X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Thu, 22 Jul 2010 14:02:11 -0400 Message-ID: X-Original-Return-Path: Received: from hrndva-omtalb.mail.rr.com ([71.74.56.122] verified) by logan.com (CommuniGate Pro SMTP 5.3.8) with ESMTP id 4403139 for lml@lancaironline.net; Thu, 22 Jul 2010 14:00:03 -0400 Received-SPF: none receiver=logan.com; client-ip=71.74.56.122; envelope-from=Wolfgang@MiCom.net X-Original-Return-Path: X-Authority-Analysis: v=1.1 cv=0ETVU5ClxBlG/P3Ky9ZuhqraSVIot/cIDNO6awFX9oc= c=1 sm=0 a=MHZY6FYWMEQOp7S43i2QIw==:17 a=rTjvlri0AAAA:8 a=Ia-xEzejAAAA:8 a=gg14p7zjAAAA:8 a=ww61Z0ubMyD4vdUO7NAA:9 a=2uDv957gpQHzehINBNcA:7 a=lfNPuJTgmf1DlHWnVKytQy33ne8A:4 a=wPNLvfGTeEIA:10 a=nQLeR2QOj2oA:10 a=Dr9Wx-Q63l4A:10 a=EzXvWhQp4_cA:10 a=lkLK8Z4honIpVo3XnTgA:9 a=w01AkdVBWHtXc2ZTijkA:7 a=zF1hWwtmndhS4J2bqGG35QOx53QA:4 a=UtLQJHOpr_gA:10 a=MHZY6FYWMEQOp7S43i2QIw==:117 X-Cloudmark-Score: 0 X-Originating-IP: 74.218.201.50 Received: from [74.218.201.50] ([74.218.201.50:1505] helo=Lobo) by hrndva-oedge04.mail.rr.com (envelope-from ) (ecelerity 2.2.2.39 r()) with ESMTP id 07/B7-23284-DF6884C4; Thu, 22 Jul 2010 17:59:29 +0000 X-Original-Message-ID: <001f01cb29c7$9b5b3e30$6401a8c0@Lobo> From: "Wolfgang" X-Original-To: Subject: CG and Pitch Sensitivity X-Original-Date: Thu, 22 Jul 2010 13:59:18 -0400 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_001C_01CB29A6.13FD52F0" X-Priority: 3 X-MSMail-Priority: Normal X-Mailer: Microsoft Outlook Express 6.00.2900.2180 X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2180 This is a multi-part message in MIME format. ------=_NextPart_000_001C_01CB29A6.13FD52F0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable After going over drawings of the Lancair 320 (Airframe Plan View - 320), = I have determined that the=20 CG range given in the POH (24.5" to 30.3") is NOT at the 15% to 30% of = MAC as stated in the manual.=20 I find that the 15% -30% of MAC range is actually 1" further forward.=20 I would like for someone to check me on this.=20 This could explain a lot of reported rear CG stability problems. Wolfgang -------------------------------------------------------------------------= ------- From: "Wolfgang" =20 Sender: =20 Subject: CG and Pitch Sensitivity=20 Date: Tue, 20 Jul 2010 18:31:09 -0400=20 To: lml@lancaironline.net=20 =20 =20 There have been a lot of terms thrown around here like CG, Neutral = Point, Stability, Aerodynamic Center and MAC but how they interact can = be better understood. I will attempt to clarify and simplify for those = that have not been around this block. In particular, stability, Neutral Point and why they work the way = they do. In fact, NP is defined as that CG condition where the airframe = will not correct itself in pitch. This is good for aerobatic and combat = aircraft but not for day to day flying. It is common knowledge that if the CG is at or behind the NP, the = airframe has zero of negative stability (if the nose goes up, it will = keep going up as the airspeed decreases unless elevator input brings it = back down and vice-versa. What is not common knowledge is why. The basis of all this is in the fact that as a typical wing = increases it's Angle of Attack, it's center of pressure (center of lift) = moves aft. This produces a moment that becomes more negative pushing the = nose back down and vice-versa. At some speed the nose will be happy at = some attitude and in steady state flight. There are some airfoils that do not exhibit this behavior and even = show the opposite behavior and are not suitable for use as main wings. = Add an elevator to a wing and positive stability behavior can be = enhanced increasing the usable selection of available airfoils if the CG = is kept forward of the center of lift. This, of course requires the = elevator to produce down force to handle the CG in front of the wing's = center of lift. Now if the nose is disturbed upward, the center of lift = moves to the rear helping the nose come back down and the elevator = experiences a less negative AoA producing less down force adding to the = restoring force bringing the nose back down. This self stabilizing type of flight is what allows one to trim = the airframe for "hands off" flight. The greater the stability, the = more "hands off" you will be. Adding reflex to a wing, any wing, will reduce the center of lift = travel with pitch changes and reduce stability. Also because reflex = moved the center of lift forward, closer to the CG, you require less = down force from the elevator leaving less margin for elevator provided = stability. Sooo . . . If you already have an aft CG and you moved your center = of lift forward closer to the CG by using reflex, you can expect less = pitch stability . . . . Want some stability back ? . . . . loose some of = that reflex until you burn off some fuel and move the CG forward. The Mean Aerodynamic Center of a wing is a point on the wing chord = which results in a constant moment when the wing angle of attack is = changed. In other words that is the point where there is no pitch = restoring force from the main wing and keeping the nose level becomes = hard work. Now, since most airfoils have a slight negative moment about = the Mean Aerodynamic Center, some elevator down force is still required = to keep the nose level. A nose up disturbance will not change the moment = of the wing (no restoring force) but there will be a reduction of down = force from the elevator and some stability will be evident. Move the CG = even further back and you get to the Neutral point, a point where the = entire airframe has NO pitch restoring force at all (making your plane a = hand full to fly). A more detailed treatment of stability can be found here; = http://www.centennialofflight.gov/essay/Theories_of_Flight/Stability/TH26= .htm Wolfgang =20 ------=_NextPart_000_001C_01CB29A6.13FD52F0 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
After going over drawings of the = Lancair 320=20 (Airframe Plan View - 320), I have determined that the
CG range given in the POH (24.5" to = 30.3") is=20 NOT at the 15% to 30% of MAC as stated in the manual. =
 
I find that the 15% -30% of MAC = range is=20 actually 1" further forward.
 
I would like for someone to check me on = this.=20
 
This could explain a lot of reported = rear CG=20 stability problems.
 
Wolfgang
lml@lancaironline.net
From: "Wolfgang"=20 <Wolfgang@MiCom.net>
Sender: <marv@lancaironline.net>
Subject: CG and Pitch = Sensitivity
Date: Tue, 20 Jul 2010 18:31:09 = -0400
To:
There have been a lot of terms thrown = around here=20 like CG, Neutral Point, Stability, Aerodynamic Center and MAC but = how they=20 interact can be better understood. I will attempt to clarify and = simplify=20 for those that have not been around this block.
 
In particular, stability, Neutral Point = and why they=20 work the way they do. In fact, NP is defined as that CG condition = where=20 the airframe will not correct itself in pitch. This is good for = aerobatic=20 and combat aircraft but not for day to day flying.
 
It is common knowledge that if the CG = is at or=20 behind the NP, the airframe has zero of negative stability (if the = nose=20 goes up, it will keep going up as the airspeed decreases unless = elevator=20 input brings it back down and vice-versa. What is not common = knowledge is=20 why.
 
The basis of all this is in the fact that = as a=20 typical wing increases it's Angle of Attack, it's center of = pressure=20 (center of lift) moves aft. This produces a moment that becomes = more=20 negative pushing the nose back down and vice-versa. At some speed = the nose=20 will be happy at some attitude and in steady state = flight.
 
There are some airfoils that do not = exhibit this=20 behavior and even show the opposite behavior and are not suitable = for use=20 as main wings. Add an elevator to a wing and positive = stability=20 behavior can be enhanced increasing the usable selection of = available=20 airfoils if the CG is kept forward of the center of lift. This, of = course=20 requires the elevator to produce down force to handle the CG in = front of=20 the wing's center of lift. Now if the nose is disturbed upward, = the center=20 of lift moves to the rear helping the nose come back down and = the=20 elevator experiences a less negative AoA producing less down force = adding=20 to the restoring force bringing the nose back down.
 
This self stabilizing type of flight is = what allows=20 one to trim the airframe  for "hands off" flight. The greater = the=20 stability, the more "hands off" you will be.
 
Adding reflex to a wing, any wing, will = reduce the=20 center of lift travel with pitch changes and reduce stability.=20 Also because reflex moved the center of lift forward, = closer to=20 the CG, you require less down force from the elevator = leaving=20 less margin for elevator provided stability.
 
Sooo . . . If you already have an aft CG = and you=20 moved your center of lift forward closer to the CG by using = reflex, you=20 can expect less pitch stability . . . . Want some stability back ? = . .=20 . . loose some of that reflex until you burn off some fuel = and move=20 the CG forward.
 
The Mean Aerodynamic = Center of=20 a wing is a point on the wing chord which results in a constant = moment=20 when the wing angle of attack is changed. In=20 other words that is the point where there is no pitch restoring = force from=20 the main wing and keeping the nose level becomes hard work. Now, = since=20 most airfoils have a slight negative moment about the Mean = Aerodynamic=20 Center, some elevator down force is still required to keep the = nose level.=20 A nose up disturbance will not change the moment of the wing (no = restoring=20 force) but there will be a reduction of down force from the = elevator and=20 some stability will be evident. Move the CG even further = back=20 and you get to the Neutral point, a point where the entire = airframe has NO=20 pitch restoring force at all (making your plane a hand full to=20 fly).
 
A more detailed treatment of stability can = be found=20 here;
http://www.centennialofflight.gov/essay/Theories_of_Flight= /Stability/TH26.htm
 
 
Wolfgang
=
 
 
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