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  1. #1
    150+ Forum Groupie Kerbo's Avatar
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    Using a real magneto start switch with FSX

    I snagged a real Cessna switch on eBay for $5 so have abandoned my scratch built design. You can see the full details on my blog.

    Once I get the wiring figured out I will update the thread with how I connected it to FSX.

    I can hardly wait to crank up my virtual Cessna.
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  3. #2
    150+ Forum Groupie Kerbo's Avatar
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    Re: Using a real magneto start switch with FSX

    The continuity tests were thoroughly confusing and I was starting to think the switch was re-assembled wrong somehow or broken. A bit of googling led me to this site: http://www.ercoupe.net/ads/76_07_12.html

    The formatting is all jumbled but near the bottom is a continuity test for the switch that I bought. It all checks out and works fine. Unfortunately the terminals don't connect in a straightforward manner when it comes to simulator input. I think with some diodes I can make some sense of it.

    This is for a switch with Cessna part number C292501-0101, Bendix part number 10-357200-12. I noted the document at the URL above states that my part number switch has a PRIME function, but it does not.

    Here are the continuity settings.
    Code:
    Position	Between Terminals
    OFF
    		R and GRD
    		L and GRD
    		L and R
    		S and PR
    R
    		L and GRD
    		R and unmarked
    L
    		R and GRD
    		R and unmarked
    		GRD and unmarked
    BOTH
    		R and unmarked
    START (twist and hold, do not push)
    		GRD and unmarked
    		S and BAT
    		L and BO
    		L and LR
    		BO and LR
    PRIME (twist, push and hold)
    		Same as above, plus
    		BAT and PR
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  4. #3
    500+ This must be a daytime job Ronson2k9's Avatar
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    Re: Using a real magneto start switch with FSX

    Probably don't need (priming) as that is a starter function. FSX doesn't simulate fuel difficulties save that of running out and pump loss.

    The control only starts the engine. Unlike a car there is no battery or other function that is controlled through the switch. You have to turn those on with the master switch on the MIP. While I'm thinking in the real plane you can have ignition from one magneto in the sim that isn't simulated.

    So you need to have Off and Both and Start.

    Have you considered taking the switch a part cut through some of the confusion? On second thought that could add to that..

    I would think though a small test rig could help out. With lights for each of the output terminals.

    In looking at the terminals on the back of the switch you have a PR.. Which would be for Prime? Atleast it's in the terminals if not in the function.

    There is the one terminal that is unmarked in there.

    Shouldn't the 'R' Right magneto also connect ground to unmarked as the left one does?

    I'm taking a wild guess here but Unmarked could be the master switch (Alt) in the cockpit. Sure would help to have a schematic of the switch itself.

    I'm thinking.

    'S' = Starter (starter motor)
    'L' = Left magneto
    'R' = Right magneto
    'PR' = Primer pump
    'Bat' = Battery
    'GRD' = Ground
    'Unmarked' = Could be Alternator (as it's a power source that is unlisted)

    This may help though.. Page 7-24 from the Cessna 172N POH .. There is quite a bit of data in there if you take a look.. I sent you the link in previous email.


    Edit: BO could = Bus Out as it shows the bus connection in the diagram to the switch.
    Last edited by Ronson2k9; 10-10-2009 at 02:04 PM. Reason: checking the drawing
    Up Up and away in my beautiful my beautiful - Amphibian

  5. #4
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    Re: Using a real magneto start switch with FSX

    I wired up real mag switches in my old Queenair sim but it's so long ago that I can't remember how I did it. I do remember though, that part of the prob is that a mag switch just connects the mag "p" cable to ground to cut the ignition.
    That's why you do a mag drop test just before shutting down, to make sure one of the "p" cables hasn't broken, thus leaving a "live" prop.

    Dean
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  6. #5
    150+ Forum Groupie Kerbo's Avatar
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    Re: Using a real magneto start switch with FSX

    I haven't hooked it up yet but it looks like I found the necessary pairs of contacts to connect to my input board for all the positions. When I verify it is working I will post what I did.

    Ron, I'm pretty sure all positions of the switch are modeled in FSX, at least with the stock 172. I get a slight RPM drop when running off just one of the mags. Has your experience been different?
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  7. #6
    500+ This must be a daytime job Ronson2k9's Avatar
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    Re: Using a real magneto start switch with FSX

    When flying - the manual says that there are the two mags one on the left and one on the right. They supply ignition coil power to the spark plugs. So with one off the engine is running with only half power.

    Only supposed to fly that way in an emergency or to test when on the ground.

    Your right about them being simulated. I had forgotten. Did you get my email I sent you BTW?
    Up Up and away in my beautiful my beautiful - Amphibian

  8. #7
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    Re: Using a real magneto start switch with FSX

    Quote Originally Posted by Ronson2k9 View Post
    When flying - the manual says that there are the two mags one on the left and one on the right. They supply ignition coil power to the spark plugs. So with one off the engine is running with only half power.

    Only supposed to fly that way in an emergency or to test when on the ground.

    Your right about them being simulated. I had forgotten. Did you get my email I sent you BTW?
    You're not running on half power, just half the spark plugs. It's a safety system.Magnetos aren't like the ignition system in your car etc. They make power and fire the plugs as long as they are driven by the physical connection to the engine, no battery power is required for them. That's why in order to shut off the ignition you have to ground a mag.
    You would expect a small drop in engine RPM when you turn off one mag.

    Dean
    AV-8B Sim Builder. If it won't hover, don't bovver

  9. #8
    500+ This must be a daytime job Ronson2k9's Avatar
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    Re: Using a real magneto start switch with FSX

    Learning every day.. So you are loosing some power not half power.. I think I get it now.. It's less efficient as some fuel is not being combusted. The engine is working harder to make nearly the same RPM.. I did a little searching and found that each cylinder has 2 spark plugs one powered by each mag so if you switch off one mag or one fails the other is still firing both cylinders. That was the missing piece of the puzzle.

    2 plugs per cylinder one mag for each one.

    I had tried that in the C206 by the way just to experiment. There was a slight loss of RPM but not that much. So your right all switch points in the mag switch are modeled in the sim.

    You can still do the switch bypass and create a new switch. Running off the other one.

    I found some more info on the mag switch
    http://www.aeroelectric.com/articles...tchOptions.pdf

    It's not the exact same switch but perhaps will be helpful?
    Up Up and away in my beautiful my beautiful - Amphibian

  10. #9
    150+ Forum Groupie Kerbo's Avatar
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    Re: Using a real magneto start switch with FSX

    Quote Originally Posted by Ronson2k9 View Post
    I found some more info on the mag switch
    http://www.aeroelectric.com/articles...tchOptions.pdf

    It's not the exact same switch but perhaps will be helpful?
    That helped some, thanks for the link.
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  11. #10
    500+ This must be a daytime job Ronson2k9's Avatar
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    Re: Using a real magneto start switch with FSX

    I did this Mag Switch bypass design for you.



    The blade has a key way and the nylon bolt is filed flat to accept the key way of the blade. That way you can turn the bolt have the blade sweep. The spring is to keep the tension on the blade as it passes by each of the contacts. From the looks of the switch it's a 1/4 turn from OFF to BOTH. I used nails as contacts in the design but if you have something that is a better conductor that would be better.

    You would need to figure out how to connect the bolt to the switch you currently have. You could use a small piece of sheet metal in a 'C' shape with one end bolted to your mag switch and the other bolted to the bypass. If you wanted to be really slick about it. You could use the same strip for the blade too. Just a thought. It would be good to have the blade free floating though so that all the contacts are positive (contact is always made).

    I used nails because they have flat thin head. You may want to counter sink them a little. so the blade is prevented from moving.

    - A small hole for the pointed end of the nail to pass through. Then you use a 'nail head' diameter bit to counter sink the head. flush as you could reverse the transit problem with a hole that is too deep.

    You could use some CA to hold the nail in place and connect the nail to the switch leads. for FS not the mag switch you have hehe.

    This should let you set up the switch fairly easily if you are having troubles getting the actual wiring working..
    Up Up and away in my beautiful my beautiful - Amphibian

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