REFLECTOR: VGs

[Al Gietzen]

Tom:
Are you using the same VGs on the canard as on the wings?  The boundary
layer at the position of the VGs is rather thin, and I think the factory VGs
(I guess about 5/16" tall) should be adequate.  Anything taller will add
drag, and the faster you go the thinner the BL and the more the drag.  For
the canard the VGs can be shorter still, I'd say 1/4" max.

On either airfoil, the VGs will reduce the stall speed by keeping the flow
attached to a higher AOA.  The VGs are closer together on the canard because
they are closer to the control surface so the vortices don't spread as far.
The placement should be roughly a bit forward of the point of tangency with
a horizontal line. 

I think the benefit of VGs, especially on the canard, can vary with the
setup of the plane; e.g., the incidence angle differential.

Al

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Tom
Sent: Wednesday, April 15, 2009 8:36 AM
To: reflector at tvbf.org
Subject: REFLECTOR: VGs

I just installed the Stolspeed VGs via the Velocity Factory templates. 
Flew this morning to test but we had a low ceiling and I couldn't get up 
high enough to do a stall series. However, first impression was not 
good. At the same power setting I generally use there was a 7 knot 
decrease in airspeed. Takeoff broke ground 3 knots lower and my approach 
speed was 4 knots slower. This seems to be way off of other folks 
experiences. Some possible issues I see:
1) the Stolspeed VGs are more streamlined than the factory VGs but they 
are also about 1/3rd taller
2) the canard is just way too busy with VGs-having 32 of them on a 
fairly small surface--additionally they appear too far forward.
The canard VGs have me baffled. Why are they needed at all?? Don't I 
want the canard to stall first anyway? Looks like way too much drag for 
minimal gain as far as the canard is concerned. Tom
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