"Axel Berger"  wrote in message
news:5C29F36E.45C5F8AB@Berger-Odenthal.De...
> Martin Gregorie wrote:
>> Hence no extra damping is needed
>
> It's good that you added the "extra". Every instrument has some internal
> damping due to inertia of some sort or another. In this case that's
> enough although we don't seem able to quantify it.

Yes, I took it as read that the mechanical nature of the dial gives its own
inertial damping. As you say, it seems from what people say that this is
enough to iron out any jitter caused by gusts. If the gauges were digital,
with no mechanical inertia, it would no doubt be a different story!

For airspeed, I can see that it is the speed of the air over the wings,
rather than the speed relative to the ground, which is the critical thing
for flying, so pitot tubes have a BIG advantage over GPS, even if GPS had
less error, which it sounds is not always the case.

But (dragging this thread kicking and screaming back on-topic!) for height
above ground, GPS or radar have the big advantage over air pressure that
they are measuring the actual height - as long as the readings are not
subject to error - and don't need constant adjustment for variations in
sea-level pressure as you move from one place to another. Obviously in the
case of GPS, you need a good terrain map to convert height above sea level
into height above ground at the place where you are, which you don't need
with radar.


It's interesting to hear people say that variations in pressure caused by
gusts are sufficiently small that they do not swamp the variation of
pressure with altitude. Without doing the sums, I don't know how much the
pressure varies with altitude, but my gut feeling (which is evidently
wrong!) would have been that it was smaller than the variations every time
there is a gust of wind. Good thing that it's not the case, otherwise
pressure-driven altimeters would be unusable.

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