need help Electrical system design

Hi all,

Having bought a 2nd hand Microair m760, I've started thinking about the electrical system. I also have a set of Savannah instruments with complete wiring harness.

Had a look at this article - http://www.microair.com.au/admin/uploads/Typicalaircraftelectricalsystems.pdf - seems like good advice to me, what do you think?

I'd be grateful for any advice about setting up the electrics. Some of these questions may be a bit stupid so please bear with me, I can wire up a powerpoint but the finer points of electrical systems escape me.

- Does the Rotax 912 usually have an alternator? As far as I can tell there doesn't seem to be one on my engine.

- Where do people mount their bus bars, fuse panels etc?

- For W&B the battery could potentially be a fair way down the fuselage. Does the extended cable run affect the current?

- Any general hints and tips for wiring? There seems to be some care needed especially with radio wiring - they mention noise filters, ferrite clamps, and capacitors for strobe systems. Any others?

- What were the traps that got you, and how did you overcome them?

Thanks all!

Cheers, Marty

Definately have 2 mag switches. I don't like key start, especially one with the mags included.

I would prefer a momentary switch for "start".

No alternator on 912, though it is a factory option. (I know someone with one for sale)

Battery/starter cable is critical with the 912. Longer length, greater diameter. Multistrand will give you more capacity again. There are calculations online. Use good quality.

From memory 16mm2 is the minimum recommended by Rotax.

My battery in behind the seat and 16mm2 multistrand is adequate. Further away and I'd probably use bigger dia.

Use industrial lugs on cable ends, not automotive. I use carbon grease on connections.

Do use a battery isolator..... one with a key.

Failure to set up correctly and you will start "doing" sprag clutches..... not an inexpensive exercise.

Use a good battery. Not only helps with good starting but I think it helps stabilise the regulator which is a known failure point.

Get rid of the radio :) POS

Oh and everything Downunder said

I would add to Downunders excellent advice - go overboard with earth/negative returns. Make sure all earth returns are well secured (as with positive connections). Attention to detail on this point will pay off "down the track" with more reliable longer lived electrical components. I never relay on body/chassis/air-frame returns alone - always run additional cable.

I think Kyle might be your Go-To man as he is in the electrical business.

Earthing and the way you do the earthing is also critical. You need to have a earth bar behind the dash..every single earth from every device needs to be connected back to that earth bar. Make sure there is a earth from the battery to the frame of the aircraft at the battery then have another large earth going from the battery to that earth bar then that main earth should go to probably onto the regulator case or at least under the starter solenoid on the firewall then onto the back of the engine..usually on the starter attachment bolt....this will solve a miriad of issues for a start

Have two distinct electrical systems. One for power the other instruments and radios. Each should have an earth bar. Connect each earth bar by one only connection. Do this and you will reduce problems with earth loops that an affect instruments and radios. Shield all instrument and radio connections earthing the shield at one end only, very important to also stop earth loops.

When using circuit brakers - group like functions eg comms, nav

So the alternator issue - what's more common, flying with or without? Assuming that if you don't have one then the battery is handling the heavy duty of starting along with all the other things without being recharged. Do you then bung it on the battery charger at the end of the day/flight?

Alternator I believe is 3kg, but guessing there's some heavy duty cables from it to the battery?

So the alternator issue - what's more common, flying with or without? Assuming that if you don't have one then the battery is handling the heavy duty of starting along with all the other things without being recharged. Do you then bung it on the battery charger at the end of the day/flight?

Alternator I believe is 3kg, but guessing there's some heavy duty cables from it to the battery?

Marty I don't think you will need an additional alternator unless you have heaps I items drawing amps. PM your email and I'll send down my wiring diagram and you can modify that to suit your setup. Like Mark and Downunder say earthing is important and 100% necessary, I have three negative posts in my build at convenient locations as these need to be 100% electrically connected. I also don't like the key switches (I had a ACS one with the kit and sold it and went classic. Recommend the single toggle double through master switch and led setup. These are hard to track down. With my wiring diagram I printed it at A3 size and then reduce to A4. (Makes it easier to see where things are going and you can revise the connections at home off the diagram. Your making nice progress.

The Rotax built in alternator is well and truly enough for your purpose..it will be fine with all analogue. The issue I have is with using lithium batteries with the original regular system. You dontneed a second alternator for dead weight

Marty. ICP before they made Savannah’s were into making automotive wiring harnesses.In my opinion /experience Savannahs have pretty good electrical systems You say you have a Savannah harness already———you are on the home straight.If you have access to a Savannah just copy everything you see.There is a Savannah wiring diagram that any builder should have in their possession.I will attempt to find /post the diagram

I have labelled all my wires to make tracing very much easier. This works for a very simple system.

Don't solder connections in any vibrating machine. It will cause wire fatigue breaks at the solder/ wire interface. Use professional crimping with PROFESSIONAL crimping tools, this ensuring a galvanic connection.

Use only Tefzel wiring. It is similar to Teflon but I'd less toxic when it burns. PVC wiring is much heavier as the insulation needs to be much thicker for the same wire cross section. A cables ability to carry current is a function of the temperature that the insulation can stand and still be functional. Teflon and Tefzel can take higher temperatures than PVC. A thinner wire will get hotter than the same current through a thicker wire, so a thinner wire will get hotter but the Tefzel can take the increased temperature. Lighter copper and insulation, good for aircraft

Marty. There is some good advice here, but I would suggest you go to the acknowledged expert.

Look up Bob Nuckolls on Google. You will see articles in Kitplanes magazine That is a good start.

What we can advise here is possibly not going to cover what you need and could even be misleading. I have wired two aircraft using Bob Nuckolls techniques.

My own words of advice are use co axial wiring where recommended around radios and also for the P leads. That is the leads from the magneto, back to the mag switches, also try to run them separately from CHT or EGT wiring. I see nothing wrong with an ignition switch such as used in GA aircraft, rather than individual mag switches. I have used both. Think about future needs and maybe run extra wires through the firewall or other bulkheads. For wires into the wings, consider disconnect plugs in case the wings need to come off.

Make sure you have a heavy enough cable to take starter loads, It could well work when too small, but the battery has to be top knotch to keep it going and it will overheat. Take great care with the earth connections, my experience tells me that a lot of hard to track faults come back to poor earths.

Consider how you will do future maintenance, you may want to be able to remove the panel. Would multi connector plugs enable you to get easier access, also how about a bit of extra wire length, for wiggle room. If you use aviation style relays, they are quite large and heavy and need to be mounted so that G loads do not cause them to contact or break contact.

Wiring at the battery can be made better if you have only two wires to it and the isolation switch right beside the battery, that is if it is in a position to be easily reached. Then you have a pos. and neg. busbar adjacent, where multiple wires can be attached,

Most commonly used aviation grade wire is white. It can be bought in about 7 or 8 colours, but you need to know exactly how much you need to prevent waste.

It is possible to get a machine to write onto wire or shrink wrap, or you can use several colours of shrink wrap in short lengths to code each end of the wire. Have a look at Jim Weir in Kitplanes, I have seen articles about it there.

Length of run of cables does affect current seriously, especially the heavy cables. Make sure the earth from the engine to the battery, which goes through the firewall cannot disconnect or come loose, if that happens all the earth return will be going through the small leads between engine and earth. Not coolUse good quality connectors and make sure there is no load on them, shrink wrap alongside any joint will reduce vibration effects.

Over the years the most annoying things about my own installations have been access to the battery and trying to add extra wires through the firewall.

So the alternator issue - what's more common, flying with or without? Assuming that if you don't have one then the battery is handling the heavy duty of starting along with all the other things without being recharged. Do you then bung it on the battery charger at the end of the day/flight?

Alternator I believe is 3kg, but guessing there's some heavy duty cables from it to the battery?

The rotax has a built-in generator on the rear of the engine, attached directly to the crankshaft.

This powers the ignition system and supplies power via the Regulator to charge the battery.

Downunder. Is the Rotax alternator really a generator. I don't run a Rotax but I thought it produced AC current which was converted to DC, rather than DC current.

Usually generators have commutators and alternators have slip rings. Which is it.

There is an optional external alternator.

On 8/30/2020 at 4:15 PM, Yenn said:

Downunder. Is the Rotax alternator really a generator. I don't run a Rotax but I thought it produced AC current which was converted to DC, rather than DC current.

Usually generators have commutators and alternators have slip rings. Which is it.

This. The manual might call it a generator but it is definitely an alternator. The AC current as far as I know gets rectified in the combo regulator/rectifier.

In a generator, the commutator acts as a 'rectifier' . 

On 8/30/2020 at 4:15 PM, Yenn said:

Downunder. Is the Rotax alternator really a generator. I don't run a Rotax but I thought it produced AC current which was converted to DC, rather than DC current.

Usually generators have commutators and alternators have slip rings. Which is it.

It is an alternator but no slip rings. The coils are fixed with permanent magnets on the rotating flywheel.  

A question for the electrical engineers,  are these alternators switched off or disconnected when the master switch is turned off in flight (smoke in the cockpit). 

52 minutes ago, Thruster88 said:

A question for the electrical engineers,  are these alternators switched off or disconnected when the master switch is turned off in flight (smoke in the cockpit).

I can say the engine will keep running. 

I would guess the regulator still receives (AC) power.....

On 8/30/2020 at 3:35 PM, Downunder said:

The rotax has a built-in generator on the rear of the engine, attached directly to the crankshaft.

This powers the ignition system and supplies power via the Regulator to charge the battery.

 

I don't think that's entirely right:

Yes, the Rotax 912 has a built in alternator on the rear.

It also has ignition coils back there that provide both the power and the timing of the ignition pulses. And those are not connected in any way to the alternator.