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-   -   Judging actual alternator need (https://vansairforce.net/community/showthread.php?t=136261)

RV10inOz 04-01-2016 07:28 AM

We have 3 screens, GTN750, stormscope, old skool strobes and lights??etc??.60A is fine.

If I could have a 70-80A I would though.

Better looking at it than looking for it!

grubbat 04-01-2016 07:41 AM

Low amps
 
I went with low amp led lights and just A 35 amp accessory pad mounted alternator. Less weight and efficient. Autoelectric is great source of info.

bill@fusion4.net 04-01-2016 08:09 AM

3 screen g3x with 650, all lights are LED, VPX, and a iphone and ipad charging on USB, and see 18 amps with all lights on. Only thing not on would be flaps/AP servos and Pitot heat. Have a 60 amp primary and 30 amp on vacuum pad. Would be fine really with the 30 amp only.

MK77 04-01-2016 12:19 PM

Quote:

Originally Posted by Carl Froehlich (Post 1066736)
Unless you are running 747 landing lights, a 60 amp alternator is fine.

Now - don't forget that you have responsibility on how you run your electrical system. The first job of your alternator is to recharge the battery. If you jump a dead battery to get the engine started and launch into IMC, even with an alternator putting out more than needed to keep up with what you have on you will have no battery reserve if the alternator fails.

Determine your reserve electrical capacity need (in flight time and then into amp/hrs) and then design and operate to always have at least that much available.

Carl

True! That reminds me of a DA42 crash several years ago. The DA42 has dual ECUs that control every aspect of the engine and prop. The pilot came out to find a dead battery so he jumped the battery and took off. When he pulled the gear up after takeoff the voltage dropped just low enough to cause an ECU failure and both engines died. After that they implemented backup batteries for each ECU.

Bevan 04-01-2016 01:08 PM

Quote:

Originally Posted by MK77 (Post 1067313)
True! That reminds me of a DA42 crash several years ago. The DA42 has dual ECUs that control every aspect of the engine and prop. The pilot came out to find a dead battery so he jumped the battery and took off. When he pulled the gear up after takeoff the voltage dropped just low enough to cause an ECU failure and both engines died. After that they implemented backup batteries for each ECU.

Really? This causes me to loose a lot of faith in the engineering of newer certified aircraft. And now it's a bandaid solution namely, "lets add another battery because we didn't predict the effects of the first one failing". What makes them think they know the failure mode and implications of the backup battery failing? i.e. how do you monitor the condition of the backup battery, how do you know it's failure mode? Will a bad backup battery cause a cascading electrical failure? Why not have a backup battery to operate the gear? Just to keep this RV related I will say, I feel my home brew electrical system is more robust and tolerant than this relatively new design you speak of.

BillL 04-01-2016 03:25 PM

I went through this exercise and it was a really eye opener looking up all the conditions and details for power draw. In the end, it will be a documentation in phase i for the various flight configurations and amp draw.

Like a strobe, use surge or continuous, will it blow a fuse during surge? I did not know, but Stein said I would be fine so I moved on. 60A PP.

Regardless of reaching a definitive answer, building the amp list and being adding checks for configurations to yield a total is a good thing to think through. It will be interesting to see how close the real numbers will be.

One plus is LED landing lights, rather than the original planned incandescent.


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