Trailing edge stuff

[John Ferrell]

Now that is what I wanted to know...!

----- Original Message -----
From: "Amir Neshati" <amirneshati at earthlink.net>
To: "NSRCA" <discussion at nsrca.org>
Sent: Tuesday, April 15, 2003 8:48 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
>
>
>
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