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Graham and others,
I see my first email did not get sent complete with photo's and
discussion.
Also, I did not include on the net the answer to Jeff.
First, I will reprint my answer to Jeff on his probable cause.
Second, I will include the design and installation considerations on the
mono hopefully addressing the over center on down design:
Answer to Jeff:
Jeff,
Nobody hurt, hopefully the engine has a clutch in the gearbox, so it
shouldn't be harmed, the prop can be replaced.
The rest is cosmetic.
Now for the gear.
It is clear from the photo's that the gear handle was in the down and
locked position.
It is also clear that the swing arm came aft with the gear handle in the
lock position and since there is not an over center lock for aft
movement (the only stop for the gear is for the LG 08 to contact the
gear frame stops with the gear handle locked down), then the arm was
fatigued until failure. Yes, it can fail (normally from twisting) just
---From sitting with the gear down on the ground and the swing arm now
unsupported by the vertical LG 08. Here is a more complete discussion:
If during the build of the mono, the landing gear retraction arm LG 08
was not positioned hard against the frame stops the gear will collapse.
When the LG 08 is against the stop, the entire retraction arm is
slightly (and it is only slightly) forward of vertical with the shock
block hinge points of LG 04 and LG 05 and against the gear frame stops
when the handle is in the handle slot in the locked position. If the
gear frame stops are not the right length, keeping the gear aft of
vertical, movement can occur with disastrous results. However, if the
stops leave the LG 08s too far forward, it has been surmised that the
gear in a bounce can actually spring the other way (aft) when the
airplane bounces up (extension) and then will collapse (especially if
the handle is loose, or the slot for the lock is sloppy fore and aft or
bushings are worn out in the system) if the gear goes over center (aft)
on the the return impact. Note that in Chapter 28M page 2 the handle
position in the locked position is set by fixing the LG 08s (even before
attaching the rest of the gear) pushed up hard against the gear frame
stop and then setting the handle into position in the lock spot. Many
hours were spent by the builders and the factory to warn that the gear
arms LG08 must be located against the stops to preclude the gear from
collapsing and from inadvertent retraction during rough field
operations.
It is interesting but necessary that the gear handle is set in the
section where we set the flaps to the full down position, and the manual
does amplify the results of the discussion but does not include warnings
in the next chapter when setting the gear and checking the alignment of
the gear LG 08 and the bock pivots below. I thought we made sure in
Annex E that the LG 08s were checked prior to initial flight testing. I
may need to readdress that.
I see by the damage that the gear collapsed by folding aft after the
gear handle finally gave up as is was the only thing that held the gear
---From collapsing.
I have one mono in the shop, and will see if I can provide pictures. I
fear that somewhere in the build or perhaps a defect in manufacture of
your gear frame may have put the gear arms slightly aft of vertical or
with the LG 08s not quite touching the stops. In the mono's I have
inspected, I see more often the three bolts on the gear retraction
handle are worn or miss-drilled allowing the gear to move. I have seen
only one gear collapse and that was due to an off field landing. The
plane was not built particularly well and the problems were very easy to
fix. The collapse was because even with the gear handle retract lever
in the lock position, you could move the gear LG08 arms fore and aft due
to a poorly installed gear handle. The gear frame was bent in the
collapse so a new gear frame was installed, and the gear reinstalled
uneventfully.
Now for the analysis of the installation and techniques with photo's not
included in my first email:
Chapter 21M gives clear instruction on the LG 09 and bushing
installation. The stops are called appropriately the over center stops.
These stops allow a slightly over center position of the gear LG08 and
shock block assembly. Again, this is only just over center.
Chapter 28M clearly indicates that for setting the flap in the down
position that LG 08 must be positioned hard against the stops built into
the landing gear frame. From the initial set of the flap, the builder
is instructed in how to install the gear handle. Note that the lower
gear swing arm is not set on the aircraft at this time.
Chapter 29M explains the install of the rest of the gear frame. No
other comments to my knowledge are made about the gear geometry.
Annex E (Mono) indicates that the LG 08 arms are to be affixed against
their stops prior to clearance for flight.
The gear handle is not the mechanism to hold the gear in the down
position. In fact, the gear handle has the following jobs to do in its
design:
a.. The handle is the lock for the over center condition of the gear
when extended (arms LG08 and rubber block lower shocks bent slightly
forward as LG 08 contacts the frame stops). As the instruction manual
indicates the LG 12 safety lock prevents inadvertent gear retraction.
Please note that technically, you can, while sitting on the ground,
retract the LG 12 safety lock and pull the gear handle out of the slot
and as long as you don't put ANY forward pressure on the handle, the
gear should not retract. I haven't tried this.
b.. The handle is the retraction mechanism, but is assisted by the
bungees in the actual retraction of the gear.
c.. The handle is the up lock mechanism only! The bungee geometry
really helps hold the gear up, but the handle does the job. So that
diagonal gear handle brace is necessary for both up and down, because
those three bolts that attach the gear handle to the LG08 are going to
wear those holes out eventually.
<http://photos.msn.com/Viewing/Album.aspx?PST=8nK2AN1B!1LEiKSdooxx9v4CF
FiF2tnPzfqiCRx5lZJ3t2ol6z9!Q9u31z2Gf34UKChg6aSaSkGODYJ1SdylXQ%24%24>
Note that the wire line shows the center pivot at the pin through
the lower LG08 arm is just forward of vertical. Typically just about
1mm when everything is bench fitted.
<http://photos.msn.com/Viewing/Album.aspx?PST=8nK2AN1B!1LEiKSdooxx9v4CF
FiF2tnPzfqiCRx5lZJ3t2ol6z9!Q9u31z2Gf34UKChg6aSaSkE!zuM!0*scuA%24%24>
I rigged this arm without bearings in the LG09 to show what a stop
that was too long would look like. Note that if the main swing arm was
put under pressure the gear would collapse.
In the the past some discussions were made that the gear leg stops on
the frame were too long (as shown in the photo above) and the gear
mechanism was not vertical. In this case the stops needed to be ground
down.
As with all installations, some fit and check operations should be done.
I know of a couple of gear frames that were incorrect in pre 2004
sales, but Roger worked out these issues and the welding subcontractor
has a superb jig to keep this from happening, but mistakes can happen.
Problems I have seen:
a.. If the gear frame is inserted into the aircraft as per the manual
before thinking about the gear and trial fitting the gear parts first to
check geometry, then very inconvenient checks must be made with your
head in the wheel well to ensure the gear geometry is correct. It is
not hard to do, but requires some dexterity.
b.. It is a main quality control point to make sure the gear stops are
installed and spaced correctly at the factory. However, I trust no
component completely and inspect components myself when setting the
flap, gear handle and gear lock mechanism. I trial fit the retract
mechanism after installation of the LG 08 / 09 in chapter 21 and prior
to gluing it in. It allows me to ensure the under aircraft work is
minimized.
c.. Next I make sure that during gear installation (Chapter 29M) the
gear handle and swing arm geometry assures that the LG 08 and shock arm
assembly is just forward of vertical on both arms a slight amount (1mm
at the joint). To do this I insert a #20 wire or similar through the LG
08/09 and pull it down to the center of the main swing arm 1/4 inch pin
LG 04. Simply look up and check the MS 20392-5 pin is slightly forward
of vertical. Finally, check that the gear handle is secure in holding
the LG08 affixed to the stop, and you are good to go.
d.. Finally, since the LG 08s are fixed together by us amatuers, one
arm may be lazy about hitting the stop, especially if the flap is
dragging or the rubber block shock mount is loose or worn or was
installed twisted slightly (really hard since it is a welded structure
but it probably could). So be really anal about this installation
getting the arms straight and the pins installed well.
Most of us are relieved and gratified when we put the gear handle down,
a distinct snap to the down position is made during retraction/extension
tests. The bungee amplifies the sound of the slight over center and
normally you cannot even get a feeler gauge between the LG08 and stop
when extended. Further, in a well built gear system, the gear handle
drops itself into the lock slot and the safety catch drops in on its
own. That is the test of the gear down lock, when the gear handle is
taken from the up lock position, and it drops itself and slams firmly
into the lock position.
Finally, if the gear over center locks are filed so they are grossly
over center, this is a major problem. If too far forward, the gear
handle geometry is going to be off. The gear handle will work against
the bungee to spring the gear from its over center then up and the
motion of the gear handle travel will be longer than normal. It also
will allow the rubber block and shock when compressed and released to
act as a spring and pull so hard on the LG 08 that if the gear handle
has any sloppiness in it, then it is possible, if the frequency of the
gear shock/block and the landing surface were just right, the LG08 may
be snapped from its contact with the uplock back to vertical, or worse
yet, rebound to slightly over center aft to the unlock side and become
unlocked. This condition is seen as an unintentional gear retraction
with bent handle as in Jeff's excellent pictures.
I believe someone has built a latch that firmly holds the LG 08s to the
stops, but this is quite a bit of more Rube Goldberg work. The existing
gear has gone hundreds of hours and landings without failure when
installed correctly and without any manufacturing defects.
Don't ask me about outriggers, I follow the instructions in the book,
and I have never installed the new bushing mods in my shop, in fact Jim
Butcher showed me on his plane. It takes me a couple tries to get one
right. And it is a comedy routine to watch me try to reassemble one
after disassembly after paint if the lock blocks were not marked with
the orientation. My hat is off to you guys that get it right the first
time.
Hope this does not add to the confusion.
Regards,
Bud
----- Original Message -----
From: GRAHAM SINGLETON<mailto:grahamsingleton@btinternet.com>
To: europa-list@matronics.com<mailto:europa-list@matronics.com>
Sent: Sunday, June 12, 2011 8:50 AM
Subject: Re: Europa-List: Re: Landing gear Failure, ATTN Bud Yerly
-------------------------------------------------------------------------
-----
Hi Graham
First I think we need to absolute clarify what you mean by O/C. My
interpretation is Over Center. In other words if you load the main tire,
it will load the swingarm which will load LG08, and if Over Center, LG08
will try and move forward, in other words towards the prop. It will be
jamming into the undercarriage mounting frame, the more you load it, the
harder it will push into the undercarriage mounting frame.
Yes, that's what I was trying to say.
In Jeffs case I think he was under center and the loading caused the
lever to bend. There is a good chance he has an ill fabricated
undercarriage mounting frame that did not allow over center, but caused
an under center situation.
I think you are probably right that Jeff's frame was inaccurate and
the LG08 could have been under centre.
So, do you mean O/C is Over center,
Yes
Thus you want 1/16" in my term Over center where when you push up on
swingarm LG08 is loaded and pushes forward. You want LG12 held by the
down indent where LG08 is held forward against the undercarriage
mounting frame stop where there is little to no slop that would allow
LG08 to go under center (in others allow LG08 to swing aft towards the
tail).
And LG12 will try and swing aft too. The nearer to dead centre the
LG08 is the less load on the stops on the frame and the less load on the
LG12 too. Also the less effect G loading will have on the position of
the LG08.
When LG08 and the shock absorber assembly pivots are all in line there
is no force trying to move them, they are in pure compression.
Bud, do you have an opinion on this? I hope I'm not confusing the
issue?
Graham
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