Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Sat, 17 Aug 2002 11:16:23 -0400
Message-ID: <redirect-1689932@logan.com>
From: "Gary Casey" <glcasey@adelphia.net>
Subject: airflow over the wing
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<<I have a question for the aero engineers/test pilots on the List.

The airflow over the top of my wings flows inboard, not outboard, or
spanwise as I have been led to believe is the normal case.  This inward
slant of the flow is indicated by the flow of residual grease from aileron
hinges and traces of lubrication I put on my gas caps.   Both wings are the
same.  The inboard angle is not great, just five or more degrees roughly if
I were to measure it from a point on the leading edge to the trailing edge.

1.  Is this normal for the 320 wing?
2.  Is it a good thing, as in less drag?
3.  Do other flyers notice the same phenomena?
4.  Why is this happening if we are to expect spanwise (outward) flow of air
over the wing?

Are these dumb questions?

George Shattuck
N320GS>>

All very good questions.
1.  Yes, it is normal for any wing.  One way to visualize it is that the
wing holds the airplane up because of a higher pressure underneath - the air
wants to "get out from under" the plane, so flows outward toward the tip.
At the same time the low pressure on top draws air from the tip inward.  The
"spanwise" flow people talk about is inward on the top and outward at the
bottom.  The angle of the flow from the centerline is a rough measure of the
induced drag of wing.  Longer wings will have smaller angles and the angle
will go from zero at the centerline to the largest angle at the wingtip.
The outward flow at the bottom and inward flow at the top meet at the
wingtip, creating the famous wingtip vortex, which is the necessary
byproduct of this flow.  It might be interesting for you to put a tell-tale
(tell-tail?) at a point just forward of the outer end of the aileron and see
if it lays smooth at all flight conditions.  Hopefully the wingtip keeps the
vortex outboard of the aileron or you might get some vibration there.
2.  It actually is a "bad" thing as described above, but is really just an
artifact of the lift.  The angle will reduce at lower angles of attack and
increase toward stall.  The REALLY good sailboat racers will look at the
inner and outer tell-tales on the sail and sail to the angle of the "V"
between them.  One could also use this angle as a pretty good AOA
indication.  Put a tell-tale  at a convenient point and then put marks on
the wing at different airspeeds as an experiment and report back (excellent
homework assignment!).  Also, if you are adding fairings for aileron
bellcranks, etc. it would be best to put them at the correct angle, not
longitudinal.  My aileron bellcrank fairing is angled at a guessed angle
from longitudinal.  This is the reason why winglets should have a little
"toe-out."
3.  Haven't really looked, but it surely is there as this angle is necessary
for the wing to work.
4.  "spanwise" means flow either inward or outward, not just toward the
wingtip.

Sorry about the verbose answer to a simple question, but aerodynamics is a
fascinating subject to me, even though I'm not trained in that technology.

Gary Casey
ES project, maximum spanwise flow to date - 60mph in the back of a truck;
now practicing inverted flight in my garage.