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How do kites work or... why do Leading edge inflatable (LEI) kites fly?

Anybody who tells you they completely understand the aerodynamics of a
leading edge kite (or any wing for that matter) is lying. The
understanding of why they work and how to design them better is, however,
increasing all the time. One complicating factor with LEI kites is that
they are aero-elastic structures. This means that the aerodynamic loading
on them is what determines their shape ! This means that for every wind
speed, every weight of rider, every line set-up, every small change, the
kite will be a different shape, and in fact is changing shape CONSTANTLY
while you ride. In stiff wings such as aircraft wings the wing doesn't
change shape (well it does, a little, but a lot less than an LEI or
ram-air foil). When kite designers design LEI's they have to design an
ideal shape for a stiff wing, and then use their knowledge of flying to
predict what those changes in shape will mean in the flying performance of
your kite. Aerodynamic computer models are not yet powerful enough to
design for these aeroelastic effects.

Some people might think that the ribs in an inflatable leading edge kite
determine the airfoil, or aerodynamic profile, of the kite. This is only
slightly true.

Where the kite has ribs, the canopy is constrained to taking the shape of
those ribs. Everywhere else, it is the cut or shaping of the panels in the
kite that determine the airfoil.

It's a strange airfoil isn't it?

Yes it is! The large tube that is the leading edge is convenient (it is
simple to make inflatable tubes) but it is not ideal in a wing sense.
There is drag created immediately behind the leading edge, and that
circular section is only an approximation over what you would really like
the shape of your airfoil to be at the nose of your kite. At low airspeeds
such as that in kite flying however this drag is not terrible and that is
why these kites work so well. At higher air speeds they would be terrible
wings.

So what happens where there are no ribs in the canopy?

When air passes over an airfoil, a low pressure area is created above the
airfoil. It is this low pressure air that sucks, or lifts, the kite up,
creating what we know as lift. The amount of lift is determined by the
speed of the air passing over the airfoil, and the shape of that airfoil.
In a fixed or stiff wing such as an aeroplane, all of this low pressure
creates lift. In a soft wing such as a LEI, the low pressure also lifts
the fabric, effectively inflating the canopy of the kite, giving it the
airfoil shape.

So why have any ribs at all?

In theory you can build a kite with no ribs at all, for example something
like the NASA-wing. The airfoil sections however are not optimal and the
fabric is drawn from all areas of the kite and makes a minimal energy
surface, in the same way a drop of water on a leaf does. This tends to
make everything spherical like that droplet. This is why nasawings have so
many bridle lines - to constrain the shape of the kite. What inflatable
ribs in LEI kites do is stretch the fabric along the length of the rib,
and therefore only allowing the canopy to develop it's profile along the
wingspan of the kite. The more ribs there are in a kite, the more
constrained the fabric will be between the ribs and the closer it will be
to the profile selected in a rib. Ribs however add weight, and to a lesser
degree add drag, to a kite. Part of the challenge in designing a kite you
want to fly is finding just the right number of ribs, with just the right
profile, and putting them in the most important parts of the wing.

Then where do I want the ribs to be?

We haven't yet talked about angle of attack. This is the angle at which
the airfoil profile attacks the wind. What a kite surfer does when they
pull in their control bar is change the angle of attack by pulling the
trailing edge of the kite towards them. If you look at the U-shape of the
kite however, and think about what happens when you do this, you increase
the angle of attack much more dramatically in the centre of the kite than
in the wing-tips of the kite. Up to a point increasing the AOA increases
the power in the wing, and the stability, it does however reduce the wing
speed, and increase the drag. This is why the extra power lasts for a
little while, but then slackens off as the kite slows down. Anyway, the
AOA is increased more at the centre of the kite than at the tips. At the
tips there is less lift for a number of reasons, partly because the
airfoil is shorter in the tips, partly because on shorter airfoils that
round leading edge is a worse approximation for a wing. There is also less
AOA at the tips. This means that there is less air-pressure sucking the
canopy into shape in the tips. That is why it is advantageous to put more
of your ribs closer to the tips and closer spaced at the tips.

So that is why we choose to have no central strut. By removing the centre
strut, we can have more struts at the tips where they count more. The
airfoil in the top central section of the kite is determined by air
pressure, as it is in the sections on either side of the central rib in a
traditional LEI. In fact, with good design, we can make the airfoil
section change in the right ways to give you more lift and more depower in
that centre section of the kite where the foil area is larger, and more
important in determining the amount of power in your kite.

But does it cause jellyfishing?

Jellyfishing is a complicated phenomena, and can be cause by a number of
things. Firstly the kite needs to have a reasonably stiff leading edge
that determines the arc shape of the kite. The stiffness of this arch
(consider it a beam) is proportional to the diameter of that leading edge.
A small increase in diameter makes a much stiffer arch as the stiffness of
the beam is proportional to the diameter more than the stiffness. In fact
the pressure in the tube has a very small effect compared to the diameter.
The junction where a rib joins a kite makes a negligble difference to the
stiffness at that connection point and can in fact weaken the arch at that
point. Jellyfishing is caused more by the wing-tips oscillating between
stall and non-stall due to their angle of attack. Again, this is why more
ribs closer to the tips is more important. There are at least two other
factors contributing to jellyfishing, one is the centre of the lift of the
profile and where it sit relative to the attachment points, the other is
the unloaded shape of the arch versus the loaded shape, ie the curve of
the arc sitting on the beach compared to what it is in the air. If the
difference is too great, the kite will oscillate between the two shapes,
particularly during gusty or stronger winds."