Re: Bernoulli's relationship

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

John Denker choked on my comments:

> At 02:04 PM 11/9/00 -0500, Hugh Haskell wrote, regarding the "static port"
> used for altimeters and such:
> >  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.
>
> Whaaaaat?????
>
> 1) I've seen dozens of different types of static ports, and I've never seen
> one angled toward the rear.  And I can't imagine why you would want that.
>
> 2) Not all of them are flush with the skin of the plane.  For instance on
> Piper Cherokees etc., it's at the end of a blade, more than 2 inches from
> the nearest skin.
>
> 3) Whether or not it's in the boundary layer has nothing to do with how
> accurately it measures the static pressure.  There are places in the
> boundary layer that are a full 1.0 Q above ambient.  There are other places
> that can get to be several Q below ambient.

Well, you're right to a degree. On small aircraft, you are likely to
see the static port almost anywhere. A Cherokee I owned some years
back had its static port flush with the skin, but I am not being
successful in remembering where it was located. And on some aircraft,
the static port is on the barrel of the pitot tube. I was thinking
about larger aircraft (primarily military) when I wrote my original
comments. On high performance aircraft (i.e., those that carry an
angle of attack indicator) the static port is usually a paint-free
plate near the cockpit, and the AOA sensor is located on the plate as
well. This area is located well away from any other structure that
could disturb the air flow in its neighborhood, so, as long as the
plane is in some semblance of normal flight, the airflow over the
static port (and past the AOA sensor) will be reasonably laminar. As
to the port being "angled backwards," this is an empirical
observation. The hole is usually flush with the surface (on some
aircraft, the port seems to sit in a little sheltered recess), and if
you look at the tube leading away from the port, it seems to angle a
bit forward, so that the port is *aimed* to the rear by about 10
degrees, give or take a bit. I have to admit, I don't know why it is
done that way. I simply assumed (not a good idea) that is was some
sort of "correction" to make the pressure at the static port more
nearly represent the correct static pressure.

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..
******************************************************