Rotax 915is and VPX installation discussion

[VictorV]

Hi Everyone,
I've been having a side discussion via email with Fred B. (Doggitz) who's also building a Kitfox with a 915 and VPX and
there have been some interesting topics so we thought we'd bring the discussion into the forums so that
we can share what we've learned and how we have approached the design/implementation. This discussion could
also be helpful to those installing a 912is since the electrical designs are almost identical.


The majority of our discussion has been regarding the best approach to start-power switching. For those unaware of
this concept, the 915 is self-contained from an electrical standpoint and supplies it's own redundant power via 2
internal generators. In order to start the engine you need to apply momentary power and ground from the battery to run
the ECU, electronic ignition, and fuel pumps. Once the engine hits 1500 rpms the internal generator takes over.
The external "start-power" must then be removed otherwise the engine controller will not be able to determine
if the primary generator has failed.


As I mentioned, we're both using the VPX as a power control center. Initially I was considering using the PPS as well
but the engine has so much functionality built-in that the PPS does not add any value. We've also heard that
running engine circuits through the VPX will void the Rotax warranty. I'm still trying to get a copy of the actual
warranty to understand what types of things will void the warranty.


Fred has chosen the GRT avionics suite and I am going with the G3X Touch. I don't believe this will have much, if any,
impact on the design of the systems supporting the engine.


Now, back to start-power....


So far I'm aware of 2 designs to deal with the start power issue:


1) A 3-position switch -- on, off (center), on-momentary. The simplified start procedure is as follows:
- Master on
- Press and hold start-power switch to momentary
- Lanes A and B on
- Wait for Lanes A and B red status lights to go out
- Fuel pump A on
- Hit the start button
- Once the engine starts, release the momentary start-power switch.


2) Use this device: https://www.stockflightsystems.com/emu-912is.html ($3900)
It has a built-in relay that turns on when power is applied and stays on until the engine hits 1500 rpm. This low-current relay is used
to trigger a higher-current relay that supports the 8 amps needed by the engine electronics during start. This is what is used in the
Kitfox SLSA as well as Trent Palmer in his Freedom Fox.


The primary flaw in #1 is that you need one hand to operate the switch and the other to operate the momentary switch which leaves
0 hands available to control the throttle. #2 is a nice solution but it's pricey and takes up panel space.


The solution we've been working on is shown below. It consists of a interval timer that's triggered by turning on either Lane A or Lane B
ignition switches. It drives a higher-current relay which controls the start-power. The "on" time is programmable from 10 to 1000 seconds.




The diode "OR" gate allows either ignition switch to enable the start power. There's also a bypass switch that will manually enable start power.


If anyone has any thoughts/ideas on this or any other topics related to 915 and VPX integration please chime in!


Victor