Master contactor question

You have to have a master contactor (relay). You wouldn't want to run
all that electrical power through a master switch. And the power for
the starter goes through the master contactor first on it's way to the
starter contactor. How would you wire a small plane without one?
If you wanted to use a switch to carry all the loads, that would be
one hell of switch to carry the juice on the way to the starter when
the engine is started. The electric hydraulic gear motor in my
homebuilt is a pretty high draw item also. And don't forget, for
planes that have 12 volt systems, they need big wires and lots of
juice running through them to power the big items.
Rich

On Sun, 1 Nov 2009 09:37:53 -0500, "Peter Dohm"
wrote:

"rich" wrote in message
.. .
My homebuilt's master contactor is going bad. Sometimes when I turn it
on it doesn't make connection. It's got 1700 hours on it, so I'd just
as soon replace it. But the way the builder wired it, he's has
positive power from the battery going through the master switch to the
small terminal on the contactor. (cole-Hersey type) But the master
contactors, such as Spruce sells, are set up to actuate with ground
power going to the small terminal. They also have plastic around their
mounting feet so their case doesn't make ground. A starter contactor
would work perfectly with the way the plane is wired. I just wonder,
are starter contactors made to withstand continous use, like a master
contactor does? And how can one tell the difference in the two, they
look identical? And if not, can the master/continuous duty type be
made to work with postitive power to the small terminal?

I've been reading this thread with modest interest and a little amusement.

At the moment, I am not entirely sure why a "typical" homebuilt would use a
master contactor and I suggest that you take a look at what the professional
designers may have done. For example, to the best of my recollection, the
Cessna 150 and 152 and also the Piper Tomahawk had starter contactors (a/k/a
solenoids) and a had master breakers that also functioned as switches; but
did not have master contactors--and I really have difficulty understanding
why a well designed aircraft in that size and weight range would need one.

I suggest that you determine whether your battery is located in an unusual
way and then ask a mechanic what was used in reasonably similar factory
built aircraft. For example: I would certainly expect a master contactor
in a Piper Cheyene; but I would not extpect to find one in a Cherokee 140.

Of course, as always, YMMV.

Peter

On Sun, 01 Nov 2009 18:13:53 -0500, rich
wrote:

You have to have a master contactor (relay). You wouldn't want to run
all that electrical power through a master switch. And the power for
the starter goes through the master contactor first on it's way to the
starter contactor. How would you wire a small plane without one?
If you wanted to use a switch to carry all the loads, that would be
one hell of switch to carry the juice on the way to the starter when
the engine is started. The electric hydraulic gear motor in my
homebuilt is a pretty high draw item also. And don't forget, for
planes that have 12 volt systems, they need big wires and lots of
juice running through them to power the big items.
Rich
If the starter solenoid is at the battery the only time the high
current conductor is live is when cranking - and virtually all the
rest of the loads can be handled by a "main switch".

The charging circuit is the only part of the wiring that can get
tricky - and there are ways around that too.

Peter Dohm wrote:
....
I've been reading this thread with modest interest and a little amusement.

At the moment, I am not entirely sure why a "typical" homebuilt would use a
master contactor and I suggest that you take a look at what the professional
designers may have done. For example, to the best of my recollection, the
Cessna 150 and 152 and also the Piper Tomahawk had starter contactors (a/k/a
solenoids) and a had master breakers that also functioned as switches; but
did not have master contactors--and I really have difficulty understanding
why a well designed aircraft in that size and weight range would need one./snip/

Peter



Hmmm...as it happens my C150 doesn't have a starter solenoid - its a
pull handle; but it does have a master contactor, like the vast majority
of airplanes, large n small.

Brian W

wrote:
...
If the starter solenoid is at the battery the only time the high
current conductor is live is when cranking - and virtually all the
rest of the loads can be handled by a "main switch".

The charging circuit is the only part of the wiring that can get
tricky - and there are ways around that too.

This is true for cars.

Brian W

brian whatcott wrote:
Peter Dohm wrote:
....
Hmmm...as it happens my C150 doesn't have a starter solenoid - its a
pull handle; but it does have a master contactor, like the vast majority
of airplanes, large n small.

My Fly Baby handles most of the 'lectric with a ordinary switch, and a
pull handle for the starter. No Master Contactor.

Adding one is on my list of things to do. It's a serious PITA to be
unable to totally kill the power to the avionics. The guy who built my
airplane put the avionics in a box above the battery.

(here's a shot showing the box on the floor:

http://www.bowersflybaby.com/tech/hhrad2.JPG

There had been no way to disconnect the battery WITHOUT removing the box
with the avionics first. I really dislike moving live electronics,
especially since it entailed first removing the stainless steel foot
panels. One of the panels slid as I was trying to get it out and
shorted the battery...exciting when there's no way to get to the battery
to disconnect it without removing the thing that's glowing red hot.

A dumb design, which I modified to an extent several years ago...now I
can disconnect the battery from below.

http://www.bowersflybaby.com/tech/BATTERY%20BOX.JPG

Unfortunately, there's still no way to easily kill the power to the
starter switch. When I bought the plane, it didn't even have rubber
boots on the bare connectors.

I added boots, but even they're not perfect. When I replaced my tach
with an electronic one, I had to safety-wire a cap over the tach drive.
It was a bear to get the wire started, so I fed in a long piece of
safety wire.

Sure enough, the back end of that piece of wire slipped under the rubber
boot on the battery hook-up to the starter switch. I heard a crackling
sound, then saw smoke rising from the safety wire. I (of course)
immediately grabbed the wire.

The very, VERY hot wire.

http://www.bowersflybaby.com/tech/tach_owie.jpg

Not only did I release it PDQ, it had apparently self-welded to the
contact so my second-degree burn was for naught. I grabbed the safety
wire pliers with my other hand and pulled the wire clear.

Having a system without a Master Contactor is certainly possible, and
probably ultimately more reliable, but I think the ability to totally
kill power to the aircraft is a big safety plus.

Ron Wanttaja

Ron Wanttaja wrote:
http://www.bowersflybaby.com/tech/hhrad2.JPG

Even at zero knots the FUN meter appears to be on high - that's quite an
aeroplane!

Jim Logajan wrote:
Ron Wanttaja wrote:
http://www.bowersflybaby.com/tech/hhrad2.JPG

Even at zero knots the FUN meter appears to be on high - that's quite an
aeroplane!

Ron also has a picture of his Flybaby being aerial refueled by a KC-10.

Dan, U.S. Air Force, retired

Dan wrote:
Jim Logajan wrote:
Ron Wanttaja wrote:
http://www.bowersflybaby.com/tech/hhrad2.JPG

Even at zero knots the FUN meter appears to be on high - that's quite
an aeroplane!

Ron also has a picture of his Flybaby being aerial refueled by a KC-10.

Well, not *my* Fly Baby, but....

http://www.bowersflybaby.com/stories/longrange.jpg

My plane is based in a valley endangered by a damaged dam; the hangar
may get flooded with one to three feet of water with about five hours
notice. A friend suggested that I build a raft and just park the plane
on it when it sits in the hangar.

I've got an antique-looking airplane, so I needed an antique-looking
raft....

http://www.bowersflybaby.com/FB_raft.jpg

Ron Wanttaja

I have it from a reliable source that changing digital photos is
impossible, so your antics truely impress me.

how did you get the aircraft on the raft without a single smudge or
dirty hand mark getting on the aircraft?

I remember seeing a photo of moonbum breaking the sound barrier so I'm
sure you could get out of there with 5 hours notice.

stealth pilot

Stealth Pilot wrote:
I have it from a reliable source that changing digital photos is
impossible, so your antics truely impress me.

how did you get the aircraft on the raft without a single smudge or
dirty hand mark getting on the aircraft?

I remember seeing a photo of moonbum breaking the sound barrier so I'm
sure you could get out of there with 5 hours notice.

stealth pilot


No dirty hand prints because he wore gloves? What I want to know is
why he needed such a large crew and why the raft is clothing optional.
It does, however, show Ron is a cheap host; notice there's no bar or
chairs. Better hope no one notices he doesn't have enough flotation
devices to go around.

Dan, U.S. Air Force, retired