To start off with, why are they multiple element
wings. If you've watched my video on vortex
generators, and by this point you probably
should have, you'll know that flow separation
occurs on surfaces when they exceed a critical
adverse pressure gradient. Now, if we consider
a wing, we can see that as this tilts up and
approaches stall this is going to start separating
off the back. Yo're going to end up with flow
separation here, and that's going to get ripped
off untill theres massive level stall and
all of this is ripped off once this is at
a big enough angle. Now we can delay the separation
by energising the boundary layer (see the
vortex generator video) and bassically any
method you use for that will allow us to push
the wing further without it stalling. You
can put vortex generators here to do that.
But where's there an area of high energy flow?
Well on the top it's getting pressurised by
the energy of the oncoming flow right? Well
what about if we were just to bleed a little
bit of air off the top and bring it to the
back. We put a little passthrough here, and
we can se now that's going to energise this
boundary layer here, and from that we can
see the flow will be more attached. But that's
not a great proifle, it's got sharp edges,
it's going to be sensitive, not generally
good for flow. SO instead lets round off the
edges a little bit, so we'll just round that
off, and that off. And we're now getting something
that looks a little more like a multi element
wing, but now let's try to extract a little
more from it. So, we use that extra energy
of the high area to act as if we're starting
a new wing at a new angle of attack out from
scratch. So, we'll put a new element starting
out from scratch. And this is a multi element
wing, and this is the reason why multi element
wings exist, because you can take the high
energy flow off here and use it to keep the
flow attached up there. And this is why at
low angles of attack, you do not need a multi
element wing, you only need it when you are
trying to push much harder.
