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Basic Electrical Architecture for review...

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steve(at)tomasara.com
Guest

Posted: Wed Sep 30, 2009 7:00 pm Post subject: Basic Electrical Architecture for review...

Greetings all,



I've attached an electrical architecture (in Bob's drawing style) for 

your review.  It's for the Longeze I'm restoring.  I'm doing the 

restoration in multiple stages.  Although it had ~1000 Hrs. on it when I 

obtained it, it's been grounded since I received it. The attached 

architecture is intended to be more robust and fault tolerant than what 

was there (which I'm sure it is...) and certainly not worse in that 

regard than a TC aircraft (though not as good as a design with an 

essential bus).



Down the road a bit, I intend to replace the vacuum system and 

associated instruments with electric, plus a backup alternator and an 

architecture with an essential bus.



I would appreciate comments on the architecture and, specifically, on 

any unmitigated faults (especially those that would not be acceptable in 

a TC aircraft).



The aircraft is VFR only.



Thanks in advance,



Steve Stearns

Boulder/Longmont, Colorado

CSA,EAA,IAC,AOPA,PE,ARRL,BARC (but ignorant none-the-less)

Restoring (since 1/07): N45FC O235 Longeze Cothern/Friling CF1 (~1000 Hrs)

Flying (since 9/86): N43732 A65 Taylorcraft BC12D

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user9253

Joined: 28 Mar 2008
Posts: 1920
Location: Riley TWP Michigan

Posted: Thu Oct 01, 2009 10:42 am Post subject: Re: Basic Electrical Architecture for review...

Steve,

Yes, the 20 amp fuse should be replaced.  A 22awg fuselink will work.

Since your plane does not have a starter, you could use a relay instead of a contactor, perhaps one like this: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=255-1830-ND

A relay weighs less and will not use as much power.

You could buy two relays, one to replace the battery contactor and one to remotely control power to the essential bus, thus having the ability to isolate all electrical power on the engine side of the firewall.

Depending on how your main power bus is constructed, you might be able to cut it in half and then connect the two halves with a diode, thus making an essential bus.  It would be relatively easy to add a switch and wire to complete the essential bus circuit.

   Dual alternators are not necessary for a VFR plane unless the engine is electrically dependent and you fly long distances over hostile terrain.

  Putting a diode between the load side of the battery contactor and main power bus along with a circuit breaker from the battery to the bus defeats the purpose of having a contactor, which is the ability to shut off electrical power at the battery without bringing a large always-hot wire into the cockpit.

Yes, the ANL fuse protects the battery and wire against alternator short circuits.

Joe

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_________________
Joe Gores

steve(at)tomasara.com
Guest

Posted: Mon Oct 05, 2009 7:47 am Post subject: Basic Electrical Architecture for review...

Bob and the group:

Quotes from previous exchange between me (>) and Bob (additional indent):

 	  Quote: 	
		   **      >- Removed the primer (As I also removed it from the aircraft.  This 

       >might also be added back later in a more appropriately wired and 

       >plumbed manner)

       

          Hmmmm . . . must be a REALLY easy-to-prop engine . . .

 **

Probably not.  At least that's not what I'm assuming.  I just won't be 
	


priming it with an *electric* primer.



 	  Quote: 	
		   **      >- Redefined the avionics ground bus to be an audio ground bus.  My 

       >Nav/Com uses a differential audio path allowing separate audio/power 

       >grounds.  All the avionics power grounds, with the exception of the 

       >low-power intercom return, terminate at the PNL Gnd bus.  All audio 

       >grounds (including the intercom power ground) terminate at the Audio Gnd bus.

       >- Reduced the number of connections tying the PNL ground to the 

       >Audio ground as it no longer carries any significant current.

       

           Can't comment on this without having understood/crafted

           the system . . .

 **

In case you were interested in following up, I've attached my detailed 
	


schematics.  They don't (yet) reflect the intended change to pull the 

regulator power though a fuse-link instead of a 20A fuse.  If it's too 

much to want to wade though that's fine (I often feel that way about it 

too).

 	  Quote: 	
		   **      >- Replaced the ANL with one sized to the existent alternator.

       

           okay . . . recall too that ANLs are VERY robust . . .

           you can go as small as ANL30 with plenty of headroom

           for 50A alternator.

       

 **

I believe my alternator is a 40A but I haven't yet pulled it off and had 
	


a shop test it.  The smallest B&C had when I ordered was 40A so I went 

with that...

 	  Quote: 	
		   **      >- Replaced the shunt with one sized to match the existent Ampmeter.

       

           You have a 50A alternator?

       

 **

I went with the 50A shunt only to match the range on the ammeter.  At 
	


some point I expect I will pull both the Ammeter and shunt and ebay them 

as a pair so I figured it made sense for them to match...  I should send 

you a picture of my original shunt.  It was home made (presumably by the 

original builder) via wire, a large brass cotter pin, solder and 

heat-shrink.  It was within 10% but I wouldn't have wanted to overload it!

 	  Quote: 	
		   **          Re-read the explanation for an E-bus in Appendix

           Z notes and chapter 17 on reliability. The E-bus

           is for maximizing a limited resource (battery energy

           stored) during alternator-out operations.  It's also

           a plan-B during battery contactor failure. Highly

           recommended.

       

 **

Ok.  Just re-read the section in Appendix Z (fresh download), will 
	


re-read chapter 17 when I get back to the hangar (and my book).  Once I 

finish that, I need to refresh my load analysis and anticipated battery 

size (including a fresh weight and balance which will be a while).  It's 

pretty typical with LongEZs (esp. with a starter which I no longer have) 

to need nose ballast and this is often accomplished with an oversized 

battery.  Not sure where mine will come in as I don't trust the last 

recorded weight and balance.



And from your other follow-up email:

 	  Quote: 	
		   **      >- Removed the Battery Bus as I have nothing that would attach to it.

       

           Correction, how about the electronic ignition. It has

           its own power switch and is an excellent candidate

           for battery bus power. If you got bad smells in

           the cockpit you can power down the whole electrical

           system without reducing engine support. Fuel pump

           might run from battery bus too.

 **

Hmmm,  Given your suggestion above, I'm now considering using a 
	


fuse-link from the always hot side of the contactor (my current "battery 

bus" such as it is...) to the electronic ignition.  This leads to an 

interesting dilemma.  The manual for my electronic ignition calls out a 

15A fuse protecting 18AWG wire.  I presume the seemingly excess fuse 

size is to provide headroom for surge or pulse current.  IF that is the 

case, I would think a fuse-link appropriately sized for 18AWG wire (i.e. 

22AWG) would be appropriate protection (and better than a 15A fuse).  

Your thoughts?



I like the idea of keeping redundant sources of ignition if I have a 

contactor failure.  However, if I decide to also put the fuel pump on 

the battery bus, then I could either use another fuse-link for it (a 

24AWG link seems appropriate) off the always hot side of the contactor 

or actually use a real fuse block battery bus for both the elec. 

ignition and fuel pump. 



OR (I don't think I need both approaches...)

I could just put in the alternate feed path to the endurance bus (which, 

in my case, is the only bus with manual load shedding).  It seems like I 

could use the 16AWG fuse-link (as on Z12) as the protection for the 

(proposed by me) panel-mounted switch/breaker for the alternate feed.  

However, this would required me to either find an isolation diode 

capable of my full electrical load (inc. lights).  And, of course, 

requires that I also install the regulator feed fuse link to the 

switched output of my contactor (as I previously pondered) rather than 

downstream of the diode.  Or, I need to bite the bullet and implement 

both a main bus and endurance bus (per Z11) so your AEC9001 would be 

enough to handle the endurance bus loads.



Given that I haven't yet re-read chapter 17 or refreshed my load 

analysis, this may change.  But as of now, I think the approach that 

best meets current objectives is to use fuse-links for the elect. 

ignition and possibly the fuel pump (not sure how I feel about the fuel 

pump switch being hot when the master switch is off. I guess I feel ok 

if I clearly indicate that on the labeling) from the always hot side of 

the contactor.  And to not implement the endurance bus approach until I 

enter the "enhancement" project down the road.



Best regards and thanks again for your input,



Steve Stearns

Boulder/Longmont, Colorado

CSA,EAA,IAC,AOPA,PE,ARRL,BARC (but ignorant none-the-less)

Restoring (since 1/07): N45FC O235 Longeze Cothern/Friling CF1 (~1000 Hrs)

Flying (since 9/86): N43732 A65 Taylorcraft BC12D

 



**      

**

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