X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Thu, 22 Jul 2010 15:33:58 -0400 Message-ID: X-Original-Return-Path: Received: from imr-ma03.mx.aol.com ([64.12.206.41] verified) by logan.com (CommuniGate Pro SMTP 5.3.8) with ESMTP id 4403234 for lml@lancaironline.net; Thu, 22 Jul 2010 15:15:46 -0400 Received-SPF: pass receiver=logan.com; client-ip=64.12.206.41; envelope-from=Sky2high@aol.com Received: from imo-da01.mx.aol.com (imo-da01.mx.aol.com [205.188.169.199]) by imr-ma03.mx.aol.com (8.14.1/8.14.1) with ESMTP id o6MJEtiG018564 for ; Thu, 22 Jul 2010 15:14:55 -0400 Received: from Sky2high@aol.com by imo-da01.mx.aol.com (mail_out_v42.9.) id q.c4b.72629119 (45481) for ; Thu, 22 Jul 2010 15:14:54 -0400 (EDT) Received: from magic-m20.mail.aol.com (magic-m20.mail.aol.com [172.20.22.193]) by cia-mc06.mx.aol.com (v129.4) with ESMTP id MAILCIAMC068-b1a94c4898ad92; Thu, 22 Jul 2010 15:14:53 -0400 From: Sky2high@aol.com X-Original-Message-ID: X-Original-Date: Thu, 22 Jul 2010 15:14:53 EDT Subject: Re: [LML] CG and Pitch Sensitivity X-Original-To: lml@lancaironline.net MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="part1_b5942.3946e4e5.3979f2ad_boundary" X-Mailer: AOL 9.5 sub 155 X-AOL-ORIG-IP: 67.175.87.113 X-AOL-IP: 172.20.22.193 X-Spam-Flag:NO X-AOL-SENDER: Sky2high@aol.com --part1_b5942.3946e4e5.3979f2ad_boundary Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit Wolfgang, Yes, using the calculator in my 7/17 email and actual main wing measurements, 15% MAC CG = 23.22 and 30% MAC CG = 29.09 or a little more than an inch forward of the Lancair published range. Scott Krueger In a message dated 7/22/2010 1:02:26 P.M. Central Daylight Time, Wolfgang@MiCom.net writes: After going over drawings of the Lancair 320 (Airframe Plan View - 320), I have determined that the 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. I find that the 15% -30% of MAC range is actually 1" further forward. I would like for someone to check me on this. This could explain a lot of reported rear CG stability problems. Wolfgang ____________________________________ From: "Wolfgang" Sender: Subject: CG and Pitch Sensitivity Date: Tue, 20 Jul 2010 18:31:09 -0400 To: lml@lancaironline.net 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.h tm_ (http://www.centennialofflight.gov/essay/Theories_of_Flight/Stability/TH26.htm) Wolfgang --part1_b5942.3946e4e5.3979f2ad_boundary Content-Type: text/html; charset="US-ASCII" Content-Transfer-Encoding: quoted-printable
Wolfgang,
 
Yes, using the calculator in my 7/17 email and actual main wing=20 measurements, 15% MAC CG =3D 23.22 and 30% MAC CG =3D 29.09 or a little mo= re than an=20 inch forward of the Lancair published range.
 
Scott Krueger
 
In a message dated 7/22/2010 1:02:26 P.M. Central Daylight Time,=20 Wolfgang@MiCom.net writes:
After going over drawings of the Lancai= r 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. <= /DIV>
 
I find that the 15% -30% of MAC ra= nge is=20 actually 1" further forward.
 
I would like for someone to check me on= this.=20
 
This could explain a lot of reported re= ar CG=20 stability problems.
 
Wolfgang
<= /TR>
From: "Wolfgang"=20 <Wolfgang@MiCom.net>
Sender: <marv@lancaironline.net>=
Subject: CG and Pitch Sensitivity
Date: Tue, 20 Jul 2010 18:31:09=20 -0400
To: lml@lancaironli= ne.net
There have been a lot of terms thrown arou= nd here=20 like CG, Neutral Point, Stability, Aerodynamic Center and MAC but= how=20 they interact can be better understood. I will attempt to clarify= and=20 simplify for those that have not been around this block.
 
In particular, stability, Neutral Point an= d why=20 they work the way they do. In fact, NP is defined as that CG condi= tion=20 where the airframe will not correct itself in pitch. This is good= for=20 aerobatic and combat aircraft but not for day to day=20 flying.
 
It is common knowledge that if the CG is&n= bsp;at=20 or behind the NP, the airframe has zero of negative stability (if= the=20 nose goes up, it will keep going up as the airspeed decreases unle= ss=20 elevator input brings it back down and vice-versa. What is not com= mon=20 knowledge is why.
 
The basis of all this is in the fact that= =20 as a typical wing increases it's Angle of Attack, it's center= of=20 pressure (center of lift) moves aft. This produces a moment that= becomes=20 more negative pushing the nose back down and vice-versa. At some= speed=20 the nose will be happy at some attitude and in steady state=20 flight.
 
There are some airfoils that do not exhibi= t this=20 behavior and even show the opposite behavior and are not suitable= for=20 use as main wings. Add an elevator to a wing and positive sta= bility=20 behavior can be enhanced increasing the usable selection of= =20 available airfoils if the CG is kept forward of the center of lift= .=20 This, of course requires the elevator to produce down force to han= dle=20 the CG in front of the wing's center of lift. Now if the nose is= =20 disturbed upward, the center of lift moves to the rear helpin= g the=20 nose come back down and the elevator experiences a less negative= AoA=20 producing less down force adding to the restoring force bringing= the=20 nose back down.
 
This self stabilizing type of flight is wh= at=20 allows one to trim the airframe  for "hands off" flight. The= =20 greater the stability, the more "hands off" you will be.
 
Adding reflex to a wing, any wing, will re= duce the=20 center of lift travel with pitch changes and reduce stability.=20 Also because reflex moved the center of lift forward, cl= oser=20 to the CG, you require less down force from the elevator= =20 leaving 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 an= d move=20 the CG forward.
 
The Mean Aerodynamic=20 Center of a wing is a point on the wing chor= d=20 which results in a constant moment when the wing angle of attack= is=20 changed. In other words that is the= point=20 where there is no pitch restoring force from the main wing and kee= ping=20 the nose level becomes hard work. Now, since most airfoils have a= slight=20 negative moment about the Mean Aerodynamic Center, some elevator= down=20 force is still required to keep the nose level. A nose up disturba= nce=20 will not change the moment of the wing (no restoring force) but th= ere=20 will be a reduction of down force from the elevator and some=20 stability will be evident. Move the CG even further back= and=20 you get to the Neutral point, a point where the entire airframe ha= s NO=20 pitch restoring force at all (making your plane a hand full to=20 fly).
 
A more detailed treatment of stability can= be=20 found here;
 
--part1_b5942.3946e4e5.3979f2ad_boundary--