This mail has been composed after consultation with a number of builders
who have been waiting in vain for some days to see reactions to the "Main
Gear" letter, mod no. ??. Is it shock, resignation, or apathy ? The
mods.in this area have been getting more severe each time at significant
expense to the factory and the builders in terms of both effort and cash.
In the beginning the bungy block, (a re-usable energy absorber) seemed
like a good idea until it turned out that in returning the energy, it
bounced the aircraft (quickie-wise) back into the air. Is this why it has
never be used before ? Although I have driven an American car with piston
mounted bumpers, modern car protection is based on crumple zones as near
the accident as possible, not on springy systems. Introduction of stops
increases the load transfer when they are reached, and a hydraulic damper
(returning the energy progressively) was apparenty is not sufficiently
large in the space available to do the job. The various mods to the frame
and its stops have resulted in isolated case(s) of buckling which means
we have an inbuilt crumple zone which might be a bit too weak.
Unfortunately in the absence of recorded data from these instances, the
actual load to which it has been subjected is not known. Quite rightly
the factory set out to remove this doubt with drop tests, which seem to
have shown that it has to be a very abnormal load to damage the frame,
but nevertheless have offered more strengthing to prevent it. But is this
the correct thing to do ? Leaving aside the difficulty and/or expense and
delay of doing the latest mod. to complete or near-completer aircraft, it
will certainly increase the possibility of passing more load beyond the
frame into the tunnel/fuselage. The tests carried out with the frame
only, cannot tell us where the buck(le) stops now. So please can the
factory repeat this test with at least a dummy tunnel and bulkhead in
place if not a real fuselage, before we do this mod. only to find that
the next time the fuselage collapses. If a problem is then found, a
number of suggestions are available, such as removing the block assembly
and replacing it with two or more hydraulic dampers as used by many
conventional undercarriages, motor cycle forks etc.
gemin
|