X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Tue, 20 Jul 2010 18:31:09 -0400 Message-ID: X-Original-Return-Path: Received: from hrndva-omtalb.mail.rr.com ([71.74.56.125] verified) by logan.com (CommuniGate Pro SMTP 5.3.8) with ESMTP id 4400498 for lml@lancaironline.net; Tue, 20 Jul 2010 17:55:29 -0400 Received-SPF: none receiver=logan.com; client-ip=71.74.56.125; envelope-from=Wolfgang@MiCom.net X-Original-Return-Path: X-Authority-Analysis: v=1.1 cv=yYDk9wvF9yILP8+WUBprz5rv2pbX1YDh8tMrnOgRP/g= c=1 sm=0 a=MHZY6FYWMEQOp7S43i2QIw==:17 a=gg14p7zjAAAA:8 a=RLlaPBF_Eu4N5VpEauIA:9 a=7rtDf68l_ffyWxb0fiYA:7 a=myu5Ivw8vZmz0PQYqBd2rjpbLEQA:4 a=wPNLvfGTeEIA:10 a=nQLeR2QOj2oA:10 a=46FmlDfxEG21Bin5hGIA:9 a=fCeioGQ0zYcqxGNzH4UA:7 a=r7Wk45CdBpLu2rrtXuY4oLeabfwA: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] ([74.218.201.50:3499] helo=Lobo) by hrndva-oedge02.mail.rr.com (envelope-from ) (ecelerity 2.2.2.39 r()) with ESMTP id 27/21-14763-F2B164C4; Tue, 20 Jul 2010 21:54:55 +0000 X-Original-Message-ID: <004601cb2856$2e0b92c0$6401a8c0@Lobo> From: "Wolfgang" X-Original-To: Subject: CG and Pitch Sensitivity X-Original-Date: Tue, 20 Jul 2010 17:54:51 -0400 MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_0043_01CB2834.A6B98E60" 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_0043_01CB2834.A6B98E60 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable 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 ------=_NextPart_000_0043_01CB2834.A6B98E60 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
There have been a lot of terms thrown around = here like CG,=20 Neutral Point, Stability, Aerodynamic Center and MAC but how they = interact can=20 be better understood. I will attempt to clarify and simplify for those = that have=20 not been around this block.
 
In particular, stability, Neutral Point and why = they work=20 the way they do. In fact, NP is defined as that CG condition where the = airframe=20 will not correct itself in pitch. This is good for aerobatic and combat = aircraft=20 but not for day to day flying.
 
It is common knowledge that if the CG is at = or behind=20 the NP, the airframe has zero of negative stability (if the nose goes = up, it=20 will keep going up as the airspeed decreases unless elevator input = brings it=20 back down and vice-versa. What is not common knowledge is = 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 = (center of=20 lift) moves aft. This produces a moment that becomes more negative = pushing the=20 nose back down and vice-versa. At some speed the nose will be happy at = some=20 attitude and in steady state flight.
 
There are some airfoils that do not exhibit this = behavior=20 and even show the opposite behavior and are not suitable for use as main = wings.=20 Add an elevator to a wing and positive stability behavior can be = enhanced=20 increasing the usable selection of available airfoils if the CG is = kept=20 forward of the center of lift. This, of course requires the elevator to = produce=20 down force to handle the CG in front of the wing's center of lift. Now = if the=20 nose is disturbed upward, the center of lift moves to the rear = helping the=20 nose come back down and the elevator experiences a less negative AoA = producing=20 less down force adding to the restoring force bringing the nose back=20 down.
 
This self stabilizing type of flight is what = allows one to=20 trim the airframe  for "hands off" flight. The greater the = stability, the=20 more "hands off" you will be.
 
Adding reflex to a wing, any wing, will reduce = the center=20 of lift travel with pitch changes and reduce stability.=20 Also because reflex moved the center of lift forward, closer = to the=20 CG, you require less down force from the elevator leaving less = margin=20 for elevator provided stability.
 
Sooo . . . If you already have an aft CG and you = moved=20 your center of lift forward closer to the CG by using reflex, you can = expect=20 less pitch stability . . . . Want some stability back ? . . . . = loose some=20 of that reflex until you burn off some fuel and move the CG=20 forward.
 
The Mean Aerodynamic = Center of a=20 wing is a point on the wing chord which results in a constant moment = when the=20 wing angle of attack is changed. In other = words=20 that is the point where there is no pitch restoring force from the main = wing and=20 keeping the nose level becomes hard work. Now, since most airfoils have = a slight=20 negative moment about the Mean Aerodynamic Center, some elevator down = force is=20 still required to keep the nose level. A nose up disturbance will not = change the=20 moment of the wing (no restoring force) but there will be a reduction of = down=20 force from the elevator and some stability will be evident. = Move the=20 CG even further back and you get to the Neutral point, a point where the = entire=20 airframe has NO 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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