>There is something else to consider in installing solenoids (besides making
>sure you have the right one in each application). You should install them
>so that gravity assists the magnetic force in holding the contacts
>together. Do not install them upside down!
I recall a story circulated around OSH about ten years ago. Seems
some show pilot landed and found a chewed up starter ring
gear and his starter contactor was stuck shut. Some arm-chair
engineering on the spot deduced that high g-loading during aerobatic
maneuvers were responsible for "teasing" the contacts in flight
and causing welding much like low battery voltage. The word
went out like wildfire . . . tho shalt mount thy contactors
UPSIDE down so that positive g-loading would not tend to close
an open relay.
The physics don't bear out any particular admonitions for orientation
in the airplane. Once energized, a contactor has about 10x the force
holding it closed than it takes to first move the contacts from a fully
open position. Since a battery contact is ALWAYS closed, it's likely
that you'll pull the wings off your airplane before you force the
contactor open during a flight maneuver.
Starter contactors (like our S702-1) have extra heavy springs
to open them (to offset sticking tendencies) and extra heavy
coils to close them (to offset contact bounce and subsequent
damage from arcing while closing the high current load). G-loading
effects on these contactors is even less significant than for
the S701-1 continuous duty part. Further, starter contactors are
normally mounted on firewall with base on vertical plane. This
orientation puts g-loading sensitivity parallel to the longtitudinal
axis of the airplane . . . don't fly into the side of a mountain, you
might cause your starter contactor to close when you didn't want it
to.
By in large, all of the stories being circulated about contactor
orientation and are not founded in the application or physics of
the matter. IF the airshow pilot was using a poorly choosen device
as a starter contactor (like our S701-1) AND it was oriented
such that gravity helps close or keep the contacts closed, then
it's thinkable that a 10g maneuver might have teased his starter
contacts closed in flight. If he were using a REAL starter
contactor installed accoding to recommendations, it would never
happen. Maneuvering g-loads in airplanes you and I like to fly
are not a risk to your various contactors.
Bob . . .
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( Knowing about a thing is different from )
( understanding it. One can know a lot )
( and still understand nothing. )
( C.F. Kettering )
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http://www.aeroelectric.com
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