X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Thu, 15 Jul 2010 19:08:18 -0400 Message-ID: X-Original-Return-Path: Received: from fmailhost03.isp.att.net ([204.127.217.103] verified) by logan.com (CommuniGate Pro SMTP 5.3.8) with ESMTP id 4394577 for lml@lancaironline.net; Thu, 15 Jul 2010 15:36:17 -0400 Received-SPF: none receiver=logan.com; client-ip=204.127.217.103; envelope-from=bbradburry@bellsouth.net Received: from desktop (adsl-85-142-250.mco.bellsouth.net[98.85.142.250]) by isp.att.net (frfwmhc03) with SMTP id <20100715193536H03002tjdne>; Thu, 15 Jul 2010 19:35:37 +0000 X-Originating-IP: [98.85.142.250] From: "Bill Bradburry" X-Original-To: "'Lancair Mailing List'" References: In-Reply-To: Subject: RE: [LML] Re: Small tail, MK II tail, CG range X-Original-Date: Thu, 15 Jul 2010 15:35:38 -0400 X-Original-Message-ID: <4BB2C97BB4EC4E459DCAFB7D7C950C30@Desktop> MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_001E_01CB2433.6075DC60" X-Mailer: Microsoft Office Outlook 11 Thread-Index: AcskMrjxS83ZBj9ITKSqD7wBg8GhxwAHo7YA X-MimeOLE: Produced By Microsoft MimeOLE V6.0.6001.18049 This is a multi-part message in MIME format. ------=_NextPart_000_001E_01CB2433.6075DC60 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable Unless we are talking about MAC and cheese, or the Mickey D kind of MAC, = the aircraft MAC is the Mean Aerodynamic Chord. This MAC is the width of = the wing when measured through the center of the wing in the forward-aft direction. On a plane like a Piper, this is just the width of the wing. With a more complicated wing design like the Lancair it is the average = of this measurement. That is where the word =93Mean=94 comes from. This measurement has nothing to do with the =93neutral point=94. It really = just describes how effectively wide the wing is. The CG (Center of Gravity) = is the point around which the airplane balances (or would balance) if it is sitting on its wheels. (Maybe that is a =93neutral point=94?) This CG = is calculated when the plane is motionless on the ground and on scales. It = is not the CG that the plane is operating with when it is in flight because = the horizontal stabilizer is usually designed to place a down force on the plane, which will have the effect of moving the CG backward in cruise. = That is why the CG is specified to be in the front 25% of the wing width = (MAC) in the specs. =20 When we determine thru the Weight and Balance calculations, the CG, we = have no idea what the CG of the plane will be in flight because as the angle = of attack moves the Center of Lift forward and aft, and the horizontal stabilizer adds and removes loads, we have no way of calculating or = knowing how these forces are moving. Hopefully the aircraft designer did all = this when he specified the CG range that we should keep the plane in when it = is on the ground and on its wheels. I suggest we stay inside these recommendations. =20 Bill B =20 =20 =20 _____ =20 From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of = Chris Zavatson Sent: Thursday, July 15, 2010 11:30 AM To: lml@lancaironline.net Subject: [LML] Re: Small tail, MK II tail, CG range =20 Wolfgang, et al The aircraft MAC (also called neutral point) relative to CG is the key = to evaluating aircraft longitudinal stability. This is independent of = whether the tail is providing an up or down force (either can be stable). Longitudinal stability is defined by the reaction of the entire airframe = to a disturbance from equilibrium. The size, location and pitching moment characteristics of each component factors in (wing, tail, fuselage = etc.). Evaluating the behavior of just the wing is not sufficient to describe = the response of the aircraft as a whole and certainly not to quantify the response. Actually, a wing section alone will be unstable as the = pitching moment is negative. It is stable when inverted - flying wings have = negative camber for this reason.=20 =20 A stable aircraft must have a positive pitching moment when in = equilibrium. In order to be stable, the pitching moment coefficient must have a = negative slope with increasing angle of attack. This provides an increasing = opposing moment to an increasing disturbance. =20 A larger tail increases the response when a disturbance occurs. It is a function of the larger area producing more restoring force for any given angular disturbance. The size of the horizontal stabilizer feeds into a quantity called the tail volume ratio - a unit-less measure of relating = tail area to wing area and wing mean wing chord to distance to the horizontal stabilizer. More area or a longer tail increase the effectiveness in = terms of stability. The neutral point is fixed by the configuration of the aircraft. Only configuration changes will move the neutral point. Lowering the flaps, = for example, changes the airfoil, relative incidence angles, pitching moment = of the wing and so on. In all configurations the neutral point must remain well behind the CG. 10% of the mean chord length is a good starting minimum. Once the neutral point is known, the incidence angles and CG = can be set. What will fall out is the trim airspeed. That is, in = equilibrium the aircraft will seek out a specific angle of attack and the = corresponding airspeed. One can play around with different combinations of incidence angles and CG locations to achieve both a stable aircraft and minimum = trim drag at any desired airspeed.=20 hope that helps, Chris =20 =20 =20 Chris Zavatson N91CZ 360std www.N91CZ.com =20 =20 =20 _____ =20 From: Wolfgang To: lml@lancaironline.net Sent: Wed, July 14, 2010 10:37:18 AM Subject: [LML] Re: Small tail, MK II tail, CG range I'm not familiar with MAC as applied to the entire airframe, can you elaborate? I think there may be a problem with that idea since the tail = is typically providing a down force which would move the "airframe MAC" to = the front, not the rear. =20 Wolfgang _____ =20 ----- Original Message -----=20 From: Chris Zavatson=20 To: lml@lancaironline.net=20 Sent: Tuesday, July 13, 2010 8:35 PM Subject: Re: [LML] Small tail, MK II tail, CG range =20 Wolfgang, et al, <> =20 A larger tail moves the MAC rearward allowing the CG to move farther aft while maintaining the same level of stability. There has been a lot of discussion about Cm. We need to be careful to distinguish between the Cm for the wing, tail and total aircraft. It is = the later that is critical to stability and this is where the larger tail influences the situation. The large tail moves the MAC to the rear = approx. 1.5 inches. For the same CG, the more rearward MAC produces a greater restoring force if the plane is disturbed from level flight. The = practical benefit for us is that it allows a lot more baggage to be thrown the = rear of the plane before suffering stability problems. You pointed out the = other benefit of increased control authority at slow speed with full flaps. =20 Chris Zavatson N91CZ 360std www.N91CZ.com =20 =20 _____ =20 From: Wolfgang To: lml@lancaironline.net Sent: Tue, July 13, 2010 2:51:23 AM Subject: [LML] Small tail, MK II tail, CG range The quest continues. =20 I'm checking further into the data on these questions and am coming to question the need for a larger tail. I'm not sure a larger tail by = itself will solve the problem. After doing some static and in flight = measurements, it looks like the tail authority is not a big problem, if a problem at = all. =20 Static measurements of N31161 have shown "vanilla" parameters. 2.5=BA incidence between the wing root at full reflex and the tail and a 1.3=BA washout. Put the flaps at 0=BA and you get an additional AoA of 1.8=BA = at the root for a total incidence of 4.3=BA . . . . not radical at all. =20 What is interesting is the POH (Dec. 1994 pg. VI-3) gives the CG range = as 24.5" to 30.3" aft of the rear face of the fire wall and the MAC at 15% = to 20% =20 . . . well . . . no . . . that range is more like a MAC range of 15% to = 30% - - - a good range made touchy only by the small size of the air frame. =20 After going over the plan view kit drawings, I come up with a CG range = of 23-1/4" to 29-1/4" for a MAC range of 15% to 30% That range is about 1-1/4" forward of the book and fits better with = first hand flight experience.=20 =20 Any more to the rear and you get negative stability at cruise and a = larger tail doesn't help much with that anyway.=20 Negative stability makes pitch control a real chore. As Scott K. has indicated, going to 0=BA flaps helps under that loading condition. =20 Too far forward and landing becomes "interesting". A larger tail can = help there . . . or don't use as much flaps. =20 I think understanding these conditions can help everyone.=20 =20 . . . The quest continues . . . Comments welcome. =20 Wolfgang =20 _____ =20 From: "Wolfgang" Sender: Subject: Small tail, MK II tail, CG range Date: Sat, 10 Jul 2010 21:01:11 -0400 To: lml@lancaironline.net The LNC2 uses the NLF(1)-0215F airfoil. A lot can be found by doing a = Google search on that number. More detail can be found by going to Google for "NASA Technical Paper = 1865". =20 I have not taken the time to reverse engineer the CG range of the LNC2 = but let me offer some observations. =20 The airfoil used has long been touted as "the greatest thing since = sliced bread" for General Aviation and it definitely has some advantages. But = it's not new. Compare this airfoil to the P-51 airfoil and you will see some close similarities. The LNC2 being composite construction instead of aluminum lets the airfoil show more of it's theoretical advantages. =20 It's a laminar shape with a good drag bucket. That bucket can be made to move to the lower Cl (lift coefficient) ranges with reflex allowing noticeably lower drag at higher cruise speeds. Along with reflex, the Cm (moment coefficient) goes positive, the center of lift of the wing = travels forward giving a nose up force requiring down trim. This is in addition = to the usual nose up force that goes with most all airfoils at high speed before considering flaps. =20 With down flap, the drag bucket will move to higher Cl's making slower flight more efficient. And, of course, the Cm goes negative giving a = nose down force requiring up trim. =20 . . . and appropriate variations in-between . . . =20 So, the rear CG limit is determined by high speed flight and available control authority, and the forward CG is determined by low speed / landing flight and = available control authority. =20 What is becoming clear here is that the center of lift does quite a bit = of traveling fore and aft which is exaggerated by allowing negative or = "cruise" flaps. Since you can't shift the CG during flight, you need a large = amount of pitch authority from the tail to cover that range of lift travel. =20 You have two choices in the LNC2, live with the limitations or install a larger tail to give that extra pitch authority. . . . A larger tail area can also help with abnormal attitude recovery. =20 Wolfgang =20 =20 ------=_NextPart_000_001E_01CB2433.6075DC60 Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable

Unless we are talking about MAC and cheese, or the Mickey D kind of MAC, the aircraft MAC is the Mean = Aerodynamic Chord.=A0 This MAC is the width of the wing when measured through the = center of the wing in the forward-aft direction.=A0 On a plane like a Piper, this = is just the width of the wing.=A0 With a more complicated wing design like the = Lancair it is the average of this measurement.=A0 That is where the word = “Mean” comes from.=A0 This measurement has nothing to do with the = “neutral point”.=A0 It really just describes how effectively wide the wing is. =A0The CG = (Center of Gravity) is the point around which the airplane balances (or would = balance) if it is sitting on its wheels.=A0 (Maybe that is a “neutral = point”?)=A0 This CG is calculated when the plane is motionless on the ground and on = scales.=A0 It is not the CG that the plane is operating with when it is in flight because = the horizontal stabilizer is usually designed to place a down force on the = plane, which will have the effect of moving the CG backward in cruise.=A0 That = is why the CG is specified to be in the front 25% of the wing width (MAC) in = the specs.

 

When we determine thru the Weight = and Balance calculations, the CG, we have no idea what the CG of the plane = will be in flight because as the angle of attack moves the Center of Lift = forward and aft, and the horizontal stabilizer adds and removes loads, we have no = way of calculating or knowing how these forces are moving.=A0 Hopefully the = aircraft designer did all this when he specified the CG range that we should keep = the plane in when it is on the ground and on its wheels.=A0 I suggest we = stay inside these recommendations.

 

Bill B

 

 

 

From: = Lancair Mailing List = [mailto:lml@lancaironline.net] On Behalf Of Chris Zavatson
Sent: Thursday, July 15, = 2010 11:30 AM
To: = lml@lancaironline.net
Subject: [LML] Re: Small = tail, MK II tail, CG range

 

Wolfgang, et al

The aircraft MAC (also called neutral point) relative = to CG is the key to evaluating aircraft longitudinal stability.  This is independent of whether the tail is providing an up or down force (either can be stable).  Longitudinal stability is defined by = the reaction of the entire airframe to a disturbance from equilibrium.  = The size, location and pitching moment characteristics of each = component factors in (wing, tail, fuselage etc.).  Evaluating the = behavior of just the wing is not sufficient to describe the response of the aircraft as a whole and certainly not to quantify the response.  = Actually, a wing section alone will be unstable as the pitching moment is negative.  It is stable when inverted - flying wings have negative = camber for this reason. 

  

A stable aircraft must have a positive pitching moment when in equilibrium.  In order to be stable, = the pitching moment coefficient must have a negative slope = with increasing angle of attack.  This provides an increasing opposing moment to an increasing = disturbance.  

A larger tail increases the response when a = disturbance occurs.  It is a function of the larger area producing more restoring force for any given angular disturbance.  The size of the horizontal stabilizer feeds into a quantity called the tail volume ratio - a unit-less = measure of relating tail area to wing area and wing mean wing chord to = distance to the horizontal stabilizer.  More area or a longer tail = increase the effectiveness in terms of stability.

The neutral point is fixed by the = configuration of the aircraft.  Only configuration changes will move the = neutral point.  Lowering the flaps, for example, changes the airfoil, = relative incidence angles, pitching moment of the wing and so on.  In all = configurations the neutral point must remain well behind the CG.  10% of the mean = chord length is a good starting minimum.  Once the neutral point is = known, the incidence angles and CG can be set.  What will fall out is the trim airspeed.  That is, in equilibrium the aircraft will seek out = a specific angle of attack and the corresponding airspeed.  One can = play around with different combinations of incidence angles and CG locations to achieve both a stable aircraft and minimum trim drag at any = desired airspeed. 

hope that helps,

Chris

 

 

 

Chris Zavatson

N91CZ

360std


 

 

From: Wolfgang <Wolfgang@MiCom.net>
To: = lml@lancaironline.net
Sent: Wed, July 14, 2010 = 10:37:18 AM
Subject: [LML] Re: Small = tail, MK II tail, CG range

I'm not familiar with MAC as applied to the entire = airframe, can you elaborate? I think there may be a problem with that idea since = the tail is typically providing a down force which would move the "airframe = MAC" to the front, not the rear.

 

Wolfgang

----- Original Message ----- =

Sent: = Tuesday, July 13, 2010 8:35 PM

Subject: Re: = [LML] Small tail, MK II tail, CG range

 

Wolfgang, et al,

<<Any more to the rear and you get negative = stability at cruise and a larger tail doesn't help much with that = anyway.>>

 

A larger tail moves the MAC rearward allowing = the CG to move farther aft while maintaining the same level of = stability.

There has been a lot of discussion about = Cm.  We need to be careful to distinguish between the Cm for the wing, tail and = total aircraft.  It is the later that is critical to stability and this = is where the larger tail influences the situation.  The large tail moves the = MAC to the rear approx. 1.5 inches.  For the same CG, the more rearward = MAC produces a greater restoring force if the plane is disturbed from level flight.  The practical benefit for us is that it allows a lot = more baggage to be thrown the rear of the plane before = suffering stability problems.  You pointed out the other benefit of increased control authority at slow speed with full flaps.

 

Chris Zavatson

N91CZ

360std

 

From: Wolfgang <Wolfgang@MiCom.net>
To: = lml@lancaironline.net
Sent: Tue, July 13, 2010 = 2:51:23 AM
Subject: [LML] Small = tail, MK II tail, CG range

The quest continues.

 

I'm checking further into the data on these questions and am = coming to question the need for a larger tail. I'm not sure a larger tail by = itself will solve the problem. After doing some static and in flight measurements, = it looks like the tail authority is not a big problem, if a problem at = all.

 

Static measurements of N31161 have shown "vanilla" parameters. 2.5=BA incidence between the wing root at full reflex and = the tail and a 1.3=BA washout. Put the flaps at 0=BA and you get an additional = AoA of 1.8=BA at the root for a total incidence of 4.3=BA . . . . not radical at = all.

 

What is interesting is the POH (Dec. 1994 pg. VI-3) gives the CG = range as 24.5" to 30.3" aft of the rear face of the fire wall and = the MAC at 15% to 20%

 

. . . well . . . no . . . that range is more like a MAC range of = 15% to 30% - - - a good range made touchy only by the small size of the air = frame.

 

After going over the plan view kit drawings, I come up with a CG = range of 23-1/4" to 29-1/4" for a MAC range of 15% to = 30%

That range is about 1-1/4" forward of the book and fits = better with first hand flight experience.

 

Any more to the rear and you get negative stability at cruise = and a larger tail doesn't help much with that anyway. =

Negative stability makes pitch control a real chore. As Scott K. = has indicated, going to 0=BA flaps helps under that loading = condition.

 

Too far forward and landing becomes "interesting". A = larger tail can help there . . . or don't use as much = flaps.

 

I think understanding these conditions can help everyone. =

 

. . . The quest continues . . . Comments = welcome.

 

Wolfgang


 

From:

"Wolfgang" = <Wolfgang@MiCom.net>

Sender:

<marv@lancaironline.net>

Subject:

Small tail, MK II tail, CG = range

Date:

Sat, 10 Jul 2010 21:01:11 = -0400

To:

lml@lancaironline.net=

The LNC2 uses the NLF(1)-0215F airfoil. A = lot can be found by doing a Google search on that = number.

More detail can be found by going to = Google for "NASA Technical Paper 1865".

 

I have not taken the time to reverse = engineer the CG range of the LNC2 but let me offer some = observations.

 

The airfoil used has long been touted as = "the greatest thing since sliced bread" for General Aviation and it definitely has some advantages. But it's not new. Compare this airfoil = to the P-51 airfoil and you will see some close similarities. The LNC2 being composite construction instead of aluminum lets the airfoil show more = of it's theoretical advantages.

 

It's a laminar shape with a good drag = bucket. That bucket can be made to move to the lower Cl (lift coefficient) ranges = with reflex allowing noticeably lower drag at higher cruise speeds. Along = with reflex, the Cm (moment coefficient) goes positive, the center of lift = of the wing travels forward giving a nose up force requiring down trim. This = is in addition to the usual nose up force that goes with most all = airfoils at high speed before considering flaps.

 

With down flap, the drag bucket will move = to higher Cl's making slower flight more efficient. And, of course, the Cm goes negative giving a nose down force requiring up = trim.

 

. . . and appropriate variations in-between = . . .

 

So, the rear CG limit is determined by high = speed flight and available control authority,

and the forward CG is determined by low = speed / landing flight and available control = authority.

 

What is becoming clear here is that the = center of lift does quite a bit of traveling fore and aft which is exaggerated = by allowing negative or "cruise" flaps. Since you can't shift = the CG during flight, you need a large amount of pitch authority from the = tail to cover that range of lift travel.

 

You have two choices in the LNC2, live with = the limitations or install a larger tail to give that extra pitch = authority.

. . . A larger tail area can also help with abnormal = attitude recovery.

 

Wolfgang

 

 

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