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What I know so far

I recently read about pitot tubes and their use in aircraft. For the basics I consulted Wikipedia, and also went through the definitions for airspeed, groundspeed, and the various other airspeed definitions (TAS, IAS, EAS, CAS). Furthermore I made myself familiar with aircraft speed indicators.

At first everything seemed logical, I thought I understood the working principle. But then a few days ago I came across the question how wind (headwind, for that matter) is accounted for on the airspeed indicator.

Question

Assume an aircraft that is standing still on the ground, experiencing a 60 kt headwind. Will the airspeed indicator show anything that is related to that headwind? Why/why not?

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    $\begingroup$ @mins Thank you for the video! I actually could have come across it myself. $\endgroup$ – pat3d3r Nov 5 '16 at 17:31
  • $\begingroup$ You can imagine your hand is the speed sensor (like when you put your hand thru a car window: You feel the wind, regardless of whether it's the car moving or the wind. If you drive into the wind the speeds add, if you drive in the direction the wind if blowing to, the speed subtract. If you are at the same speed than the wind and in the same direction, then your hand feel no wind, even if actually the car is moving. An airspeed sensor (pitot probe) works exactly on this principle. $\endgroup$ – mins Nov 5 '16 at 19:55
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The pitot-static system (and the airspeed indicator) doesn't care about the speed of the aircraft with respect to the ground. It is concerned only with the speed of air with respect to the aircraft (that is why it is called airspeed and not groundspeed indicator).

So, if your aircraft is on ground and experiences a headwind of 60 kt, the airspeed indicator will show an airspeed of 60 kt, in an ideal case.

This is important because the wing generates lift depending on the airspeed, not the ground speed (forgetting about stuff like ground effect, etc.). This means that if an aircraft is parked on the ground and there is a headwind of sufficient velocity, it will try to lift the aircraft up.

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  • $\begingroup$ So if the aircraft is accelerating into this headwind (for take-off purposes) the airspeed indicator (IAS) will start at 60 kt, go down until the aircraft exceeds 60 kt and then go up again? $\endgroup$ – pat3d3r Nov 5 '16 at 17:16
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    $\begingroup$ @pat3d3r Not quite. If the aircraft is sitting stationary on ground in a 60 knot headwind, the airspeed indicator (ASI) will start at 60 knots (there are 60 knots of air moving over the aircraft with a groundspeed of 0 knots). As the aircraft is accelerated to 20 knots of ground speed, the ASI will show 80 knots (as there are 80 knots of air moving over the aircraft). If the aircraft accelerates to a ground speed of 100 knots, the ASI will show 160 knots (as there are 160 knots of air moving over the aircraft). $\endgroup$ – J Walters Nov 5 '16 at 17:43
  • $\begingroup$ @JonathanWalters Ok, that makes sense, especially in respect to the differentiation of airspeed and groundspeed. Thank you! $\endgroup$ – pat3d3r Nov 5 '16 at 18:15
  • $\begingroup$ @pat3d3r What you described in your first comment is sortof what would happen if you had a 60 kt tailwind though. Because of the design of the pitot tube (pointing forward), it would actually start at 0, remain at 0 until reaching a ground speed of 60kt, and then accelerate as the aircraft continued to accelerate. Note that in many cases due to the designed of the aircraft and the length of the available runway, it may not be able to even takeoff with such a strong tailwind. $\endgroup$ – Lnafziger Nov 5 '16 at 23:42
  • $\begingroup$ @Lnafziger I still have trouble understanding the physics behind it. Taking the tailwind situation: During acceleration of the aircraft the air mass of the tailwind is keeping the incoming air (due to acceleration) out of the pitot tube until incoming air speed exceeds the tailwind? $\endgroup$ – pat3d3r Nov 7 '16 at 9:12

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