Re: Bernoulli's relationship

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

Skip Kilmer 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.

Since the airspeed indicator uses the difference between dynamic and
static pressure to calculate airspeed, and the altimeter uses static
pressure to determine altitude, the measuring point for static
pressure is usually the same for both instruments. One might think
that the interior of the aircraft would be a good place for that,
but, of course in modern pressurized aircraft that won't work, since
the "cabin altitude" is seldom much higher than 5,000 feet,
regardless of how high the plane itself is. So the "static port" has
to be on the outside skin of the plane, and it is usually located on
the side of the plane, just below the cockpit. The opening is usually
angled a little toward the rear of the plane and set flush to the
skin of the plane, so it is essentially measuring the pressure in the
boundary layer, where the air is not moving, relative to the plane.

Even in non-pressurized aircraft, the interior pressure is not always
a reliable indicator of the static pressure, since various
ventilation ducts to the interior can distort the static pressure
inside the plane. However, I have heard of pilots who lost their
airspeed indicator due to the static port being plugged with ice or
some other foreign substance, regaining at least an approximate
airspeed indication by breaking the glass on the altimeter and
letting the cabin pressure get to the static line. Obviously, this
only works on unpressurized aircraft.

Hugh
--

Hugh Haskell
<mailto://haskell@ncssm.edu>
<mailto://hhaskell@mindspring.com>

Let's face it. People use a Mac because they want to, Windows because they
have to..
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