Robert M. Gary wrote:
I understood that. I just way over estimated how much air went through
the pitot tube. Any tube that has a hole in the front and a smaller
hole in the back would do the job. The net pressure to the A/S
indicator would be less but that's just a calibration issue. So I
understood the concept but over estimated the flow.

Mea culpa. There was actually something you didn't know (or didn't
consider) but it was so obvious to me that I never even thought about
it. And really it's much less obvious than the physics - unless you
happen to routinely build and design stuff. I say this by way of
explanation and apology.

You're right - you could in theory have any tube with a bigger hole at
the front and a smaller one at the back, and the indication would be
less but could be calibrated out. There's a reason it's not done that
way, and it has nothing to do with the physics and everything to do
with manufacturing practices and standards.

ASI's are interchangeable, and it would be a maintenance nightmare not
to have them interchangeable. Imagine that every pitot tube had to
have its own matching (calibrated) ASI. Often the pitot tube is made
by the manufacturer. Fortunately they rarely fail, because
replacements are sometimes very expensive or even impossible to find.
Piper made its own pitot tubes for a long time. Go try to find one for
a short winger.

ASI's will all fail eventually. They're made with brass bellows that
are soldered together. The solder joints move and hold (just a little)
pressure. Eventually they are guaranteed to leak. The leak will show
up as a too-low airspeed indication.

So for practical purposes, you really need a pitot tube that supplies
approximately the right ram pressure.

Michael