I take your point - the stop buffers on my aircraft were 1 mm different. We
have just rectified that with two 'top-hats' of 1mm differing thickness
installed onto cut-off buffer-tips. Now full, bilateral contact and a 2mm
over-centre action. My friends tell me I shall no longer need my little
latch-spring.... I'm not sure about that yet! H
----- Original Message -----
From: "Kingsley Hurst" <hurstkr@growzone.com.au>
Subject: Europa-List: Gear Unlatching
<hurstkr@growzone.com.au>
>
> > 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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