Static ports open directly to the outside of fuselage. When aircraft is flying, there will be air flowing outside of static port. How does this not cause a vacuum effect inside the static port. Due to the Bernoulli Effect, as the speed of air flowing outside of fuselage increases, the pressure inside static port should decrease. This means, on same altitude, depending on my aircrafts speed, I will read different values on my altimeter. What prevents this ?
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"Due to the Bernoulli Effect, as the speed of air flowing outside of fuselage increases, the pressure inside static port should decrease." Who told you that? That's not true. – Tanner Swett Dec 18 '18 at 22:03
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When aircraft is flying, there will be air flowing outside of static port.
No, that is not true if the aircraft flies without sideslip and at constant altitude (which it should do most of the time). Instead, there will be a pressure equilibrium between the air in the lines leading to the static port and the static pressure outside. The position of static ports is carefully chosen so the airflow around them is neither accelerated (which would lower local static pressure) nor retarded (which would increase local static pressure). Also, static ports are on both sides of the fuselage so the ram effect of the windward static port in a sideslip is equalised by the suction on the leeward static port. Note that in this case there is indeed some air flowing from the windward to the leeward side through the static line connecting both.
Now to the Bernoulli effect. It says in essence that the sum of static and dynamic pressure is constant (if we disregard the height term, which is not needed in this context). Since you want to measure the same pressure that a stationary observer would measure, you need to pick a place along the airframe where the dynamic part of the pressure is zero because all flow is tangential to the port opening. Small errors can be corrected by calibrating measurements in flight test.
Peter Kämpf
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This is a really good question! The Bernoulli Effect requires a decrease in the size of the space through which air is traveling to obtain a decrease in pressure. Just having airflow moving past the static port will not induce the effect.
A static port is essentially just a small hole somewhere outside the aircraft that provides a source of ambient air pressure for instruments that use it to provide indications of altitude and rate of climb/descent for example.
Also, static ports are usually located on the side of the fuselage, where the boundary layer will provide a relative smooth airflow from where to sample that ambient air pressure.
Juan Jimenez
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Thanks for your answer. So you say, the air which flows outside the static port, does not cause a vacuum effect inside of static port tubes. – utku Dec 18 '18 at 19:53
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If the port is placed correctly on the aircraft's exterior, yes. If it caused a vacuum it would make any instruments read incorrectly. For example, on older aircraft you may find a venturi tube on the outside of the aircraft. The tube produces a decrease in air pressure in the middle from Bernoulli's effect that is used to drive gyroscopic instruments. Putting the static port there as well would make it useless. – Juan Jimenez Dec 18 '18 at 19:56
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Not in my experience. Relative wind is more applicable to angle of attack than to something like this. – Juan Jimenez Dec 18 '18 at 20:03