Posted this in the coursework forum as well but it doesn't appear any aerospace engineer visits that one :-) I'm supposed to help someone with this for his essay but I don't completely understand it myself. I understand how a pitot tube measures the total pressure, and how a static pressure orrifice measures the static pressure. Assuming incompressible flow Bernoulli's equation provides a relation between these two, e.g. the fluid velocity at the orrifice location's becomes v = sqrt ( 2 * (total pressure - static pressure) / rho ). But how can you make the connection between the measurement of the fluid velocity at this point and it being the aircraft's true speed? The pressure varies constantly over and under the airfoil, which means the fluid velocity changes constantly as well. Which also means different placements of the static port would give different fluid velocities. How do you know even where to put this, in order to get the aircraft's V?