Trailing edge stuff

[Amir Neshati]

Hi all....below is what I've saved from Goerge Hicks a while back and it's
kinda
relevant to this topic...I know I keep my carpet tassels kinda blunt too ;-)

Enjoy,
Amir......

Hi Everyone,
For the past few years I've had the opportunity to tag on some
testing of various trailing edge shapes during some of the wind tunnel tests
I've conducted.  A few of the trailing edge devices I've examined were:
squared-off edges, gurney flaps, & T-strips ....They all work on the same
principle.  Here's some of the things I've found.

1- (Trailing Edge fixes work better at low Reynolds numbers(low-airspeeds,
small chord lengths)) ...This is why aerodynamicist often refer to these
trailing edge fixes as "Tunnel Tricks".  If air had no viscosity it wouldn't
matter too much what type trailing edge shape you used.  The viscosity of
air causes a layer of slower air to form very near to airplane's
surface....this layer of retarded flow is referred to as the boundary layer.
The lower the Reynolds number the greater the boundary layer's thickness
will be...most of these trailing edge devices act as a boundary layer siphon
which virtually pumps away the boundary layer at the trailing edge.  This
thinning of the boundary layer often makes the aerodynamics much more linear
and often reduces drag during high-lift situations.

2-(Aerodynamic effects)...When you measure the lift and drag of a
squared-off trailing edge airfoil section and compare it to a sharp trailing
edge you basically find two major effects.

First you find that the squared-off trailing edge linearizes the lift curve
slope around zero angle of attack.   The lift curve is simply a plot of how
lift changes with angle of attack and is typically linear until the airfoil
section stalls.  FWIW, It is very common to have joggles in the lift curve
near zero angle of attack on most symmetrical airfoils when tested at low
Reynolds numbers because of boundary layer separation near the trailing
edge.  This is one of the reasons you can play around with wing incidence
(to a small degree) on pattern airplanes and fix the downlines...on the
giant-scale IMAC ships the Reynolds number is quite a bit larger and this
effect isn't as noticeable.

The second thing you see is that the maximum lift coefficient on the
squared-off tip is typically a little higher than the sharp trailing edge.
The reason for this is because the pumping action seen at the trailing edge
delays flow separation on the wing's upper surface to a higher angle of
attack.  Since the wing can now operate at a higher angle of attack without
flow separation occurring, the lift generated increases.

A small side effect of the squared-off trailing edge is to reduce drag at
high angles of attack.  This occurs because the wing's wake is smaller due
to the boundary layer thinning effect mentioned above.  In IMAC we really
aren't that concerned with drag though.

3-(Control Surface Effectiveness)- This typically increases because of all
the reasons mentioned above.  Is this better???  Who knows, from a handling
qualities point of view....some pilots like it, some don't... some can't
tell the difference.  Either way you can dial in the feel you prefer with
your radio.

4-(Hinge Moment Increase)-  One thing you don't hear about much is the
increased servo torque required to deflect a control surface that has a
squared-off trailing edge.  When you make the trailing edge more effective,
by squaring it off, you increase the overall loading which the trailing edge
of the wing can produce.  Since the trailing edge is the furthest point from
the hingeline, a small change in the trailing edge loading can cause a
noticeable effect on the hingemoment.  Can't get something for nothing :o)


5-(Flutter prevention??????)   Maybe not, There is no reason why the
squared-off trailing edge should be considered as a flutter suppression
device.  The squared-off trailing edge actually creates a vortex whose
strength and frequency is a function of airspeed and angle of attack... this
vortex could actually promote flutter.  Either way flutter is much more
dependent on the mass and moment of inertia of the control surface and the
natural frequency of the control surface and attachment structure than on
the frequency of the aerodynamic forcing function.  Any airplane will
flutter if the airspeed gets high enough because the magnitude of the
aerodynamic forcing function grows proportional to the airspeed
squared...The only thing that is a sure-fire cure for flutter is mass
balancing...if you don't like to mass balance make sure your servo gears can
handle the aero loads without stripping...therefore use metal gears on big
birds...

George R. Hicks
Sino Swearingen Aircraft Corporation
 (Aerodynamics/Performance Group)

1770 Sky Place Blvd.
San Antonio, TX   78216
(210) 258-6052

Visit our website at:
www.SJ30jet.com



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