Return-Path: Sender: "Marvin Kaye" To: lml@lancaironline.net Date: Wed, 26 Jan 2005 07:59:46 -0500 Message-ID: X-Original-Return-Path: Received: from wind.imbris.com ([216.18.130.7] verified) by logan.com (CommuniGate Pro SMTP 4.2.8) with ESMTP-TLS id 622609 for lml@lancaironline.net; Wed, 26 Jan 2005 03:08:58 -0500 Received-SPF: none receiver=logan.com; client-ip=216.18.130.7; envelope-from=brent@regandesigns.com Received: from [192.168.1.100] (wireless-216-18-135-19.imbris.com [216.18.135.19]) (authenticated bits=0) by wind.imbris.com (8.12.11/8.12.11) with ESMTP id j0Q88MZV005761 for ; Wed, 26 Jan 2005 00:08:22 -0800 (PST) (envelope-from brent@regandesigns.com) X-Original-Message-ID: <41F74FF5.6000108@regandesigns.com> Disposition-Notification-To: Brent Regan X-Original-Date: Wed, 26 Jan 2005 00:08:21 -0800 From: Brent Regan User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.2) Gecko/20040804 Netscape/7.2 (ax) X-Accept-Language: en-us, en MIME-Version: 1.0 X-Original-To: Lancair Subject: Re: Alert NACA 64-212 Content-Type: multipart/alternative; boundary="------------050508020505050607060603" This is a multi-part message in MIME format. --------------050508020505050607060603 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Deja vu all over again! Robert's application of theoretical aerodynamics are an interesting exercise but I am having difficulty agreeing with his "Locked in Spin" theory. Alas, real world conditions can rarely be reduced to a wiggly line. While he is basically correct within the limits and scope of his consideration, there are larger issues which dominate. A few facts to reason with: The airfoil section plots apply to a homogenous (straight, no taper, no blending) wing of infinite length (no tip or fuselage effects). They are intended as a rough guide to airfoil selection. The performance curves will not accurately predict the characteristics of the last outboard inch of the wing. The inverse kinematic viscosity coefficient that he uses to calculate Reynolds numbers is applicable for an altitude of roughly 5000 feet, or more specifically, a temperature of approximately 44 degrees Fahrenheit. As the temperature drops the RN gets larger. The 64-212 airfoil was selected for its high lift coefficient AND GOOD RESPONSE TO AILERON INPUT. We cannot conclude from the data presented that the hysteresis effect that is present at a RN of 3 million is present BELOW that number. One point does not a trend make. Airplanes have mass and things with mass have inertia. As a IV enters a stall the wing root stalls first and then the stall moves outboard until there isn't enough total lift to support the plane. One could argue that it takes more than a foot or two of wing to hold the plane in the sky, so you could conclude that there is a significant chunk of wing tip still flying just before the plane starts accelerating towards terra firma. As the stall marches out the wing it reaches that point where there isn't enough wing to keep the plane flying and the plane starts accelerating downward. Inertia resists changes the pitch (and roll and yaw) attitude so the AOA increases due to the increasing vertical wind component which in turn results in less lift as more of the wing stalls. This process continues until the horizontal stabilizer produces enough lift for long enough to pitch the airplane downward and the drag from the high AOA goes away and you really start accelerating downward (falling) until you regain enough airspeed to make enough lift to pull out of the dive without a secondary stall. During the last part of the stalling process the AOA at the wing tip increases at an accelerating rate and then, as the nose drops, rapidly goes to near zero. The pilot does NOT "push the stick forward to unstall (the wing)" The plane unstalls the wing as it pitches forward, the pilot pushes the stick forward to keep the wing from stalling again (allowing the airspeed to build). All of you who have "Been there, done that" will understand. Even if the last couple of feet of the wing were prone to the lift hysteresis theorized, the transition through that region of AOA would happen very quickly and, even if the forces were asymmetrical, they would not be applied long enough (even with a high roll moment) to produce a significant roll RATE. As described, stall recovery does not happen from a state of high AOA. After the stall the pointy end pitches down and the aircraft is accelerating at a low AOA, no where near the feared hysteresis. Roberto is correct in advising against aileron use during stall but not because it would exacerbate the theorized hysteresis but rather because ailerons, like flaps, change the camber and AOA of the effected wing section. Aileron input "twists" the wing, causing one wing to stall before the other and potentially inducing a spin. Flight training 101, remember??? The effect of the ailerons would be many times greater than the theorized lift hysteresis during stall development therefore the actions of the pilot are of far greater consequence. Roberto is not the first to advance this unsafe wing theory. When the Lancair IV was first introduced there was another engineer who's ego exceeded his talent. Harry Riblett circulated a letter (EAA, NASA, anyone with a pulse and could spell aerodynamics, etc.) claiming that the IV was designed with an "unsafe" wing. You see, Harry was the engineer who did not get the design contract. He seemed sure of his position, so sure in fact that he subsequently used the exact airfoil in one of his subsequent designs, the Rigel AA300. HHMMmmmmm. Flight testing further discredited Harry. Aint life a bitch. Roberto writes: "and also because of the hysteresis effect the setting of angle of incidence is not just important IT'S CRITICAL." It should be obvious to most that this statement is flat wrong. The wing incidence is NOT critical. What I would hope he meant was that the difference in the incidence of the two wings is important. It is so important in fact, it is set at the factory when the mounting holes are drilled in the wing while it rests in a machined jig. Roberto's point about how workmanship effects flight characteristics is well taken but should not be limited to wing thickness or leading edge radius. If you build a piece of crap, do not be surprised if it flies like crap. I sincerely believe that Roberto's "Alert" is much ado about nothing. Having said all this I am sure that I deserve to be called an idiot who has no understanding of the finer points of aerodynamics, so, I will save the effort and freely admit it now. I would only ask that, before any more bandwidth be sacrificed, Roberto consult with an expert, say, Mr. Hollmann, who has forgotten more than I will ever know about this topic and is the author of the oft quoted M.A.D., to opine as to the validity of the theory as presented. I would, however, remind Mr. d'Italia that Mr. Hollmann subsequently used the identical airfoil on the Stallion. Q.E.D. It is the builder/pilot, stupid, not the airplane! Regards Brent Regan PS. One good flight test is worth 10,000 novice aerodynamicist opinions. BR --------------050508020505050607060603 Content-Type: text/html; charset=us-ascii Content-Transfer-Encoding: 7bit Deja vu all over again!

Robert's application of theoretical aerodynamics are an interesting exercise but I am having difficulty agreeing with his "Locked in Spin" theory.  Alas, real world conditions can rarely be reduced to a wiggly line.  While he is basically correct within the limits and scope of his consideration, there are larger issues which dominate.

A few facts to reason with:

The airfoil section plots apply to a homogenous (straight, no taper, no blending) wing of infinite length (no tip or fuselage effects). They are intended as a rough guide to airfoil selection. The performance curves will not accurately predict the characteristics of the last outboard inch of the wing.

The inverse kinematic viscosity coefficient that he uses to calculate Reynolds numbers is applicable for an altitude of roughly 5000 feet, or more specifically, a temperature of approximately 44 degrees Fahrenheit. As the temperature drops the RN gets larger.

The 64-212 airfoil was selected for its high lift coefficient AND GOOD RESPONSE TO AILERON INPUT.

We cannot conclude from the data presented that the hysteresis effect that is present at a RN of 3 million is present BELOW that number. One point does not a trend make.

Airplanes have mass and things with mass have inertia.

As a IV enters a stall the wing root stalls first and then the stall moves outboard until there isn't enough total lift to support the plane. One could argue that it takes more than a foot or two of wing to hold the plane in the sky, so you could conclude that there is a significant chunk of wing tip still flying just before the plane starts accelerating towards terra firma. As the stall marches out the wing it reaches that point where there isn't enough wing to keep the plane flying and the plane starts accelerating downward. Inertia resists changes the pitch (and roll and yaw) attitude so the AOA increases due to the increasing vertical wind component which in turn results in less lift as more of the wing stalls. This process continues until the horizontal stabilizer produces enough lift for long enough to pitch the airplane downward and the drag from the high AOA goes away and you really start accelerating downward (falling) until you regain enough airspeed to make enough lift to pull out of the dive without a secondary stall. During the last part of the stalling process the AOA at the wing tip increases at an accelerating rate and then, as the nose drops, rapidly goes to near zero. The pilot does NOT "push the stick forward to unstall (the wing)" The plane unstalls the wing as it pitches forward, the pilot pushes the stick forward to keep the wing from stalling again (allowing the airspeed to build).  All of you who have "Been there, done that"  will understand.

Even if the last couple of feet of the wing were prone to the lift hysteresis theorized, the transition through that region of AOA would happen very quickly  and, even if the forces were asymmetrical, they would not be applied long enough (even with a high roll moment) to produce a significant roll RATE. As described, stall recovery does not happen from a state of high AOA. After the stall the pointy end pitches down and the aircraft is accelerating at a low AOA, no where near the feared hysteresis.

Roberto is correct in advising against aileron use during stall but not because it would exacerbate the theorized hysteresis but rather because ailerons, like flaps, change the camber and AOA of the effected wing section. Aileron input "twists" the wing, causing one wing to stall before the other and potentially inducing a spin. Flight training 101, remember??? The effect of the ailerons would be many times greater than the theorized lift hysteresis during stall development therefore the actions of the pilot are of far greater consequence.

Roberto is not the first to advance this unsafe wing theory. When the Lancair IV  was first introduced there was another engineer who's ego exceeded his talent. Harry Riblett circulated a letter (EAA, NASA, anyone with a pulse and could spell aerodynamics, etc.) claiming that the IV was designed with an "unsafe" wing. You see, Harry was the engineer who did not get the design contract. He seemed sure of his position, so sure in fact that he subsequently used the exact airfoil in one of his subsequent designs, the Rigel AA300. HHMMmmmmm. Flight testing further discredited Harry. Aint life a bitch.

Roberto writes: "and also because of the hysteresis effect the setting of angle of incidence is not just important IT'S CRITICAL."
It should be obvious to most that this statement is flat wrong. The wing incidence is NOT critical. What I would hope he meant was that the difference in the incidence of the two wings is important. It is so important in fact, it is set at the factory when the mounting holes are drilled in the wing while it rests in a machined jig.

Roberto's point about how workmanship effects flight characteristics is well taken but should not be limited to wing thickness or leading edge radius. If you build a piece of crap, do not be surprised if it flies like crap.

I sincerely believe that Roberto's "Alert" is much ado about nothing.

Having said all this I am sure that I deserve to be called an idiot who has no understanding of the finer points of aerodynamics, so, I will save the effort and freely admit it now. I would only ask that, before any more bandwidth be sacrificed, Roberto consult with an expert, say, Mr. Hollmann, who has forgotten more than I will ever know about this topic and is the author of the oft quoted M.A.D., to opine as to the validity of the  theory as presented. I would, however, remind Mr. d'Italia that Mr. Hollmann subsequently used the identical airfoil on the Stallion.

Q.E.D.

It is the builder/pilot, stupid, not the airplane!

Regards
Brent Regan

PS. One good flight test is worth 10,000 novice aerodynamicist opinions.
BR



 --------------050508020505050607060603--