[aerohead]

I ran across a VG-related article from KITPLANES,Vol. 15,No.4,April,1998,Pgs 60-67, by Terrence O'Neill.
O'Neill had been in the market for a safe aircraft for he and his wife.
He narrowed the field to a Viking Dragonfly,tandem-wing,with low canard,high rear-wing.He added VGs to the canard after re-reading a 1993 Barnaby Wainfan article in KITPLANES on tandem-wing aircraft.
Piper Advanced Technology-1 designer George Meade was killed in this aircraft after experiencing what NASA termed a 'trim' (would remain stalled) event if C.G. was too far aft and power on.Prop blast held the canard at the stalled angle of attack.It was non-recoverable.
O'Neill had installed a set of Bacon-Saver angle of attack (AOA) indicators on his Dragonfly.On his first flight he experienced no funny business up to 12-degrees AOA,then at 13-degrees AOA the canard went bobbing into partial stall.
I've no data on the Dragonfly canard but I looked at a Clark Y I have data on.
At 0-degrees AOA,its upper contour goes to 11-degrees down-slope.
At 12-degrees 'clean' flying,the upper surface is at 23-degrees,Rolf Buchheim's recommended slope maximum for automobiles,and 1-degree steeper than what W.A.Mair has recommended.
As it happens,one of my fluid mechanics texts has a 3-photo montage of a Clark Y 'flying' in the wind tunnel.
*@ 12-degrees AOA there is perfect flow attachment on top over the 23-degree max. slope.
*@ 17-degrees AOA,88% of the wing is in separated flow with no re-attachment.
*@ 19-degrees AOA,the wing is stalled over the entire upper contour.
All this in spite of the presence of VGs.
On the You-Tube video from the old NACA film you can watch the stall progress as a function of AOA.
PS I found the image in the phil knox photo albums under Book Illustrations,page-1,line-5,far right image.(needs to be rotated)