Hi all, having been following this discussion here's my random thoughts!
ASSUMING THE MECHANISM IS CONSTRUCTED AND ADJUSTED CORRECTLY then with
the gear down and locked the over centre lock cannot possibly "unlock"
itself (ie travel back undercentre) UNLESS SOMETHING BREAKS OR BENDS. An
overcentre lock is exactly that - a lock.
With the gear down the LG8 arms will be tight against the stops and will
be held there in place by the retraction lever which will be tight
against the FORWARD face of the DOWN gate. (That's important - any gap
here and the lever could travel forward a small amount in the gate thus
moving the LG8's back off the stops slightly and critically REMOVING the
overcentre).
With the LG8's correctly locked against the stops as above, then the
overcentre lock cannot possibly "collapse" undercentre. It doesn't need
much overcentre. Even a tiny amount will be enough to create a "safe"
lock. As long as there is "some overcentre" present it will stay
overcentre.
However, studying the diagrams again I then started to worry - and the
following is pure speculation! If due to a very unfortunate series of
oscillations (bounces!) in conjunction with undulationg ground, could a
freak occurance happen such that the rubber block compresses so much
that the swinging arm would then be "fired" downwards again with such
force that (as the shock absorber became extended again) the arm would
continue down with such force that it pulled on the LG8 arms enough to
actually pull them back a little off their stops? (Of course, something
would have to bend or break - probably the retraction lever - for this
LG8 movement to happen). It wouldn't need something to bend much, just
enough for the LG8's to move back to a small undercentre geometry. The
next ground contact would then finish the job. (One thing is sure, if
there is any "undercenter" present for any reason then nothing will hold
the plane up - the gear will collapse immediately). Sadly, any
"evidence" of damage caused by the first event would be completely lost
by the continuing damage caused during the subsequent gear collapse.
Could this scenario happen? I don't know but I feel it is most unlikely.
It needs someone with much more engineering knowledge (and knowledge of
energy stored in rubber blocks!) than myself to speculate, but I feel
that compared to the whole gear assembly the rubber block is tiny and
the swinging arm will have a lot of inertia. (The wheel at the end is
very heavy though.) Also the angles involved, even with correct
overcentre geometry are very small. Any "pull" downwards from the shock
absorber will be almost straight down and not much in the direction
required to pull the LG8's off their stops! Ironically, too much
overcentre could make this scenario worse?? - the angle will then be
increased and any pull downwards by the shock absorber will have a
greater pull on the LG8's in a rearward direction thus straining the
retraction lever. (Zero overcentre and the pull would be straight
downwards so no risk - but no safety margin either!). During development
one presumes that Ivan, Andy or Nev will have swung and bounced up and
down at great length to prove this can't happen!
A while ago in the UK the PFA issued a document detailing mandatory
undercarriage inspections at each annual which includes a check of the
geometry amongst other things. Compliance with this should be good for
peace of mind! If owners from outside the UK don't have this then it may
be worth getting a copy as it's good reading.
I love my monowheel and it's flying characteristics and think it's a
great design! I will continue with the (completely unproven) thought
that in the small number of uncommanded retraction occurances that have
happened (with the greatest respect and sympathy to the owners)
something must have been wrong somewhere. However I will be checking my
geometry carefully and will be trying not to bounce too much!!
Regards,
Jon Smith
G-TERN
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