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What do you think of this idea?

DaleB

DaleB

Well Known Member
I really want a keyed ignition/start switch that will disable all the bits needed to start the plane, but I'm not wild about paying over a hundred bucks for the usual mag/start switch, which has features I simply don't need. I plan to use P-mags, so I have a separate plan for the power and P-lead wires.

So I'm thinking this may be a solution. A marine type 3-position keyswitch (OFF/IGN/START) controls the main battery contactor and functions as a MASTER switch. The main bus from the master contactor supplies power via fuse blocks (or breakers if you prefer) to everything else. The starter contactor is controlled by the START position on the keyswitch.

There would be no traditional MASTER rocker or toggle switch. Just an ALTERNATOR switch. In case of an in-flight electrical emergency (like a fire) you could turn the keyswitch off, dropping power to everything -- but the P-mags would still fire under their own power until switched off.



Anyone see a problem with doing it this way? Anything I missed or got horribly wrong? This is by no means complete or finished, I am many moons out from doing the panel and FWF wiring. Just hashing out how I want things to work. I got a lot of ideas from the Nuckolls book, but I see those diagrams as a starting point rather than a final solution. This looks awfully simple, but I can't see that I'm giving up anything. And for the record -- the mission is day VFR (maybe day/night VFR depending on rules changes), and I'll have backup instrumentation for airspeed, altitude, and heading that don't rely on ship's power.

I'm open to comments and suggestions.
 
Hidden APRS

I used a marine type 3-position keyswitch (OFF/IGN/START) with a traditional master. The IGN position controls the APRS transmitter.
 
Another option:

Separate toggles for each mag, momentary toggle or pushbutton for start, and either one or two toggles for Master and Alternator(as fits your opinion of whether they need to be separate or not). If you're concerned about security or safety when parked, a hidden (behind panel somewhere) "master enable" switch could kill everything. And: no need for another key on your keychain.
 
Snowflake said:
Another option:

Separate toggles for each mag, momentary toggle or pushbutton for start, and either one or two toggles for Master and Alternator(as fits your opinion of whether they need to be separate or not). If you're concerned about security or safety when parked, a hidden (behind panel somewhere) "master enable" switch could kill everything. And: no need for another key on your keychain.
Click to expand...

Thanks for the comments. Originally I had the two toggles for the mags (as it is now), one for master/alt, and a start button. I really don't mind having a key, and it would live on its own key ring with the hangar key - which worked perfectly for me when I was in a flying club. It wouldn't be more of a hassle than groping around for a hidden switch.

I'm not terribly concerned about security, since I haven't heard anything about a rash of RV thefts, but it's a factor. I"m more thinking about safety when parked in a spot where I may not be with it constantly. I know some guys don't bother with a keyed switch, but if something did happen - either safety or security related - I'd hate to spend time explaining why I thought something as basic as a keyed ignition switch was unnecessary.

Another spin I had on this idea was to use a 3-position toggle OFF/BATT/ALT master switch, then put the engine related bits (boost pump, mag power and alternator field) on a separate bus with the keyswitch controlling a 30A relay for that stuff. That way you could have the EFIS and cabin lights on without needing the key, but would still need the key to do anything dangerous.

And John - I like that idea! I honestly haven't decided whether to bother with APRS or not. Weird thing for a ham, I know, especially considering what's financing my build. :)
 
I used a marine ignition (Sea-Doo) switch as master, a push button start switch, and toggles for the rest.

The key ring also holds the key for the forward baggage so there less chance of taking off with the baggage door unlocked.

I make it a policy to never take the keys out of the baggage door unless it's locked...

The other plus is that the plane keys quick connect to my car key ring, making it less likely to go home and leave the master on.
 
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There be dragons here.

Your design is a potential fire hazard. You are using unprotected thin gauge 12V wiring to switch on the Master contactor and Starter contactor.

This could be overcome, but it is really an architectural problem. Conventional practices switch the ground wire to the Master contactor coil (not needing protection) and the starter contactor coil is fused before running through a starter switch.

The marine starter switch will not give you this directly. You could use it to switch the ground to the master contactor, then use a a relay connected to the other pole to switch a protected +12V to the starter contactor.

Something like this: http://www.vx-aviation.com/sprocket/photos/panel_elec/schematics/MASTER.pdf, but with the marine switch controlling the master contactor and the starter contactor rather than the toggle/pushbutton combo I have shown. Both functions switch to ground, which is what you want.

My example is a bit complicated because I have a starter interlock function, but just ignore it for your application.
 
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vlittle said:
There be dragons here.

You design is a potential fire hazard. You are using unprotected thin gauge 12V wiring to switch on the Master contactor and Starter contactor.

This could be overcome, but it is really an architectural problem. Conventional practices switch the ground wire to the Master contactor coil (not needing protection) and the starter contactor coil is fused before running through a starter switch.

The marine starter switch will not give you this directly. You could use it to switch the ground to the master contactor, then use a a relay connected to the other pole to switch a protected +12V to the starter contactor.

Something like this: http://www.vx-aviation.com/sprocket/photos/panel_elec/schematics/MASTER.pdf, but with the marine switch controlling the master contactor and the starter contactor rather than the toggle/pushbutton combo I have shown. Both functions switch to ground, which is what you want.

My example is a bit complicated because I have a starter interlock function, but just ignore it for your application.
Click to expand...

Good point, Vern, but wouldn't a simple in-line fuse straight off the battery on the single wire going inside the cabin to the switch provide the necessary protection? Say a 5A fuse (which should be more than enough power to activate both contactors at once) and 18ga wire to the switch and both contactors?
 
vlittle said:
There be dragons here.
Click to expand...
Good points, thanks for pointing them out. I revised the diagram.

  • START relay coil is now on a fuse.
  • ALT switch allows starting only with alternator field switched off
  • MASTER coil supply is 22 AWG (could use an in-line fuse just as easily)
  • Master/Start switch now switches to ground
 
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I built mine without a key switch. After all, who in their right mind would steal a homebuilt? The electronics maybe but not the plane.
 
Mich48041 said:
A momentary key switch can be used only for the starter.
Click to expand...
Yes, I looked at those. In fact I spent quite a bit of time looking at everything DK and Mouser had to offer in the way of keylock switches. None would do what I wanted -- which was to use a key to enable the master as well as the starter. With the key switch for only the starter, you could still have the ignition "hot" without the key. I'd be much more comfortable knowing that if the key's not in it, it's not starting, period.
 
A starter contactor is usually energized by a switch in the hot wire. If that wire shorts to ground, the result is a blown fuse and no engine start, but nothing dangerous.
If the starter contactor is energized by a switch connected to ground, then if that wire shorts to ground (unlikely but possible), the engine will crank unexpectedly, which is a dangerous scenario.
How about using a keylock switch that supplies power to a separate small fuse block? That fuse block will power the P Mags and a start push button. The master switch can still be turned on to play with the avionics, but there is no way to start or crank the engine without the key being turned on.
 
Mich48041 said:
A starter contactor is usually energized by a switch in the hot wire. If that wire shorts to ground, the result is a blown fuse and no engine start, but nothing dangerous.
If the starter contactor is energized by a switch connected to ground, then if that wire shorts to ground (unlikely but possible), the engine will crank unexpectedly, which is a dangerous scenario.
How about using a keylock switch that supplies power to a separate small fuse block? That fuse block will power the P Mags and a start push button. The master switch can still be turned on to play with the avionics, but there is no way to start or crank the engine without the key being turned on.
Click to expand...
You know, my first crack at this had the key switch energizing an automotive type relay to supply power to a fuse block for the P-mags, fuel pump and starter relay. I reworked it to simplify things, but maybe a little too much. I was a little leery of switching ground to the starter relay also, for the reason you mentioned. If (for example) vibration caused an in-flight short that engaged the starter it could ruin a guy's whole day. BUT... in this case, you'd have to have the alternator shut off for that to happen. Power is supplied to the start relay by the ALT switch in its OFF position.

Edit: Here we go.

 
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Dale,
You latest version looks OK.
The relay coil should have an arc suppression diode across it like the contactors do. Some relays are available with a built-in diode.
It would be better to protect the over voltage circuit with a breaker instead of a fuse because O.V. devices have been known to nuisance trip. Bob Nuckolls indicated that he will be soon selling a new and improved O.V. protective device controlled by a microprocessor. It is supposed to be less likely to nuisance trip.
Have you considered an endurance bus supplied by an alternate path from the battery?
 
Endurance bus? What's that? I've heard of an essential bus, an emergency bus, but not an endurance bus.
Click to expand...
In the latest edition of Bob Nuckolls' book, he has renamed the bus to "ENDURANCE BUS", which is a good descriptive name. When operating on the Endurance Bus without an alternator, the battery energy will be conserved by shutting off the master contactor and unnecessary loads. Losing the alternator is not necessarily an emergency. And there could be essential loads that are not connected to the Endurance Bus. Those essential loads can be turned back on with the master switch just before landing. Bob's book is available for FREE DOWNLOAD.
 
"Endurance" is the new "Essential". :) At least that's what I gather. It's a bus designed to give you the ability to stay flying if the alternator decides to take a vacation.

In my case, the "endurance" plan is that once airborne, I could theoretically stay flying as long as I have fuel. The P-mags don't need battery power once the engine is running. My backup instrumentation will be EITHER mini-EFIS with its own battery, OR a steam altimeter, airspeed indicator and compass. So... I could, if needed, just shut off the master and keep flying. If it's just an alternator failure, most of the non-essential stuff can just be turned off to save power. I'm about halfway to talking myself into having a switched avionics bus. I think I'll probably put that decision off, though, until I make the EFIS/radio/transponder decisions.
 
Hmmm, potato, potatoe.

For me, I'll have an A&B bus set up with dual screens. A feeds the pilots by the main alt and bat; B the other side by the standby alt in the vac pump location and LiPo bat. An A&B tie feeds both in normal circumstances off the main alt and bat. "Off/Or" switch gear for the alts; off/and/or switch gear for the bats.

At least that's the concept, subject to more daydreaming....

Probably go with impulse mag and something electronic in the other hole. 8.7-1 and it should be hand-propable on autofuel....
 
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DaleB said:
Good points, thanks for pointing them out. I revised the diagram.

  • START relay coil is now on a fuse.
  • ALT switch allows starting only with alternator field switched off
  • MASTER coil supply is 22 AWG (could use an in-line fuse just as easily)
  • Master/Start switch now switches to ground
Click to expand...

Hi Dale, getting close. The gotcha in your new diagram is that the R terminal on the starter contactor does not go to the coil, so you can't connect it to ground. In fact, it connects to the main contacts and is used as an indication to a lamp that the starter is on. The actual coil ground is internally connected to the contactor case.

You can use this design, however, just don't ground the R terminal unless you like smoke.



Vern
 
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