There is one vortex generator on every rib and false
rib except the most inboard rib. The forwardmost vg measures 10.5"
back, the next measures 11.5" back, and the rearmost measures
13" back. This staggered distance pattern repeats itself
out to the last available rib. (All measurements are from the
centerline of the leading edge to the front edge of the vg, measured
with a thin flexible measuring tape - different than we did the
MKIII. Actual placement was by stretching a string from tip to
tip and marking them from the straight line)
During flight tests we tried various locations,
both forward, rearward and staggered. No matter where we put the
vg's, they worked at least to some extent to make the stall slower,
the climb better, and the handling just above the stall much better.
We even came up with some very unconventional arrangements, but
this placement works very well overall.
Climb with vg's is 200' a minute better than without,
slow flight at 40 mph is good, you can do brisk coordinated turns
and the airplane does not feel like it is on the edge, it feels
solid. Power off stall is around 35 mph with a clean, gentle break
when it lets go, with a normal recovery by easing the stick slightly
forward and letting the airspeed increase to 40 mph. Power on
stall at 4500 rpm is slightly slower, at around 32 mph, and typically,
it will sort of nod in and out of a stall, if you hold the stick
still it just bobs along, breaking and then catching itself with
no tendency to fall off to either side. With the stick further
back, the stall break is clean and recovery is good. Stall in
a coordinated turn is straightforward, no tendency to fall off
to either side. Have not done any cross control stalls, because
I have no spin training, and I'm sure it would.
The biggest plus seems to be that it now has that
elusive quality that is hard to describe, but you know it when
it's there. Without the vg's, it flew good. Now it flys sweet.
The results are well worth the trouble.
Update - April 2011
Following the complete rebuild of N582EF, we decided
to try STOLSPEED VG's this time around, and are entirely satisfied.
Following guidelines from the company's website, we put them at
10% of the chord, or 6" back from the leading edge. Stall
speed dropped 5 mph, and slow flight just above the stall is very
comfortable. Handling is good, stalls are gentle and the airplane
recovers with just a slight forward stick. Based on the simplicity
of simply buying and installing these, (plus not having to worry
about slicing a finger when washing the wing) along with the improvement
in looks and a bit less weight, I would never again consider making
my own VG's.
VG's are 6" back from the leading edge, measured
to the front of the VG. Here is the tool we used to determine
the 6" chord distance:
Recently got an email asking the question - why
were the VG's on my MKIII at 10" back, the original vg's
on this one between 10.5 and 13" back, and these ended up
at 6"? I suspect that it was because our home made VG's were
a lot bigger than the Stolspeed vg's. Here is a copy and paste
from the Stolspeed web page:
The VGs need to be positioned
such that they have the best ‘bite’ at the airflow when
the wing is at the stall angle of attack. If they are too far
aft they risk being buried in the thickening boundary layer and
the start of separation, so they lose effectiveness at stall.
If they are too far forward they are reputed to slightly increase
drag at cruise, but I have never been able to meaure such - better
to be farther forward than too far aft. So, with the wing positioned
at the stall angle of attack, the VGs should be slightly forward
of the highest point of the airfoil.
Historically, the margin
is considered to be 8-12% of wing chord back from the leading
edge to the highest point of the VG, with most experience settling
on 10% as the most appropriate. There’s lots of talk around
the hangar fliers of finding the ‘sweet spot’, but my
experience doesn’t show any dramatic difference at any one
point. My testing indicates that 15% can be too far back, so
that at a very high angle of attack the VGs can become ineffective.
From 8-12% I could detect no difference in stall performance
at all, and couldn’t measure any change in drag at cruise
speed.
Those figures are to the
highest point of the VG, but since it's easiest to measure to
the front tips of the VGs, I recommend starting with the tips
at 7% of chord aft of the leading edge - this puts the highest
point of the VG at about 9%. We use the measurement to the tips
of the VGs to mark the line on the wing because it's easier to
place the VGs accurately to this line. If you try other positions
and get significant results with your aircraft, please let us
know so that others can benefit.
So for the FSII, we figured thusly: 6" on a
60" chord wing is 10%. 10" on a 60" chord wing
is 16%. But, since my home made VG's were twice as big as the
Stolspeed plastic ones, it is somewhat of an apples and oranges
situation. Which is why we went more with what Stolspeed recommended,
and moved them further forward, because as their web site says,
better to be a bit too far forward than too far aft. This would
probably apply to any store-bought VG's that are smaller than
our home made ones. Assuming that any brand of store-bought vg's
are small like the Stolspeed ones, I would guess that 10"
to the aft edge would probably be too far back. Before we put
them on, Ed & I were discussing it, and I thought 7",
he wanted 6". Since it is his airplane we went with 6"
and they worked out fine. Would 7" have worked as well? No
opinion. But I do find myself wondering that perhaps big home
made vg's are not going to behave the same as small store bought
ones and anyone who buys small vg's and puts them 10" back
could find themselves very annoyed.
