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The pitot tube located near the nose could be affected by the boundary layer and could increase the error. Then why is the pitot tube located near the nose?

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  • $\begingroup$ Downvoted because even a bit of prior research would show that not all, or even most, pitot tubes are located on the nose. For these two examples, I would suspect that it's a location with undisturbed airflow, a short tubing run, and out of the way of ground traffic. $\endgroup$ – jamesqf Dec 2 '20 at 17:46
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There are several reasons to have the pressure sensors in the front section of the fuselage.

  1. The further aft the more turbulent the airflow is and the thicker the boundary layer caused by the fuselage. This would cause strong fluctuations of the measured pressure and the reading could be inaccurate. Far forward the boundary layer is the thinnest because the boundary layer thickness grows with the distance traveled along the surface.
  2. The position on the nose should be less sensitive to angle of attack and side slip angle changes. Having the sensors on the side of the fuselage would cause significant changes in pressure measurements depending on the aircraft attitude relative to the incoming air.
  3. The pressure sensor in the nose is close to the cockpit instrumentation and avionics computers. Pressure tubes can be short and cannot be disconnected by mechanical forces that easily.
  4. There is no ground equipment moving close to them. Having them further aft could easily damage the sensors during baggage or passenger loading/unloading.
  5. During winter months airplanes have to be sprayed with de-icing fluids. Up front they are not flooded by these substances and are spaced far away from the affected areas.

Some aircraft have the sensors located on the wings. The Cessna 172 pitot tube is one example that comes to mind.

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    $\begingroup$ I would add the following reference: E. Haering, “Airdata measurement and calibration,” NASA TM 104316, December 1995. $\endgroup$ – afcdesign Dec 1 '20 at 13:27
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    $\begingroup$ The probe is aligned with the local flow,so is normally parallel-ish to the local skjn surface. The pressure sensed varies from the freestream flow away from the body, and so the Air Data Computers have to be calibrated against the true freestream flow with corrections applied. This is Calibrated Airspeed.To get the true freestream flow data, the certification aircraft is equipped with a test boom that places the pitot probe way out in front away from the influence of the fuselage shape, and differences between the test probe and fuselage probe recorded in calibration tables that the ADC uses. $\endgroup$ – John K Dec 1 '20 at 15:34
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    $\begingroup$ The probe needs to be pointing straight into the local flow because the air needs to enter the probe as close to straight on (from the probe's perspective) as possible. If the air is entering from an angle, the effective opening size of the inlet hole is smaller, so the dynamic pressure sensed will be lower. There will be some angle error at pretty much all conditions except one (the one where the probe is straightest into the local flow), so the calibration tables have to account for this error as well. $\endgroup$ – John K Dec 1 '20 at 15:54
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    $\begingroup$ I imagine pretty much all planes with nose-mounted propellers have the pitot tube on a wing to keep it out of the propwash. $\endgroup$ – Fred Larson Dec 1 '20 at 20:12
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    $\begingroup$ Or on the tail believe it or not. My own plane (a Pazmany PL-2)) has the pitot at the top of the vertical fin, above the propeller stream. Thorp T-18s also do that. The problem with it is the pitot comes into the prop wake just above the stall speed (the ASI starts fluctuating; how much depends on the power setting), so it should be about 6 in higher, although Pazmany may have designed that on purpose, because it makes a good visual pre-stall indication, the airplane not having any kind of stall warning system. $\endgroup$ – John K Dec 2 '20 at 2:04
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The boundary layer grows in thickness as you move aft on a fuselage. It is thinnest near the nose. A pitot tube near the nose only has to protrude a small distance away from the fuselage skin in order to extend out of the boundary layer and into undisturbed air.

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    $\begingroup$ Thank you for your answer, but could you prove your claim by an authentic source? $\endgroup$ – Roh Dec 1 '20 at 7:50

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