X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Sat, 04 Aug 2007 10:56:39 -0400 Message-ID: X-Original-Return-Path: Received: from imo-d05.mx.aol.com ([205.188.157.37] verified) by logan.com (CommuniGate Pro SMTP 5.1.11) with ESMTP id 2241896 for lml@lancaironline.net; Sat, 04 Aug 2007 07:34:09 -0400 Received-SPF: pass receiver=logan.com; client-ip=205.188.157.37; envelope-from=Sky2high@aol.com Received: from Sky2high@aol.com by imo-d05.mx.aol.com (mail_out_v38_r9.2.) id q.c09.1eb75e45 (39952) for ; Sat, 4 Aug 2007 07:33:27 -0400 (EDT) From: Sky2high@aol.com X-Original-Message-ID: X-Original-Date: Sat, 4 Aug 2007 07:33:27 EDT Subject: AOA Q & A X-Original-To: lml@lancaironline.net MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="-----------------------------1186227206" X-Mailer: 9.0 Security Edition for Windows sub 5365 X-Spam-Flag: NO -------------------------------1186227206 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit Marv, Thanks for putting out those great AOA information points. Here they are collected together for reference: How does the PSS/AFS AOA determine the actual AOA? The algorithm that Jim used to calculate AOA accurately is what made his product patentable. Up until he started his research the data that was being used from similar probes (what they use on the Dynon, and a couple other AOA offerings) was simply the input from an alpha probe. The alpha probe is a lot like a pitot tube but instead of being pointed directly into the relative wind to collect ram air in a single hole, it has angled upper and lower surfaces (imagine a right angle shaped wedge tilted 45 degrees so the upper surface looks up, the lower surface down) with a port in each surface and depending on the relative angles between those ports and how the relative wind struck them you'd get a differential pressure. Unfortunately, that sort of probe only provides 2 of the data points required to accurately calculate AOA... Jim brought on the next step by incorporating dynamic pressure (the difference between pitot and static) and discovered that if he divided the wing differential pressure by the overall airframe dynamic pressure the results correlated exactly to the AOA. During the calibration process the data points that are being stored are actually points on the L/D curve.... the zero-g maneuver identifies the zero degree angle of attack, and by flying and then recording the port indications at 15% above the stall the instrument learns that (15%) data point on the curve. Then, using those 2 data points the rest of the curve is calculated and the results displayed on the panel indicator. That's about it. Does the PSS/AFS AOA account for different wing loading and flap deployments? I don't believe the Chelton AOA indication has any way of determining actual wing loading, so it's going to be pretty hard pressed to give you accurate AOA information at anything but the single wing loading sample that was used to develop its calibration data. If that AOA info came from a lightly-loaded set of data points it would be suicidal to attempt to use it when flying at gross. If, OTOH, the data came from a heavily loaded wing loading situation the AOA warnings are going to start long before you are anywhere near the aircraft's critical AOA. To further complicate the issue, as soon as you lower the flaps you have effectively installed a different airfoil on the airplane and it has its own L/D curve with its own set of AOA data... that's why the PSS/AFS AOA instrument looks at the flap settings and actually requires a separate calibration to account for that different airfoil. Are there any other benefits for a true AOA? Another advantage of the AOA instrument... if you're flying at cruise and the noisemaker goes silent the first step is to fly the airplane. This means setting up for engine out glide, which is L/D max. The AOA instrument actually shows you where that's at, so from cruise you'll pull back on the stick (stick controls AOA, throttle ! controls airspeed) to raise the nose to the L/D max AOA. In the process you'll trade some of that cruising airspeed for altitude, buying yourself a bit more glide. Once the airspeed has bled off you simply maintain (trim to) the L/D max AOA indication and get headed to the nearest airport. The rules are the same for L/D max as those that we live by during the approach... when lightly loaded L/D max will be at a lower airspeed than what would be required when at full gross. That's the problem with "best glide speed"... there are an infinite number of speeds that will yield L/D max, all dependent on wing loading. L/D max AOA, however, happens at only one AOA... fly that AOA and the best glide speed will follow. ************************************** Get a sneak peek of the all-new AOL at http://discover.aol.com/memed/aolcom30tour -------------------------------1186227206 Content-Type: text/html; charset="US-ASCII" Content-Transfer-Encoding: quoted-printable
 Marv,
 
Thanks for putting out those great AOA information points.  Here t= hey=20 are collected together for reference:
 
How does the PSS/AFS AOA determine the actual AOA?
 
The algorithm that Jim used to calculate AOA accurately is what made hi= s=20 product patentable.  Up until he started his research the data that was= =20 being used from similar probes (what they use on the Dynon, and a couple oth= er=20 AOA offerings) was simply the input from an alpha probe.  The alpha pro= be=20 is a lot like a pitot tube but instead of being pointed directly into the=20 relative wind to collect ram air in a single hole, it has angled upper and l= ower=20 surfaces (imagine a right angle shaped wedge tilted 45 degrees so the upper=20 surface looks up, the lower surface down) with a port in each surface and=20 depending on the relative angles between those ports and how the relative wi= nd=20 struck them you'd get a differential pressure.  Unfortunately, that sor= t of=20 probe only provides 2 of the data points required to accurately calculate AO= A...=20 Jim brought on the next step by incorporating dynamic pressure (the differen= ce=20 between pitot and static) and discovered that if he divided the wing=20 differential pressure by the overall airframe dynamic pressure the results=20 correlated exactly to the AOA.  During the calibration process the data= =20 points that are being stored are actually points on the L/D curve.... the ze= ro-g=20 maneuver identifies the zero degree angle of attack, and by flying and then=20 recording the port indications at 15% above the stall the instrument learns=20= that=20 (15%) data point on the curve.  Then, using those 2 data points the res= t of=20 the curve is calculated and the results displayed on the panel indicator.&nb= sp;=20 That's about it.
 
Does the PSS/AFS AOA account for different wing loading and f= lap=20 deployments?   
 
I don't believe the Chelton AOA indication has any way of determining=20 actual wing loading, so it's going to be pretty hard pressed to give you=20 accurate AOA information at anything but the single wing loading sample that= was=20 used to develop its calibration data.  If that AOA info came from a=20 lightly-loaded set of data points it would be suicidal to attempt to use it=20= when=20 flying at gross.  If, OTOH, the data came from a heavily loaded wing=20 loading situation the AOA warnings are going to start long before you are=20 anywhere near the aircraft's critical AOA.  To further complicate the=20 issue, as soon as you lower the flaps you have effectively installed a diffe= rent=20 airfoil on the airplane and it has its own L/D curve with its own set of AOA= =20 data... that's why the PSS/AFS AOA instrument looks at the flap settings and= =20 actually requires a separate calibration to account for that different airfo= il.=20
 
Are there any other benefits for a true AOA?
 
Another advantage of the AOA instrument... if you're flying at cruise a= nd=20 the noisemaker goes silent the first step is to fly the airplane.  This= =20 means setting up for engine out glide, which is L/D max.  The AOA=20 instrument actually shows you where that's at, so from cruise you'll pull ba= ck=20 on the stick (stick controls AOA, throttle ! controls airspeed) to raise the= =20 nose to the L/D max AOA.  In the process you'll trade some of that crui= sing=20 airspeed for altitude, buying yourself a bit more glide.  Once the airs= peed=20 has bled off you simply maintain (trim to) the L/D max AOA indication and ge= t=20 headed to the nearest airport.  The rules are the same for L/D max as t= hose=20 that we live by during the approach... when lightly loaded L/D max will be a= t a=20 lower airspeed than what would be required when at full gross.  That's=20= the=20 problem with "best glide speed"... there are an infinite number of speeds th= at=20 will yield L/D max, all dependent on wing loading.  L/D max AOA, howeve= r,=20 happens at only one AOA... fly that AOA and the best glide speed will=20 follow. 
 
 



Get a s= neak peek of the all-new AOL.com.=
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