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  1. #1
    150+ Forum Groupie mach7's Avatar
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    Mechanical Drum readout

    Hello;

    I have been considering building my own engine instruments for my Bae146 simulator. I have already successfully built a fully operational APU using .9 step stepper motors MOSFET controlled through a programmed 12F629 PIC chip. It works so well that I am considering constructing the N1, TGT, N2 and WF indications (x4).

    I am considering a bit of an unorthodox set up of.....having the thrust levers control MSFS thrust settings whilst having a second tapping off the levers to operate the stepper motors to parrellel the flight simulator power setting(s). Eliminating any usb connection from the step motor gauges to flight sim. In other words...(assuming standard atmosphere)....60% n1 on FS2004 would correspond to 60% n1 step motor indication...it may take some setting up, but I THINK it might work.

    The reasons I am doing this are simple....I do not like the way MSFS displays the engine start sequence...just not realistic...MSFS has light off occur and a 'surge' of power while the N1, N2 and TGT sort of 'peak' then stabilize at the required ground idle values.

    I also find that the the MSFS engine indications 'lag' with thrust lever input. I realize this will still occur, and when I set "80%' on the step motors it will take some time for the 'actual' MSFS power setting to stabilize to this value.

    Now to my question....I do not anticipate a problem making and programming the anolog 'needle' gauge. My question is more of a mechanical one.

    I have searched the internet but could find nothing on construction of a digital drum readout. This readout is located directly below the N1 and TGT gauge needle and corresponds to the mechanical digital readout.

    Has anyone ever ventured in this area? and if they have...is it very difficult to construct....

    I have considered fpurchasing surplus gauges from the actual aircraft, then modifying them....but it will probably be too cost prohibitive.

    Thank you for your time..

    Jim

  2. #2
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    The drum displays in aircraft instruments are constructed like an old style odometer. There is an input shaft which rotates the lowest order digit. (see http://auto.howstuffworks.com/odometer1.htm) In an A/C instrument, this shaft is driven by an electric motor. The motor also drives a reduction gear train which in turn typically drives a synchro transformer. The synchro transformer provides the error signal for the instrument electronics. The electronics powers the motor to zero the synchro transformer output voltage.

    The reduction ratio of the gear train inversely matches the range of the drum counter. For example, if the drum counter ranges between 0 and 199, the gear ratio will be 200 to 1. The drum counter will take 200 rotations of the input shaft to span its full range, but the synchro transformer is limited to a single rotation.

    I looked into building a specialized stepping motor to act as a single digit drum display, but was not successful. (See May 11, 2008 entry http://www.mikesflightdeck.com/old_stuff4.htm) I'm unlikely to get back to that particular project for a while.

    Another possibility is building an instrument similar in construction to a real one using parts from mechanical odometers or impulse counters
    (http://www.goldmine-elec-products.co...?number=G15343)

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  4. #3
    75+ Posting Member cscotthendry's Avatar
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    Re: Mechanical Drum readout

    I've encountered another type of drum readout as well. I have an old tuning mechanism and an old VOR gauge that both have tape loops in them. The mechanics are similar to the drum system, but the digits are printed on a tape loop that runs on the drums. This allows for odd number sequences like when a compass goes from 359 to 000 or in the VOR frequency steps.
    If you PM me I'd gladly send pics of these mechs.
    Regards: Scott Hendry
    www.scotthendry.com

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