brian
Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA
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 Posted: Fri May 26, 2006 8:41 am Post subject: A mechanical model of the electrical system (was: Grounding |
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On May 26, 2006, at 8:12 AM, Mickey Coggins wrote:
| Quote: |
matronics(at)rv8.ch>
>> In my case in my RV Super 8 with my Batt in the back, I grounded it
>> locally & ran a 12ga to the front, from the battery, for
>> grounding of
>> all the 'stuff' to a single point. I have a perfectly quiet
>> electrical
>> system. For all practical purposes, only the starter &
>> alternator, are
>> using the airframe ground.
>> Mike
>
> Not a good deal. That long ground isn't really a ground. Please
> consider using the single point ground block mounted on the
> firewall
> as depicted in the Connection and making all cockpit and forward
> accessory grounds at that point limiting your local airframe
> grounds
> to the list cited in my posting of last night.
I'm confused. Why wouldn't Mike's 12ga wire extended to an
isolated forest of tabs be a ground?
I considered doing the same, but decided to ground the
forest of tabs to the front of the aircraft as well.
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I tend to be a visually-oriented person. I am going to try to paint a
mental picture for you that will let you see how all this stuff works
the way it does with the picture in my mind.
Since it is hard to visualize what electrons do when they flow
through a conductor I came up with a graphic physical representation
that models things pretty well. I think of wires and conductors as
stretchy things that go between the floor (ground) and the ceiling
(source of power or positive bus). The distance between the floor and
ceiling is the voltage. Think of a wire as a sort of bungee cord that
gets longer the harder you pull on it. The pull is current and how
far it stretches is the voltage drop. The amount of stretch for a
given current is the resistance. Fat wires don't stretch as much as
thin ones for a given pull (current). If you pull too hard on a
bungee it breaks (wire burns through).
If the wire "bungee" is connected to ground (the floor) the more I
pull on it (pass current through it) the more it stretches away from
the floor (ground potential) and it has some voltage above ground.
Likewise if I attach a wire to the ceiling (positive bus) the harder
I pull on it the more it stretches toward the floor. The end that is
attached to my load, e.g. light, radio, whatever, is below the level
(voltage) of the ceiling. How much it is below the level of the
ceiling (voltage drop) depends on the current draw and the thickness
of my wire.
So your airplane is a forest of these things stretching from floor
(ground) to ceiling (positive bus). Each wire is represented by a
bungee whose thickness is a function of wire thickness and each load
is a thinner bungee that stretches without breaking. In a perfect
world the load bungees would make it all the way from floor to
ceiling with no added distance but we have the wires which stretch a
little bit themselves so the stretch of our load isn't quite the full
distance.
I hope this is making sense so far and everyone can see the picture
in their minds.
Let's use this to model Mike's 12awg wire from ground to his isolated
forest of tabs. The 12awg wire is itself a bungee. One end is
attached to the floor and the other end has an eye bolt to which we
attach all the other "ground" bungees from all our loads. Each of our
loads adds more pull to our ground wire as it is attached. Our ground
wire "stretches" a little more each time another load is attached.
Now imagine we have a load that pulls and lets go (current increase
and decrease such as a flashing light) over and over. If we look at
our ground wire it will be bouncing up and down just a little bit as
the pull changes. This "bouncing" will be transmitted to all the
other ground wires and therefore to their loads. Everything will
start bouncing up and down in time with the load that is switching on
and off. The only way we can reduce the bouncing is to make the
ground wire fatter so it doesn't stretch as much when you pull on it.
This is why we use "fat" ground wires where we can. By the way, this
bouncing is current-induced noise in the ground.
Now to make the model a bit more realistic I am going to change our
"floor" and "ceiling" to be more realistic. Even the floor and
ceiling have some "stretch" and "bend" to them. Think of our ground
and positive buses as being like the cantilever spar of our wing and
anchored at one point each -- the positive and negative terminals of
our battery. As we attach our loads farther and farther out on our
buses (spars) there is more flex when we pull on them. Other loads
that we attach to the spar will also "feel" the flex of our buses as
we draw current from them.
I need to run and do some other stuff today but I will come back and
post again using this model to explain how varying current in one
place can be picked up as noise in a different place. I will also use
this model to explain the issues of using the airframe as our ground
bus and how a single-point ground will help eliminate noise.
OTOH the teacher invites others in the classroom to think about this
as homework and come back and explain how attaching things to our
ground "spar" (bus) can induce noise in our avionics if you don't use
a single point ground. 
Brian Lloyd 361 Catterline Way
brian-yak AT lloyd DOT com Folsom, CA 95630
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
I fly because it releases my mind from the tyranny of petty things . . .
Antoine de Saint-Exupry
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_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery |
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