So, how does it expand? (aneroid wafer - intend to the airspeed indicator) I have two “theories” (from what I’ve learned) which are :

  1. The ram air entering through the pitot tube physically hitting, pushing the aneroid wafer forcing it to expand - giving us the actual indication in the cockpit. faster airspeed—> more air hitting, pushing the aneroid wafer at higher velocity —> greater expansion?

  2. The ram air rapidly filling the aneroid wafer making the air more dense —> more pressure, therefore forcing it to expand due the pressure differential between the aneroid wafer and the sealed case, which contain the outside atmosphere pressure. Faster airspeed —> more air —> increasing pressure inside the aneroid wafer —> larger pressure differential—> greater expansion?

Which one is true ? or both maybe ?

  • $\begingroup$ Is there really a difference? It's definitely a pressure thing, for sure. Related to the dynamic pressure at the pitot tube. Which could be construed as being a result of "physically hitting" on some scale. Well, I'll let someone else tackle an actual answer on this one. $\endgroup$ Commented Jun 26, 2019 at 17:51
  • $\begingroup$ Check out this link: allaboutairplanes.wordpress.com/2011/09/24/… $\endgroup$ Commented Jun 26, 2019 at 17:55

2 Answers 2


The aneroid wafer is just a metal balloon. Add air into its interior, above ambient pressure, and it expands.

The outside of the wafer is exposed to ambient static pressure. The inside of the wafer is plumbed to the pitot inlet. Ram air into the pitot raises the pressure within the pitot plumbing to above ambient, which being connected to the interior of the wafer, inflates it. The movement of the inflating wafer is geared to the airspeed indicator and voila, ya got yer speed indication.


The irony here is that aeronautical engineers build models of air flow based on a misapplication of data from pressure taps fed into the Bernoulli Equation to calculate airspeed at the surface of the wing at various points, claiming without any proof whatsoever that the results are accurate, while pilots use a device that does not use the Bernoulli Effect at all to accurately measure airspeed, which is critical to their survival.

Air has mass. Mass implies momentum. Changes in momentum constitute acceleration. When a closed tube (a pitot tube) is thrust into an airstream, the air that goes into that tube is quickly accelerated into the direction of the pitot tube. That acceleration creates weight, measured by the aneroid wafer. The greater the differential between the speed of the ambient air and the pitot tube, the more acceleration, and the more weight.

That weight compresses the air, so yes, it is denser in the tube than it is in the ambient air. That air would decompress out through the mouth of the tube to match the pressure of the ambient air, except that this is disallowed by the acceleration of more air striking the mouth of the tube.

The constant stream of oncoming air maintains the pressure in the tube, so as the relative speed of the air and tube change, so does the pressure, and so does the pressure reading, which is translated into airspeed.

The Bernoulli Equation was invented to measure blood flow in a blood vessel in the 1700s. It assumed an incompressible fluid flowing through a vessel with parallel walls, neither converging nor diverging.

Einstein ignorantly developed the "Longer Path" theory of lift generation of wings based on a misapplication of the Bernoulli Effect. Orville Wright argued that he was wrong, and Einstein accepted that he was ignorant and should not have posited the theory, but because of hero worship for Einstein's intellectual prowess, engineers really, REALLY want to use the Bernoulli Equation to measure airspeed.

Pressure taps measure pressure. They do not measure airspeed. Models of airspeed at the surface of wings based on data from pressure taps are deeply flawed, and yet they dominate aeronautics.

If the Bernoulli Equation could give you an accurate airspeed, then airplanes would use a device different from a pitot tube with an aneroid wafer to accurately measure airspeed. Engineers proclaim that air is "incompressible" at subsonic speeds, and yet here, inside the pitot tube is compressed air. If it wasn't compressed, the instrument would not measure airspeed.

And yet, it does.

  • $\begingroup$ "Engineers proclaim that air is "incompressible" at subsonic speeds" - I've never heard that before. Where is that from? $\endgroup$ Commented Jun 26, 2019 at 22:29
  • $\begingroup$ "engineers really, REALLY want to use the Bernoulli Equation to measure airspeed." No, we really, really want people with not understating of engineering to stop trying to guess at what we do or want to do. $\endgroup$ Commented Jun 27, 2019 at 8:15

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