ballpark. Thanks for the heads up. The forces
could easilly be much higher with a little side load on the gear as well.

Cable strengths run about 900 lbs for the MAM recommended 3/32" cable.
2000 lb for 1/8" (recommended by Geert)
2800 lb for 5/32
4200 lb for 3/16

The currently recommended method of using a loop would theoretically double the
strength but stress concentration from wrapping it around a 3/8" bolt without a
thimble probably cancels that benefit. If I use the MAM method I will probably
at least use a double loop of 1/8" to insure the cable is not the weakest link.
I do like your idea of not using the same attach point as the bungees though.

BobP's recommendation that the bungees be kept tight enough so that they don't
move during normal taxiing should eliminate most wear on the cable and the pivot
bolts.
All the problems that I've heard of seem to be related to a worn cable and/or
loose bungees ??? (Except for Geert who had LG-5's that were also too short)

Ken

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[Wayne G. O'Shea]

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Your following message has been delivered to the 156 members of
the list murphy-rebel@dcsol.com at 22:49:20 on 19 Apr 2000.
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Ken,

As for my analysis, I actually paid attention in "Statics" for engineers
class (if I wasn't in the school pub!) and understood what was going on at
the time! 19 to 20 years later I don't necessarily remember a damn thing
though and usually just let common sense rule!!

Don't forget that the mentioned tension forces would be double in the strut
when you land on one wheel or ski, unless you do it at such a steep angle
that the forces become compressive in nature!!!!

Good luck getting the bungees tight enough so the strut doesn't move while
taxiing. Haven't seen a set yet, that were this tight. Remember you would
need over 1200lbs of preload at gross weight to keep them tight and
non-moving!

Wayne

-----Original Message-----
From: klehman@albedo.net <klehman@albedo.net>
To: (Murphy Rebel Builders List) <murphy-rebel@dcsol.com>
Date: Wednesday, April 19, 2000 10:51 AM
Subject: Re: conventional gear

Wayne

Your analysis seems to be in the ballpark. Thanks for the heads up. The

forces

could easilly be much higher with a little side load on the gear as well.

Cable strengths run about 900 lbs for the MAM recommended 3/32" cable.
2000 lb for 1/8" (recommended by Geert)
2800 lb for 5/32
4200 lb for 3/16

The currently recommended method of using a loop would theoretically double

the

strength but stress concentration from wrapping it around a 3/8" bolt

without a

thimble probably cancels that benefit. If I use the MAM method I will

probably

at least use a double loop of 1/8" to insure the cable is not the weakest

link.

I do like your idea of not using the same attach point as the bungees

though.

BobP's recommendation that the bungees be kept tight enough so that they

don't

move during normal taxiing should eliminate most wear on the cable and the

pivot

bolts.
All the problems that I've heard of seem to be related to a worn cable

and/or

loose bungees ??? (Except for Geert who had LG-5's that were also too

short)

Ken

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[klehman]

Yeah it's been over 20 years since I studied this stuff too but my number was
about 1050# tension per strut static in the 3 point attitude at 1650# gross. The
geometry gets worse as the gear flexes so 2g (on the gear) more than doubles
that force even with no crab or sideslip.

"Wayne G. O'Shea" wrote:

snip<
Good luck getting the bungees tight enough so the strut doesn't move while
taxiing. Haven't seen a set yet, that were this tight. Remember you would
need over 1200lbs of preload at gross weight to keep them tight and
non-moving!
Wayne

OK, I did a quick and dirty little experiment to see how difficult this is going
to be.

After the first little bit of movement, the force exerted by my unused 1992
vintage MAM bungee cord is approximately linear with elongation (F=15x+20
approximately) up to 60% elongation. After that the force increases rapidly to
about 150# at 70%. I have found a reference to a test strength rating of 200#
and another for 400#. Don't know which is correct but the ramping up of the
force at 70% makes me wonder if this stuff should be stretched much more than
that.

MAM's method of using 5 wraps or loops per side of each bolt would require about
53 to 60# of tension on the bungee cord as it is installed (about 20 or 25%
elongation) and I can see how that would be difficult.

Ron's directions for making Bungee Rings recommends about 30% elongation which
would give about 70# of preload per strand. So one ring is 2 x 70 = 140#. Eight
rings per strut =1120#. (Six rings might be a bit low) This is in the ballpark
of our required static strut tension. I presume these things loose some tension
as they age.

My bungee attachment bolts are 7.7" apart. Using 30% pre-elongation, an
additional 3" of strut extension before the safety cable tightens, would give
about 85% elongation (The bungee cord would be 1.85 times longer than it was
before it was installed). Only fools extrapolate but that should handle more
than 2g on the gear before loading the safety cable.

Does this sound reasonable? Has anyone been unable to get sufficient preload
using bungee rings?

Bob, you seem to favour 6 rings per strut but with more preload. I couldn't pull
hard enough in my little test to see what the result of that would be at the 3"
max strut deflection that MAM now calls for. The force seems to be ramping up
fast though in that range which would be a good thing as long as rubber fibres
don't start breaking.

There is some room for confusion here about elongation. It appears that the cord
is manufactured with 20% or more elongation before we even purchase it. The
above comments ignore that however.

Ken
just call me "Stretch"

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[Wayne G. O'Shea]

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Your following message has been delivered to the 157 members of
the list murphy-rebel@dcsol.com at 13:34:51 on 21 Apr 2000.
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Ken (aka Stretch), your 1050lb's is probably right. Like I said I pulled the
number out of my head sitting at the computer. I don't know were why
compression spring strut "folder" is right now, as I am in the middle of
moving my work shop from the plastics plant to home. When I find it I will
have a look and see what number I had originally come up with.

Looks like you will just need to find 8 more friends to help you pull your
bungees tight!

Blue skies,
Wayne

-----Original Message-----
From: klehman@albedo.net <klehman@albedo.net>
To: (Murphy Rebel Builders List) <murphy-rebel@dcsol.com>
Date: Friday, April 21, 2000 9:14 AM
Subject: Re: conventional gear

Yeah it's been over 20 years since I studied this stuff too but my number

was

about 1050# tension per strut static in the 3 point attitude at 1650#

gross. The

geometry gets worse as the gear flexes so 2g (on the gear) more than

doubles

that force even with no crab or sideslip.

"Wayne G. O'Shea" wrote:

snip<
Good luck getting the bungees tight enough so the strut doesn't move

while

taxiing. Haven't seen a set yet, that were this tight. Remember you would
need over 1200lbs of preload at gross weight to keep them tight and
non-moving!
Wayne

OK, I did a quick and dirty little experiment to see how difficult this is

going

to be.

After the first little bit of movement, the force exerted by my unused 1992
vintage MAM bungee cord is approximately linear with elongation (F=15x+20
approximately) up to 60% elongation. After that the force increases rapidly

to

about 150# at 70%. I have found a reference to a test strength rating of

200#

and another for 400#. Don't know which is correct but the ramping up of the
force at 70% makes me wonder if this stuff should be stretched much more

than

that.

MAM's method of using 5 wraps or loops per side of each bolt would require

about

53 to 60# of tension on the bungee cord as it is installed (about 20 or 25%
elongation) and I can see how that would be difficult.

Ron's directions for making Bungee Rings recommends about 30% elongation

which

would give about 70# of preload per strand. So one ring is 2 x 70 = 140#.

Eight

rings per strut =1120#. (Six rings might be a bit low) This is in the

ballpark

of our required static strut tension. I presume these things loose some

tension

as they age.

My bungee attachment bolts are 7.7" apart. Using 30% pre-elongation, an
additional 3" of strut extension before the safety cable tightens, would

give

about 85% elongation (The bungee cord would be 1.85 times longer than it

was

before it was installed). Only fools extrapolate but that should handle

more

than 2g on the gear before loading the safety cable.

Does this sound reasonable? Has anyone been unable to get sufficient

preload

using bungee rings?

Bob, you seem to favour 6 rings per strut but with more preload. I couldn't

pull

hard enough in my little test to see what the result of that would be at

the 3"

max strut deflection that MAM now calls for. The force seems to be ramping

up

fast though in that range which would be a good thing as long as rubber

fibres

don't start breaking.

There is some room for confusion here about elongation. It appears that the

cord

is manufactured with 20% or more elongation before we even purchase it. The
above comments ignore that however.

Ken
just call me "Stretch"

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[Bob Patterson]

"Stretch" & Wayne,

You're both right !! It is <almost> impossible to pull the
single bungee cord supplied by the factory tight enough to get a good
preload. There is also the problem of different tension on each wrap,
depending on the distance to the shop wall (:-) ) - this almost always
causes bungee "sag" after a few flights, even if the original appears
satisfactory. That is why we decided to go with the rings - PLUS, they
are MUCH easier to install ! Even if you let go of one, you don't lose
all of your work to that point.

'Way back there, I spoke to the company that makes the rings,
looking for the 'right' size for the Rebel - seems the #9044 is the
closest standard ring. They recommended approx. a 40 to 50 % pre-load
for best operation. Something you CAN do that will help the operation
of the bungees AND the durability of the bungee bolts, is to add a
sleeve to the bolts, between the gear tube and the penny washer. This
can be as simple as a piece of clear fuel line, or, better still, a
piece of Delrin machined to a 'spool' shape. The bolt would tighten
against this spindle, loading it in tension, which would help it
resist the side loads from the bungees. Even a small tube will let
the bungee slide over it easier than the bolt - they tend to get flat
spots at the bolt, which means each side of the loop is working alone,
rather than spreading the load over both... and the bungee is weakened.

| |
|\ ( )( ) /|| |
=| \(__)(__)/_||______|__
bolt head----> = __________|______|__ <--- bungee bolt
=| /( )( )\ || |
|/ ( )( ) \|| |
| | | | |
| | | | |
V V V |
spool bungee V
(spindle) rings gear tube


Part of the function of the bungees is to absorb energy, not store it
like a spring. The internal friction of all those little strands of
rubber sliding over each other dissipates some of the energy. The spindle
allows the strands more freedom to slide around the whole ring rather
than just stretching on one side.

There might be some advantage to making your own rings using the
bungee supplied, as it is thicker than the #9044, which is 9/16". Less
preload would still give a stronger bungee - but perhaps with less
'give'......

In any case, you don't want the bolts to be hitting the cable
EVER, in normal use. It's there in case the bungees fail.

......bobp


----------------------------orig.---------------------------------------
At 09:11 AM 4/21/00 -0400, you wrote:

Yeah it's been over 20 years since I studied this stuff too but my number was
about 1050# tension per strut static in the 3 point attitude at 1650#

gross. The

geometry gets worse as the gear flexes so 2g (on the gear) more than doubles
that force even with no crab or sideslip.

"Wayne G. O'Shea" wrote:

snip<
Good luck getting the bungees tight enough so the strut doesn't move while
taxiing. Haven't seen a set yet, that were this tight. Remember you would
need over 1200lbs of preload at gross weight to keep them tight and
non-moving!
Wayne

OK, I did a quick and dirty little experiment to see how difficult this is

going

to be.

After the first little bit of movement, the force exerted by my unused 1992
vintage MAM bungee cord is approximately linear with elongation (F=15x+20
approximately) up to 60% elongation. After that the force increases rapidly to
about 150# at 70%. I have found a reference to a test strength rating of 200#
and another for 400#. Don't know which is correct but the ramping up of the
force at 70% makes me wonder if this stuff should be stretched much more than
that.

MAM's method of using 5 wraps or loops per side of each bolt would require

about

53 to 60# of tension on the bungee cord as it is installed (about 20 or 25%
elongation) and I can see how that would be difficult.

Ron's directions for making Bungee Rings recommends about 30% elongation which
would give about 70# of preload per strand. So one ring is 2 x 70 = 140#.

Eight

rings per strut =1120#. (Six rings might be a bit low) This is in the ballpark
of our required static strut tension. I presume these things loose some tension
as they age.

My bungee attachment bolts are 7.7" apart. Using 30% pre-elongation, an
additional 3" of strut extension before the safety cable tightens, would give
about 85% elongation (The bungee cord would be 1.85 times longer than it was
before it was installed). Only fools extrapolate but that should handle more
than 2g on the gear before loading the safety cable.

Does this sound reasonable? Has anyone been unable to get sufficient preload
using bungee rings?

Bob, you seem to favour 6 rings per strut but with more preload. I couldn't

pull

hard enough in my little test to see what the result of that would be at the 3"
max strut deflection that MAM now calls for. The force seems to be ramping up
fast though in that range which would be a good thing as long as rubber fibres
don't start breaking.

There is some room for confusion here about elongation. It appears that the

cord

is manufactured with 20% or more elongation before we even purchase it. The
above comments ignore that however.

Ken
just call me "Stretch"

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