Return-Path: Sender: (Marvin Kaye) To: lml Date: Mon, 16 Dec 2002 07:21:52 -0500 Message-ID: X-Original-Return-Path: Received: from imo-m05.mx.aol.com ([64.12.136.8] verified) by logan.com (CommuniGate Pro SMTP 4.0.2) with ESMTP id 1912314 for lml@lancaironline.net; Sun, 15 Dec 2002 21:59:32 -0500 Received: from Sky2high@aol.com by imo-m05.mx.aol.com (mail_out_v34.13.) id q.17d.139bfe9c (30960) for ; Sun, 15 Dec 2002 21:59:28 -0500 (EST) From: Sky2high@aol.com X-Original-Message-ID: <17d.139bfe9c.2b2e9b90@aol.com> X-Original-Date: Sun, 15 Dec 2002 21:59:28 EST Subject: Re: [LML] Airspeed Calibration - Methods X-Original-To: lml@lancaironline.net MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="part1_17d.139bfe9c.2b2e9b90_boundary" X-Mailer: AOL 8.0 for Windows US sub 4103 --part1_17d.139bfe9c.2b2e9b90_boundary Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit Naf, Mike, et al, Each of the other methods utilize some interesting assumptions and complex math. I am using the equipment I have and simple arithmetic. Here are my assumptions: TAS - True speed through undisturbed air. This is calculated by adjusting indicated air speed (IAS) for temperature (related to speed), pressure altitude and compressibility (related to speed). This would be ground speed (GS) if there were no wind or movement of the airmass within which the airplane is flying. Rocky Mountain Instrument's (RMI) TAS is calculated using all three components noted above and its accuracy is dependent on, among other things, its OAT probe. My OAT probe is located just ahead of the left wheel well door and its support and wiring is in the wheel well. This area is unaffected by sun or heat generated by the airplane or cockpit. IAS - Measured by the difference between the ram air collected by a pitot tube and static (ambient) air pressure. I utilize two static ports located as recommended by the factory. The static lines were checked and exhibit no leakage. The pitot is a standard heated "L" shaped tube located on the left aileron bellcrank door sufficiently far from the wing and propeller blast to eliminate any peculiar disturbance. The pitot line was also pressure checked. IAS is cross checked with RMI and standard airspeed indicator and are within 1 Kt. GS - Readout from Garmin 430, 12 satellites, Selective Availability turned off meaning 49 feet RMS position error and .1 Kt RMS speed error in a steady state. Stabilized flight - Otto will do the flying in altitude hold, heading control (autopilot DG bug) after stabilized hand flight is trimmed. This sometimes takes ten minutes to set up just right. Otto is so much better than me. This is done in smooth air. There is no precession of the DG and no altitude hunting. The power is set and leaned, heading selected, flight stabilized, auto-pilot engaged and, when observation indicates stabilization, readings of IAS, TAS and GS are taken. The heading bug is turned 120 degrees and, when stabilized on the heading, readings are taken. This is repeated for the third leg. Note that this technique gets a reliable three legged data set and keeps you over the same terrain to boot (unless the winds are hellacious). CAS or calibrated airspeed can be reasonably determined by averaging the data and applying the difference of GS and TAS to IAS. Here is my recent test: Overcast, wind at the ground was 180@11 (Mag Var = 0), 2500 ft, 7 deg C, 30.05 hg. Test 1 - 2500 rpm, 25" DEG IAS TAS GS 270 176 180 168 150 176 180 183 030 176 180 194 ------------------------------ 538 540 545 Avg 176 180 181 CAS=177 Test 2 - 2290 rpm, 18.1", 10 degrees of flap DEG IAS TAS GS 270 114 116 102 150 115 117 115 030 113 115 125 ------------------------------ 342 348 342 Avg 114 116 114 CAS=112 All of this is close enough. Even the other techniques are off by a Kt or two when crosschecked. Scott Krueger N92EX --part1_17d.139bfe9c.2b2e9b90_boundary Content-Type: text/html; charset="US-ASCII" Content-Transfer-Encoding: 7bit Naf, Mike, et al,

Each of the other methods utilize some interesting assumptions and complex math.  I am using the equipment I have and simple arithmetic.

Here are my assumptions:

TAS - True speed through undisturbed air.  This is calculated by adjusting indicated air speed (IAS) for temperature (related to speed), pressure altitude and compressibility (related to speed).   This would be ground speed (GS) if there were no wind or movement of the airmass within which the airplane is flying.  Rocky Mountain Instrument's (RMI) TAS is calculated using all three components noted above and its accuracy is dependent on, among other things, its OAT probe.  My OAT probe is located just ahead of the left wheel well door and its support and wiring is in the wheel well.  This area is unaffected by sun or heat generated by the airplane or cockpit. 

IAS - Measured by the difference between the ram air collected by a pitot tube and static (ambient) air pressure.  I utilize two static ports located as recommended by the factory.  The static lines were checked and exhibit no leakage.  The pitot is a standard heated "L" shaped tube located on the left aileron bellcrank door sufficiently far from the wing and propeller blast to eliminate any peculiar disturbance.  The pitot line was also pressure checked.  IAS is cross checked with RMI and standard airspeed indicator and are within 1 Kt.

GS -  Readout from Garmin 430, 12 satellites, Selective Availability turned off meaning 49 feet RMS position error and .1 Kt RMS speed error in a steady state.

Stabilized flight - Otto will do the flying in altitude hold, heading control (autopilot DG bug) after stabilized hand flight is trimmed.  This sometimes takes ten minutes to set up just right.  Otto is so much better than me.  This is done in smooth air.  There is no precession of the DG and no altitude hunting.

The power is set and leaned, heading selected, flight stabilized, auto-pilot engaged and, when observation indicates stabilization, readings of IAS, TAS and GS are taken.  The heading bug is turned 120 degrees and, when stabilized on the heading, readings are taken.  This is repeated for the third leg.  Note that this technique gets a reliable three legged data set and keeps you over the same terrain to boot (unless the winds are hellacious).

CAS or calibrated airspeed can be reasonably determined by averaging the data and applying the difference of GS and TAS to IAS.

Here is my recent test:

Overcast, wind at the ground was 180@11 (Mag Var = 0), 2500 ft, 7 deg C, 30.05 hg.

Test 1 - 2500 rpm, 25"

DEG IAS TAS  GS
270  176  180   168
150  176  180   183
030  176  180   194
------------------------------
       538  540   545
Avg  176  180   181   CAS=177

Test 2 - 2290 rpm, 18.1", 10 degrees of flap

DEG IAS TAS  GS
270  114  116   102
150  115  117   115
030  113  115   125
------------------------------
       342  348   342
Avg  114  116   114   CAS=112

All of this is close enough.  Even the other techniques are off by a Kt or two when crosschecked.

Scott Krueger
N92EX

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