Why is there a misreading in the altitude gauge at Mach1?

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saberrider

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Unread post08 Oct 2020, 19:31

There is a little jump of the needle on the altimeter reading on the F-16 (or other jets) just at Mach 1.0? Why's that?
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Unread post08 Oct 2020, 20:14

Thinking logically (i.e. I have not studied this or been told but instead am just using my education) the pressure shift hitting the static port causes erroneous readings.
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jetblast16

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Unread post09 Oct 2020, 02:34



Original video source: https://www.youtube.com/watch?v=p1PgNbgWSyY

Around 12 minutes in he explains it. And, you'll be smarter everyday from watching his (great) videos :wink:
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boilermaker

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Unread post16 Oct 2020, 05:47

saberrider wrote:There is a little jump of the needle on the altimeter reading on the F-16 (or other jets) just at Mach 1.0? Why's that?


THe shock wave is a discontinuity in pressure, and works much like a tap, with high entropy and drop in pressure but not in flow velocity (for water in a pipe). However, outside a pipe in the air, it is a velocity drop that occurs. Ahead of the shock the air is supersonic, and behind the shock the air is subsonic with a pressure increase. This is a way also to create ram jet air compression by the way without the need of mechanical compressor means to increase pressure.

The shock wave thickness is very thin. Within the shock wave you have unsteady air at the highest state of disorder. It is similar to the wall of a hurricane but much smaller and much more violent. You can only read the correct situation either behind or in front of the shock wave, but not accross it, or you will be comparing apples and oranges. It is when the system is comparing apple and oranges due to its geometry and the geometry of the appearing and evolving shockwave that you get this erroneous reading.

There is a specific physical distance between the ram air pressure tap and the static air pressure tap. The shock wave forms at the tip (ram air pressure) side first and then travels down the tube on top of the static port and then once it is behind the static port, both ram and static are in the same zone of dynamic/pressure reading (for a well designed "shockless" pitot tube as some physical shapes allow the shock to form at the tail of an aircraft and not its front). Whereas there is a moment when the ram air pressure reading is in the supersonic side of the shock wave while the static pressure reading is behind the shock wave at a much higher pressure and still in the subsonic part of the airflow behind the shock wave.

So long a shock is not formed below mach 1 air velocity (it can go to mach 1 on the wings when flying subsonic), the ram pitot and static system provides the input values for the Bernoulli equation mechanically tuned mach meter. Once a shock is formed, depending on its position, the inputs to the equation are erroneous, plus or minus the compressibility factors that the mach meter computes for.

Needless to say, when the shock wave itself is on top of the static pressure tap, it is impossible to get any accurate or computable reading at all because you are not sensing real situation.
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saberrider

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Unread post17 Oct 2020, 16:36

Thank you .

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