----- Original Message -----

From: J H Webb

To: lml@lancaironline.net

Sent: Saturday, September 27, 2008 8:21 AM

Subject: [LML] Re: Legacy Static Port Fix

Dennis,

 

   Another nice side effect is to reduce the amount of water that gets into the static system, when the airplane is overnighted out in the rain.

 

Jack Webb

----- Original Message ----
From: Dennis Johnson <pinetownd@volcano.net>
To: lml@lancaironline.net
Sent: Friday, September 26, 2008 12:10:35 PM
Subject: [LML] Legacy Static Port Fix

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:

http://www.reacomp.com/true_airspeed/

 

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