VSI Metering Unit

[Bodie]

Can anyone help with this one please.

I'm reading up on the VSI - the Instruments module for Bristol GS. Now I grasp the very simple idea that there is a pressure difference which provides the reading. What I am unclear on is the Metering Unit, which seems so very simple but makes no sense.

Can anyone explain in plain english??

Thanks in advance
Bodie

[misterbonkers]

Right, i'll try, but first, have you tried cross checking with their CD?

The metering unit meters air going into the VSI chamber.

If you just had a gaping hole in the instrument, the pressure differential would not really occur with height change because the pressure would equalise to quickly.

The metering unit restricts air flow so the differential can measured and seen by the pilot.

i.e., climb, into lower pressures, so capsule inside has greater pressure inside as a pressure differential is created, this compresses the capsule, which moves the needle to show an ascent at an appropriate rate which is based on the rate of change of pressure, i.e. metering unit is only allowing a set equalisation of pressures, so the original pressure inside instrument is kept until a higher height (which is at a lower pressure, so great ROC is shown).

To be fair, you are unlikely to get many if any questions on this! Gyros and EFIS are the big cookie, and, if your having problems with Gyros, don't worry, Baz will sort you out with his Bongo in the Congo.

If ya get stuck on anything else and you found my help of any use, feel free to email me with any more queries and i'll see what I can do! Alternatively - use the school your paying! they dont miund if you email queries or drop them a line.

[mono]

The metering unit does just that, it meters the amount of air entering the vsi so that a predetermined lag exists between the pressure inside the VSI and the ambient pressure. In this way the capsule inside the VSI expands or contracts in a measureable way and therefore its output can be calibrated.

The metering unit comprises a number of restrictions in its construction, the airflow increasing then slowing down, becoming turbulent, then laminar. These have been incorporated to ensure that the lag is more or less constant for varying pressure/temperature variations.

The next problem is that with a deliberate lag it is possible to climb tens, or even hundreds of feet before the instrument indicates a climb/descent. This problem is solved by incorporating an inertial dashpot inside the (I)VSI, after the metering unit which acts on the capsule such that the inertial forces acting on the a/c will cause an almost instant indication of climb or descent, after which the normal pressure differential action provides the indication. Hence the term instantaneous (or inertial) VSI (IVSI).

:D