Savannah airbox signal wire hookup and other wiring questions

[Bodie]

Hello fellow Savannah Builders,  

 

I have 4 wiring questions.  After scouring the manuals (both Rotax and Savannah) and the forum I can't seem to find the answer.  

 

1)  The first pic is the backside of the airbox and outside air temp guage.  Can someone tell me to which points the two signal wires attach?

2&3)  The second pic is the wiring near the ECU.  I think I have the brown wires correct?  and, where do the two blue wire go?

4)  There is a black wire coiled up near the ECU.  Where does it go?

 

Thanks guys.

 

-steve

 

 

Edited February 10, 2021 by Bodie

[Blueadventures]

Maybe ignition switches?

[IBob]

Hi Bodie, looks like you are getting to the exciting bits!

 

1. Airbox/air temp gauge. Each sensor has 2 wires. Attach the pair for one sensor in the LH pair of sockets, the other sensor in the RH pair of sockets. Your gauge has a switch on the front, to switch beteen sensors, right? So you then need to power up the gauge with 12v, warm one of the sensors and figure out which way the switch is so that you can mark it. I have mine so when the switch is left, it is airbox, when it is right it is outside air. This makes sense to me as my outside air sensor is tucked up in the RH wing root/windscreen join.

 

2,3 and 4. Those two blue wires come from the starter solenoid. Originally, they would have been plugged into the the two ignition modules to provide soft start (timed ignition retard).
BUT Rotax now supply the engine with an 'Easy start unlock cable', which is your coiled black cable there.
All you need to do is connect either of those blue cables to the black cable, and that's it, you will have soft start. (I connected both blue cables to the black, just to tidy things up.)

Here is the applicable service instruction: https://teveso.cz/content/m_dokumentace/SI-912-028_Advanced_Start_System_for_ROTAX_Engine_Type_912_Series.pdf

See item 3.1.1 "The single wire of this harness (see fig. 1) must be connected to the momentary 12V start power signal at the starter solenoid........"

 

Hope that all makes sense, let us know how you go!

[IBob]

PS:

1. The airbox/air temp sensors are just resistors that vary in value depending on temperature. So there is no right or wrong polarity for them.

What I have noticed with my airbox temp probe is that it is quite slow to respond when I turn on carb (air) heat. Provided the exhaust is hot, the carb (air) heat will be almost immediate, but the airbox and the sensor take a while to heat up, giving the impression that the carb (air) heat is slow acting.

2, 3 &4. Attached is pic of my ignition wiring. The 2 blue wires attached to the engine black wire are the ones you can see cable tied to the white upper engine mount tube.

[Bodie]

Great IBob!  That all makes absolute perfect sense.  If only the manual were so!  I'm getting to the point on this build that I can see the end.  I don't know what I would do if it weren't for this site and all the help.  Right now I'm waiting for some parts that were missing from the kit,  a couple that I inadvertently buggered up, and some parts from Aircraft Spruce.  I need some decent weather for my "Big Sky Shop Paint Booth" (relatively warm, bug free day).  

[IBob]

Yep, Bodie, every one of us has their moments with the manual, believe me!

But when it came to it, I found the electrical very straightforward. The wiring looms they supply save a huge amount of work.

Also, when it comes to the sensors etc on the engine and to the instruments, the wire ends are colour coded with little bits of coloured sleeve. And the wiring diagram (in the back of the manual, in the POH, I think) shows those colours.

 

What I did do (and lots of other builders do too) is run a fat negative cable from the battery to the starter relay* mount bolts, then jumper from there to the starter motor (and back to a common instrument ground point on an internal side flange just behind the RH of the panel). This is an extra, but just seems to be good practise, rather than relying on the aircraft shell as a negative path:

With a Rotax, it is important to get good starting, as poor starting risks expensive clutch damage. For good starting you want the engine to turn over quite fast, and to get that you need a good battery (and starter relay) and good sized cables between battery and starter motor.

 

* I as fitted the short battery negative cable to chassis, as supplied.

Edited February 11, 2021 by IBob

[Jabiru7252]

Many thermistors have slow response times, lag times between 15 seconds and a few minutes. A semiconductor junction is faster than a resistor and I'd be surprised if the sensors are in fact 'straight resistors'. The smaller the mass of the sensor the faster the response (all other stuff being equal). Lot's of info on google...

[IBob]

Hi Jabiru7252. Yes, that was poorly worded: I was mainly trying to convey the idea that polarity is not an issue.

And I would be confident the lag time in the carb (air) heat sensor is due to the thermal mass of the sensor: it is potted into a small bolt, which is then screwed into an aluminium socket in the airbox. The rise in air temperature is not great, it will necessarily take time to raise the temperature of all this.

 

As some sort of comparison, I have commissioned a great deal of industrial refrigeration, where the standard sensor is a Pt100 in a 6mmSS pocket. Initial calibration would be carried out in a 0'C ice water slurry. Heat transfer in a slurry is far more rapid than in air, but we would still allow a full 2mins for a probe to stabilise.

[Bodie]

Thanks IBob and Jabiru7252.  I noticed on the Savannah S I was learning in that the carb heat was slow in recording change during run up.  That makes sense.

Also, I hear what you're saying on not relying on the airframe as the ground.  On all our ag equipment the most common cause of electronic problems short of failure are related to insufficient or corroded grounds.  What gage wire do folks use for the "additional" ground? 

[IBob]

Yep, the negative is literally the other half of the circuit.

And I'm referencing an Australian Rotax guru (I think) who commented that the aircraft that seemed to suffer starting and clutch problems were more often those without a dedicated negative cable.

 

Um, not good on my wire gauges. Same size as the positive cable that goes forward?

[Bodie]

My S kit has an aluminum positive cable.  Are the older kits copper or aluminum?  Easy enough to cross reference the carrying capacity I guess.

[IBob]

I'm pretty sure mine was copper. Certainly my (added) negative cable was.
Maybe PM Mark at Kyle Communications here, he can probably give you a size if you're not sure.

[Jabiru7252]

Aluminum is nowhere near a good conductor as copper (about 60%) but is lighter. That's the only reason (and a poor one) I could see for using it in electrics in a plane.

[onetrack]

Get rid of aluminium cables, they are all too prone to corrosion, and they're of lower tensile strength than copper.

 

You need a much bigger diameter aluminium cable than a copper cable, to carry a specific current load. This means more insulating material is required as well, adding to weight.

 

The sole reason manufacturers use aluminium wire, is simply cost-saving. Aluminium is dirt-cheap as far as metals pricing goes.

 

Copper is a precious metal and has always been regarded as such. Accordingly, its value virtually remains high, due to industrial demand, and jewellery and coinage demand. 

 

[Kyle Communications]

Yes get rid of all Alu cables and replace with copper...also the only earth to the frame for the battery negative ICP do at the battery and they rely on the alu fuselage to provide the path...this is just bad !!! run another earth wire from the battery negative as well as keeping the original...run the other negative to the engine...bolt on the back of the start or somewhere else..it has been proved that not having a direct earth will/can stuff the sprag clutch in the engine as the engine doesnt turn just quite as fast as it can

 

[Bodie]

Thanks guys!  Kyle Communications, what size cables are you using in your Sav?  Also, I'm getting to where I'm going to plumb the fuel header tanks, and I like your 4 valve setup.  Was that a 1/4" npt block you used?  With 1/4" stubby ball valves?  I looked through the other posts and couldn't seem to find the size.  The Sav kit does a great job of switching between metric and sae (kind of like our John Deere farm equipment!  

[IBob]

Hi again Bodie. I copied Mark's 4 valve setup, which works very well for me. The block we use in NZ, and in Australia where Mark is, is a 14"BSP air manifold, with ball valves. You will have some local equivalent.

If I were building again, I would also run the return line to that manifold (rather than to the RH inner tank). It's no biggy, but it would be one less penetration/fitting in that tank. And it would remove the possibility of fuel loss if flying with that tank topped right off: as it is, if I see very high level on that tank in the sight glass, I switch to that tank for 10mins or so and burn some off.

 

To run the LH fuel lines across to the valve block on the R of the fuselage, I made a tray that screws on in front of the cabin frame member. Some builders run the lines under and to the rear of that frame member, but this puts an undulation in the lines where air may sit, resulting in differential fuel flow. Where at all possible, avoid undulations in the upper fuel lines, and run them steadily downwards.

For those LH fuel lines, you can fit solid pipe (with hose at the last part down to the block). In my case, since I do not have solid pipe there, I recently laced those lines to a piece of light aluminium angle across the width of the tray. This has removed all undulations and the fuel tanks are now self-levelling on the ground, and feed more evenly in the air.

 

PS I believe ICP may now have amended the manual in recent kits, specifying how the 4 tank option should be plumbed. Builders of those kits may have to follow the amended manual for their build to be compliant.

 

Attached is a pic showing the tray carrying those LH lines.

 

 

 

[Kyle Communications]

Later on after fitting that fuel manifold I got a right angled barb fitting and put it into the right side end of that block and fitted another tap to it and bought my fuel return to it. That way I could turn the fuel return on and off and because it went to the fuel block it did not fill up one tank it just went as usuable fuel

 

The main wire from the battery which in mine was right down the back of the aircraft was quite heavy...maybe 4 or 5  gauge in your terminology. It was a long run so I had to make sure it was heavy enough. The positive was larger than the negative as the negative was a parallel really because it was connected to the frame at the battery. 

 

I will look to see if I have any pictures..as I sold the aircraft late last year. This new rebuild I am doing I will only have 2 taps for fuel as I have totally remade the tanking system even though I have 4 tanks but the fuel return will come back to the manifold like I did later to stop the overflow of the tank

The new tank system you can see on the S rebuild blog

 

 

Mark

 

[Bodie]

Thanks again, gentlemen!  The aluminum is in the scrap bin!

 

IBob,  I'm a bit confused here being on the other side of the world and the other side of the equator.  Did you mean 1/4"BSP?  Which I would guess would equate to 1/4" NPT...  I like the looks of that setup and will do the same.  I have 4 tanks and only plan on using the two inboard.  I bought stainless steel tubing and ran that through the wing to the cabin.  Except for the short hose connecting the tank hose barb to the tubing, I have no rubber hose running through the wing ribs .  I figured it would be easy to replace a short than the whole piece.  

[IBob]

Bodie, sorry, yes 1/4"BSP. Over here the manifold blocks are sold at auto paint shops etc, as they are principally used for compressed air lines.

 

I only occasionally use the outboard tanks here in NZ. But it's handy to be able to go there and back on some flights without having to source mogas.

Your solid tubing in the wing sounds good. Just be aware that the tanks may swell and shift a bit in use, so your coupling to the tank needs to be able to accommodate that: it sounds as if yours will.