Trailing edge stuff

[wgalligan]

Amir,
You just tickled my tassle...   ;-)'

I like my square edges.

Wayne


-----Original Message-----
From: Amir Neshati <amirneshati at earthlink.net>
To: NSRCA <discussion at nsrca.org>
Date: Tuesday, April 15, 2003 7:44 PM
Subject: Trailing edge stuff


>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
>
>
>
>=====================================
># To be removed from this list, send a message to
># discussion-request at nsrca.org
># and put leave discussion on the first line of the body.
>#
>

=====================================
# To be removed from this list, send a message to 
# discussion-request at nsrca.org
# and put leave discussion on the first line of the body.
#