X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Fri, 26 Sep 2008 12:10:35 -0400 Message-ID: X-Original-Return-Path: Received: from smtp1.av-mx.com ([137.118.16.57] verified) by logan.com (CommuniGate Pro SMTP 5.2.8) with ESMTP id 3187340 for lml@lancaironline.net; Thu, 25 Sep 2008 19:16:55 -0400 Received-SPF: pass receiver=logan.com; client-ip=137.118.16.57; envelope-from=pinetownd@volcano.net Received: from DennisDell (63-170-118-207.dsl.volcano.net [63.170.118.207]) (Authenticated sender: pinetownd@volcano.net) by smtp1.av-mx.com (Postfix) with ESMTP id AACF0290520 for ; Thu, 25 Sep 2008 19:16:05 -0400 (EDT) X-Original-Message-ID: From: "Dennis Johnson" X-Original-To: "Lancair Mailing List" Subject: Legacy Static Port Fix X-Original-Date: Thu, 25 Sep 2008 16:16:02 -0700 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_00B1_01C91F2A.0166C690" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.5512 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 This is a multi-part message in MIME format. ------=_NextPart_000_00B1_01C91F2A.0166C690 Content-Type: text/plain; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable 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.=20 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. =20 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 ------=_NextPart_000_00B1_01C91F2A.0166C690 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable
As most of the Legacy pilots know, if the Legacy's static ports are = flush=20 with the outside skin of the fuselage, the indicated airspeed will most = likely=20 be erroneously low.  I thought I was doing the right thing by = making mine=20 flush when I installed them, but when I started flying, I noticed that I = almost=20 always had a tailwind!  Flying the four-way GPS groundspeed check, = my=20 indicated airspeed was 13 knots too low.  That error was enough to = make the=20 Chelton computed winds meaningless.
 
Here's the link for the four-way GPS airspeed calculator:
http://www.reacomp.com/tru= e_airspeed/
 
I experimented with temporarily taping button head rivet heads over = the=20 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=20 existing, flush, static ports.  I flew the four-way GPS groundspeed = check=20 this morning and my IAS is now three or four knots too high, which is an = error=20 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=20 face that's .067" thick at the center, tapering down to a thin (but = not=20 sharp) edge.  The hole in the center is .063" in diameter.  I = painted=20 them the color of the fuselage and bonded them in place with=20 epoxy/micro/Cabo-Sil.  The 3/8" diameter is much larger than it = needs to=20 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)=20 air along the fuselage skin at the location of the static ports, which=20 causes flush static ports to read erroneously high pressure = (and=20 therefore low airspeed).  The new buttons moved the static = ports far=20 enough away from the fuselage skin that it is outside the disturbed = air. =20 It surprised me that .067" is far enough to get outside the boundary = layer, but=20 it works, so I guess it must be. 
 
If you don't have a machinist friend, button head rivet heads, with = the=20 shaft punched out, work fine.  Double-sided clear Scotch tape = will=20 hold them on for at least one test flight.
 
Fly safe,
Dennis
Legacy, 198 hours
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