X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Wed, 04 Jun 2008 07:50:38 -0400 Message-ID: X-Original-Return-Path: Received: from [66.64.141.200] (HELO lucky.dts.local) by logan.com (CommuniGate Pro SMTP 5.2.3) with ESMTP id 2952200 for lml@lancaironline.net; Tue, 03 Jun 2008 20:01:14 -0400 Received-SPF: none receiver=logan.com; client-ip=66.64.141.200; envelope-from=cjensen@dts9000.com Content-class: urn:content-classes:message MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----_=_NextPart_001_01C8C5D5.C1D5D442" X-MimeOLE: Produced By Microsoft Exchange V6.5 Subject: RE: [LML] Re: Selecting an EFIS X-Original-Date: Tue, 3 Jun 2008 19:58:45 -0400 X-Original-Message-ID: <8984A39879F2F5418251CBEEC9C689B3AFCE1A@lucky.dts.local> In-Reply-To: X-MS-Has-Attach: X-MS-TNEF-Correlator: Thread-Topic: [LML] Re: Selecting an EFIS Thread-Index: AcjF0cY90QHjrNCVQ/KUitrt701tTAABEFrA From: "Chuck Jensen" X-Original-To: "Lancair Mailing List" This is a multi-part message in MIME format. ------_=_NextPart_001_01C8C5D5.C1D5D442 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable If I ever decide to fly in circles for a long time, I will be sure to = buy a Chelton. =20 Chuck Jensen=20 -----Original Message----- From: Lancair Mailing List [mailto:lml@lancaironline.net]On Behalf Of = Brent Regan Sent: Tuesday, June 03, 2008 7:32 PM To: lml@lancaironline.net Subject: [LML] Re: Selecting an EFIS Rick writes: "... some EFIS systems (especially the inexpensive ones) derive their = attitude depiction data from GPS and pitot data. In other words, if you = lose GPS or your pitot (blockage, icing, etc.), your ATTITUDE depiction = will become INACCURATE. The most startling thing that I found out = during my research is that the FAA CERTIFIED, Chelton system does have = the problem of inaccurate attitude data with the lost of pitot input. = Their manuals state that the attitude indication can be off as much as = 2.5 degrees when pitot data is not available....>>> 2.5 degrees ??!!??!! Horror of horrors! Rick's post proves two things. One is that a little knowledge is a = dangerous thing. The other is that if you start with a conclusion you = can find facts that "support" said conclusion.=20 Now for some real facts and reason. Most mechanical AH gyroscopic = instruments rely on the gravity to determine which way is up. The = influence of this "gravity vector erection" is small compared to the = forces that maintain the gyroscope "rigid in space". Imagine a pendulum = hanging in a jar of honey. If you roll the jar, the pendulum will point = in a new direction for a while but will, over time, return to pointing = straight down. Likewise, if you roll into a coordinated turn with a = mechanical gyro AH and hold the turn for several minutes the AH will = eventually show level flight when you are still turning. Your inner ear = also has this property, but for different reasons. When your are in a turn the centripetal force vector and the gravity = force vector combine to create an apparent gravity vector, which is not = vertical (with respect to the earth). Mechanical gyro AHs, and apparently GRT, rely on the assumption that, on = average, aircraft fly straight and level. Now suppose you were a cleaver guy and you knew the velocity of the = aircraft and the rate of turn (degrees per second) you could calculate = the centripetal force vector and then subtract that from the apparent = gravity vector to get the actual gravity vector. You would know which = way is up regardless of how long you maintain the turn. Chelton uses a = proprietary algorithm to do this very thing and delivers the resulting = superior performance. The other benefit of this is that the Chelton = ADAHRS is the only low cost solid state AHRS that does not have a "kill = maneuver" that will cause significant attitude errors (e.g. low rate = climbing turn). Will the Chelton attitude accuracy degrade with loss of air data? Yes, = but in it's degraded state it still has better performance that its = competitors over its entire operating range. Rick's assertion that his = GRT AHRS is superior to the CFS ADAHRS because it does not have an air = data "problem" is like saying a car without wheels is superior to a car = with wheels because it does not have a potential flat tire "problem".=20 The bottom line is that the Chelton system has undergone extensive = certification testing and has proven its performance. All the other = wannabes can claim to be "just as good or better" but until they prove = it empirically you should not take the claims as fact. Having spent the last 14 years designing, building and flying cockpit = flight information systems and having the experience of having thousands = of these systems deployed I can say that it is never a good idea to fly = actual IMC without mechanical backups and that most experimental EFIS = systems only work great when you don't need them, although many fail = that test as well. Regards Brent Regan ------_=_NextPart_001_01C8C5D5.C1D5D442 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
If I=20 ever decide to fly in circles for a long time, I will be sure to buy a=20 Chelton.
 
Chuck Jensen
-----Original Message-----
From: Lancair Mailing = List=20 [mailto:lml@lancaironline.net]On Behalf Of Brent = Regan
Sent:=20 Tuesday, June 03, 2008 7:32 PM
To:=20 lml@lancaironline.net
Subject: [LML] Re: Selecting an=20 EFIS

Rick = writes:

"... some=20 EFIS systems (especially the inexpensive ones) derive their attitude = depiction=20 data from GPS and pitot data. In other words, if you lose GPS or your = pitot=20 (blockage, icing, etc.), your ATTITUDE depiction will become = INACCURATE.=20 <snip> The most startling thing that I found out during my = research is=20 that the FAA CERTIFIED, Chelton system does have the problem of = inaccurate=20 attitude data with the lost of pitot input. Their manuals state that = the=20 attitude indication can be off as much as 2.5 degrees when pitot data = is not=20 available....>>>


2.5 degrees = ??!!??!!=20 Horror of horrors!

Rick's post proves two things. One is that a = little=20 knowledge is a dangerous thing. The other is that  if you start = with a=20 conclusion you can find facts that "support" said conclusion. =

Now for=20 some real facts and reason. Most mechanical AH gyroscopic instruments = rely on=20 the gravity to determine which way is up. The influence of this = "gravity=20 vector erection" is small compared to the forces that maintain the = gyroscope=20 "rigid in space".  Imagine a pendulum hanging in a jar of = honey.  If=20 you roll the jar, the pendulum will point in a  new direction for = a while=20 but will, over time, return to pointing straight down. Likewise, if = you roll=20 into a coordinated turn with a mechanical gyro AH and hold the turn = for=20 several minutes the AH will eventually show level flight when you are = still=20 turning. Your inner ear also has this property, but for different=20 reasons.

When your are in a turn the centripetal force vector = and the=20 gravity force vector combine to create an apparent gravity vector, = which is=20 not vertical (with respect to the earth).

Mechanical gyro AHs, = and=20 apparently GRT, rely on the assumption that, on average, aircraft fly = straight=20 and level.

Now suppose you were a cleaver guy and  you = knew the=20 velocity of the aircraft and the rate of turn (degrees per second) you = could=20 calculate the centripetal force vector and then subtract that from the = apparent gravity vector to get the actual gravity vector. You would = know which=20 way is up regardless of how long you maintain the turn.  Chelton = uses a=20 proprietary algorithm to do this very thing and delivers the resulting = superior performance.  The other benefit of this is that the = Chelton=20 ADAHRS is the only low cost solid state AHRS that does not have a = "kill=20 maneuver" that will cause significant attitude errors (e.g. low rate = climbing=20 turn).

Will the Chelton attitude accuracy degrade with loss of = air=20 data? Yes, but in it's degraded state it still has better performance = that its=20 competitors over its entire operating range.  Rick's assertion = that his=20 GRT AHRS is superior to the CFS ADAHRS because it does not have an air = data=20 "problem" is like saying a car without wheels is superior to a car = with wheels=20 because it does not have a potential  = flat=20 tire "problem".

The bottom line is = that the=20 Chelton  system has undergone extensive certification testing and = has=20 proven its performance. All the other wannabes can claim to be "just = as good=20 or better" but until they prove it  empirically  you should = not take=20 the claims as fact.

Having spent the last 14 years designing, = building=20 and flying cockpit flight information systems and having the = experience of=20 having thousands of these systems deployed I can say that it is never = a good=20 idea to fly actual IMC without mechanical backups and that most = experimental=20 EFIS systems only work great when you don't need them, although many = fail that=20 test as well.

Regards
Brent=20 Regan


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