Originally Posted by adouglas
They're called vortex generators and they're a whole different ball of wax. Their purpose is to re-energize a boundary layer that has become turbulent.
I'm talking about surface contamination at the leading edge, such as might be caused by ice, bugs, or even a transverse paint stripe. Or surface roughness or waviness. We've all seen spam cans with wavy surfaces, dinged up leading edges, etc. etc. etc. and they fly just fine without serious degradation of performance.
I'm sure I've seen stuff about how important it is to get the leading edges of your airfoil clean, smooth and perfect, or degradation of performance will occur.
But this seems to be an area of debate, so I decided to do some digging. I did a couple of searches in my PDF of compiled Canard Pushers and found some references to the importance of having a very smooth canard surface, but they specifically refer to the GU canard. I don't know if the Roncz canard on the MkIV is less susceptible to surface roughness. I also found reference to paint trim on the leading edge tripping the laminar boundary layer and causing speed losses of as much as 11 knots. Tripping the boundary layer on a conventional airplane will have an effect, but not that much. And conventional aircraft do not display any trim change with leading edge contamination (not just rain), which canards do, to one extent or another. This is common knowledge. This might be where I got the idea that canard aircraft are a lot more sensitive to leading edge contamination than spam cans.
Anyone have anything useful to add?
From my experience with my dragonfly(a derative of rutan's quickie), with the GU airfoil that was chosen for the canard, many, but not all were succeptable to contamination and moisture in that they would loose lift and require more back elevator to remain at constant altitude. Something as small as a spanwise paint line had been known to trip the boundry layer with bad effects. Vortex generators were added to the canard, much further back than would be used to increas lift, to reattach the airflow and make the GU airfoil handle normally with contamination (such as rain).. as a matter of fact, without the VGs, a pilot could tell when he/she would be in rain before any moisture was visible because the canard would loose some lift and require the appropriate stick movement. The VGs eliminated this problem.
It was my understanding that the Roncz airfoil switching from the original GU on the EZ/CZ canard was to eliminate that problem.