Starting solnoids.

[Jack Wilhelmson]

I am posting on this subject to get it started and also to get some onions.

 

Several builders have ask me this question about the type and connections of the starting relay and I have found that some "experts" are giving answers that I don't agree with.

 

The first is that it is ok to route the starting current through the master power relay.

 

I disagree with this because the starting current can exceed the relay rating. Also the staring current drain may be so high that the battery voltage drops below the hold in voltage of the master relay. In this case the master relay will drop out under high current and ARC the contacts. The starting relay should have it's own feed from the battery. The master relay feed should have a high current "fuse of last resort(50 amp)" in it's primary feed.

 

The second is that it is ok to mount the battery on one side of the firewall and the staring relay on the other side.

 

The starting relay should be very close to the battery and on the same side of the firewall with the battery. Never run a unfused primary lead through the firewall. It is ok to run the heavy wire from the starting relay through the firewall because it is only hot during starting.

[John Slade]

Very timely info, Jack. Thanks.

I'm in the process of screwing things to the foreward side of the firewall. So far I have quite a list:

 

17ah rg main battery

main battery contactor

main battery bus

17ah rg alternate battery

alt battery contactor

alt battery bus

starter contactor

ground bus

60 amp current limiter

alternator field contactor

EC2 fuel injection & ignition computer

 

I've attached a picture.

Any comments or suggestions on this layout would be very welcome.

John Slade

[Jack Wilhelmson]

John:

 

I looked at your firewall picture and honestly I can't understand the hookup at all. Could you send me the full schematic you are working from?

 

Questions:

1. Why are the two master solonoids (normal and emergency) coils connected the the left and right ignition switchs?

2. It appears that the normal and emergency buses are direct wired together?

 

Have you considered putting the emergency power battery in the nose to help the weight and balance? I assume you would not use the emergency battery for starting current except as a booster for the other battery. I think that I will consider doing this on my next airplane. The advantage is that the wiring to the front battery can be lighter wire because all the thngs the emergency battey needs to run (except the ignition and fuel pumps) are up front.

[John Slade]

>honestly I can't understand the hookup at all.

Hmmm. I'm not so sure I understand it either.

I'm working from Bob Nuckolls's book drawings Z-11, 29 & 30.

 

>1. Why are the two master solonoids (normal and emergency) coils connected the the left and right ignition switchs?

That was my idea. I wanted to be able to select (and test) battery 1, battery 2 or both. With the ingition switch off I'd get essential bus power only.

 

>2. It appears that the normal and emergency buses are direct wired together?

No. The two busses you see on the firewall are always on battery busses - i.e. for hobbs, clock, fuel pumps (via switchs) etc. The main and essential busses are under the right armrest. See http://www.kgarden.com/cozy/chap22.htm for details and pictures.

 

>Have you considered putting the emergency power battery in the nose to help the weight and balance? I assume you would not use the emergency battery for starting current except as a booster for the other battery.

 

I considered it, but I'd like to use either 1, 2 or both for starting. I'm hoping the weight & balance will work out, but will move one or both batteries if necessary. These little 17ah rgs are pretty light anyway.

[Mevans]

Jack;

 

I agree, I run my starter selonoid directly from the battery, I did not want the starter to draw juice through any other part of the systems. There is an inline (50 AMP) fuse in the master relay to keep to much draw from going through the master relay. I will run one battery and one alternator. I like John's set up, but I have less space in my Long.

 

Matt

[John Slade]

Jack,

Given my immediate interest in these issues I asked Bob Nuckolls to comment on the above. He responded in the Aeroelectric list, but for the sake of continuity I'll summarize his message here in my own words...

 

1. Battery contactor is already closed when the starter is engaged. It can stand the amps without a problem.

 

2. We can avoid significant battery voltage drops by using well maintained RG batteries.

 

3. The biggest problem with wiring the starter contactor direct is the potential for a stuck starter contactor with no way to shut the starter down. Starter contactors stick more often than battery contactors.

 

4. Bob doesnt think that a 50 amp fuse in the primary feed is justified.

 

5. He says there's no reason not to run hot unfused fat wires through the firewall if its done right.

 

I don't know enough to argue with either of you guys - just relaying the information.

John Slade

[Jack Wilhelmson]

John:

 

I know that the "expert" you mentioned recomments this. I still disagree. There are several other minor reasons for not putting the starter solnoid on the engine side of the firewall and the battery on the other side. If a ammeter that is designed to read both inflow and outflow from the batery is used then the shunt for the ammeter, the main feed for the master solnoid, and the sensing wires for the ammeter will all have to penetrate the firewall or the alternator power lead will need to penetrate the firewall. These are all minor complications and proper fusing and protection can be provided with these realitive low current circuits. In any case, my question is, why should this be done, when it is easier and safer to

place all this on the other side of the fire wall. Stuck starter solnoids are rare, however, they do occur, but they occurr during startng and not during flight.

 

IF, the fat wires must run unfused through the firewall, then the usual rubber gromets are not adequate for protection because they tend to become brittle with age and split. The hole through the firewall should be insulated with a hard insulator bushing (nylon, Teflon or similar material.

[John Slade]

Thanks for the comments, Jack. My starter solenoid will be on the cabin side of the firewall for some of the reasons you outlined, and so I can connect it to the battery contactor with a brass strip.

 

Even on the ground a runaway starter can be a real problem, so on balance, or until I fry my first battery contactor, the starter current will go through the battery contactor.

 

I just ordered some teflon bulkhead pass throughs for the fat wires.

 

All I have to do now is screw it all together and see if it works.

John Slade

 

PS - Bob Nuckolls shouldn't have quotes around the term "expert". His knowledge and experience in aircraft electronics is ledgendary. This doesnt mean he's infallible, but he sure knows more than I do.

[niceez]

Jack,

 

One thing that has happened on more then one occasion is the aircraft pulling enough G’s to cause the solenoid to engage in flight. If you were trying to avoid something in your flight path it would be very possible for this to happen. Mounting the solenoid so the contactor is sideways will help.

 

Another point. Going through the firewall with any amperage is not a concern if accomplished using normal practices. I can think absolutely no reason for concern here. Maybe you could bring shed some food for thought here.

 

Having two batteries for starting I think is all right for a ground vehicles but nonsense in an airplane. All that is required is to switch positions of the two batteries to make the airplane start and in reality, how often is that going to happen? What single engine (piston) certified airplane uses two batteries?

 

Dale

 

PS Ol' Bob K. does know his stuff.

[John Slade]

Jack,

 

Dale,

>One thing that has happened on more then one occasion is the aircraft pulling enough G’s to cause the solenoid to engage in flight.

 

This was discussed and generally described as "an old wives tale" in the aeroelectric list fairly recently. I forget the details, but I think "Ol' Bob K." said something about it taking more G than you or you're airplane will ever take. As I remember, the concensus was that this is not an issue.