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Questions: 17-5 & Sizing

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Phil.Perry(at)netapp.com
Guest

Posted: Mon Dec 28, 2009 6:05 pm Post subject: Questions: 17-5 & Sizing

Hi,  

   

I’ve been a lurker for awhile, mainly because I haven’t had to think too much about the electrical system of the airplane yet.  There is no doubt that I’ll be covering some well plowed ground with these questions, but maybe you can help get me off high-center.  

   

I am building an all glass RV-10 and I don’t plan on using Electronic Ignition.  

   

My first question involves figure 17-5 (Dual Alternator, Dual Battery Electrical System).  The narrative discussion on the crossfeed contactor is a little light and I’m having a hard time getting a handle on that diagram.

 

   

<![if !supportLists]>1)      <![endif]>I’d like to understand how the cross feed functions.    

<![if !supportLists]>a.       <![endif]>Is it an automatic switch?    

<![if !supportLists]>b.      <![endif]>Is it closed during normal operations?  

<![if !supportLists]>c.       <![endif]>I’m guessing it’s just like any other contactor and requires ~1 Amp current to maintain closure?

 

   

<![if !supportLists]>2)      <![endif]>Still on 17-5, couldn’t there be a diode placed between the “Main Battery” and the “Main Bus”?  For the cost of a diode, you could add a secondary path that is independent of any contactor.  The chances of needing to use it is highly unlikely, but it’s a really cheap way to save 2-3 Amps (Main Bat Contactor, Aux Bat Contactor, Cross Feed Contactor) if you ever need to shed off load.

 

   

<![if !supportLists]>3)      <![endif]>Finally I’m not finding much in the way component sizing.  Meaning I’m trying to figure out which specific diodes I will need.  Which specific model numbers/sizes of contactors I will need.  

   

   

Thanks for the help.    

As I get the architecture laid out, I’ll share it for review.  

   

Thanks,  

Phil  

   

      [quote][b]

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user9253

Joined: 28 Mar 2008
Posts: 1921
Location: Riley TWP Michigan

Posted: Tue Dec 29, 2009 7:36 am Post subject: Re: Questions: 17-5 & Sizing

Phil,

This is the way that I understand it.

1A.  The cross feed switch is not automatic.  You have to manually operate it.  Center is off.  Up (momentary spring-back) starts the engine using both batteries.  Down is on.



1B.  You would only move the switch into the down position if one of the alternators failed.



1C.  Probably



2.  If a diode connects the battery to the Main Power Bus, then how would you shut off the power in case of smoke in the cockpit?  You can accomplish your goal of adding a secondary path that is independent of any contactor by using an essential bus switch.  See Bob Nuckolls' drawing Z-13/8.

  I do not have figure 17-5 that you refer to.  From your description, it sounds similar to Z-14.  I think that Z-14 is desirable for flight over very long distances over hostile terrain.  But for travel within the contiguous U.S.A., Z-13/8 is more than adequate.  Using Z-13/8, your RV-10 will have a more reliable electrical system than most type certificated small planes.  And it will be lighter, less expensive, and simpler than Z-14.



3.  The diodes used for arc suppression are 1N5400 available at

http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/1N5400/?qs=sGAEpiMZZMtEwUVCuofpuI3LdHjUmCjoSwNVLo%2fSo6g%3d

The AeroElectric Connection sells a diode for the essential bus at 

https://matronics.com/aeroelectric/Catalog/AECcatalog.html  (Scroll down page)

Here is another suitable diode for the essential bus with a slightly higher voltage drop:

http://www.mouser.com/ProductDetail/Fairchild-Semiconductor/GBPC2510/?qs=sGAEpiMZZMtcLAek5QF0i29EdKsLUhPgdV8BgFVVYoM%3d

Joe

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_________________
Joe Gores

nuckolls.bob(at)aeroelect
Guest

Posted: Tue Dec 29, 2009 10:04 am Post subject: Questions: 17-5 & Sizing

At 07:59 PM 12/28/2009, you wrote:

Hi,



I've been a lurker for awhile, mainly because I haven't had to think 

too much about the electrical system of the airplane yet.  There is 

no doubt that I'll be covering some well plowed ground with these 

questions, but maybe you can help get me off high-center.



I am building an all glass RV-10 and I don't plan on using Electronic Ignition.



My first question involves figure 17-5 (Dual Alternator, Dual Battery 

Electrical System).  The narrative discussion on the crossfeed 

contactor is a little light and I'm having a hard time getting a 

handle on that diagram.



1)      I'd like to understand how the cross feed functions.

a.       Is it an automatic switch?

b.      Is it closed during normal operations?

c.       I'm guessing it's just like any other contactor and requires 

~1 Amp current to maintain closure?



    Yes, it's just like the battery contactors. It is closed for

    engine cranking so that both batteries can be used to start

    the engine. It's open for all normal operations. IF one

    alternator fails and IF your plan-b operations protocols

    call for sharing the output one alternator with the entire

    system, then you can close it manually for remainder of flight.



2)      Still on 17-5, couldn't there be a diode placed between the 

"Main Battery" and the "Main Bus"?  For the cost of a diode, you 

could add a secondary path that is independent of any contactor.  The 

chances of needing to use it is highly unlikely, but it's a really 

cheap way to save 2-3 Amps (Main Bat Contactor, Aux Bat Contactor, 

Cross Feed Contactor) if you ever need to shed off load.



    Why are you considering 17-5? The same system is described

    in more detail in the Appendix Z as figure Z-14. This system

    is much more expensive, heavier and takes up more volume in

    the airplane than say Z-13/8 . . .



3)      Finally I'm not finding much in the way component 

sizing.  Meaning I'm trying to figure out which specific diodes I 

will need.  Which specific model numbers/sizes of contactors I will need.



    You need to conduct a load analysis and then craft

    how you plan to use installed systems under the

    various failure modes.



    Figure Z-13/8 is probably everything that 99% of OBAM

    aircraft needs. It's lighter, simpler, a whole lot less

    expensive and offers system reliability on a par with

    a King Air.



    You won't find detail sizing information as the drawings

    are intended to be starting points for an architecture.

    Exact sizing of breakers, wire, and indeed electrical

    energy sourcing for each system still needs to be

    developed by the builder.



    How do you perceive that Z-13/8 will not meet your

    needs?

    Bob . . .

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