Jon, I think you are on the right lines here. Although a lock is a lock it
is clearly capable of being unlocked! If the plane takes off abruptly
having gone over a significant bump the unloading of compression will
encourage the swinging arm to continue downwards, putting tension on the
overcentre elements thus tending to bring them back into a straight line and
indeed travel on into an 'undercentre' unlocked position if the movement is
violent enough, and if the gear lever set up is loose enough.
From this point of view it might be argued that being further over
centre in the downlocked position might increase the hazard as more momentum
(of the overcentre locking elements) will be developed when the swinging arm
unloads downwards, increasing the likelihood of forcing itself undercentre.
If it starts significantly overcentre it effectively (like a good back row
forward) takes a run at it and gets up a fine head of steam by the time it
reaches the neutral centre position, and is inclined to carry on taking
anything in its path with it!
The fragility of the joint between the gear selector lever and the
rest of the system was brought home to me when I found after flying through
some fairly severe turbulence in rotor, that the gear lever stopped 1/4
inch short of the detent on putting the gear down. The inertial effects on
the swinging arm and wheel had been enough to shift the alignment of the
joint where the gear lever joins LG08S with three bolts. Others of course
have reported similar things after trailing the plane with the gear locked
in an up position.
My conclusions are:
(1) that the system is best set up being only just overcentre
(2) that the Gear lever/ LG08S joint has to be as solid as possible, ie
holes precisely to size and faces bonded, and the reinforcing mod added)
(3) that the detents for the gear lever should allow no significant
movement
Regards, David Joyce, G-XSDJ
----- Original Message -----
From: "Jon Smith" <jonsmitheuropa@tiscali.co.uk>
Sent: Wednesday, February 28, 2007 8:51 AM
Subject: Europa-List: Re. mono gear collapse
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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