Thorp Air Command - T18.net

Hello, I am in the process of moving my battery from behind the baggage compartment to the firewall. The Master solenoid was mounted to the battery box with a peice of hard plastic. Is the master solenoid supposed to be isolated from grounding?

The starter solenoid is mounted on the firewall so it is not isolated.

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Matt Smith
Des Moines, IA
55RC

Matt,
Yes, it needs to be isolated, for that reason most have plastic mounts. Grounding the master is what closes the circuit. In other words, the master switch is just a switch that just grounds and in turn closes the solenoid.

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

Thanks Lee,

So if I have this right, if the solenoid was not isolated it would ground through the case and the master solenoid would be "on" all the time, rendering the master switch useless? Or would it short out if the master switch is turned on?

How come the starter solenoid is not isolated?

Sorry for the probably rudimentary questions.

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Matt Smith
Des Moines, IA
55RC

Matt,
If the master solenoid was grounded you'd have a hot master at all times ... I.e. no "master off" (battery would not last long). I doubt turning on or off the master switch would do anything at all.

The reason the starter solenoid needs to be grounded is that unlike the master switch it is energized via a positive current and needs to be grounded to close.

Of course one is continuous duty (master) and the other momentary (starter) but thats not really relevant to this conversation.

Mount your starter solenoid sideways if youre mounting them fresh again ... As they get older the springs weaken and a few g's can engage the starter when you don't want it to be engaged.

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

Thanks again Lee,

I am not planning on moving the starter solenoid. I am not sure where I will mount the master solenoid, looking for room. I did read the old newsletter about not mounting it upside down but problem ended up being a connection in the dash. Any advantage to mounting it sideways?

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Matt Smith
Des Moines, IA
55RC

What style of starter are you using? Most of the new light weight starters have a built in solenoid and do not require a second solenoid.

Matt,
I don't see any advantage to putting the master solenoid sideways in our airplanes. The aerobatic guys in my neighborhood mount theirs sideways (along with the starter solenoid) to keep from engaging the starter (or shutting the master off) when pulling a lot of g's (+ or -). That being said I've mounted both of mine sideways on the box, on the firewall, very similar to RB's setup. Mostly for space-related reasons.

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

I don't see why we use two solenoids when the skytec has one mounted on the starter itself.
I have seen the starter solenoid "bounce" and engage the starter with a hard landing, as for the master, mount it any direction. Space reduction is accomplished by eliminating the starter solenoid.

I of course completely agree Willie but in Matt's case he not moving his stater sol. Just the battery and the master. As to why I have both, chalk that up to me being a cheap-azz and using what's on the shelf ... Lead brick bendix.

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

I was not aware the SkyTec had a
built in" solenoid ?

RB

It is an automotive-style design with a solenoid that engages the gears, as well as closing heavy-duty contacts to energize the motor. Keep in mind that the current draw of this solenoid is greater than that of a "normal" separate starter solenoid that only has to close electrical contacts. IMHO, you would do well to use a separate heavy-duty switch for your "start" function" if not using a separate start solenoid. That's what I did. The Bendix style mag/start switch will take a beating, even with a diode to suppress arching.

The "start" solenoid design varies from the "master" solenoid, in that the energizing coil terminates with one side on an insulated/isolated terminal, and the other grounded to the case. That way, a single "hot" wire (when switched on) energizes the coil, with the other side of the coil "grounded". The "master" solenoid generally has two separate, isolated coil terminals. One is connected directly to the battery, and the other is "grounded" through the master switch.

By the way, The manufacturer of the start & master solenoids that most aircraft use, does not recommend horizontal mounting. Here is a link to a White Rodgers site that specifies the recommended mounting position as vertical, with the dome or cap down.
http://www.emersonclimate.com/Docume.../R-4003web.pdf

Many of us have reasoned that the start solenoid should be mounted so that a hard landing, for instance, won't cause the contacts to "make", and the master just the opposite, so that gravity is working with it to keep the points closed. I don't personally think that that is too important. BTW, they do make solenoid switches that are designed to be mounted horizontally. The ones we use will usually work fairly reliably horizontally, but not always... If you take one apart & see how they're constructed, I think you'll mount it the way the manufacturer says.

There you go... everything you didn't want to know about solenoids.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

You're right Bill... space, weight, & an extra set of electrical contacts are all good things to consider if you use a starter like a SkyTec

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

To answer Rich, that small round cylinder that is attached along side the motor is a solenoid. Mostbuilders use a jumper from it to the main battery terminal. You can eliminate the second solenoid by removing the jumper and using the start switch to engerize the solenoid/starter. Most starter shops recommend doing it the later as it prevents solenoid lag when dis-engaging and extends the life of the starter.

In regards to using two solenoids with Sky-tec starters, Go to the Sky-tec web site for their explanation. Looks like starter switch capability has a lot to do with it.

Click troubleshoot, click tech talk, click experimental NL starter wiring option.

Hey John,
You said solenoids work "fairly reliably" mounted horizontally, "fairly" does not sound good. As you are probably the most meticulous person I know, can you elaborate on that a bit. Most of the guys in the local chapter swear they have to be mounted horizontal, I even pulled one apart a few years ago and from what I saw, that plunger weighs nothing, the spring seems more than capable of putting enough pressure on the plunger to keep it from laying down and binding. But again, I will bow to your expertise on this ... As I said anyone who knows you or has seen your work knows ... John doesn't half azz anything!

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Lee Walton
Houston, TX
N51863,N118LW
KEFD