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Electrical System Finished (mostly)

I gave up onĀ  logging hours. As I don’t have a 51% requirement, it’s a renovation not a kit build, there is no regulatory need to show amateur effort percentage. Of course, other than the engine rebuild, it’s 100% my effort anyway.

Panel completed

The panel is ready for installation. All its bits and pieces are installed, the instrument harness and pitot static tubing are all connected up.

Electrical system wired up to switches

The electrical system is based on a Bob Nuckolls design. Unlike the traditional Cessna master and avionics-master, it had two separate buses with no single point of failure. This is achieved by having a separately switched cross-feed from the battery to the “Essential Bus” and the traditional contactor-driven master switch sends power to the “Main Bus”. There is a feed from the main to the essential bus through a diode that prevents back feed from the eBus to the main bus.

All that sounds more complicated than it really is. In practice the critical (or essential) systems run off the eBus and everything else off the main bus. In the event of an alternator failure you switch off the master: that’s it. This leaves only the cross-feed connected to the battery, thus all your heavy loads are off and the critical stuff is on. The endurance on eBus only with an Odyssey PC680 is over 4 hours which is similar to the fuel endurance of the aircraft.

The idea here is instead of dead alternator = land immediately (pan pan, I have 20 mins to dead battery) it becomes: continue to destination / appropriate airfield, switch on the master when on approach and land as normal. To achieve this endurance every incandescent bulb has been replaced with LEDs and the only real current hog is the pitot heater.

Annunciator wired and tested

How then do I know the alternator bit the dirt? What if the cross-feed isn’t turned on?

Annunciator – in need of labels.

It started out in the schematic drawings as a couple of warning lights and ended up with these five. It was fairly straightforward until I realized that three of them are activated by a +ve signal and the other two -ve. That means the three have no voltage until the switch is on, the other two have voltage always but should only be lit when the ground connection is made by the failed device.

The warnings and cautions are like this:

  • Alternator OFF (Red)
  • Pitot Heat FAIL (Red)
  • eBus Feed OFF (Red)
  • Starter ON (Amber)
  • Fuel Pump ON (Amber)

Once all that was wired up (on paper) I came to the conclusion that I need to be able to check that the lamp itself is working during preflight. While they are LEDs and should last nearly indefinitely the circuit might fail somewhere, better to know. I also needed a push-button for start as I’m not using the Off-Left-Right-Both-Start key switch; mostly because the one that was in it is in rough shape and I’m too cheap to buy a new one. The key-switch I used from a reputable UK manufacturer is intended as an industrial panel lock out switch and was 20% of the price of the aviation one. The mags are on a pair of toggle switches just because. It has three positions – centre is off, left momentary (springs back to the middle) is for test and right momentary powers the starter contactor.

In the test position all the LEDs are powered to make sure they are still alive!

The test position lights the lamps and grounds all the test LEDs on the fuse panel showing if any of the fuses are dead.

So far so good. The problem I discovered as I was wiring it up was that when I powered up the test circuit to make all the annunciator LEDs come on it would then connect power to the +ve signal devices so the fuel pump, starter contactor and eBus would get power via the 22gauge test circuit: not the plan.

The result was a set of diodes. I found a couple of circuit diagrams of “common cathode” and “common anode” lamp test modules online which it turns out equate to the +ve and -ve signals mentioned about above. I used KiCAD (open source electronic CAD package) to magic up a PCB and uploaded it to aisler.net and along came the solution.

Annunciator lamp test module

This allows the key switched lamp test to work and made wiring up the annunciator lights, signals etc. much easier.

Wired for pitot heat

With the addition of 12AWG aircraft wire the supply to the main bus and the wiring for pitot heat is in. I put an IP67 rated automotive disconnect at the wing root as with everything else that routes into the wings. That should make the wing installation literally plug and play.

USB Charging

With everyone using tablets and phones in flight these days there is always a need to plug them in. There will be no smoking, so no need for the cigar lighter socket with a converter jammed in and wires all over. I was unwilling to use the expensive certified charging sockets, especially as I wanted 5 charging points. The result is a “transport category” isolated DC-DC converter that turns 12V into 5V which in turn is fed to simple USB sockets.

This is all run through a single piano key, so the whole USB charging system can be shut off.

Ready to install

The picture shows why I gave up logging hours. The panel is relatively simple from the front despite its capabilities. I wanted it to look sort of period, the purists will of course object to any kind of glass, while having the capabilities and safety of modern avionics especially solid state AHRS. The complexity is all behind the scenes.

The panel is now tested and almost ready to go into the aircraft. I intend to add a door on the fuse panel to look a bit like the glovebox on the original. Once the new heating ducts are bolted in I will install and connect up the panel.

Panel Progress

The CAD file has finally been applied to metal. 6061-T6 Al .0625in thick.

The work was done at Watermark Engineering in Dublin; the owner is an avgeek who is plans-building a Fiesler Stork replica. Using their punch press we (the operator with looking on) punched out the panel and the shock-mount instrument sub-panel.

First up was the shock mount which has the multi-function display in the middle. I’m trying to keep the spirit of the original panel while adding the safety of engine monitoring and electronic AHRS. While I’m using it as an MFD, the unit (MGL iEFIS MX1) is actually a full EFIS with primary flight display, engine management and even an autopilot. The beauty of this unit is the level of configuration possible for the screens. I have designed several custom screens and started work on interactive checklists.

This is the stock Primary Flight Display Screen. It packs a lot onto a small screen and is not how I intend to use it. The checklists are able to show the relevant data directly on the list. The fuel level is manually entered at the start of the flight (as the tanks don’t have electronic sensors and I’m not opening the wings). The system then uses flow rate to calculate and display the remaining fuel in real-time.

The shock mount below is ready for paint.

From left to right will be: Airspeed, AHRS, Altimiter, EFIS, VSI, DI/AHRS (backup function), RPM (Tach). So if the 6-pack is supposed to be in front of the Pilot this is missing VSI, DI and Slip/Skid. However, the AHRS has the DI built in, so heading is shown on the DI. Thus all that’s missing is VSI and skip/skid.

The iEFIS let me build a screen that has all the engine management display, the VSI, slip-skid, AOA, and G-meter so rounding out and exceeding the 6-pack, while also allowing me to control the COM, NAV, and Transponder from the top bar of the screen.

All of that shock-mounts into the panel below which retains the piano keys and hides the fuse panel behind the glove-box door. The slot on the left is for my glasses and the (backup) paper chart I don’t tend to look at as I mount SkyDemon on an 8″ tablet on the yoke.

The punch press “nibbles” the shapes using various round and rectangular punch and die pairs. There was quite a bit of filing and sanding to do to finish it up. It certainly beat hand cutting it!

The result so far is shown below with the piano keys test fit.

If the panel looks too deep (tall), that’s because it is. There are two bends to be made along the bottom to reinforce by forming a u-channel along the whole bottom of the panel. The shock mounting points (tabs) need to be folded also. That’s beyond what I can do here, so it should be done some evening this week at Watermark.

From the back you can see the rebuilt piano key mount with new switches and the (seriously overbuilt) electrical system. Attached to the map box are a stratux (ADSB-in) and 5V power supply for 5 USB charging points to power cameras and tablets. Of course that’s about as tidy as it is ever going to look as the wiring will be a bit of a mess. Have to finish up the folding and painting before I get to that point.

3D CAD for your panel? – Recommended!

Learning a CAD package is non-trivial. I have spent many hours in Solidworks and on occasion thought to myself, “this is a waste of time, I should be working!” However, I have now found 4 separate errors in the panel that would have resulted in having to recut it. Given I have to get 6061-T6 from spruce in the USA, shipped by UPS, it comes out to about $150 per 2’x’4 sheet delivered. Then the cutting it out labour on top of that means remaking it would not be cheap!

Solidworks is very expensive – I just bought it for work and a single seat in the UK is GBP4K + support! Joining EAA gets you a student engineer license as part of your membership. I recommend it!

As I can’t get to the aircraft at the moment due to the Covid-19 lockdown, I decided to “waste” some time tracking down 3D models of instruments to stick into this model. It turned out not to be a waste of time at all. I had left plenty of space for each instrument based on the hole patterns until I tried “installing” them in Solidworks and they snagged on the shock mounts and the edge of the cut-out for the subassembly.

I didn’t have a model for the MGL Blaze unit I’m using as DI and AHRS so was going to assume it must be the same as the other instruments – they’re all standard, right? Wrong. I knocked up the Blaze part and installed it in the subassembly which required more tweaking to get everything clear of snags and obstructions.

In short – all the time spent in CAD has saved money, aggrevation and the delay that would have occurred if I had to buy more Al in from the US.

Still Here!

The work has continued and progress has been made. (We’ve had an election so the passive voice seems to be in fashion.) This is the results of moving the panel from 2D CAD (QCAD) into SolidWorks (thanks to the EAA for the low-cost licence).

Much more to come and as I’m in documentation mode, I’m planning to update this over the next few days!

DRAFT Panel Design

I’m attempting to strike a balance between keeping the quirky nature of a 170 panel and leveraging modern avionics – while NOT spending a fortune.

To that end, the venturis and gyros are gone and this is the draft of her new panel. I’m planning to print it at scae and stick it to the old panel and decide what I like/don’t.

Again the hours logged against this are a guesstimate over several days of on and off research and sketching.

The panel is bare

Everything is now out of the panel. Figured out how to get all the controls out of the panel and despite being aware of the ball bearing in the carb heat control, still managed to lose it. Oh well!

The suction system is removed and a secondary static line that wandered up the A-post and connected the altimeter only has been removed (and retained in case it turns out to be needed.)

I’m starting the CAD work for the new panel and the wiring diagram(s). Away all weekend and next week on familty vacation, bringing laptop to continue research and drawings.

 

Piano Keys

Everything to date has been tearing stuff out for inspection and replacement. All necessary and interesting but I hit a snag with the last few controls on the panel so I decided to work on some cosmetics.

The piano keys are a trademark feature of the 120/140/170 family. The 6 switches protrude through the panel and some designer went a bit art deco with them. I’m keeping them (of course) so they had to be smartened up. This restored Cessna 140 has them aong the bottom of this photo.

Removed last weekend, they looked like this (except where I had started cleaning:

The switches behind them were all different and mostly trashed. One died during extraction.

Now they look like this:

The brushed look is intentional (not original) there was too much pitting to just clean and polish them up. Several hours with grinder and polisher and they look and feel much nicer. Not sure why someone drilled holes in the centre piece; and couldn’t get them symmetrical.