X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Sun, 09 Dec 2007 16:46:32 -0500 Message-ID: X-Original-Return-Path: Received: from imo-d21.mx.aol.com ([205.188.144.207] verified) by logan.com (CommuniGate Pro SMTP 5.2c1) with ESMTP id 2571726 for lml@lancaironline.net; Fri, 07 Dec 2007 21:19:20 -0500 Received-SPF: pass receiver=logan.com; client-ip=205.188.144.207; envelope-from=Sky2high@aol.com Received: from Sky2high@aol.com by imo-d21.mx.aol.com (mail_out_v38_r9.3.) id q.c73.1fd10ac9 (65100) for ; Fri, 7 Dec 2007 21:18:38 -0500 (EST) From: Sky2high@aol.com X-Original-Message-ID: X-Original-Date: Fri, 7 Dec 2007 21:18:37 EST Subject: Re: [LML] Computing TAS from GPS tracks X-Original-To: lml@lancaironline.net MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="-----------------------------1197080317" X-Mailer: AOL 9.0 VR sub 5006 X-Spam-Flag: NO -------------------------------1197080317 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit In a message dated 12/7/2007 6:14:47 P.M. Central Standard Time, mnewman@dragonnorth.com writes: My true airspeed as computed from my airspeed indicator is about 14 knots faster that the measured result. Mike, Uh, I would agree that your indicated TAS is 14 Kts higher than a computed TAS and if the TAS is off, so is the IAS. I would not blame the dial on your airspeed indicator (correcting for altitude and temperature), although the airspeed device could be a problem with some sort of internal leak. Since it is hard to get pitot pressure higher than is should be theoretically, the culprit for too high an airspeed indication is almost always a too low static (ambient) pressure sensing. The bad part of this is that altitude would be off also. Ground tests may not completely provide and answer to the source of the problem but they can certainly eliminate many problems. There are some you can do yourself. 1. Pitot side. Using a home built manometer, you could check: H2O" Airspeed indication in Knots 2.41 70.0 4.00 78.2 6.00 95.6 8.37 113.0 2. Static side. Note that a static system leak into the cockpit at cruise will give a false higher altitude and a faster airspeed. In my airplane at 180 KIAS, such a leak would have the altitude indicate about 300 feet higher and airspeed at about 20 Kts faster than actual because of about a 4" H2O difference. These errors are reduced at slower speed since the lower pressure in the cockpit is less. You might run you TAS experiment at 110 KIAS and see if the correction is smaller - assuming you won't fall out of the sky. You might also consider the last static system check (at least within the last two years) and any "dickering" done since then. On the ground, you could seal one static port (You do have two, don't you?) and use the manometer or a slight suction device on the other static port and see how long the altitude is held as a leak indication (be gentle as the VSI and airspeed indicator will react also). I would definitely want to know why the error is so great. Luckily, if there is a static leak to the cockpit,as you slow to level flight stall speed, the error gets smaller as the ambient to cockpit pressure differential gets smaller. Scott Krueger AKA Grayhawk Lancair N92EX IO320 SB 89/96 Aurora, IL (KARR) Darwinian culling phrase: Watch This! **************************************Check out AOL's list of 2007's hottest products. (http://money.aol.com/special/hot-products-2007?NCID=aoltop00030000000001) -------------------------------1197080317 Content-Type: text/html; charset="US-ASCII" Content-Transfer-Encoding: quoted-printable
In a message dated 12/7/2007 6:14:47 P.M. Central Standard Time,=20 mnewman@dragonnorth.com writes:
<= FONT=20 style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size= =3D2>
My true=20 airspeed as computed from my airspeed indicator is about 14 knots faster t= hat=20 the measured result.
=
Mike,
 
Uh, I would agree that your indicated TAS is 14 Kts higher than a=20 computed TAS and if the TAS is off, so is the IAS.  I would not bl= ame=20 the dial on your airspeed indicator (correcting for altitude and temperature= ),=20 although the airspeed device could be a problem with some sort of internal=20 leak.  Since it is hard to get pitot pressure higher than is should be=20 theoretically, the culprit for too high an airspeed indication is almos= t=20 always a too low static (ambient) pressure sensing.  The bad part of th= is=20 is that altitude would be off also.
 
Ground tests may not completely provide and answer to the source o= f=20 the problem but they can certainly eliminate many problems.  There are=20= some=20 you can do yourself.
 
1. Pitot side.  Using a home built manometer,  you could=20 check:
 
H2O"  Airspeed indication in Knots
2.41    70.0
4.00    78.2
6.00    95.6
8.37  113.0
 
2. Static side.  Note that a static system leak into the cockpit a= t=20 cruise will give a false higher altitude and a faster airspeed.  I= n=20 my airplane at 180 KIAS, such a leak would have the=20 altitude indicate about 300  feet higher and airspeed at about 20=20= Kts=20 faster than actual because of about a 4" H2O difference.  These er= rors=20 are reduced at slower speed since the lower pressure in the cockpit is=20 less.  You might run you TAS experiment at 110 KIAS and see if the=20 correction is smaller - assuming you won't fall out of the sky.
 
You might also consider the last static system check (at least within t= he=20 last two years) and any "dickering" done since then.
 
On the ground, you could seal one static port (You do have two, don't y= ou?)=20 and use the manometer or a slight suction device on the other static=20 port and see how long the altitude is held as a leak indication (be gen= tle=20 as the VSI and airspeed indicator will react also).
 
I would definitely want to know why the error is so great.  Luckil= y,=20 if there is a static leak to the cockpit,as you slow to level flight stall=20 speed, the error gets smaller as the ambient to cockpit pressure differentia= l=20 gets smaller.=20
 
Scott Krueger=20 AKA Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL=20 (KARR)

Darwinian culling phrase: Watch=20 This!



Check o= ut AOL Money & Finance's list of the h= ottest products and top money wasters of 2007.
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