Date: Tue, 21 Aug 2001 08:07:07 -0500 From: David Domeier Subject: COZY: Re: [c-a] True True Airspeed II Ken, re "I was 165 mph at 2200 with -6 mph correction and was 191 mph with -3 mph correction at 2600 rpm. The lower the speed the more the error. ... " I assume you are converting the GPS calculated TAS to calibrated airspeed and finding an error with indicated airspeed, and that error is different as speed is changed. I believe the error is induced by changing static pressure and not the GPS system. It is most difficult to accurately measure static pressure inflight. Certainly the pressure measured at the static port on the ramp is different than at 200 mph or at any speed. It has to be because once air is moving, its static pressure changes. The challenge with any airplane is finding a location to measure static pressure that is least effected by this phenomena. The error usually is not great, but there is an error and it generally is on the high side because of a pressure drop at the port. At one point last year, after experimenting with moving the port and installing a small fence aft it, I had CAL airspeed within a knot of IAS. Yesterday, I ran a GPS ground track and found a 6 knot error in the indicator once again. Now that is frustrating. Nothing has changed from last year with the static port. Perhaps there IS an error in the GPS ground speed calculations but I am inclined to believe the error is else where. The next time you run a test, use http://www.reacomp.com/true_airspeed/index.html to compute your TAS in addition to your method. It would be interesting to see if the TAS number is the same. The error may be in computing TAS and not GS. dd Cozy MKIV N10CZ From ???@??? Wed Aug 22 20:24:37 2001 Return-Path: Received: from mx05.mrf.mail.rcn.net ([207.172.4.54]) by mta02.mrf.mail.rcn.net (InterMail vM.4.01.03.27 201-229-121-127-20010626) with ESMTP id <20010822013428.MXSB10747.mta02.mrf.mail.rcn.net@mx05.mrf.mail.rcn.net> for ; Tue, 21 Aug 2001 21:34:28 -0400 Received: from alum.mit.edu ([18.7.21.81]) by mx05.mrf.mail.rcn.net with esmtp (Exim 3.32 #2) id 15ZMua-0000qi-00 for marc.zeitlin@rcn.com; Tue, 21 Aug 2001 21:34:28 -0400 Received: from twc2.betaweb.com (majordomo@ns.betaweb.com [216.231.140.250]) by alum.mit.edu (8.9.3/8.9.3) with ESMTP id VAA04010; Tue, 21 Aug 2001 21:34:27 -0400 (EDT) Received: (from majordomo@localhost) by twc2.betaweb.com (8.9.3/8.9.3) id VAA04730 for cozy_builders-list; Mon, 20 Aug 2001 21:18:25 -0400 X-Authentication-Warning: twc2.betaweb.com: majordomo set sender to owner-cozy_builders@canard.com using -f Received: from smtp01.mrf.mail.rcn.net ([207.172.4.60]) by twc2.betaweb.com (8.9.3/8.9.3) with ESMTP id VAA04724 for ; Mon, 20 Aug 2001 21:16:03 -0400 Received: from 207-172-109-67.s67.tnt1.war.va.dialup.rcn.com ([207.172.109.67] helo=hotrod) by smtp01.mrf.mail.rcn.net with smtp (Exim 3.32 #2) id 15ZMme-0007XF-00 ; Tue, 21 Aug 2001 21:26:17 -0400 From: "DL Davis" To: , "Cozy Builders Mailing List" Subject: COZY: RE: [c-a] True True Airspeed II Date: Tue, 21 Aug 2001 21:30:22 -0400 Message-ID: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit X-Priority: 3 (Normal) X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook IMO, Build 9.0.2416 (9.0.2911.0) Importance: Normal In-reply-to: <3B825CFB.F50B2914@swbell.net> X-MIMEOLE: Produced By Microsoft MimeOLE V6.00.2479.0006 Sender: owner-cozy_builders@canard.com Precedence: bulk Reply-To: "DL Davis" Yes, I believe there will be a difference in the calculated TAS with those two methods. There seems to be a common misconception about how to use GPS to check your true airspeed. I have heard many pilots talk about the method of flying equal times in three headings, 0, 120, and 240, and averaging the three recorded GPS groundspeeds to find a true airspeed that has been corrected for winds. This averaging of the three groundspeeds will tend to minimize the windspeed errors, but it does NOT truly cancel the windspeed error as many pilots believe. This method is not accurate enough to resolve minor differences of a few knots, if the wind is significant, and I wouldn't trust this method to calibrate an airspeed indicator. Try this thought experiment. Suppose the wind were blowing at 10,000 mph from the North. You fly the three headings, one hour each. It doesn't really matter what headings you have flown because you are going to end up somewhere South of the equator and your GPS will say you averaged about 10,000 mph getting there, no matter which way you have pointed your airplane. Obviously this method is not going to cancel out windspeed errors. I think David is right about using the link below to get a reasonable groundspeed calculation. A simple average won't do if the winds are significant and you are looking for accuracy to the nearest knot. BTW, I also agree that static pressure errors are often overlooked, as well as updrafts/downdrafts. Its very hard to calibrate the airspeed indicator accurately. You need a calm day and very careful measurements. DLD COZY 24DL -----Original Message----- From: David Domeier [mailto:david010@swbell.net] Sent: Tuesday, August 21, 2001 9:07 AM To: Ken Brimmer Cc: canard-aviators@yahoogroups.com; Cozy Builders Mailing List Subject: Re: [c-a] True True Airspeed II ...Perhaps there IS an error in the GPS ground speed calculations but I am inclined to believe the error is else where. The next time you run a test, use http://www.reacomp.com/true_airspeed/index.html to compute your TAS in addition to your method. It would be interesting to see if the TAS number is the same. The error may be in computing TAS and not GS. dd Cozy MKIV N10CZ To unsubscribe from this group, send an email to: canard-aviators-unsubscribe@yahoogroups.com Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/