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Thread: Vacuum System design using Turn and Bank indicators

  1. #1

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    Vacuum System design using Turn and Bank indicators

    I am installing two TNBs (each requires about 1.8"Hg) in a aircraft using a 4" venturi. I have researched design information but Tony B does not mention this type of installation. My local chapter guru's are torn between series and parallel installations. Do I require a regulator, needle valve, series, parallel plumbing, what about hose diameter? Anything else I forgot? Thanks for your help. There are no other vacuum instruments in the plane.

    Mark

  2. #2

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    First, I suggest a regulator as the performance is a function of flow. Too much pressure/flow and the gyro will spin faster than it is supposed to. Not sure how that affects its operating life. Should not affect performance.

    The series vs parallel question boils down to, do you want the failure of one instrument to stop the other also? When the gyro bearings die I believe that the flow resistance goes up. In a series connection this means that the flow to both instruments is negatively affected. So connect them in parallel.

    For what its worth, gyro's run on flow. We have a pressure gauge simply because it is easier to measure pressure in the system than flow. But pressure is only an approximation. Oh, and you want a filter on the system air inlet (the venturi is the outlet). Dirt will kill your gyro's performance.

    Now with a venturi system, one thing you should know is that the gyros really won't spin up until about 90mph on takeoff. So don't go into a cloud for a couple of minutes if you need those gyros to stay upright. The flow through the venturi caused by the prop wash as you taxi only starts the gyros spinning.

    Having flown with a venturi, I don't "get" why two T&B's. I would vote for no T&B and one artificial horizon.

    Hope this analysis helps.

    Wes
    N78041
    Last edited by WLIU; 04-29-2013 at 06:57 AM.

  3. #3

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    Wes: this is some good info.

    I see these T&B cordinators come in electric or vacuum. which is better or is there a better or worse?

  4. #4

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    Series plumbing won't work. You'll need 3.5 - 4.5" to power both instruments, the 4" venturi should be perfect with 100 kts cruising speed.

    I would "T" the line from the venturi and connect to each instrument ( I presume you are installing one in each cockpit of a front/back seat airplane). On the other side of the instrument, plumb both lines to another "T" fitting and put a filter on the single line end. That's it.

    You can always test the setup with a vacuum gauge to ensure proper operation but I doubt you'll need a restrictor valve (if that's what you mean by needle valve). The setup should work fine.

  5. #5

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    I have to say that it did not occur to me that the airplane might be a tandem cockpit design where the builder wants a T&B in both cockpits.

    Electric vs vacuum. The venturi on the outside of the airplane will knock up to 5mph off your speed. They are REALLY draggy. That's a negative. And they take a while to get up to speed.

    Electric units spin up as soon as power is applied and are almost always ready to go when your start your takeoff roll. But you have to have an electrical system. If you do, I will suggest that the positives of an electric T&B make it a vastly better choice than a venturi powered T&B. When you figure in the weight of the venturi, the regulator, hose, and filter, the electrically powered system is lighter. And it is easier to run wires than plumbing. And no filter to periodically replace.

    Of course, if you go electric and have some $$, there are great high tech instruments that fit in the same hole in the panel and provide the T&B function as well as an artificial horizon and DG function. But I assume that you are trying to keep the panel budget under control.

    Best of luck,

    Wes
    N78041

  6. #6

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    To all.. thanks. Yes, two T&Bs one forward one aft in a tandem, high drag, aircraft. It is my understanding the T&Bs like the Century model require only 2" Hg. as per Spruce Cat Part # AN5820-1 description. Electric would be great but we have these and they are in the panel ready to "fly".

    So... to Marty: I thought the T&B have a filter on them already, those small tophats on the top of the instrument. I am not at the hangar now but I believe these are the instrument's filter and all that is required. The E&M manual has a single 2" venturi going directly to a single instrument, no added filter. Does this sound right regarding the filter?

    Restriction: In the case of an instrument failure and the system closes all flow would be directed to a single instrument, all 4" of the venturi. Now we run into possible overspeed and damaging the instrument. This is the reason for my consideration of a restrictor on each line so that no line ever draws more than 2.1".

    Hose size: Considering 3/8 inch line. Does this sound correct? The venturi has 1/4NPT on it which has a smaller ID than the 1/4 inch. Aeroquip 306 hose is my first choice.

    Thanks all.

  7. #7

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    Mark, you won't damage the T&B gyro from overspeed. If it runs other than "on speed" it won't indicate precise rates but I don't think you'll notice unless you get out a stop watch and measure. That coupled with odds of a failure mode where one gyro fails and blocks it's respective source line is ~ 1 in 7.5 milion? Certainly no harm in including the system as you stated.

    I'd also go with the intergral filter. It sounds like you have this all figured out.

    Just curious, how far behind the c.g. is the rear panel where the second T&B will be mounted?

  8. #8

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    I wish I had a good answer for that, the datum is the nose and the W&B uses index units for the arm/load. Loading calcs are for index units at a certain location and the "safe zone" needs to be 205 to 225 index units. I need more time to cogitate.

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