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Electrical System Drawing

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stickandrudder1(at)comcas
Guest

Posted: Sat Jul 16, 2011 6:44 pm Post subject: Electrical System Drawing

Attached is a draft electrical schematic based primarily on Bob Nuckolls' Z-13 electrical system drawing. Note that I plan on using an internally regulated Plane Power 30A backup alternator, which is supposed to be out by Oshkosh. If not, I'll use the B&C SD-20. The reason for the second 7.2 AH battery is to insure field current to the backup alternator in the event of main battery failure. I have thought of using a second full sized battery as the AUX, which I would connect in parallel with the main battery for more cranking power.  



 Questions I have are:

 

 Is an Avionics bus & switch unnecessary or is it still a good idea? I like the idea, and I know the GTX 327 is supposed to be powered off during engine start. Also the GTN 6xx install doc says it should be on an Avionics bus. Others have suggested (and even said that Bob Nuckolls doesn’t think much of the idea in his book – although I couldn’t find anywhere he discussed it) that the avionics bus is unnecessary with modern avionics, adds complexity and additional potential points of failure. Bob doesn’t show it on the drawings I’ve studied, so I’m wondering.   



 What are the failure modes of the EBUS Diode? Will it ever fail open? If it fails and allows reverse current, what is the best way to detect the failure?

 

 My schematic has a battery bus drawn, but I really don't see a need for it. Other than a clock and interior lights, is there something that I have placed on another bus that really should be on a battery bus?

 

 

 What about the location of the backup alternator?

 I show it connected to the essential bus instead of the un-switched side of the battery contactor, in the event of a dead short in the battery. I'm not sure if that's a failure mode I need to even consider.  

   

Another thing I’ve though of is although I haven’t done a load analysis yet, but I’m pretty sure with a 30A backup alternator, I could combine the Avionics and E-Bus busses (the only things that would be added to the E-BUS would be Com2, Transponder and audio panel.  

   

Any comments and criticism of the design would be greatly appreciated.  

   

The aircraft I am building is an RV-7A. The mission is day/night IFR.  

   

   

Thanks Paul

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Drawing7.pdf

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stickandrudder1(at)comcas
Guest

Posted: Sat Jul 16, 2011 6:50 pm Post subject: Electrical System Drawing

Attached is a draft electrical schematic based primarily on Bob Nuckolls' Z-13 electrical system drawing. Note that I plan on using an internally regulated Plane Power 30A backup alternator, which is supposed to be out by Oshkosh. If not, I'll use the B&C SD-20. The reason for the second 7.2 AH battery is to insure field current to the backup alternator in the event of main battery failure. I have thought of using a second full sized battery as the AUX, which I would connect in parallel with the main battery for more cranking power.  



 Questions I have are:

 

 Is an Avionics bus & switch unnecessary or is it still a good idea? I like the idea, and I know the GTX 327 is supposed to be powered off during engine start. Also the GTN 6xx install doc says it should be on an Avionics bus. Others have suggested (and even said that Bob Nuckolls doesn’t think much of the idea in his book – although I couldn’t find anywhere he discussed it) that the avionics bus is unnecessary with modern avionics, adds complexity and additional potential points of failure. Bob doesn’t show it on the drawings I’ve studied, so I’m wondering.   



 What are the failure modes of the EBUS Diode? Will it ever fail open? If it fails and allows reverse current, what is the best way to detect the failure?

 

 My schematic has a battery bus drawn, but I really don't see a need for it. Other than a clock and interior lights, is there something that I have placed on another bus that really should be on a battery bus?

 

 

 What about the location of the backup alternator?

 I show it connected to the essential bus instead of the un-switched side of the battery contactor, in the event of a dead short in the battery. I'm not sure if that's a failure mode I need to even consider.  

   

Another thing I’ve though of is although I haven’t done a load analysis yet, but I’m pretty sure with a 30A backup alternator, I could combine the Avionics and E-Bus busses (the only things that would be added to the E-BUS would be Com2, Transponder and audio panel.  

   

Any comments and criticism of the design would be greatly appreciated.  

   

The aircraft I am building is an RV-7A. The mission is day/night IFR.  

   

   

Thanks Paul

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Drawing7.pdf

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Jul 17, 2011 7:09 am Post subject: Electrical System Drawing

At 09:46 PM 7/16/2011, you wrote:

Attached is a draft electrical schematic based 

primarily on Bob Nuckolls' Z-13 electrical system 

drawing. Note that I plan on using an internally 

regulated Plane Power 30A backup alternator, 

which is supposed to be out by Oshkosh. If not, 

I'll use the B&C SD-20. The reason for the second 

7.2 AH battery is to insure field current to the 

backup alternator in the event of main battery 

failure. I have thought of using a second full 

sized battery as the AUX, which I would connect 

in parallel with the main battery for more cranking power.



Questions I have are:



Is an Avionics bus & switch unnecessary or is it 

still a good idea? I like the idea, and I know 

the GTX 327 is supposed to be powered off during 

engine start. Also the GTN 6xx install doc says 

it should be on an Avionics bus. Others have 

suggested (and even said that Bob Nuckolls 

doesn’t think much of the idea in his book – 

although I couldn’t find anywhere he discussed 

it) that the avionics bus is unnecessary with 

modern avionics, adds complexity and additional 

potential points of failure. Bob doesn’t show it 

on the drawings I’ve studied, so I’m wondering.



    Go to AeroElectric.com and do a search on avionics

    master.  The original ideas behind the birth of the

    avionics master was a figment of the designer's

    imagination. The enduring rationale for the AV

    master is not based on physics or facts.

What are the failure modes of the EBUS Diode? 

Will it ever fail open? If it fails and allows 

reverse current, what is the best way to detect the failure?



    They don't fail.

My schematic has a battery bus drawn, but I 

really don't see a need for it. Other than a 

clock and interior lights, is there something 

that I have placed on another bus that really should be on a battery bus?



    Do you have an electrically dependent engine?

    Those items should be on a battery bus.



    If you're going to have two alternators and

    two batteries, suggest Z-14 instead.



What about the location of the backup alternator?

I show it connected to the essential bus instead 

of the un-switched side of the battery contactor, 

in the event of a dead short in the battery. I'm 

not sure if that's a failure mode I need to even consider.



Another thing I’ve though of is although I 

haven’t done a load analysis yet, but I’m pretty 

sure with a 30A backup alternator, I could 

combine the Avionics and E-Bus busses (the only 

things that would be added to the E-BUS would be 

Com2, Transponder and audio panel.



Any comments and criticism of the design would be greatly appreciated.



    I know there are compelling notions of "doing

    it a little bit better" but as your list of

    questions illustrates, making changes generates

    new uncertainties. The Z-figures have already

    been subjected to the same questions over the

    last 20+ years. By now they are 99.9% free

    of unknowns.



    Suggest you study Z-14 and develop a list

    of questions that address and perceptions

    you have of not meeting design goals for

    your airplane. It's much more comfortable

    to start with a proven recipe for success

    and then see if serious design goals are

    not being met.



    If it were my airplane, I'd save a LOT of

    dollars, weight and worry with Z-13/8. That

    system offers overall reliability that a

    King Air doesn't have. Only in the last 20

    years has Z-12 found its way onto type certificated

    aircraft in large quantities. The vast majority

    of TC aircraft use the same architecture as

    a 1946 C-140A Your concerns about failures of diodes

    and batteries suggest that you're not

    comfortable with the idea that reliability of

    these things is very high when used with

    understanding.



    It's far better that you have a SIMPLE

    system you understand than a NOT SO SIMPLE

    system born of mis-understanding.

   Bob . . .

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nuckolls.bob(at)aeroelect
Guest

Posted: Sun Jul 17, 2011 11:00 am Post subject: Electrical System Drawing

At 09:46 PM 7/16/2011, you wrote:

  	  Quote: 	
		   Attached is a draft electrical schematic based primarily on Bob Nuckolls' Z-13 electrical system drawing. Note that I plan on using an internally regulated Plane Power 30A backup alternator, which is supposed to be out by Oshkosh. 	


    Do you really NEED that much back-up power?

    What are the load analysis numbers for the

    minimum loads to get you to airport of intended

    destination while saving the battery for

    approach to landing?



  	  Quote: 	
		    

 Another thing I’ve though of is although I haven’t done a load analysis yet, but I’m pretty sure with a 30A backup alternator, I could combine the Avionics and E-Bus busses (the only things that would be added to the E-BUS would be Com2, Transponder and audio panel. 	


    Loan analysis needs to be first . . . before

    you size alternators. Also, keep in mind that

    intermittent loads like landing lights, transmit

    current are not continuous running loads.



    Can you get your ENDURANCE loads down to 8A

    or less?



    If you do a search of unhappy events in the cockpit

    leading to bent airplanes and/or bleeding people,

    the percentage of total incidents involving electrical

    system failures is exceedingly small. A majority of

    those instances could have been mitigated by

    better pilot understanding and good preventative

    maintenance.



    It's comforting to add redundancy on top of band-aid

    on top of worry issue . . . but all of these have

    costs of ownership.  I passed a big pile of first-aid

    kits in the pharmacy at Walmart last week. I think they

    were $5. If Wallyworld sells them, they must move out

    the door in significant numbers.  I've been driving

    for over 50 years, owned dozens of cars and probably had

    at least a half-dozen first-aid kits of one form or another.

    I can recall no single instance of ever using the contents

    of such kits . . . nor finding myself in a position of

    wishing I had one.



    You are far more likely to enjoy a high order of

    reliability by careful study of what's need should

    any single device fail . . . and developing a plan-B

    for that failure. The goal is not to keep everything

    lit up all the time, only those things needed for

    getting to airport of intended destination so that

    the battery alone will get you on the ground with

    lights a blazing both inside and outside the cockpit.



    Every serious accident we've studied here on the list

    that grew of out electrical systems issues had

    root cause in poor craftsmanship and/or poor judgement

    and lack of understanding of the system. Adding more

    hardware will generally not do much to offset those

    human frailties.



 

 

   Bob . . .    [quote][b]

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