REFLECTOR:Vortex Generators on XL

[Tom Martino]

This is by far the best explanation I have read to date.  Bravo!

Now the 64-thousand dollar question ... how many and where should they
be placed on the Velocity?



-----Original Message-----
From: steve korney [mailto:s_korney@hotmail.com] 
Sent: Sunday, December 07, 2003 2:48 PM
To: reflector@tvbf.org
Subject: RE: REFLECTOR:Vortex Generators on XL

Maybe this will help to answer some questions...Note that VG's are a 
BAND-AID for inproper design.

If you've ever opened up the stopper in a sink full of water and watched
the 
water swirl down the drain, you know what a vortex is. Simply put, it is
an 
energetic swirling mass of fluid. Vortices are quite common in
aerodynamics. 
Probably the most well-known and significant of these are the trailing 
vortices that are seen coming off the tips of wings in flight. These 
vortices are not desirable because they create a type of drag known as 
induced drag, or that drag induced by a surface generating lift (like a 
wing). Aerodynamicists often spend considerable effort trying to reduce
the 
adverse effects of such vortices. However, vortices very similar to
trailing 
vortices can also be used to produce beneficial effects, and one of the 
methods used to create beneficial vortices is the vortex generator. In
previous questions, we have discussed the concept of flow separation. 
When an aircraft flies at high angles of attack, the airflow over the
wing 
can become detached, or it stops following the shape of the wing. When
this 
happens, the lift produced by the wing will suddenly and rapidly
decrease, 
and the wing is said to be stalled. When the flow separates from the
wing, 
it usually means the air is moving too slowly, or there isn't enough
energy 
in the flow to keep it moving. Since vortices are energetic, they can be

used to put energy back into the flow to keep it moving in the desired 
direction. This is what vortex generators are designed to do.

Vortex generators are simply small rectangular plates that jut above the

wing surface. They look like tiny little wings jutting up perpendicular
to 
the wing itself. As air moves past them, vortices are created off the
tips 
of the generators just like the trailing vortices mentioned earlier.
These 
vortices interact with the rest of the air moving over the wing to speed
it 
up and help reduce the possibility of separation. 

Ineffective control surfaces: The separation problem becomes even more 
significant since control surfaces like flaps and ailerons are usually 
located along the trailing edge of a wing. When the flow seperates from
the 
wing, these control surfaces have little or no air flowing over them and

they become ineffective. Thus, not only will the aircraft lose lift when
the 
wing stalls, but the pilot may not be able to control the orientation of
the 
aircraft. In this case, vortex generators are often placed shortly
before 
the control surfaces to create a faster flow of air over the surfaces
making 
them more effective. 

Vortex generators are not the only method used to delay wing stall. Wing

fences, thick trailing edges, dogtooths or sawtooths, drooped leading
edges 
or slats, and leading-edge notches produce similar effects. Each method
has 
its drawbacks, most notably increased drag, and they are typically only
used 
as a last resort when re-designing the entire wing is just not
practical. 
For these reasons, they are sometimes referred to as the "vacuum
cleaners of 
the aerodynamicist" since they are used to clean up after previous
mistakes.




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