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Re: Bernoulli's relationship



At 10:41 AM 11/9/00 -0600, Kilmer, Skip wrote:
Of course, airplane manufacturers go to some pains to place the alitmeter
port in an area where the air remains static even while the plane is moving.

The phrase "an area where the air remains static" is not something one is
likely to find in aerodynamics books.

Those words are highly open to misunderstanding. I'm sure many readers
will assume the words mean something like "an area where the air is held
stationary (relative to the aircraft)". And such an inference would be
100% wrong.

In fact, the air rushes sideways past the altimeter port at 100% of the
true airspeed.

As usual, a thoughtful application of Bernoulli's formula to this situation
gives the right answer. And just as usual, a thoughtless application gives
the wrong answer.

Here's another scenario to motivate arranging the altimeter to get the
right answer independent of airspeed. On an ordinary ILS (Instrument
Landing System) approach in bad weather, you need to follow the ILS
glideslope to within 200 feet (sometimes 100 feet) of the ground. You must
break off the approach if you can't see the runway at a predetermined
point: the point where the glideslope crosses a certain altitude. The
situation would be quite unpleasant if your altimeter were off by hundreds
of feet depending on airspeed.