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instantaneous VSI

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instantaneous VSI

Old 16th Dec 2000, 18:29
  #1 (permalink)  
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Post instantaneous VSI

i don't work in aviation but am facinated by it. recently i obtained an instantaneous VSI as a souvenier. it was formerly from a B747. While i think i understand how an ordinary VSI works , how does an "instantaneous" one differ mechanically? I have heard that it has accelerometers ? is that true ...The unit i have is from aerosonic i think. the unit i have has a port for the static . it also has a three pin power socket. does the "instant " function need power ..or is it a pure mechanical thing. furthermore , the needle seems to indicate a 250 feet climb at rest and there is no zero correction knob ..why is this so ? is my instrument defective (i.e those in operation never have an zero error ?)why isn't there a zero error knob ?

a lot of questions ...sorry !
Old 16th Dec 2000, 18:50
  #2 (permalink)  
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I would also like to know if you have any websites that might explain the mechanism .....
Old 16th Dec 2000, 19:37
  #3 (permalink)  
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The IVSI get their info primarily from the IRS. This signal is then damped by inputs from the static ports.
Old 17th Dec 2000, 03:09
  #4 (permalink)  
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Thumbs up

Culled from my ATPL theory notes:

An IVSI contains accelerometers mounted in the vertical axis connected to pistons. These pistons give an instantaneous pressure increase (descent) or decrease (climb) in the static supply before the choke as soon as a vertical acceleration is sensed by the accelerometer. This causes the VSI needle to move immediately upon climbing /descending, removing the large instrument lag with an ordinary VSI. Once the IVSI has hit the steady state in the climb/descent, it behaves just like an ordinary VSI measuring the pressure differential across a choke in the static line.

As with all ATPL stuff, don't know how closely this relates to real life, regarding the mention of a link to the IRS mentioned above

(typos corrected)

[This message has been edited by foghorn (edited 16 December 2000).]
Old 17th Dec 2000, 06:22
  #5 (permalink)  
Jim lovell
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Found from my ATPL notes also:

IVSI( also known as an inertia lead VSI)is similar to a VSI but incorporates a dashpot(cylinder) which is mounted on a spring loaded piston. Upon say entering a climb, the vertical acceleration forces the piston down the dashpot and in doing so sucks air out of the capsule. This, in turn makes the capsule contract, which moves the VSI needle to show a climb. The reverse happens should a descent be iniated. The normal VSI sensing system eventually catches up and continues to show the climb or descent as would a conventional VSI.
Old 17th Dec 2000, 15:05
  #6 (permalink)  
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thank you very much. once i get some tools , i will open it up and look inside ..... don't worry , it will not fly again !

Old 18th Dec 2000, 21:00
  #7 (permalink)  
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Regarding the input from the IRS above: Electronic IVSIs will feed in from the IRS's. These are used on nearly all a/c with an IRS system.
On a/c without an IRS system, the abovementioned static system with the dashpods is indeed used (which is the system that comes up in the ATPL theory).

The IRS method is by far the most accurate, and has very, very minimal lag (virtually zero).

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