Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml@lancaironline.net
Date: Sat, 26 Jun 2004 20:56:01 -0400
Message-ID: <redirect-227840@logan.com>
From: tednoel <tednoel@cfl.rr.com>
Reply-To: tednoel@cfl.rr.com
Subject: Rationale for flap fences
User-Agent: KMail/1.6.2
MIME-Version: 1.0
Content-Disposition: inline
Content-Type: text/plain;
  charset="us-ascii"
Content-Transfer-Encoding: 7bit

To all,

The flap fences were my idea (It's my airplane!).  In researching various 
aerodynamic ideas, I ran across a NACA paper 
(http://techreports.larc.nasa.gov/ltrs/PDF/1999/mtg/NASA-99-swing-mgm.pdf) 
that talked about fences for the underside of flaps. The major idea was to 
reduce noise at flap deployment on transport category A/C. But it triggered 
some thinking.

Winglets function by harnessing the wingtip vortex to create a functionally 
longer wing with a lower aspect ratio. Winglets can be on the upper or lower 
surface of the wing. They work best at low speeds and low Reynolds numbers. 
At best, at cruise, they generally disappear aerodynamically.

The flap fences as originally conceived were nothing more than winglets on 
flaps. They come into play when the flap is at low airspeed and high angle of 
attack. Thus, they are fundamentally no different in concept than the 
winglet. It's just on a different wing. 

One of the issues in the L-IV is flap area. We would like to have as much as 
possible to reduce stall speed on approach as much as possible. But doing 
that would reduce aileron area, so a compromise was reached in design. By 
applying fences to the lower surface (winglets) we hope to add to the 
effective area of the flap. This should then lower stall speed in the dirty 
configuration.

 If you look carefully at the outboard edge of the flap, you will see that the 
fence is not at the absolute edge, since that would place it out of the 
streamline in the retracted position. 

We haven't flown, so much remains to be discovered. Stay tuned...

Ted Noel
-- 
PK