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I have been reading some about helicopter phyisics and what not before I hopefully start training in around august. The book I am reading had a diagram of how an airspeed indicator worked, I understand the basic principle that pressure blows into a tube the bubble of whatever you want to call it at the other end will expand, pushing gears and moving the needle. However I dont understand exactly how static pressure is stopping the dynamic pressure from expanding the bubble all the way out. I mean if you keep blowing air into a balloon it will keep expanding until it pops. Perhaps someone can explain this too me. Thanks in advance guys!

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hope this helps, the ASI works off of differential air pressure





Airspeed indicators work by measuring the difference between static pressure, captured through one or more static port(s) and total pressure due to "ram air", captured through a pitot tube. This rise in pressure due to ram air is called impact pressure.


The static ports are located on the exterior of the aircraft, at a location chosen to detect the prevailing atmospheric pressure as accurately as possible, that is, without any disturbance from the passage of the aircraft. Some aircraft have static ports on both sides of the fuselage or empennage, in order to more accurately measure static pressure during slips and skids.


Pitot tubes face forward, in the direction of flight. Icing is a problem for pitot tubes when visible moisture is present in the atmosphere, as when flying through clouds or precipitation. Electrically heated pitots are used to prevent clogging with ice.

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Is some of the dynamic pressure air released through the dyaphram like those little whistle thing on a tea kettle? It just seems to me that the air keeps getting pushed in through the pilot tube, the pressure keeps building, and the dyaphram keeps expanding unless there is some release of the pressure?

Edited by slick1537
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the construction of the "BRASS" diafram allows for only so much movement, as the ram air is coming into the pitot tube travels and thru the piping to the back of the instrument, it becomes more of a static pressure than a fluid pressure due to the constriction of the tubing it has to travel thru.


it's like if you have a syringe, replace the needle with a psi gauge, place the plunger into the back of the syringe, now push the plunger with a stready pressure towards the gauge compressing the air, the plunger represents the ram air, it is only going to travel so far before it takes more pressure that is currently being applied to move it further, the faster you fly the greater the pressure, but the pressure being applied is not stronger than the instrument itself.


forgot to add, the static air port on the instrument is so that the expanding diafram doesn't compress the ambiant air inside the gauge causing an eronious reading.




clear as mud now :blink:

Edited by 67november
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