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curious questions

 
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aurbo(at)ak.net
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PostPosted: Sat Apr 14, 2007 7:35 am    Post subject: curious questions Reply with quote
Hello,

A friend of mine that occasionally helps me work on my RV-9 has asked me some questions that I can't answer satisfactorily. He is a great guy and has a very curious mind, he owns and works on boats, cars, etc, but doesn't have a history with aircraft. These are the questions in his own words posted below. Any help will be gratefully accepted.


In the course of helping a friend construct an RV 9 I had the opportunity to examine the electrical system design. I was puzzled at the use of a master relay (solenoid) to switch the entire electrical system. A number of questions occurred to me regarding this design:<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />
The questions: How much amperage draw does the solenoid consume simply being energized? At what voltage does the Solenoid release? And why this design?

It seems to me inserting the master relay (solenoid) into the electrical system has no benefit and some detriment.
The detriments are: 1. constant draw on battery when energized. 2. greater complexity. 3. increased weight (all 6 oz of the solenoid) 4. failure of the complete electrical system if a) the solenoid fails, or b) if you have alternator failure and voltage drops low enough to allow the solenoid to not close/open (we have all heard the clicking of the starter solenoid on a run down car battery- but the radios still work).
Contrast this with the benefit- uh- what benefit? There is the same amount of current flowing to the panel from the battery whether it comes via the relay or via a straight run to a master switch. Starter current continues to utilize the starter solenoid. Alternator output current can be routed directly to the battery since the field voltage can be turned off via the master switch (and as I understand the design, inserting a fuse in the alternator output line).

My limited knowledge may be impeding my appreciation of the function of the master relay- but I have been pondering this- and can’t come up with a good rationale. On large aircraft, where you have significant electrical loads that are distant from the cockpit, it would make sense to use a relay so that you minimize voltage drop and/or weight by eliminating long wiring runs. But this doesn’t seem to be the case here. The argument regarding electromagnetic fields near radios does not hold up since the loads are the same whether switched through a relay or not.
I guess the bottom line is- There are some VERY smart people building these aircraft. Surely there is a good reason for the master relay design- I just want to know what it is?
In the course of helping a friend construct an RV 9 I had the opportunity to examine the electrical system design. I was puzzled at the use of a master relay (solenoid) to switch the entire electrical system. A number of questions occurred to me regarding this design:
The questions: How much amperage draw does the solenoid consume simply being energized? At what voltage does the Solenoid release? And why this design?

It seems to me inserting the master relay (solenoid) into the electrical system has no benefit and some detriment.
The detriments are: 1. constant draw on battery when energized. 2. greater complexity. 3. increased weight (all 6oz of the solenoid) 4. failure of the complete electrical system if a) the solenoid fails, or b) if you have alternator failure and voltage drops low enough to allow the solenoid to not close/open (we have all heard the clicking of the starter solenoid on a run down car battery- but the radios still work).

Contrast this with the benefit- uh- what benefit? There is the same amount of current flowing to the panel from the battery whether it comes via the relay or via a straight run to a master switch. Starter current continues to utilize the starter solenoid. Alternator output current can be routed directly to the battery since the field voltage can be turned off via the master switch (and as I understand the design, inserting a fuse in the alternator output line).
My limited knowledge may be impeding my appreciation of the function of the master relay- but I have been pondering this- and can’t come up with a good rationale. On large aircraft, where you have significant electrical loads that are distant from the cockpit, it would make sense to use a relay so that you minimize voltage drop and/or weight by eliminating long wiring runs. But this doesn’t seem to be the case here. The argument regarding electromagnetic fields near radios does not hold up since the loads are the same whether switched through a relay or not.
I guess the bottom line is- There are some VERY smart people building these aircraft. Surely there is a good reason for the master relay design- I just want to know what it is?
Blue Skies,

Mike Ice
RV-9, electrical almost complete, just laid up fiberglass skirts for the canopy last night,
next up: motor mount, install gear legs, then engine

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lm4(at)juno.com
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PostPosted: Sat Apr 14, 2007 4:40 pm    Post subject: curious questions Reply with quote
In the course of helping a friend construct an RV 9 I had the opportunity to examine the electrical system design. I was puzzled at the use of a master relay (solenoid) to switch the entire electrical system. A number of questions occurred to me regarding this design:
[quote]
The questions: How much amperage draw does the solenoid consume simply being energized? At what voltage does the Solenoid release? And why this design?
______________________________________________________________________________
1. I can't say for sure but solonoids of this type usually draw between 700ma and 1A.
2. They don't release. They stay on as long as the master switch is on.
________________________________________________________________________________

It seems to me inserting the master relay (solenoid) into the electrical system has no benefit and some detriment.
The detriments are: 1. constant draw on battery when energized. 2. greater complexity. 3. increased weight (all 6 oz of the solenoid) 4. failure of the complete electrical system if a) the solenoid fails, or b) if you have alternator failure and voltage drops low enough to allow the solenoid to not close/open
Consider this. You want to connect a battery to your entire airplane,except the ignition system on certified acft. This will supply as much as 60 amps to the acft systems. If you can't disconnect it your battery will go flat overnight. So what will you use to connect/disconnect. How about a little 6oz. solonoid and a tiny, lightweight, 5 amp switch. I think that would be much better than a big old 50 amp switch in the cockpit along with the heavy cable run to the switch and then to the main buss. Now consider starting the engine. with a single wire system , and with any nav equipment left on, when you hit the starter the 200 to 300 amps that will be pulled from the battery on a cold day will cause a voltage drop that will finish off your nav equipment. So what to do ? Run a cable to a Master solonoid and run very light wires thru the firewall to the master switch. Then run a jumper cable from the first contact point of the master solonoid to the current carrying terminal of a second solonoid. The starter solonoid. And from there to the starter. And then run a couple of very light wires thru the firewall to a very light starter switch. Save a few lbs.. Save a lot of panel real eatate. HTH
Larry Mac Donald
lm4(at)juno.com (lm4(at)juno.com)
Do not archive

[b]

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N1DeltaWhiskey(at)comcast
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PostPosted: Sun Apr 15, 2007 9:02 am    Post subject: curious questions Reply with quote
I'll give this a shot - specific response embedded below.

Doug
[quote] ---

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aurbo(at)ak.net
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PostPosted: Sun Apr 15, 2007 4:19 pm    Post subject: curious questions Reply with quote
Doug,

Thanks for the very good reply. I forwarded this to my friend and I am sure he will like it better than my usual, "Because that is the way it says to do it on the Z-11 drawing".

Mike Ice
[quote] ---

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nuckollsr(at)cox.net
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PostPosted: Mon Apr 16, 2007 4:58 am    Post subject: curious questions Reply with quote
At 04:18 PM 4/15/2007 -0800, you wrote:

Quote:
Doug,

Thanks for the very good reply. I forwarded this to my friend and I am
sure he will like it better than my usual, "Because that is the way it
says to do it on the Z-11 drawing".

Good job Doug.

Bob . . .

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