Since speed is related to dynamic pressure, then why is static pressure used instead of dynamic pressure in ASI to measure speed?

Since speed is related to dynamic pressure, then why is static pressure used instead of dynamic pressure in ASI to measure speed?

Why is dynamic (or pitot) pressure not sufficient?

• What you have in mind is a pitot-static tube aka Prandl tube, a sensor delivering the stagnation pressure information. For the simple Pitot tube, we need the static port information. Difference. – mins Dec 19 '18 at 15:49

Since speed is related to dynamic pressure, then why is static pressure used instead of dynamic pressure in ASI to measure speed?

It's not. The reading on the airspeed indicator is directly determined by the dynamic pressure. The ASI uses the static pressure for the sole purpose of determining the dynamic pressure.

Why is dynamic (or pitot) pressure not sufficient?

Dynamic pressure is sufficient, but the pitot tube doesn't give you the dynamic pressure; it gives you the total pressure, which is the sum of static pressure and dynamic pressure. In order to determine the dynamic pressure, an airspeed indicator has to find the difference between static pressure (from the static tube) and total pressure (from the pitot tube).

The ASI uses both, dynamic pressure from the pitot tube, and static pressure from the static ports. If just dynamic pressure was used, then at some point the system becomes pressurized and no change is seen. Having the static ports involved introduces effectively a controlled leak so that changes in dynamic pressure can be seen.

• Your second sentence is not correct. If you seal the static port, then the ASI will still work correctly for a single altitude and barometric pressure. But, will be wrong if either of those change. Essentially, the static port acts to calibrate for changes in pressure. – Adam Dec 12 '18 at 15:08
• And how useful is single altitude/barometric pressure? Not at all. That is why planes have multiple exterior static ports (my plane has 2 for instance, one on each side of the fuselage), and if those should become blocked (ex. taped over for washing and then not uncovered - reference the airliner that crashed off the coast of Brazil I believe) there are alternate static ports inside the plane that can be used. Data will not be as accurate (as the port is inside vs being exposed to external air), but it's better than nothing. – CrossRoads Dec 12 '18 at 15:23
• "dynamic pressure from the pitot tube" - Doesn't the pitot tube give total pressure, not dynamic pressure? That's why the airspeed indicator needs to know the static pressure as well. If the pitot tube gave dynamic pressure, then the airspeed indicator could be connected to only the pitot tube and not the static port. – Tanner Swett Dec 13 '18 at 5:49