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Date: Mon, 24 Jan 2005 10:46:12 -0500
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From: "Gary Casey" <glcasey@adelphia.net>
Subject: Re: [LML] Alert. NACA 64212
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<<1.  Since the ES and IV wings are so different in size and loading, is
this tip stalling effect more pronounced in one plane or the other?
2.  Does CG have any impact on the chart or does it merely impact the
ability to reduce AOA enough to recover once the plane departs?
3.  How much of the wing is considered the "tip"?  Outboard of the ailerons
or including them?
4.  Do the tips stall in advance of the wing root and if so, by how much?
5.  How much effect would some vortex generators ahead of the ailerons have
on this graph?>>

A very interesting subject.  I am by no means an airplane aerodynamics
expert, but I have had some experience with flow in general.  I'm not
surprised that there is a bimodal (two stable, steady state results
available at the same conditions) flow condition after the stall, and I'll
bet that other airfoil exhibit this same effect, perhaps to a lesser extent
and maybe harder for the experimenter to document.  The following comments
and observations are to be taken "in general" as I'm sure there are
exceptions to any of them:

In order to improve the chances for laminar flow over a larger area, The
thickest portion of the wing has to be moved as far aft as possible.  This
provides the larges area where the velocity is gradually increasing and
pressure decreasing, which is the driving condition for laminar flow.  As
soon as the velocity starts to decrease and pressure increase laminar flow
is essentially impossible as the local conditions are disrupting the
boundary layer.  The bad thing about moving the thickest section aft is that
it forces one to reduce the leading edge radius.  And usually a stall begins
at the trailing edge with the high pressure air in the stalled area pushing
forward until it gets to a natural barrier - generally the leading edge.
After the initial break in lift coefficient(CL) the CL  goes down with
increasing angle of attack.  However, once the flow separates right at the
leading edge it is unlikely to re-attach without a dramatic reduction in
AOA, producing the "hysteresis loop."  If the stall is only partial a small
change in AOA will move the point of separation fore and aft in a
progressive and repeatable manner.

So what to do?  I can't answer question 1, but I think, regarding 2, the CG
primarily affects the ability of the tail to push the wing to a lower AOA.
as for 3, I'm sure the wing profile changes gradually, but looking at mine
the transition seems to be essentially at the flap/aileron junction, putting
most or all of the aileron in the 64212 profile.  Regarding question 4, I'm
always a little puzzled at the very slight twist put in most wing designs,
maybe 2 degrees.  I'm not convinced that very small difference will allow
the tip to keep flying after the root stalls.  My reasoning is that as soon
as any part of the wing stalls the lift decreases and the plane starts to
"fall", immediately increasing the AOA of the rest of the wing.  A 2 degree
difference doesn't seem like enough to make a difficult wing magically okay
with regard to stall behavior.  My non-expert opinion is that that adding
vortex generators so distance aft won't help much since they will be
engulfed in the stalled air flow and will have no effect AFTER the stall,
although they will probably tend to prevent the stall initiation a little.
I'm guessing that the vortex generators I see on airliner wings are
primarily there to improve aileron sensitivity, not to change the stall
characteristics.  Putting a vortex generator at the very front of the wing
would likely help the situation (I'm reminded of the Glastar) at the cost of
preventing laminar flow.

Incidentally, I was just looking at the stall warning vane on a Mooney, a
plane that has a similar airfoil.  The stagnation point (the Continental
Divide of airflow, where all the air on one side goes over the wing and on
the other side goes under) at stall for any of these sharp-leading-edge
wings is well behind the leading edge, something like 2 inches or so.  You
can imagine that the air has to wrap itself all the way around that small
radius, so it will let go at some point very suddenly and once gone will not
reattach easily.  A drooped cuff seems to be the accepted cure for this
problem, as done by both Lancair and Cirrus.  During my wing construction I
was tempted to take a mold off the leading edge and the slide it forward on
the upper surface to create my own cuff design, but then I figure I would be
going to far into the "experimental" realm, so I didn't.

Gary Casey