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Marv,
Here is the latest on over center link mounted switches.
--For those on this list who have not seen the article to which Marv referred
yesterday, I have attached a copy (unfortunately minus the pictures) below.--
The over center link mounted switches are doing well. Adding them was
incredibly easy since no landing gear parts had to be altered. This new set-up
has been flying for a little over 100 hours and I have not had any problems it.
I am still flying with both sets of landing gear indicators. They are two
completely independent systems. Since the new system has been installed I did
again receive an intermittent indication from one of the original switches. The
O/C link mounted switch indicated a locked condition and the gear was actually
locked.
Yes, the wires to the O/C link mounted switches must go through some bending
when the gear is cycled. The trick is to keep the radius large and wire gauge
small. I installed the wires three times before finding a path that I liked,
one that minimized bending and would prevent the wires from getting caught on
any of the moving parts in the wheel well. The bending seen by these wires is
now no worse than the mic switch wires coming out of the control sticks and they
are cycled far less often. I will most likely never remove the old system
simply because of the redundancy it offers.
Chris Zavatson
N91CZ
christopher_zavatson@udlp.com
--------------
3 Green and Landing Gear Micro Switches.
When flying aircraft with retractable gear, one of the greatest moments of
anticipation occurs while you wait for those three little green lights to
illuminate upon selecting gear down. I don't know what the GA rate of gear
system failure is, but I have had five gear incidents in my 300 hours of retract
time: One was in a certified plane, four in experimentals, three of which were
in my 360. Only in the certified plane was there a true system failure, the
others where all indication problems.
The three incidents in my 360 where traced to the method by which the down and
locked position is sensed by the micro switch on the main gear. To determine if
the links have actually gone over center, one needs to detect the position of
the two links relative to each other. On the 360, the micro switches monitor
this only indirectly. They are positioned such that they measure the relative
position of the upper over center links to the aircraft structure. Assuming the
relative position of this link to the fuselage is always the same on all axes
when the gear is down and locked, this approach would be quite adequate and
simple to install. In our case, a roll pin in the link depresses the lever on
the micro switch. The problem with this set-up lies in the fact that both ends
are supported by bearings that allow rotation about a line drawn between the
bearings. Rotation about this line, when the gear is down, will raise and lower
the position of the roll pin relative to the micro switch. A large washer on
the mounting bolt of the upper over center link is used to prevent this rotation
or at least most of it. If you adjust your micro switches to trip only when the
links are truly over center, not much movement is required to indicate an unsafe
condition. This little bit of movement can occur by forcing the linkage to
rotate as described above. The quickest remedy is to adjust the switches such
that they activate a little earlier, possibly just before the links go over
center. By doing this, any amount of pushing or pulling on the linkage will not
be able to give a false unsafe condition. The draw back is that you get green
lights while the gear is not yet locked. From a practical standpoint, if the
gear has gone down 95% of the way to trip the switch a little early, chances are
it will travel the remaining 5% and actually lock.
For some time I toyed with the idea of mounting the switches in such a way that
they could only activate when the links were truly over center, completely
independent of the above effects. I like solutions that are inexpensive, simple
to build, and reversible. That way if it doesn't work I haven't lost a lot of
time or money and I can have my old parts back. The enclosed picture shows the
set-up I am now testing. It only requires an aluminum plate with four
properly located holes and some hardware. I removed the levers from some spare
switches, essentially converting the switches to push button operation, and
mounted them on the upper links using the aluminum plates. The actuator is an
AN3 bolt, some washers and a nut installed in an existing hole in the lower
link. This configuration only shows green when the links are truly over center.
It is very precise and makes the locked indication completely independent of
any movement or rotation of the linkage relative to the airframe when the gear
is down. I have not removed the original switches or wiring, but instead added
an additional set of lights for testing this new set-up.
The entire landing gear system, as designed, is mechanically simple which helps
make an actual system failure a rare event. During my false indication
incidents, I trusted the gear more than the indication, but I took the
opportunity to buzz the control tower anyway.
Chris Zavatson
Woodland, CA
[Thanks for the update, Chris... just goes to show you, there's always
more than one way to get something done. Keep us advised if there's any
change in the operational status of those new switches. <Marv> ]
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LML homepage: http://www.olsusa.com/Users/Mkaye/maillist.html
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