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Thread: Rotax 915is and VPX installation discussion

  1. #11
    Senior Member
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    Default Re: Rotax 915is and VPX installation discussion

    Quote Originally Posted by VictorV View Post
    ...........Eric, you implied that it's the former but is there a way to verify this? Maybe someone that has a 912is or 15is can measure
    what's happening on their system.

    Victor
    Victor - I think Eric was taking your original drawing at face value where you show one side of the Lane switch to be a ground. What Eric stated was correct for an electrical circuit based on what's on the drawing. You are right to try and figure out what Mother Rotax is doing for real since the Lane switch is nothing more than completing continuity between those two pins on HIC a and B.

    Since I am going to use a 912is and the VPX I am watching this with a lot of interest.

    G
    Gary (Geek) Phenning
    Leavenworth (Not the Prison), WA
    Kitfox STi in Building Stage

  2. #12

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    Default Re: Rotax 915is and VPX installation discussion

    Assuming that the switch just connects one pin to ground I believe the corrected circuit should be something like this....I think

    NAND-Based Circuit.jpg
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    Building SS7
    915iS
    Garmin G3x

  3. #13

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    Default Re: Rotax 915is and VPX installation discussion

    Another question....

    Even though I'm installing a 915is I have attached the wiring diagram from the 912is since it's exactly the same and is easier to read

    The 915is installation manual differentiates between EMS ground and airframe ground. The EMS ground is supposed to be isolated from
    the rest of the plane. During start the EMS ground should be connected to the airframe ground when applying start power.

    I understand what they're getting at but I'm struggling with the actual implementation. My plan for grounding is to have a ground bus
    behind the panel and behind the seat. They're connected with a heavy wire and connected to the "-" of the battery. I'm not planning on
    using the fuselage as any sort of ground.

    The ground connections shown in the diagram are as follows:

    Airframe Ground
    - Starter motor
    - Regulator Plate B
    - Start power switch
    - Battery backup switch
    - Master switch

    EMS Ground
    - Regulator Plate A
    - Start Power switch
    - Battery backup switch

    I just want to verify that I only need to connect the airframe ground (connected to "-" of the main battery) to Regulator Plate A via either the Backup Battery Switch or Start Power Switch?

    rotax wiring.jpg

    Victor
    Building SS7
    915iS
    Garmin G3x

  4. #14
    Senior Member Eric Page's Avatar
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    Toledo, WA
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    Default Re: Rotax 915is and VPX installation discussion

    Quote Originally Posted by VictorV View Post
    I just want to verify that I only need to connect the airframe ground (connected to "-" of the main battery) to Regulator Plate A via either the Backup Battery Switch or Start Power Switch?
    Almost. I think you mean the right thing, but to be pedantically correct:

    ...I only need to connect the airframe ground (connected to "-" of the main battery) to Regulator Plate A via the Battery Backup Switch and Start Power Switch.
    This allows Regulator A to power the engine's electronics completely independent of the airframe electrical system, except during engine start or battery-only operation (dual alternator or regulator failure).

    This can be accomplished with a single wire from Regulator Plate A to one of the switches, then a jumper to the other switch, and a similar arrangement from the two switches to airframe ground.

    Personally, I would wire both Reg A and airframe ground directly to Battery Backup, then jumper them to Start Power. The Operator's Manual (section 4.5, Failure of the EMS power supply) states that if both Alt/Reg A and B fail the engine will quit, then the pilot should close Battery Backup and restart the engine. So, by wiring directly to Battery Backup and using jumpers to Start Power, the weight and complexity of wiring them separately is eliminated and the additional failure points of the jumpers are moved to the less important switch.

    Obviously, I'm operating on a literal interpretation of the Operator's Manual. It says, "Remedy: Switch “ON“ the Battery Backup Switch (In this case the power supply is provided by the aircraft battery). Restart engine." If this guidance actually means to restart the engine using normal start procedures (i.e. Start Power must be closed), then both switches are equally important in a dual Alt/Reg failure and it makes little difference whether they're wired separately or which one is wired via jumpers.

    Does anyone know if a Rotax i-Series engine will start with Battery Backup closed and Start Power open?
    Eric Page
    Building: Kitfox 5 Safari | Rotax 912iS | Dynon HDX
    Member: EAA Lifetime, AOPA, ALPA
    ATP: AMEL | Comm: ASEL, Glider | ATCS: CTO
    Map of Landings

  5. #15

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    Default Re: Rotax 915is and VPX installation discussion

    Thanks for the feedback Eric. It sure would be easier if Rotax just took care of the start power and just had a single aux power input.

    Victor
    Building SS7
    915iS
    Garmin G3x

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