X-Virus-Scanned: clean according to Sophos on Logan.com X-SpamCatcher-Score: 10 [X] Return-Path: Sender: To: lml Date: Sun, 11 Feb 2007 12:11:06 -0500 Message-ID: X-Original-Return-Path: Received: from wind.imbris.com ([216.18.130.7] verified) by logan.com (CommuniGate Pro SMTP 5.1.6) with ESMTPS id 1830392 for lml@lancaironline.net; Sun, 11 Feb 2007 11:58:18 -0500 Received-SPF: none receiver=logan.com; client-ip=216.18.130.7; envelope-from=brent@regandesigns.com Received: from [192.168.1.138] (cbl-238-80.conceptcable.com [207.170.238.80] (may be forged)) (authenticated bits=0) by wind.imbris.com (8.12.11/8.12.11.S) with ESMTP id l1BGvMdq008265 for ; Sun, 11 Feb 2007 08:57:28 -0800 (PST) (envelope-from brent@regandesigns.com) X-Original-Message-ID: <45CF4AF1.5000804@regandesigns.com> X-Original-Date: Sun, 11 Feb 2007 08:57:21 -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 Mailing List Subject: Re: [LML] More on Lift and flow around wings Content-Type: multipart/alternative; boundary="------------070706030608030600090707" This is a multi-part message in MIME format. --------------070706030608030600090707 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Thank you Fred for the informative links and thank you Rick for giving a very good explanation for vortex generation and pointing out that changing your frame of reference can help in understanding the dynamics. To further illustrate the latter, I will relate a conversation I had years ago regarding actual flight versus wind tunnel testing. "Conversation" is code for argument and engineers hate arguments like pigs hate mud. My friend was arguing that because the air was moving past a stationary wing it had different dynamic properties, presumably because of its kinetic energy, than if the air was stationary and the wing was moving through it. I told him of a time during my flight training when the winds at my airfield were 25 Kts on the ground (same heading as the runway) and 45+ Kts above 500 feet. My instructor had me put the airplane (C172) in slow flight configuration right after takeoff and the little Cessna climbed steadily with zero ground speed. Above about 500 feet we actually started "backing up" towards the landing threshold end of the runway. I eased of power a little and dropped the nose and, for a few minutes, hovered stationary with zero VSI and zero ground speed. Spooky! Clearly the airplane was stationary relative to the ground (as in a wind tunnel) and the air was moving past it. I asked my friend "At what point during this flight did the aerodynamic "rules" transition from those of an airplane moving through the air to those of the air moving past the airplane?". I took his silence as agreement that there was no difference. Air is a fluid and the only way to mechanically act on a fluid is with pressure and pressure is a force per unit area. Integrating all the pressures on all the surfaces of the aircraft with respect to the gravity vector gives the total net vertical force (AKA lift). If the total vertical force is equal to the mass of the aircraft times the local acceleration off gravity then level flight is achieved. Exactly how are all those pressures generated on all those surfaces? Paul is right in saying that nobody knows and, if we believe Hiesenberg, nobody CAN know. The overall net effect on the air (all the air there is) surrounding the airplane is a change in vertical velocity which, if integrated, would obey F=MA. Saying that "Lift occurs because flow is turned downward creating downwash." is like saying that milk comes from cartons. While technically correct, it doesn't tell the complete story of how the milk got into the carton. Regards Brent Regan --------------070706030608030600090707 Content-Type: text/html; charset=us-ascii Content-Transfer-Encoding: 7bit Thank you Fred for the informative links and thank you Rick for giving a very good explanation for vortex generation and pointing out that changing your frame of reference can help in  understanding the dynamics. To further illustrate the latter, I will relate a conversation I had years ago regarding actual flight versus wind tunnel testing. "Conversation" is code for argument and engineers hate arguments like pigs hate mud.

My friend was arguing that because the air was moving past a stationary wing it had different dynamic properties,  presumably because of its kinetic energy, than if the air was stationary and the wing was moving through it. I told him of a time during my flight training when the winds at  my airfield were 25 Kts on the ground (same heading as the runway) and 45+ Kts above 500 feet.  My instructor had me put the airplane (C172) in slow flight configuration right after takeoff and the little Cessna climbed steadily with zero ground speed. Above about 500 feet we actually started "backing up" towards the landing threshold end of the runway. I eased of power a little and dropped the nose and, for a few minutes, hovered stationary with zero VSI and zero ground speed. Spooky!

Clearly the airplane was stationary relative to the ground (as in a wind tunnel) and the air was moving past it. I asked my friend "At what point during this flight did the aerodynamic "rules" transition from those of an airplane moving through the air to those of the air moving past the airplane?". I took his silence as agreement that there was no difference.

Air is a fluid and the only way to mechanically act on a fluid is with pressure and pressure is a force per unit area.  Integrating all the pressures on all the surfaces of the aircraft with respect to the gravity vector gives the total net vertical force (AKA lift). If the total vertical force is equal to the mass of the aircraft times the local acceleration off gravity then level flight is achieved. Exactly how are all those pressures generated on all those surfaces? Paul is right in saying that nobody knows and, if we believe Hiesenberg, nobody CAN know.

The overall  net effect on the air (all the air there is) surrounding the airplane is a change in vertical velocity which, if integrated, would obey F=MA.

Saying that "Lift occurs because flow is turned downward creating downwash."  is like saying that milk comes from cartons. While technically correct, it doesn't tell the complete story of how the milk got into the carton.

Regards

Brent Regan
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