What is the difference between a Pitot tube and a Pitot-static tube? If an aircraft fitted with Pitot tube and static holes on the fuselage, is it necessary to consider the static pressure measurements to calculate the airspeed?
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3 Answers
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A pitot probe strictly measure ram air pressure, commonly called "pitot pressure" or Pt pressure. A pitot tube measures pitot pressure and pitot pressure only. A separate static port is required to measure static pressure (Ps) and will usually be located on a section of the fuselage where airflow is minimized.
A pitot-static port is a combination pitot AND static port. That is, the tube itself has an opening for detection of Pt pressure and one or more openings for static pressure. In this case, there generally is no static port on the fuselage since it's built into the pitot-static probe.
TLDR: the only difference between the two is that a pitot-static probe is a pitot probe with static ports whereas a pitot tube is strictly just a pitot tube. :-)
Edit: Not sure if the original question was edited after my answer or if I missed the second part in my original response... Static pressure is required to know indicated airspeed because as altitude increases, airspeed increases due to less air resistance.
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$\begingroup$ Also called Total Pressure: both static and dynamic pressure combined. $\endgroup$– KoyovisCommented May 30, 2017 at 8:24
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I haven't heard of that specific item, but Piper used to design the pitot-static systems on their light aircraft with a single blade-like mast which contained both a stagnation tap on the front face and a static port on the aft side as well as a drain hole on the bottom to remove moisture from the unit. It was also electrically heated for deicing.
Supposedly the Piper design had an advantage over conventional, separate pitot tubes and static ports as installed in Cessnas, Mooneys, etc. in that the indicated airspeed more closely matched that of calibrated airspeed on the pitot-static blades as opposed to the separate tube and port design.
answered May 31, 2017 at 1:41
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A pitot tube is designed for measuring ram air pressure, but a static port is designed for measuring static pressure.
There are differences between them.
Ram air pressure is the sum of the dynamic pressure and static pressure.
You can't only measure the dynamic pressure because the air always has static pressure.
So you have to measure the ram air pressure which is the sum of the dynamic and static pressure and then remove the static pressure which is measured by static port.
