Return-Path: Sender: (Marvin Kaye) To: lml@lancaironline.net Date: Mon, 30 Aug 2004 12:14:55 -0400 Message-ID: X-Original-Return-Path: Received: from hagus.bright.net ([209.143.0.74] verified) by logan.com (CommuniGate Pro SMTP 4.2) with ESMTP id 384760 for lml@lancaironline.net; Mon, 30 Aug 2004 11:21:41 -0400 Received-SPF: none receiver=logan.com; client-ip=209.143.0.74; envelope-from=airmale@bright.net Received: from bright.net (paul-bryn-breeze-122.wireless.bright.net [216.201.20.123] (may be forged)) by hagus.bright.net (8.12.10/8.12.10) with ESMTP id i7UFKS5N025514; Mon, 30 Aug 2004 11:20:49 -0400 (EDT) X-Original-Message-ID: <413345AD.80407@bright.net> X-Original-Date: Mon, 30 Aug 2004 11:20:13 -0400 From: J H Webb User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.4) Gecko/20030624 Netscape/7.1 (ax) X-Accept-Language: en-us, en MIME-Version: 1.0 X-Original-To: beech1@airmail.net, Lancair Mailing List Subject: VG's Content-Type: multipart/alternative; boundary="------------060206030909020404080100" This is a multi-part message in MIME format. --------------060206030909020404080100 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Hi Michael, In my position as Chief Engineering Test Pilot for a Major GA manufacturer we tried several ways to lower stall speed. This can be very important beyond what may come to mind. The stall speed controls the maximum useful load that a multiengine plane can handle. This is a restriction in the regulations that limits the maximum Gross weight (and useful load indirectly) by specifying a minimum Rate of climb at 5000 ft MSL. This rate of climb is obtained by a complex formula based on stall speed. i.e. higher stall speed lower gross wt. Understanding the relationship of stall and load carrying capacity shows the best economic advantage of vg's wing cuffs and etc. That being said, we tried many various attempts to lower stall speed without adding additional wing and found the some were effective in lowering stall speed BUT all of these caused a reduction in cruise speed in twin engine airplanes of between 8 to 15 mph. The only system that did not reduce speed but required additional HP was active BLC (actively sucking down the Boundary layer through small holes in the wing). If you took the power from the engine and used BLC in cruise there was a trade off of how much drag reduction and how much power used. Too much power to BLC and too little drag reduction equals speed reduction. But after all of those words I guess that you must realize that vg's will reduce your cruise speed. Looking at this in a practical way you know if you lower your landing gear the airplane slows down or anything extended into the airflow over the airplane will slow it down. VG's are really very little speed brakes. Recently I flew a C310 several times before and after the addition of vg's and it is slower by about 10 kts, but it will carry more load. If you don's have a flight test boom and very accurately calibrated airspeed and static system, the best way to compare changes is to do passing times. This is to take a base airplane that you do not change and compare the changed airplane to this standard. This can be a friends airplane. You fly side by side and with a stop watch measure the time for the fastest or slower airplane to pass at MCP with props synced by looking through your prop and syncing it with the other planes prop. This produces identical RPM's for both airplanes and with clean airfilters the power should be the same. With a turbo charged airplane you must use a very accurate MP gage and carefully check the induction system for leaks. By using this system you can measure very small changes and see easily the speed increases or decreases. You can easily see 1/2 kt changes (very long passing times). The bottom line is there is not anything for nothing. Passive BLC is your best bet. i.e. removing bugs and clean airplane as possible for speed. vg's, slots, slats, and etc. are for slow speed. Jack Webb L 360, L IV ATP, CFI Airplanes & Instruments, Multi, & Sea Many Type Ratings Experienced Experimental Test Pilot both multi and single engine Former Chief Engineering Test Pilot for a Major Manufacturer AeroSpace Engineer BSAE Ohio --------------060206030909020404080100 Content-Type: text/html; charset=us-ascii Content-Transfer-Encoding: 7bit Hi Michael,
    In my position as Chief Engineering Test Pilot for a Major GA manufacturer we tried several ways to lower stall speed.  This can be very important beyond what may come to mind.  The stall speed controls the maximum useful load that a multiengine plane can handle. This is a restriction in the regulations that limits the maximum Gross weight (and useful load indirectly) by specifying a minimum Rate of climb at 5000 ft MSL. This rate of climb is obtained by a complex formula based on stall speed. i.e. higher stall speed lower gross wt. 
    Understanding the relationship of stall and load carrying capacity shows the best economic advantage of vg's wing cuffs and etc.
    That being said, we tried many various attempts to lower stall speed without adding additional wing and found the some were effective in lowering stall speed BUT all of these caused a reduction in cruise speed in twin engine airplanes of between 8 to 15 mph.  The only system that did not reduce speed but required additional HP was active BLC (actively sucking down the Boundary layer through small holes in the wing). If you took the power from the engine and used BLC in cruise there was a trade off of how much drag reduction and how much power used. Too much power to BLC and too little drag reduction equals speed reduction. 
    But after all of those words I guess that you must realize that vg's will reduce your cruise speed.  Looking at this in a practical way you know if you lower your landing gear the airplane slows down or anything extended into the airflow over the airplane will slow it down.  VG's are really very little speed brakes.
    Recently I flew a C310 several times before and after the addition of vg's and it is slower by about 10 kts, but it will carry more load.  If you don's have a flight test boom and very accurately calibrated airspeed and static system, the best way to compare changes is to do passing times. This is to take a base airplane that you do not change and compare the changed airplane to this standard.  This can be a friends airplane. You fly side by side and with a stop watch measure the time for the fastest or slower airplane to pass at MCP with props synced by looking through your prop and syncing it with the other planes prop. This produces identical RPM's for both airplanes and with clean airfilters the power should be the same.  With a turbo charged airplane you must use a very accurate MP gage and carefully check the induction system for leaks.
    By using this system you can measure very small changes and see easily the speed increases or decreases.  You can easily see 1/2 kt changes (very long passing times).
    The bottom line is there is not anything for nothing.  Passive BLC is your best bet. i.e. removing bugs and clean airplane as possible for speed. vg's, slots, slats, and etc. are for slow speed.

Jack Webb
L 360, L IV
ATP, CFI Airplanes & Instruments, Multi, & Sea
Many Type Ratings
Experienced Experimental Test Pilot both multi and single engine
Former Chief Engineering Test Pilot for a Major Manufacturer
AeroSpace Engineer BSAE
Ohio



  
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