No Master/Starter Solenoid - Alternate way to wire
 

I was thinking of a better way to wire the aircraft. It seemed over kill to have three solenoids under the cowl:

FW Master: Has to be large to handle up to 300 amps flow thru to the starter

FW Starter: It is redundant.

Starter: Modern aircraft starters (SkyTec) all have their own solenoid

If you wire the battery to the starter direct you will by pass the firewall starter relay, which is not longer needed. Also since you no longer have to feed the starter current thru the master you can use a small relay (solid state) to supply the "switch" master buss in the cockpit. The mess on the fire wall has always looked terrible and has many large connections. This alternate way will save +2 lbs and almost 1 amp of wasted current (12 watts) in wasted power to the master contactor. Picture below:



I think this will work with no loss in function or safety. I suppose you could argue that the starter engages and back-drives as a generator. What is the chance of that? Slim or nil. Well if it is a concern add the firewall starter relay back. I see no reason to have a big old firewall master relay.

George

Her is a little more detail with NipponDesno ALternator (I-VR) wired in:

Even if you choose to use a starter with a built-in solenoid, it would be a good idea to include a "battery master" solenoid. If the starter solenoid gets "stuck" in the engaged position, you need a way to disconect the battery from the starter.

Dave Cole
RV-7 wings

George,
I looked into this idea when I wired my -6 back in the early '90s. I cannot recommend this method of wiring the starter. Besides the possibility of the starter solenoid sticking with no way to disconnect, you will have a rather large wire running through the maze of engine tied directly to the battery. If this wire broke or had the insulation rubbed away and shorted to ground, you have the potential of a great fire without any way of removing power. I've been an A&P for over 30 years and you'd be surprized how often this happens.
Just my $.02.
Mel...DAR

Starter worries
 

Yes thanks for the comments.

It seems the big hang up is having a big #2 wire hot all the time and fire. I guess that is the reason for the fuse. The main worry to me would be fuel. If you route, secure and protect the starter wire on the front and right side (away from the fuel lines, gascolator, fuel pump) the risk should be small, but point taken. Also if you are forced landing you will turn the fuel off, pump off and master off. There is no guarantee you will not have a fire with any wiring design as long as there is fuel on board. Looking at "standard" wiring, electrically HOT connections from the battery to master relay and jumper to starter relay can cause a short or spark.

The second hang up (no pun intended) is the starter sticking. This is something I did not think about, but I did not think that could happen, at least with modern staters with out a Bendix drive. If somthing like that happened during start, it stays stuck (during start), shut the engine down and don't turn the master/alt switch off?

How is the starter going to engage by itself and stick?
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What skytec says:

[i]Stuck firwall solenoids: "What about the Bendix? Maybe it stuck.

Since Sky-Tec starters do not use mechanical Bendix drives to actuate the starter, this is actually nearly impossible for a Sky-Tec starter to keep itself engaged with the aircraft ring gear. Sky-Tec starters are electromechanically engaged therefore requiring voltage to engage the starter's drive pinion gear with the ring gear. Without voltage, the pinion simply cannot remain in the flywheel. A spring and a helical return will both force the drive pinion back out of the ring gear and into the rest position.

If utilizing a Bendix starter, then yes, this very well may have caused the problem (and likely did - it is a very common failure mode of starter Bendix drives).[i]

(ref http://www.skytecair.com/Cessna_Solenoids.htm)

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So I am not sure, could you guys educate me a little about stuck starters. It may be an old issue based on old starter designs? I know it is bad when the starter is engaged in flight, such as if you hit the starter button or the firewall solenoid (if you have one) gets pulled down/on during high G maneuvers (that is why we mount them upside down). If this happens it can act as a generator and cause electrical havoc. Based on the starter design I doubt this is an issue.

Any more info would be great. I guess you could always add the starter firewall solenoid back in, while leaving the master off. The master is the power wast since it is on all the time (about .8 amps or 12 watts). The FW starter solenoid should give you protection from the starter, if you are worried. Also it eliminates any worry about hot big wire causing fire, although I think there is a slim chance of that. With this you could leave off the +300 fuse and still leave off the master relay.

This is what Skytec has to say about run-on:
http://www.skytecair.com/Wiring_diag.htm
http://www.skytecair.com/StarterRunOnLights.htm
http://www.skytecair.com/images/Starter_RunOn_Doc.pdf

Apparently the R22 chopper has no FW starter relay.

Thanks for the input George