IVSI question

Discussion in 'Flight Following' started by RotaryWingBob, Apr 12, 2006.

  1. RotaryWingBob

    RotaryWingBob En-Route Gone West

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    Can someone please help me understand how an IVSI works and why it doesn't have a lag like a regular VSI?

    The explanations I've found so far are about as clear as mud to this old head :dunno:
     
  2. MauleSkinner

    MauleSkinner Final Approach

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    Inside the IVSI, in addition to the normal plumbing, is a weighted valve of some kind...it's suspended by springs in its neutral position inside a tube. The "top" side of the tube is ported into the plumbing of the IVSI, and the "bottom" side is open to the cabin. This setup allows the inertia of the pull-up or push-over to affect the VSI indications.

    Maybe if you imagine a syringe, with the injection end up, and plumbed into the VSI, and the plunger suspended with the springs.

    When the airplane begins a descent, the mass in this valve initially moves upward within the instrument (just like when you get light in the seat when you push over into a descent). The way it's plumbed, the weight then pushes a small amount of air into the instrument, simulating an increase in static pressure, resulting in an "instantaneous" descent indication. Once the descent stabilizes, the springs allow the mass to move back to center, and the VSI indication is based solely on static pressure change.

    Conversely, when the airplane begins a climb, the valve makes an initial movement downward, drawing air out of the VSI, indicating an initial climb. Again, when the climb stabilizes, and inertial forces are removed, the mass moves back to center, and climb rate indication is based solely on static pressure change.

    An instrument shop checks the operation of an IVSI by flipping it upside-down, and noting a temporary climb indication.

    Hope this helps!

    David
     
  3. RotaryWingBob

    RotaryWingBob En-Route Gone West

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    Yes it does. The explanation I came across kept talking about accelerometers which didn't make sense to me -- I guess they meant the weighted valve which you described. Thanks!
     
  4. Steve

    Steve En-Route

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  5. RotaryWingBob

    RotaryWingBob En-Route Gone West

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    Thanks, Steve!
     
  6. Steve

    Steve En-Route

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    No problemo, Bob.

    Btw, we had 2 Bell -47s in the warbird area at Sun 'n Fun this year. Cavanaugh Flight Museum in Addison, TX brought them. They were decked out in M.A.S.H colors and one had a litter on the side. According to Mr. Cavanaugh they did one complete restoration in 61 days and 2500 man hours just prior to SnF. When they hover taxied in and out they hardly made a breeze. [sorry for the thread hi-jack]
     
  7. gismo

    gismo Touchdown! Greaser!

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    David, your explanation sounds correct except that AFaIK there are no "valves" in an IVSI. There are weighted pistons supported by springs in cylinders and they do the opposite of what you say the "valves" do. With an increase in positive G the pistons add some air pressure to the case making the gauge read an immediate climb and a decrease in positive G causes the opposite actions (decrease internal pressure and immediate descent indication).
     
    Last edited: Apr 12, 2006
  8. RotaryWingBob

    RotaryWingBob En-Route Gone West

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    No prob! There was a 47 at KLNS last sunday -- that is a really classic ship!
     
  9. RotaryWingBob

    RotaryWingBob En-Route Gone West

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    Oh man, you guys are really trying to confuse this old fart:yes:
     
  10. Bill Jennings

    Bill Jennings Touchdown! Greaser! PoA Supporter

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    OK, based on "Here's what IVSI does for you", why don't we have them??? Sounds like it is helpful on instrument apps.
     
  11. RotaryWingBob

    RotaryWingBob En-Route Gone West

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    Cost, for one thing, Bill. We're considering one for an R44 that we would like to buy. On a new R44 IVSI adds almost $900 even with the exchange of the regular VSI. On helicopters they're used to help the pilot maintain altitude in a hover.
     
  12. ScottM

    ScottM Taxi to Parking

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    Thanks between your and David's explination I finally understand these things too.
     
  13. MauleSkinner

    MauleSkinner Final Approach

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    I stand corrected...the "valve" terminology was the result of serious brain fade.

    Changing the case pressure rather than the bellows pressure probably makes more sense, too...You're changing the pressure of a small chunk of air, rather than the entire atmosphere ;)

    Fly safe!

    David
     
  14. gismo

    gismo Touchdown! Greaser!

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    Maybe the simplest explanation is that there are weighted pistons that cause the pointer to indicate a climb when the g-force is increased.
     
  15. RotaryWingBob

    RotaryWingBob En-Route Gone West

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    I like that, Lance, and it makes sense to me. Thanks!
     
  16. ScottM

    ScottM Taxi to Parking

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    BTW those weighted pistons are the acelerometers that are reffered to in the texts.