Vertical Speed/Velocity Indicator
A VSI shows Rate of Climb/Descend - How?
The VSI (like the Altimeter) is only connected to the Static Port (Line).
That being said, it is also a pressure differential instrument. The fluctuations/movements of the needle is nothing else but the result of the pressure difference between the direct Static-Pressure within the Diaphragm and that received in the case through the 'Calibrated Leak'. In other words, the leak delays airflow into the case. See picture below.
The delay causes the instrument to actually display 2 types of information:
1. 'a Trend' ... then … 2. 'a Rate'
So, 2nd (rate) follows the 1st (trend) in a manner of a few seconds.
Trend -VS- Rate - What's the difference?
Trend = first quick fluctuation or jump of the needle as the Pilot either pulls/pushes the yoke/stick. <> PULL = needle goes UP / PUSH = needles goes DOWN
Rate = If you keep your airplane in a given pitch-up/pitch-down attitude, needle will stabilize after its first reaction (trend) at a given FPM number (300/400fpm ...)
This is the value you would mention in response to the 'What is your Rate Of Climb' question. - Not the trend! ;-)
In case you care about what happens in inside!?
Example of Operation
Current Situation: Level-Flight | VSI-Indication: steady @ 0ft/min - Following Action: Start climb
As you start to pitch-up (climb initiation), pressure changes (it decreases).
In turn, the static pressure from the Static-Line (within the Diaphragm) also becomes less, resulting in a contraction of it. Meanwhile, there is still some higher pressure in the instrument's case acting upon the Diaphragm and helping it to squeeze further.
Why is this pressure still higher?
The Calibrated Leak has a much smaller whole (orifice) which lets the air within the case escape at a slower rate compared to the air leaving 'the Diaphragm'. In other words,
the air that leaves the Diaphragm is faster than the air that leaves the case.
This delay is about 5-8 sec.
The above described process leads to the first indication:
- A trend
After the pressures have equalized, which means, no more difference in pressure between the inside of the case and the Diaphragm, the needle also becomes stabilized at a given FPM, let's say 500ft/min. - This leads to the second indication <> Rate of Climb.
Even the smallest movements of the yoke/stick are probably recognized by the VSI-needle.
It is very sensitive. - Another reason it should not be your 'Primary Pitch instrument', unless it comes to 'Constant-Rate & Level-Climbs/Descents'.
During a climb...
During a descent...
If your control input will be very smooth/slow, you can almost avoid the trend information.
It wil take some practise.
What is an IVSI?
An Instantaneous Vertical Speed Indicator (IVSI) will directly indicate a Rate of Climb.
IVSI -vs- VSI?
The IVSI incorporates acceleration pumps within the case that speed up the air exchange process between the Diaphragm and the Case. This process will remove the initial trend information, giving you a 'direct and reliable' Rate of Climb/Descend.
What Error is the VSI subject to?
A Static-Port (Line) blockage will result in a frozen VSI instrument as air (pressure) would then be stuck in the line. So there would be no more exchange of air between the Diaphragm and the instrument's case.
What can I do if this happens?
You can break the instrument's window if time permits. That would let some cabin air pressure enter the case. BUT!, dont forget, your cabin air pressure is always a bit lower than your outside (ambient) air pressure.
This would result in lower pressure inside the case, which will give you 'slightly' incorrect indications during climbs and descends.
How do I figure out whether my plane is equipped with a VSI or an IVSI?
You can you find that information right on the face of the instrument itself.
It either says Vertical Speed or IVSI (Instantaneous Vertical Speed).