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.