> Talking about latches....
> Yesterday, with the gear down and latched..................I had a violent
drop
> ..............and the gear unlatched and with turbulent air the flaps
pushed the
> gear up.
Hedley,
Your posting and others I have heard of with the gear unlatching prompts me
to raise a problem I had with the gear plus a few other points.
When I installed my undercarriage mechanism, I found it quite difficult to
insert the pins which join the retraction arms to the damper assy. On
investigation, I discovered that although both retraction arms were firmly
against the over centre stops, when viewed from the side, both arms were not
parallel to each other. In other words, the holes in the ends of each
retraction arm were not exactly aligned with each other. The misalignment
was in the order of 2 mm and it was obvious that the over centre stops were
not the same length. On checking the same items on another builders frame,
I found that his were also in error so I suspect there is more than two
undercarriages around the world with this problem.
The fix - I shortened the longer stop and repinned the arms after turning
the main retraction shaft 90 degrees. The improvement both in ease of pin
insertion and U/C retraction was very noticeable. The U/C is now as smooth
as silk to extend and retract and I can insert the pins with my fingers at
any point of the retraction cycle.
Now to my first point. If others have this same problem unbeknown to them,
there is a good chance at some time an uncommanded retraction may occur
because it is quite conceivable that one retraction arm might get slightly
over centre the wrong way and fight the other one trying to do the right
thing by us all. Remember we are talking about very little movement here.
When the arms are over centre, they are only about 2mm over if my memory
serves me correctly and if my measurements were accurate.
The other points I would like to make in general about the retraction
mechanism are:-
1 The retraction lever should be such that it wants to naturally spring
into the detent at the down end of its travel. This means that when the U/C
is locked down, even if the locking latch is raised, the lever should not
want to exit the detent without positive force being applied by the pilot
albeit it only a relatively small force.
2 With the U/C extended and the lever locked in the down detent, the
retraction lever should have no perceivable movement for and aft. It is
necessary to check this with no weight on the main wheel otherwise the
forces applied by the over centre locking will give a false impression of no
play when in fact there could be.
3 Of course, it goes without saying that there should be no relative
movement between the retraction lever and the retraction arms.
If the over centre geometry of the retraction arms is correct, and the above
three points are complied with, IMHO, uncommanded retractions should be non
existent. Others more intelligent than I may be able to shoot me down here
so I will sit back and wait for the flack.
For the benefit of any builders who may never have had any experience with
over centre devices, I would like to make one final comment.
The purpose of the U/C lever locking in the down detent is to hold the
retraction arms fully against the over centre stops AT ALL TIMES. It is not
meant to take any loads as a result of the wheel trying to retract as the
wheel is trying to do when the aircraft is sitting on the ground. In the
down position, the only time the lever should feel any load is when the
aircraft bounces for any reason because the downward travelling wheel tends
to want to pull the over centre mechanism straight and in doing so wants to
make the lever move in the retraction direction. Hence the need for point 2
above.
AND notwithstanding what I have just said, I WILL BE installing a little
grasshopper spring on the latching pawl of the U/C lever "Just In Case".
Apologies to those I may have bored or if I am only stating the obvious.
Regards
Kingsley Hurst
Mono Classic 281 in Oz
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