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Undercarriage Overcentre Locking

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gcsmith(at)flyer.co.uk
Guest

Posted: Fri Feb 23, 2007 2:30 pm Post subject: Undercarriage Overcentre Locking

Greetings,

  

 Having  suffered an undercarriage collapse during the takeoff roll on my monowheel, I am  now pondering what corrective actions to take to prevent a re-ocurrence. As far  as I can figure out, the overcentre locking went 'under centre' when the  aircraft went over a large 'ski-jump undulation' at just below flying speed on  takeoff from a grass strip. Having spent a lot of time and money cleaning up the  mess and fitting a new prop etc, I'm reluctant to go flying again, even if I  confine my operations to an absolutely glass-smooth runway, until I understand  why it happened in the first place and  have some degree of confidence that it won't happen again.  

  

 I've looked  for any possible 'builder variations' in this area to see if there is anything  that I could have screwed up, but it seems that the geometry in this area is  totally determined by the manufacture of the landing gear frame and the u/c  components, what's more, this area has been rigorously inspected on several  occasions by some very fastidious inspectors. (although my landing gear frame  was modified in the factory to add reinforcing following some very early landing  frame failures, it's possible that the geometry was affected during this mod).  Are any other mono owners aware of the  amount of overcentre on their installations, it would be interesting to see if  there is much variation.

  

 On my aircraft, the  degree of overcentre in the 'down and locked' position is 1/64  of hardly anything at all, (i.e. very small)  and it occurs to me that shaving a few mm off the stops on the landing  gear frame would be a good way of increasing the security of the overcentre  locking.

  

 Question: has anyone  tried this? Can anyone think of a reason why not?

  

 Regards,

 Graeme  Smith

  

 No  26

  [quote][b]

rparigor(at)suffolk.lib.n
Guest

Posted: Fri Feb 23, 2007 3:41 pm Post subject: Undercarriage Overcentre Locking

Hello Graeme



I am not flying my XS mono yet (A-265 picked up April 2003). When building

I mounted my gear on a fixture and was able to study and fool with till I

was satisfied I fully understood it's operation.



First off you said you measured the over center, how did you measure it?



First off you need to make sure that the rubber spring block is compressed

to proper dimension. The more you tighten, the more overcentre you will

see. If for some reason the gap becomes less than called out for, you will

start ooching towards undercentre. I used a thin straight edge resting on

the edge of 3 pins that are the same diameter. The top pin is the pivot

pin that attaches to LG08s, the bottom pin is LG04, and the middle pin is

a little tricky, I made an insert that fits in the middle of LG08 tube

with a pin protruding the same size as the prior 2 mentioned. Now I have 3

points and can measure overcentre.



To my horror, 1 side was under centre and the other just bout neutral! I

ground down stops to where there was just a tad over 1/16" overcentre. You

need to be very careful you don't over do this as it will begin to put

much stress on UCMF. I forget the exact number that Andy said, but perhaps

1/8" was the limit.



The top of stop was gone when I achieved 1/16" on 1 side and the other

side was paper thin. What I did was grind down to 1/8" overcentre, and

inserted with Redux/Flox a pretty hard and stiff piece of polyurethane I

got from McMaster Carr, and pinned it after cure with a bolt. The

overcentre is just a tad over 1/16" on the polyurethane. Andy gave my

kluge a thumbs up.



Please don't overlook the connection between LG08P and the retraction

lever. I was careful to ream these 3 holes and scuff sand and Redux this

joint, and installed the mod that reinforces this connection as well. If

this joint has any movement when the lever is in the down and locked

position, it may not be ramming and jamming the LG08s against the UCMF

stops.



I also made a spring return for the down stop lock where it is very

positive in keeping it where you want it.



Once all was assembled on aircraft, the stops were not hitting absolute

the same time. Splitting hairs here, but I put a piece or 2 of some

Stainless Steel tape as a shim. You can get small pieces of that tape from

Lockwood Aviation, they sell it as prop lead edge tape, or

www.mcmaster.com sells it by the roll. 



Hope this helps.



Ron Parigoris

rparigor(at)suffolk.lib.n
Guest

Posted: Fri Feb 23, 2007 4:18 pm Post subject: Undercarriage Overcentre Locking

OOOOOPPPPSSS



"If for some reason the gap becomes less than called out for, you will

start ooching towards undercentre"



I meant to say if the distance between the top and bottom shock absorber

plate increases, for instance the M8 x 100 bolts stretch, loosen or for

whatever reason distance increases from distance called out for (79mm or

3.1"), there will be less overcentre. Increase distance = less overcentre,

decrease distance = more overcentre. 



Ron Parigoris

Paul Boulet

Joined: 09 Jan 2006
Posts: 57
Location: Malibu, California

Posted: Fri Feb 23, 2007 9:59 pm Post subject: Undercarriage Overcentre Locking

For what it's worth, I had the same exact experience.  My solution was to convert to Tri Gear.  Frustrating and expensive indeed

Paul Boulet N914PB



---

gcsmith(at)flyer.co.uk
Guest

Posted: Sat Feb 24, 2007 3:30 pm Post subject: Undercarriage Overcentre Locking

Paul,

  

 Sorry to hear that. I considered the same solution, but  have the impression that nose gear collapses are almost as common as mono  collapses??

  

 Graeme



   From: owner-europa-list-server(at)matronics.com  [mailto:owner-europa-list-server(at)matronics.com] On Behalf Of Paul  Boulet

Sent: 24 February 2007 05:59

To:  europa-list(at)matronics.com

Subject: Re: Undercarriage  Overcentre Locking

 

  For  what it's worth, I had the same exact experience.  My solution was to  convert to Tri Gear.  Frustrating and expensive indeed

Paul Boulet  N914PB



 -----  Original Message ----

From: Graeme Smith <gcsmith(at)flyer.co.uk>

To:  europa-list(at)matronics.com

Sent: Friday, February 23, 2007 2:26:29  PM

Subject: Undercarriage Overcentre Locking



   Greetings,

  

 Having  suffered an undercarriage collapse during the takeoff roll on my monowheel, I am  now pondering what corrective actions to take to prevent a re-ocurrence. As far  as I can figure out, the overcentre locking went 'under centre' when the  aircraft went over a large 'ski-jump undulation' at just below flying speed on  takeoff from a grass strip. Having spent a lot of time and money cleaning up the  mess and fitting a new prop etc, I'm reluctant to go flying again, even if I  confine my operations to an absolutely glass-smooth runway, until I understand  why it happened in the first place and  have some degree of confidence that it won't happen again.  

  

 I've looked  for any possible 'builder variations' in this area to see if there is anything  that I could have screwed up, but it seems that the geometry in this area is  totally determined by the manufacture of the landing gear frame and the u/c  components, what's more, this area has been rigorously inspected on several  occasions by some very fastidious inspectors. (although my landing gear frame  was modified in the factory to add reinforcing following some very early landing  frame failures, it's possible that the geometry was affected during this mod).  Are any other mono owners aware of the  amount of overcentre on their installations, it would be interesting to see if  there is much variation.

  

 On my aircraft, the  degree of overcentre in the 'down and locked' position is 1/64  of hardly anything at all, (i.e. very small)  and it occurs to me that shaving a few mm off the stops on the landing  gear frame would be a good way of increasing the security of the overcentre  locking.

  

 Question: has anyone  tried this? Can anyone think of a reason why not?

  

 Regards,

 Graeme  Smith

  

 No  26

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clevelee(at)earthlink.net
Guest

Posted: Sun Feb 25, 2007 11:47 am Post subject: Undercarriage Overcentre Locking

Graeme,

  

 I'm building  A198 - now moved with me to Market  Harborough, and I've been studying/worrying about the overcenter (OC) position  and have come to the same conclusion about the marginal amount of factory designed  overcenter provided. I believe this might be a compromise - as too far  overcenter and you fight the shock cord tension to raise the gear.   While the machining of the gear components  is quite precise, I believe the stops are part of a weldment - which is  considerably less so. 

  

 Overcenter toggle clamps used for holding products  being machined or assembled in industry have adjustments that control the degree  of tension against the arm that puts pressure against the OC condition.   There is an adjustable nut to control this - Locking pliers the same way.   Hit them hard or vibrate these devices and they will open if the  adjustment isn't such that the lock is firm.   The gear lock in  the cockpit does not put pressure into the joint, so while it helps, it's really  a 'late' way to save the day.  It does not modify the geometry of the  parts by forcing increased pressure into the components.

  

 When there is no  weight on the wheel, pressure into the OC condition is held in the static state  by the shock cord.  I would submit that the degree of tension on the shock  cord has a significant bearing on the integrity of the system.  And the  criteria for shock cord tightness, is only the degree of  'effective  assistance to raise the wheel' .

  

 I am  considering going further OC on my ship,  however, that will require repositioning the gear handle half the four bolt  holes, or less, so will likely require pinning.  It also means that  'popping' it out of the OC locked position will be more difficult when trying to  raise the gear.  Of course a better  solution is to put pressure into the system.  This could possibly be  done inside the cockpit with an OC clamp on the gear lever - with the slot modified to  allow it to be pulled rearward..  The clamps I referred to above  come in a J-hook pull style, which could  keep rearward pressure on the Gear lever in the aft condition.  Giving correct meaning in the Mono to 'Gear down  and Locked'.

  

 Cleve  Lee



 A198 Mono XS Jabiru  3300

 Wings closed, top  on, sanding...sanding...sanding...

    [quote][b]

ami(at)mcfadyean.freeserv
Guest

Posted: Sun Feb 25, 2007 12:15 pm Post subject: Undercarriage Overcentre Locking

Merely holding the lever back doesn't necessarily  work, as the 'undercentre' forces, once they take over, will just bend the gear  retraction lever. 

 This can be exacerbated by slackness in the bolt  holes that secure the lever to the retraction arms, despite the bolts being  tight.

  

 Duncan McF.

 [quote]   ---

gcsmith(at)flyer.co.uk
Guest

Posted: Sun Feb 25, 2007 2:18 pm Post subject: Undercarriage Overcentre Locking

Ron,



Thanks very much for the info and your experience with the over centre

locking -- very valuable!



When I say I measured the overcentre, I really meant that I eyeballed it by

sighting along the centres. It was over centre but only by a very small

amount (maybe 1/16"). There is nothing in the manual to say how much past

centre it should be. You say Andy said 1/8" would be the limit, that's good

to know. I have now ground away the stops, another constraint is contact

between the LG08P and the LG mounting frame. There is a scalloped area in

LG08P to provide clearance against the frame, removing more than a couple of

mm from the stops quickly brings LG08P very close to the frame.



I don't understand the bit about overloading the landing gear frame, surely

this must be designed to take the full landing load which would mostly pass

through the reaction plates to the stops, with some load being taken by the

top attach point on the LG frame??



Regards,

Graeme



--

rparigor(at)suffolk.lib.n
Guest

Posted: Sun Feb 25, 2007 4:36 pm Post subject: Undercarriage Overcentre Locking

Hello Graeme



Lets see if can help you get to the bottom of your bottoming.



First and foremost, you can not use your "Eyechrometer" to measure

overcentre ---- period.



I will match my calibrated "Eyechrometer" with any Europa builder. I can

work to a few 1/10,000" on torque tube drive pin fit, and work with

1/100,000" fitting together a Cox .020 model aeroplane engine.



You need to take precise measurement, I initial tried to sight, no way can

you eyeball 1/16".

I fooled for some time with my gear on a solid fixture, with shock

absorber out and the tyre moved up and down precise with a hydraulic pump

jack. Very enlightening. 



I suggest especial if you have your bungi out, take tyre off, and shock

absorber out but bolts in place and run swingarm up and down through its

travel, while you measure overcentre on the 3 points I initial described.

You need a thin straight edge, either thin high strength aluminium or

stainless. A thin ruler may work, but keep checking that straight remains

true.



You first need to get in your head the fact the further down the swingarm

goes the less overcenter things will be when LG08s are against stop, and

the further up the swingarm goes, the more overcentre things will become.



I have no doubt that pushing a pencil on paper, someone calculated loads

and birth of a undercarriage mounting frame was hatched. Remember that

this was unacceptable, as the overcentre load on the stops was failing the

frame. The reason for this failure you will see if you check as I

described, compress shock absorber and overcentre increases. OK then when

real world proved not strong enough, beef was added. Nothing wrong with

this.



On paper lets say 1/32" or 1/16" overcentre is called for, and on paper it

is proved that this is just fine. Is real world really the desired amount

of overcentre? Mine was not for sure. Was my LG08s really as per drawing

as far as placement of holes and position of stops? I have a fair share of

time welding 4130 tubing. Was proper fixtures used, correct length

components and order of tacking and welding proper, and calibrated sledge

hammer used to keep things on track really used to produce stops and pivot

points in the right place? Was my frame checked against a go no/go fixture

after it was completed?  Also the top and bottom actuation plates, how

much bow are in these when you are trying to achieve 3.1", and then when

there is a bow, what point do you measure?

I needed to make a spacer or spacers (I forget exact but will check if you

need) to space 1 or both LG08s from UCMF. In addition I needed to releive

some of LG08 (perhaps both) to achieve acceptable clearance from UCMF. And

final I needed to grind the head down, quite substantial from one of the

bolts so it didn't hit the UCMF on the port side. Bottom line is all

components need to equate to overcentre. Any one of the mentioned can

skunk you into too little. My UCMF came in with a pretty poor excuse for

white  powder coating on it, scratches and flaking abounded. Stripped,

sandblasted with Al/Ox, prmed and painted.



You will also notice that when you push the swingarm up, that clearance

between the downstop on tunnel and the lever develops. This is normal due

to increasing overcentre action. I anticipated that a bounce will grow

this clearance, and a relaunch as you describe will bang the lever hard

against this stop. I have my lever and stop angle tainted where the lever

will be forced inboard by a minor angle, and have lever bent so that there

is spring action pushing the lever inboard.



If you do not have access to obtain a proper measuring tool, E-mail me

with dimensions you need and I will make one for you, then you can give it

to the Europa Club for loan out to anyone who wants to measure theirs.



I could have welded a cap on my UCMF, but I already had it painted, so the

urethane inserts was a neat way to get precise length. I also did not like

the bang I was able to create when the LG08s hit the stops. The Urethane

helped out with bang. I first ground and filed stops till I had 1/8"

hitting LG08s precise, then made Urethane inserts to bring it to 1/16".

Worst case is it will be 1/8", probably OK, I would not want to try 1/4",

forces grow awful fast. I actual did calculation, and adding just an extra

1/16" increases stop loads on UCMF when shock absorber is compressed by a

lot.

I made a somewhat structural upstop for swingarm. It is a piece of 3/4"

marine plywood just ahead of the tyre and aft of the retracted shock

absorber. I have it glassed to the tunnel sides, and also screwed and

resting on a extruded aluminium channel that is drilled to look like swiss

cheese and glassed to the tunnel sides as well. 



If your joint is with no movement between gear lever and LG08 because of

tight fitting bolts, Redux and Mod to stiffen this joint, and you truly

had some overcentre, then my dart throw would be your 3.1" grew to make it

undercentre.



That said I suspect you simply did not have enough overcentre, and

probably undercentre at least on 1 stop like mine, and you know the rest.



Please do a thorough inspect of your gear, make sure no broken or bent

components, please post exact over or under centre you find. Check to make

sure there is a secure mount to swingarm for the bottom of shock absorber

stuff.



I have my gear slot set so I need to pull pretty hard on lever without

bungee but with full undercarriage weight installed, this is the time I

set my 3.1" and also set my overcentre.



Good Luck.



Ron Parigoris

rparigor(at)suffolk.lib.n
Guest

Posted: Sun Feb 25, 2007 7:09 pm Post subject: Undercarriage Overcentre Locking

Hello Graeme



Thinking about overall system, if one wing was slight low before main

wheel took full force just before ski jump occurred, if that low wing

outrigger latch decided to disengage, any rearward force on the outrigger

would equate to flap up force, and flap up force would equate to force on

starboard LG08 to be forced aft, or undercentre??



Was either of your outriggers collapsed?



Ron Parigoris

SteveD

Joined: 09 Jan 2006
Posts: 50

Posted: Mon Feb 26, 2007 3:05 pm Post subject: Undercarriage Overcentre Locking

All I can say is listen to Ron, he kept after me to check my gear about a year ago and it wasn't even close to being right. I don't think I would have made it through taxi testing.



I've added some marked up drawings. see links.



I made a centering tool per Rons instructions for the swing arm. 3/4 inch socket one wrap of tape, and a cut off piece of drill bit the same size as the lower rod LG04.



I used the back of a long hack saw blade with a 1/16 drill bit taped to it as a straight edge/ measuring tool. Just don't bend it while measuring.



Lots of grinding down of the stops, using Prussian blue to insure flat contact face. From the factory the stops were too long and did not have a flat contact face. I blued the stop, dropped the gear, raised the gear and the markers on the forks were thin "C" shapes.



Checks were done with complete gear, wheel, and bungee in place.



Once again,

Thanks, Ron.

Visit -  www.EuropaOwners.org



Attachments: 

http://www.europaowners.org//zfiles/tool_138.jpg

http://www.europaowners.org//zfiles/gear2_598.jpg

http://www.europaowners.org//zfiles/gear1_124.jpg

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