Dan wrote:
john smith wrote:

In article .com,
" wrote:

A friend is restoring a Fairchild 24. It did not have a static
port... instruments just vented to cabin.
He's wondering where one might install a pair, one per side.
Any experience on this? It isn't real fast so it might not matter.


Does anyone know why the sides of the fuselage, mid-way between the
cabin and the tail surfaces is uses on many aircraft?


When choosing a site for a static port the most important
consideration is turbulence at and near the port. In the aircraft you
have seen that's the smoothest area.

Mounting static ports on opposite sides of the aircraft "averages" the
pressures sensed which greatly reduces errors. Airliners do it, but I
can't see it in home builts. If you have a noticeable error with a
properly installed static port you have bigger problems.

Dan, U.S. Air Force, retired
I'm a tad surprised this question received so little feedback. Here's a
bit gained from 30+ years' worth of a sailplane guy's perspective...

In a nutshell, "What Dan said."

That noted, many (not all) sailplanes obtain their ROC pneumatic info
from pitot-static ports, and the developers of variometers and
total-energy compensators have learned a great deal of how very
difficult it is to obtain accurate static pressure.

For sailplanes, sensing static pressure in the cockpit doesn't cut it
for total energy compensation - it typically varies from 'real' static
due to pressure or suction effects, depending on speed, setting of
vents, etc.

Similarly, getting static from nose ports or underwing ports generally
suffers from speed-dependent flow-field effects (pressure/suction).

In the 1970's it was learned that fuselage ports somewhere between 50%
and 70% of the way from the wing T.E. to the vertical fin L.E. tended to
be pretty good on many sailplanes, but flap effects and the nature of
the method of making the port holes (by then, usually in fiberglass)
couldn't be ignored. Probably most common today are static ports on a
vertical-stab-mounted probe perhaps 12-18 inches ahead of the fin L.E.

I can see why in a draggy, relatively low-powered ship (e.g. Fairchild
24) using cockpit static would be acceptable (especially given the
ship's vintage and knowledge existing when it was built). In the
absence of a free-stream probe for sensing static, my guess is aft
fuselage side statics would likely be a decent compromise in your
friend's restoration. Of course, if the ship isn't experimental, there
might be type certification issues...

Regards,
Bob Whelan