|
As most of the Legacy pilots know, if the Legacy's static ports are flush
with the outside skin of the fuselage, the indicated airspeed will most likely
be erroneously low. I thought I was doing the right thing by making mine
flush when I installed them, but when I started flying, I noticed that I almost
always had a tailwind! Flying the four-way GPS groundspeed check, my
indicated airspeed was 13 knots too low. That error was enough to make the
Chelton computed winds meaningless.
Here's the link for the four-way GPS airspeed calculator:
I experimented with temporarily taping button head rivet heads over the
static ports, which seemed to solve the problem. A friend machined
some nice "buttons" that I bonded to the fuselage over the top of the
existing, flush, static ports. I flew the four-way GPS groundspeed check
this morning and my IAS is now three or four knots too high, which is an error
that's too small for me to chase down. I'm declaring it fixed.
The buttons are machined aluminum, 3/8" in diameter, with a convex
face that's .067" thick at the center, tapering down to a thin (but not
sharp) edge. The hole in the center is .063" in diameter. I painted
them the color of the fuselage and bonded them in place with
epoxy/micro/Cabo-Sil. The 3/8" diameter is much larger than it needs to
be, but that size was easy to machine and easy for me to bond in place.
It appears there is a very thin boundary layer of disturbed (high pressure)
air along the fuselage skin at the location of the static ports, which
causes flush static ports to read erroneously high pressure (and
therefore low airspeed). The new buttons moved the static ports far
enough away from the fuselage skin that it is outside the disturbed air.
It surprised me that .067" is far enough to get outside the boundary layer, but
it works, so I guess it must be.
If you don't have a machinist friend, button head rivet heads, with the
shaft punched out, work fine. Double-sided clear Scotch tape will
hold them on for at least one test flight.
Fly safe,
Dennis
Legacy, 198 hours
|