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Z-12 Questions

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jpx(at)qenesis.com
Guest

Posted: Mon Nov 12, 2007 11:04 am Post subject: Z-12 Questions

I have now read my new Aero Electric Connection cover to cover.  Even  

for someone with a university education in electronics, it was still  

fabulously enlightening.  Thanks Bob !

One of the things I have already learned is that Bob has designed  

everything as it is for a particular and often unapparent reason.

So I have questions before I change anything 



I am building a night VFR aircraft which will have a Dynon as the main  

instruments and engine monitoring.  I will probably have a real ASI  

and altimeter, since I find them much easier to read and I can happily  

fly without the Dynon then.  We take long distance trips, so I am more  

concerned with failures that make it difficult to fly 10 hours home  

than flight safety to get back on the ground, since I am usually  

flying in nice weather.



In my 400 hours of flying, I have already had an alternator failure in  

a rental aircraft.  Although it was uneventful really, I would not  

want to fly a long trip home without any electrics.



So Z-12 looks like a good fit for me, with an SD-8 as the backup  

alternator (brilliant idea this product).



Figure 17-4 shows an SD-8 charging the battery directly, whereas Z-12  

is like 17-8 with the two alternators are in parallel, and thus the  

SD-8 cannot charge the battery with the contactor open.  I don't think  

the B&C SB1B-14 regulator is intended for the SD-8.  So wire the SD-8  

like figure 17-4 ?



It is easy to put a long list of items on the endurance bus.  However,  

maybe that doesn't matter.  The likely failure modes need to be  

analyzed.  In the event of an alternator failure, switches can be used  

to turn off unnecessary devices, so it doesn't matter which bus they  

are really on (just contactor hold current).  In the event of an  

electrical fire, the battery contactor will be switched off, so only  

the necessary items for an immediate emergency landing need to be on  

the endurance bus.



Maybe an electrical fire is far less likely since I will be using  

fuses almost entirely.  Unlike a circuit breaker that cannot be  

trusted to pop, the fuse will likely blow.  No fire, smoke or smell,  

so usually no need to turn off the battery contactor ??



The diode tying the main bus to the endurance bus seems unnecessary.   

The main bus could easily be wired to pin 3 of the e-bus alternate  

feed switch.  This implies the diode is more reliable than the switch  

(which is only switched once to test at each run-up).  If it is a  

reliability concern, why not wire the switch and keep the diode ?  Or  

is there another reason I haven't even guessed at ?



I will appreciate everyone's input.



Jeff Page

Dream Aircraft Tundra #10

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nuckolls.bob(at)cox.net
Guest

Posted: Tue Nov 13, 2007 6:53 am Post subject: Z-12 Questions

At 02:00 PM 11/12/2007 -0500, you wrote:



 	  Quote: 	
		  



I have now read my new Aero Electric Connection cover to cover.  Even

for someone with a university education in electronics, it was still

fabulously enlightening.  Thanks Bob !

One of the things I have already learned is that Bob has designed

everything as it is for a particular and often unapparent reason.

So I have questions before I change anything 


	


    Good thinking . . .

 	  Quote: 	
		  I am building a night VFR aircraft which will have a Dynon as the main

instruments and engine monitoring.  I will probably have a real ASI

and altimeter, since I find them much easier to read and I can happily

fly without the Dynon then.  We take long distance trips, so I am more

concerned with failures that make it difficult to fly 10 hours home

than flight safety to get back on the ground, since I am usually

flying in nice weather.



In my 400 hours of flying, I have already had an alternator failure in

a rental aircraft.  Although it was uneventful really, I would not

want to fly a long trip home without any electrics.



So Z-12 looks like a good fit for me, with an SD-8 as the backup

alternator (brilliant idea this product).


	


   Why not 13/8? Z-12 is a standby alternator retrofit

   philosophy for airplanes already flying. Not really

   intended for new design.

 	  Quote: 	
		  Figure 17-4 shows an SD-8 charging the battery directly, whereas Z-12

is like 17-8 with the two alternators are in parallel, and thus the

SD-8 cannot charge the battery with the contactor open.  I don't think

the B&C SB1B-14 regulator is intended for the SD-8.  So wire the SD-8

like figure 17-4 ?


	


    Let's not mix/match features across architectures.

    Concentrate on lowest parts count (which translates

    to minimized weight, $time$ to install and cost of

    ownership) that meets mission goals.

 	  Quote: 	
		  It is easy to put a long list of items on the endurance bus.  However,

maybe that doesn't matter.  The likely failure modes need to be

analyzed.  In the event of an alternator failure, switches can be used

to turn off unnecessary devices, so it doesn't matter which bus they

are really on (just contactor hold current).  In the event of an

electrical fire, the battery contactor will be switched off, so only

the necessary items for an immediate emergency landing need to be on

the endurance bus.


	


   It does matter . . . you need to set you own

   design goals for performance in the endurance mode

   of en route flight. With one alternator, battery

   capacity is used to support en route loads so we

   strive to keep them very low . . . say 2-3 amps

   so that there are some reserves in the battery for

   approach to landing.



   As soon as you add the SD-8, NOW endurance loads

   can be up to 8A and hold the battery completely

   in reserve for approach to landing.

 	  Quote: 	
		  Maybe an electrical fire is far less likely since I will be using

fuses almost entirely.  Unlike a circuit breaker that cannot be

trusted to pop, the fuse will likely blow.  No fire, smoke or smell,

so usually no need to turn off the battery contactor ??


	


   I've never seen a breaker refuse to open on a

   hard fault. But they DO allow more energy to be

   driven into the fault condition than their faster

   cousins. But yes, circuit protection of either

   variety can be expected to avoid bad smells in the

   cockpit.

 	  Quote: 	
		  The diode tying the main bus to the endurance bus seems unnecessary.

The main bus could easily be wired to pin 3 of the e-bus alternate

feed switch.  This implies the diode is more reliable than the switch

(which is only switched once to test at each run-up).  If it is a

reliability concern, why not wire the switch and keep the diode ?  Or

is there another reason I haven't even guessed at ?


	


   The diode IS more reliable than a switch. Further,

   the modification you suggest runs BOTH power paths

   for the e-bus through the single component . . . i.e.

   single point of failure for power to the e-bus.

 	  Quote: 	
		  I will appreciate everyone's input.


	


   If I were building an airplane today, Z-13/8 is my

   architecture of choice for kind of aircraft I would be

   building and the manner in which I plan to use it. I

   suspect that Z-13/8 would be most adequate for 98%

   of the OBAM aircraft being licensed every year.



   Do your load analysis. Decide which devices will

   reside on which busses. Then starting with Z-13/8,

   deduce what failure mode or design goal is not

   being accommodated.



   Bob . . .

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