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Consider the following situation, also depicted in the picture bellow (and please excuse my lack of Paint skills):

  • A helicopter is flying at a constante altitude, and hovering the same ground spot. The helicopter is not air tight, and it even has an open window.

  • On the inside of the helicopter there is a box, with a small hole on the side. Except for this hole the box is air tight. Inside the box there is an air pressure sensor.

  • A plane, flying at a higher altitude than the helicopter, drops a box exactly like the one on the helicopter.

When the two boxes are at the same height (one sitting still on the chopper and one falling) how do the pressure sensor readings differ from each other, considering the Bernoulli principle? If the pressure readings differ, can they be used to estimate the fall velocity of the box?

enter image description here

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  • $\begingroup$ So in synthesis, your question is, is there a predictable change of pressure on the box that could lead someone to determine its velocity given its altitude? $\endgroup$ – Neil Apr 18 '16 at 12:06
  • $\begingroup$ Yes, that would be the first question. The follow-up question would be: if there was no helicopter, would it be possible to attach another sensor to the falling box (in some way or position) that would allow to estimate the fall speed based on the pressure diference between inside/outside of the box? $\endgroup$ – AmiguelS Apr 18 '16 at 12:24
  • $\begingroup$ My guess is that it would be something like trying to determine the difference between a weight upon a sensitive scale or a fan blowing on it. While you might get the same "weight", one would vary far more. You could use that to "guess", just like I'm sure the pressure would vary more with higher velocities that you could use to guess. However, it would probably not be a very accurate predicter. $\endgroup$ – Neil Apr 18 '16 at 12:37
  • $\begingroup$ I don't know if you realize that the presence of the air flow around the helicopter changes the pressure in the vicinity - I suspect you would like the question answered if the helicopter was replaced with a hot air balloon (no air flow in the vicinity). More opportunities to practice your drawing skills? $\endgroup$ – Floris Apr 18 '16 at 13:22
  • $\begingroup$ I think what you mean is this: en.wikipedia.org/wiki/Pitot_tube. Pilot tubes are used to measure the airspeed of the aircraft and their correct function is extremely important to keep aircraft within their safe operating envelope (a stall is a very bad thing!). Together with a static pressure measurement altitude and airspeed can be determined fairly reliably, at least for purposes of aviation. The downwash of a helicopter is significant, but only within a few rotor diameters, i.e. within the distance where ground-effect is of importance. $\endgroup$ – CuriousOne Apr 18 '16 at 18:10

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