X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml@lancaironline.net Date: Mon, 23 Sep 2013 12:26:18 -0400 Message-ID: X-Original-Return-Path: Received: from nm13-vm4.bullet.mail.gq1.yahoo.com ([98.136.218.223] verified) by logan.com (CommuniGate Pro SMTP 6.0.7) with ESMTPS id 6486521 for lml@lancaironline.net; Mon, 23 Sep 2013 11:00:31 -0400 Received-SPF: none receiver=logan.com; client-ip=98.136.218.223; envelope-from=chris_zavatson@yahoo.com Received: from [98.137.12.56] by nm13.bullet.mail.gq1.yahoo.com with NNFMP; 23 Sep 2013 14:59:45 -0000 Received: from [98.136.164.75] by tm1.bullet.mail.gq1.yahoo.com with NNFMP; 23 Sep 2013 14:59:45 -0000 Received: from [127.0.0.1] by smtp237.mail.gq1.yahoo.com with NNFMP; 23 Sep 2013 14:59:44 -0000 X-Yahoo-Newman-Id: 985784.44825.bm@smtp237.mail.gq1.yahoo.com X-Yahoo-Newman-Property: ymail-3 X-YMail-OSG: NhHkjqcVM1km9lk0cvlwFSZkZwtCEgIJTWGy7_Tf6xphx9s KvkcmIlrd5T2vBw9YFoHBUYJY7WpTXuj2lgVcUJohL7.rT2CLw.OE.sHiA3V qE4AiUFgwsMKXuAOsHcJPhyoNeby_jpBit_EgaVv7Tdjz3mZJ99VZs2JFmm0 6E.UWR4qxGJN3bHrV8oBgzsgIBwSMIRI8Wf8hKvV6Ytg7q2K2hUZ9fjnGi_t JFVyjfgeKzIqMpttsMkQtEMoZCo7LR6YwO36KDPTLIbnCJ1EePhHS99Rpj8N y0a0H5.TzVcnGDoBUuN66IKasFxKOjakF0f6ANVwbnfLVaHCyOV26MV3FEtl wn367QelwGCJy3THaZZ6Pdia4jQveSA050bNWi3D26QSTNdf7OS1YIFDAYy. TwLcuJWhFl3H8atyaT9Pp63fPvKfPglHhDGe7TdsZmQ2BNzxZmFWgE_vyQbO U.b3a.AambkLP0NxQ58s0M9EmNDr6TyEykZmzUVYAN5FNS3F1DGxKhhJt_GS 2r8MqUwQmaShXFFUNuWaW.Uo3mOXXBhVBBlIroQovx7YHnkICuck_hiqDV39 YlvZ4i4OXay8xlc0q1hGqJeQx.ckS2fS1C1YiGoCg57Sm6P8JaYk9Vf_XWBE eLByb7bah37CgQTOhviJpOLV02thgANZ9XvdrA9eZOSemEsKUDa3XPQ-- X-Yahoo-SMTP: 076hgjCswBC.G6e0vm7vgvZ9JJ0zmeBo_Oyw X-Rocket-Received: from [192.168.0.111] (chris_zavatson@50.66.84.166 with ) by smtp237.mail.gq1.yahoo.com with SMTP; 23 Sep 2013 14:59:44 +0000 UTC Subject: Re: [LML] Re: Lancair 320/360 performance and stability References: From: Chris Zavatson Content-Type: multipart/alternative; boundary=Apple-Mail-22D380A1-DFF1-4C2E-925A-7E01C1CD055E X-Mailer: iPad Mail (9B206) In-Reply-To: X-Original-Message-Id: X-Original-Date: Mon, 23 Sep 2013 07:59:30 -0700 X-Original-To: Lancair Mailing List Content-Transfer-Encoding: 7bit Mime-Version: 1.0 (1.0) --Apple-Mail-22D380A1-DFF1-4C2E-925A-7E01C1CD055E Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=us-ascii Bob, There are a couple of things going on at high AOA when the tail enters the w= ake. Unfortunately some of the component contributions are more easily isol= ated in the wind tunnel. The low energy air reduces the natural restoring force or the force to push t= he nose down. The elevator also sees this lower energy flow. Given a fixed e= levator position this would result in less downforce as you describe. The lo= w energy flow also reduces the hinge moment. If the pilot is holding a cons= tant pressure on the stick, the elevator will actually deflect more. You ca= n see an increase in elevator deflection per unit force in Figure 10. The cu= rve steepens quite a bit. =20 The springs add another interesting element. I would expect them to add som= e degree of stability. I am not familiar with all the geometric differences between the 235 and 320= airframes, but it certainly sounds like the published aft CG limit has no m= argin on the 235. Thanks for sharing your observations. Chris Sent from my spiffy iPad On Sep 19, 2013, at 4:43 AM, bob mackey wrote: >=20 > Chris:=20 > First of all, thanks for an excellent writeup and a fine demonstration of s= cientific measurement-based engineering. You've done a great service to the L= ML community by your work and by reporting it so well.=20 >=20 >=20 > I fly a small-tail 235/O-320. At the aft CG limit published by Lancair, st= atic stability is just about gone. I flew there one time and choose not to r= aise the landing gear for fear of the CG moving a tiny bit further aft. Main= taining level flight was difficult. Dropping the flaps about 1/2 inch restor= ed some pitch stability. Fortunately my passenger was able to move some gear= from behind his head to the footwell. Without that, landing would have been= challenging. As soon as I landed, I revised the CG limits in the operator's= manual 0.5 inch forward of Lancair's suggested value.=20 >=20 > This airframe is also known to oscillate when slipping with full rudder an= d full aileron. In a left slip, the nose describes clockwise circles with ab= out 1-2 second period. Relaxing the slip just a bit stops the oscillation. T= hat is probably a rudder blanking effect.=20 >=20 > I cannot say that I have observed stick *force* reversal at high AoA, but r= ather stick position reversal. The trim springs may be masking a true force r= eversal at the elevator hinge. What I do know is that as nose comes up in a f= lare, the stick must move forward to stabilize at the higher AoA. Holding a f= ixed stick position in the flare would have the "nose pitching up on its own= " as you say. I do not think that is a masking effect. If the stabilizer wer= e moving down into the wing's low-energy wake, I would expect to have to pul= l more to continue raising the nose. The opposite is true, so either the sta= bilizer is normally in the wake and drops into higher energy air at high AoA= , or some other effect is responsible. I have assumed that it is an increase= in the wing's pitching moment at high AoA, but haven't measured it. =20 >=20 >=20 >=20 > -bob mackey >=20 >=20 >=20 >=20 >=20 >=20 >=20 >=20 > From: > Chris Zavatson > Sender: > Subject: Re: [LML] Re: Lancair 320/360 performance and stability > Date: Wed, 18 Sep 2013 23:16:35 -0400 > To: lml@lancaironline.net >=20 >=20 >=20 > Scott, > Our little Lancairs with a wing aspect ratio around 6.6 are definitely in t= he Hawk category - so you do have an appropriate call-sign. > Horizontal stabilizers have a few other issues. They live in a terrible p= lace aerodynamically speaking. First there is the intersection and proximit= y to the fuselage, then the downwash off the wing that diminishes their effe= ctiveness. Then, at high angles of attack, they can enter the wake of the w= ing. This low energy air further diminishes their effectiveness and hinge m= oments start to fall off. It is this region that is of most interest. This= is where all the anecdotal reports of stick force reversals in the flare ge= t started. The stories about the nose pitching up on its own, told at the L= ancair fly-in after folks arrive with the plane packed full of people and ba= ggage .=20 > I think the most important chart in all of the reports is Figure 12 in the= stability and control write-up. It shows a well behaved increase in AoA wi= th stick force throughout, even with an aft CG.=20 > Chris > =20 > Chris Zavatson > N91CZ > 360std > http://www.n91cz.net/ --Apple-Mail-22D380A1-DFF1-4C2E-925A-7E01C1CD055E Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=utf-8
Bob,
There a= re a couple of things going on at high AOA when the tail enters the wake. &n= bsp;Unfortunately some of the component contributions are more easily isolat= ed in the wind tunnel.
The low energy air reduces the natural rest= oring force or the force to push the nose down.  The elevator also sees= this lower energy flow. Given a fixed elevator position this would result i= n less downforce as you describe. The low energy flow also reduces the hinge= moment.  If the pilot is holding a constant pressure on the stick, the= elevator will actually deflect more.  You can see an increase in eleva= tor deflection per unit force in Figure 10. The curve steepens quite a bit. &= nbsp;
The springs add another interesting element.  I would e= xpect them to add some degree of stability.
I am not familiar w= ith all the geometric differences between the 235 and 320 airframes, but it c= ertainly sounds like the published aft CG limit has no margin on the 235. &n= bsp;Thanks for sharing your observations.
Chris

Sent from= my spiffy iPad

On Sep 19, 2013, at 4:43 AM, bob mackey <n103md@yahoo.com> wrote:

Chris: 
First of all, thanks for an excellent writeup and a fine dem= onstration of scientific measurement-based engineering. You've done a great s= ervice to the LML community by your work and by reporting it so well. <= br>

I fly a small-tail 235/O-320. At the aft CG limit published by Lanca= ir, static stability is just about gone. I flew there one time and choose no= t to raise the landing gear for fear of the CG moving a tiny bit further aft= . Maintaining level flight was difficult. Dropping the flaps about 1/2 inch r= estored some pitch stability. Fortunately my passenger was able to move some= gear from behind his head to the footwell. Without that, landing would have= been challenging. As soon as I landed, I revised the CG limits in the opera= tor's manual 0.5 inch forward of Lancair's suggested value. 

This airframe is also known to oscillate when slipping with full rudder a= nd full aileron. In a left slip, the nose describes clockwise circles with a= bout 1-2 second period. Relaxing the slip just a bit stops the oscillation. T= hat is probably a rudder blanking effect. 

I cannot say that I have observed stick *force* reversal at high AoA, bu= t rather stick position reversal. The trim springs may be masking a true for= ce reversal at the elevator hinge. What I do know is that as nose comes up i= n a flare, the stick must move forward to stabilize at the higher AoA. Holdi= ng a fixed stick position in the flare would have the "nose pitching up on i= ts own" as you say. I do not think that is a masking effect. If the stabiliz= er were moving down into the wing's low-energy wake, I would expect to have t= o pull more to continue raising the nose. The opposite is true, so either th= e stabilizer is normally in the wake and drops into higher energy air at hig= h AoA, or some other effect is responsible. I have assumed that it is an inc= rease in the wing's pitching moment at high AoA, but haven't measured it. &n= bsp;



-bob mackey








=
From: Chris Zavatson <chris_zavatso= n@yahoo.com>
Sender:<marv@lancaironline.net>
Subject: Re: [LML] Re: Lancair 320/360 performance and stability
Date:Wed, 18 Sep 2013 23:16:35 -0400
To: lml@lancaironline.net
 3D"Message
Scott,
Our little Lancairs with a wing aspect r= atio around 6.6 are definitely in the Hawk category - so you do have&nb= sp;an appropriate call-sign.
Horizontal stabilizers have a few other issues.  They live in= a terrible place aerodynamically speaking.  First there is the interse= ction and proximity to the fuselage, then the downwash off th= e wing that diminishes their effectiveness.  Then, at high angles o= f attack, they can enter the wake of the wing.  This low energy air fur= ther diminishes their effectiveness and hinge moments start to fall off= .  It is this region that is of most interest.  This is where= all the anecdotal reports of stick force reversals in the flare get st= arted.  The stories about the nose pitching up on its own, to= ld at the Lancair fly-in after folks arrive with the plane packed full of pe= ople and baggage . 
I think the most important chart in all of the reports is Figure 1= 2 in the stability and control write-up.  It shows a well behaved incre= ase in AoA with stick force throughout, even with an aft CG. 
Chris
 
Chris Zavatson
N91CZ
360std
<= div>http://www.n91cz.net/
= --Apple-Mail-22D380A1-DFF1-4C2E-925A-7E01C1CD055E--