P-mags and wiring

There has been lots of discussion recently on Vansairforce, as to how to wire P-mags.

I didn't really want to depart from E-mags wiring approach, but I did want to minimise the number of switches. I also wanted to keep it as similar as possible to the switching of normal Slick or Bendix mags for ergonomic reasons.

I decided to wire it exactly as E-mag proposed, but combined the functions of their two switches, into a single 2-50 type (expensive) switch.



The way it works is that when you are ready to start the engine, first you turn the master on. Now the electronics of both the P-mags are live. Then you move the levers for both the left and right P-mags from the lower position to the middle. Now the 'P-leads' are live. Then you press the starter.

If you want to test your mag drop. You run up to say 1800rpm (keep the stick back), or whatever speed suits you, and push the left lever to the bottom position. Monitor the drop. The electronics remains live but the 'p-lead' is off. Repeat for the right mag. This is just like the past so far.

To test the self generation you turn the 'p-lead' off on one mag, say the left, and lift the switch on the right mag into its upper position. This cuts the external power to the right mag. As long as the engine keeps running the right mag is generating. Vica versa for the left mag. The upper positions are spring loaded, so the switch will not stay up there by accident.

To test for the rpm where the self generation ceases, you do the same thing, except holding the switch in its upper position, you very slowly throttle back until it quits. Don't save it, let it come to a halt. I only do this from time to time at the end of the day when I am shutting down anyway.

If for any reason on the ground you want power to the P-mags removed, pull the fuses.

This has worked well for me for 27 hours now. The learned habits of the past are continued and the switching is simple. I told Brad at E-mag what I proposed to do and while he did not actively support it he did not appear to have any problems with it either.

If you want to make your wiring really complicated, there are all sorts of things you could do, but why not keep it simple? This way its just like the switches from the past and as long as you put the switches out of the way, you wont knock them off in flight. Just like the past!

The plug leads are standard automotive equipment that E-mag provide with adapters, so they fit the larger holes in the engine block. They are easy to make up and test if you just follow E-mag's instructions.

The electrics are nearly complete.

I am always disappointed that I cant make the electrics look more tidy. I am an EE but I guess I need the skills of an electrician, which I don't have, to do this well. Despite those draw backs, I thought it might be worth including some pictures so others can get my approach to the location of the various electrical building blocks.

Power comes through the firewall on the RHS of the fuselage to a fuse panel which is attached to the sidewall. There are nut plates for this inside the cowl cheek for this to attach to.

The intercom is just below the fuse block.


All of the wiring to the various engine sensors transits the firewall on the LHS.

The LHS is pretty empty except for the cables going aft to the stick and wing leveler.



I have used the area below the role bar to house pilot and P2 headset jacks, switches and indicators for electric seats, and a PTT for P2. I haven't quite finished the wiring in this picture.

The fuse block is becoming populated.

The fuse block is steadily becoming populated. I think only three feeds missing, the radio, transponder and intercom.

The intercom will be located just below the switches you see for the p-mags, about where the cleco is.

I like the idea of having it here. While easy to get to it is out of the way. Once set, it should not need attention, and therefore does not need to clutter the panel.

If it sits there it is between the radio and the head set jacks so the wire runs should be minimised. Wire is deceptively heavy, so I have tried to make the runs short.

The wire needs a bit of a tidy up.

Suddenly there is wire everywhere!

For me this is one of the exciting points of putting my RV together. I enjoy the rapid progress that can be made installing the wiring.

Having ordered an AFS3400 I benefit from three very comprehensive harnesses which plug into the back of it. First step therefore is to thread all the wires in the general direction they have to go, and make sure they are combed out . That is what is going on in this first picture.
A last minute decision which I am very pleased about is that I ordered this very neat little firewall penetration part from Safeair. A bit pricey for what it is by the time it is shipped to the UK, but I am please I did. Almost all the wires that have to penetrate the firewall are installed. Just the electric mags outstanding, so it can clearly cope with a fully monitored engine.

The power penetrates the firewall by means of an insulated stud off camera on the RHS of the fuselage.

DigiFlight II, trim, AoA and other wiring runs.

I have started on the wiring as a change from working on the baffles. In this first picture you see the wire to the roll servo, and the flap position switch for the AoA.





















The wires you see here, are from the trim and from the pitch servo. I have tried to leave a little slack in the bundle in case I ever have to remake the plug.





















All of the wires come together at a junction underneath the LH step. The tab strip is now fully populated except for the wires from the stick. The bundle going forward - top left hand of the picture - contains all the wires from the pitch and role servos, trim, fuel pump, flaps, PTT, and A/P disconnect. This bundle at the last count contains 24 conductors in total. And I am building a simple RV!

The trim speed switches between fast and slow automatically, depending on flap position. In order to keep it simple, I decided not to have an additional manual switch controlling this. If it fails I will just trim at the speed that remains.

This is all of the wiring on the LHS of the aircraft with the exception of the loom for a magnetometer in the LH wing tip. I had forgotten that.

The strobe power, and headset wiring, will be on the RHS.

The first building blocks of the electrical system are installed.

I installed the first two electrical components recently. In this picture you can see a 20 way fuse block. This had to go in now since it was necessary to install the cowl cheek extension for painting. Power will come from inside the cowl cheek extension.

If a fuse blows I will manage without whatever I loose. Playing with breakers and fuses in the air is pointless.
Under the left hand step, next to the area containing the stick I am installing some electrics. There are three relays and the speed control unit for the trim. I have mounted them on a removable panel so I could ease the task of wiring everything up.

Two of the relays control the trim. Strictly speaking I could drive the trim from the stick switch, but I prefer it this way.

The other relay reads a signal from a micro switch on the flaps. When it senses the flaps are up it pushes the trim power through the speed control unit. Otherwise it bypasses it. I have no manual override, but this way I have one less switch to play with. More time for 'eyes out of the window'.

I really like push on terminals. This is a 12 way unit of pairs of tabs. It will be fully populated (24 connections) when all this comes together, and I am all for simplicity. They are made by Western Electric. I am always surprised they are not more popular.

You can just see the end of a bit of conduit poking through the fore and aft bulkhead. My intent is to use this to take a very few wires through the centre of the bulkhead where the control column passes as a way to get behind the spar.

The relays are Potter & Brumfield part # T9AP5D52. They are easy to buy from any of the big electronics suppliers. The terminal strip is here.

http://www.weco.ca/1_0/1_4/1_4_tech_photo_search.asp?level_id=6

Look at the second item down.