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How the Airspeed Indicator Works


Rotortramp
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If you're ever stranded on an island with Anne Heche and need to outfit your plane with pontoons and fix your airspeed indicator, you'll now know how it works.

 

I always use the language of "it compares the ram pressure to the static pressure", but how exactly does it "compare" it?

 

http://youtu.be/HM7ZMPpbeDA

 

 

374892889_1299aba915.jpg

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As iChris said in the other thread, it reads Total Pressure (TP) - Static (S).

 

TP = Ram + Static.

 

So your Airspeed = TP - S

 

What I'd like to know is how the static is subtracted from the equation. Is it really a "mechanical subtraction device" or is that one of the reasons why the static port goes into the casing? If it's a mechanical subtraction device, how does that work?

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I always use the language of "it compares the ram pressure to the static pressure", but how exactly does it "compare"

What I'd like to know is how the static is subtracted from the equation. Is it really a "mechanical subtraction device" or is that one of the reasons why the static port goes into the casing? If it's a mechanical subtraction device, how does that work?

374892889_1299aba915.jpg

 

 

Note from the diagram above, it's essentially a pressure chamber enclosing a diaphragm. This pressure chamber or case is vented to static pressure that exerts a constant pressure on the diaphragm that’s vented to pitot pressure. In order for the diaphragm to expand it must exert a pressure in excess of the static pressure pressing against its expansion.

 

As an example, if the static pressure inside the case is 14 psi and the total pressure at the pitot tube is 34 psi, The diaphragm will only begin to expand after the pressure inside the diaphragm exceeds 14 psi. Therefore, the movement of the diaphragm is only proportional to the remaining 20 psi, Which is the difference between the 34 psi at the pitot tube and the 14 psi in the case. That in a nutshell is how the subtraction is accomplished.

 

Yes, that is the reasons why the static port goes into the casing.

Edited by iChris
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