Aerodynamics Videos

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A wing lifts when the air pressure above it is lowered. 

It’s often said that this happens because the airflow moving over the top, curved surface has a longer distance to travel and needs to go faster to have the same transit time as the air moving along the lower, flat surface. 

But this is wrong! 

When this video is paused, it’s clear that the transit times above and below the wing are not equal: the air moves faster over the top surface and has already gone past the end of the wing by the time the flow below the aerofoil reaches the end of the lower surface.

What actually causes lift is the introduction of a shape into the airflow, which curves the streamlines and introduces pressure changes – lower pressure on the upper surface and higher pressure on the lower surface.This is why a flat surface like a sail is able to cause lift – here the distance on each side is the same but it is slightly curved when it is rigged and so it acts as an aerofoil.

In other words, it’s the curvature that creates lift, not the distance.

Fluids flowing near a surface tend to follow the shape of the surface. It is known as the Coanda Effect and its explanation depends on viscosity, the frictional forces between the molecules of a fluid (be it liquid or gas). The Coanda effect is the culprit behind many everyday incidents as well as being the reason why air bends around the wing aerofoil.

When the air bends around the surface of the wing it tries to separate from the airflow above it. But since it has a strong reluctance to form voids, the attempt to separate lowers the pressure and bends the adjacent streamlines above. The lowering of the pressure propagates out at the speed of sound, causing a great deal of air to bend around the wing. This is the source of the lowered pressure above the wing and the production of the downwash behind the wing.

When looking at a typical airfoil, such as a wing, from the side, several design characteristics become obvious. You can see that there is a difference in the curvatures (or camber) of the upper and lower surfaces of the wing. The camber of the upper surface is more pronounced than lower surface, which is usually somewhat flat.  The chord line is a reference line drawn from the center of the leading edge straight through the wing to the trailing edge. The distance from this chord line to the upper and lower surfaces of the wing shows the amount of upper and lower camber at any point. Another reference line, drawn from the leading edge to the trailing edge, is the mean camber line. This mean line is equidistant at all points from the upper and lower surfaces.  Different airfoils have different flight characteristics. The weight, speed, and purpose of each aircraft dictate the shape of its airfoil. The most efficient airfoil for producing the greatest lift is one that has a concave, or “scooped out” lower surface   

The airflow arrives at the kite at the leading edge and divides to either go over or under. When moving underneath the kite, it will be pressed downwards and there will be an increase in air pressure. Airflow over the top surface acts as though it were passing through a kind of bottleneck, similar to a venturi tube, between the free-stream (undisturbed air) and the surface. Therefore its velocity increases and this is accompanied by a corresponding decrease in static pressure.  Since the air pressure underneath the kite is now higher than over the kite, the kite gets pushed or sucked upward towards the lower pressure area.

It is also the downward acceleration of the air that causes lift. The reason we arrive at this conclusion is that an infinitely thin wing would still generate lift, even though the air would remain at a constant speed. i.e. momentum conservation still applies, but Bernoulli's principle does not (or maybe I should say that it has no effect). For example, Paper airplanes fly just fine, and they have extremely thin wings 

The kite and airflow interact to produce lift and drag forces. A resultant aerodynamic force is created that is orientated in the opposite direction to the riders weight. It defines the lines direction and is the force we transmit through our legs to the board, making us able to ride. Tension is the force exerted by the total weight of the pilot, equipment and resistance of the board in the water. The kite needs to pull less than the riders weight (unless you wish to jump) but more than the resistance to the ground or water.

The Angle of Attack is the angle between the chord of an airfoil and the airflow direction. Changing the Angle of Attack influences the way the airflow and the airfoil interact. The greater the angle the greater the lift that is generated. You change this angle by using the bar, the trim system fixed on the center line, or the connection knots on the bridles.

The greater the surface area (kite size) the more power the kite will have. Kite power is proportionate to the projected size of the kite.

The greater the airspeed over the airfoil the more power is generated. If you multiply the velocity by 2, the power will be multiplied by 4.

The greater the air pressure, the greater the power generated. Warm, humid air produces less power than cold, dry air.

An angle of attack can become too large. With too much angle, the drag becomes stronger and the kite is pulled backwards.The result will be a loss of power and change in lift direction causing the kite to back-stall.

There also needs to be a minimum amount of lift for the kite to fly. Reaching too small an angle makes the kite front stall as the airflow on the upper surface can become inactive (zero or negative angle). The back lines are too long compared to the front lines and the kite will not respond to steering. 

 Best trimming is usually to set the angle of attack between 5 and 20 degrees. This is normally the range given by the designer of the kite bar.  

To ensure the kite is correctly trimmed, stabilize it at 10:30 or 1:30, pull the bar all the way down and wait 10 sec. The kite must stay stable and react to steering.

What is backstalling? It happens when there is too much Angle of Attack (AoA). The back lines

are too short compared to the front lines and vice versa). The kite will start to move backwards trailing edge first, lose all its power and crash.

What to do when this happens?

1. Push your bar away to immediately reduce the AoA so the kite starts to fly normally.

2. Then either pull on the trim strap to reduce the front lines’ length or, if already at its

maximum, land the kite and modify the line connection to make front lines shorter or back

lines longer.

How to be sure a kite is trimmed properly?   (Do this each time you launch a kite)

1. Launch the kite and bring it to 11 or 1 o’clock.

2. Pull the bar slowly and carefully all the way down (sheet in at its maximum).

3. Keep this position for 10 seconds.

a. If the kite is stable and you can steer it normally, it is trimmed properly.

b. If the kite backstalls the back lines are too short (or front lines too long).

c. If the kite front stalls or is hard to steer (no steering reaction) the back lines are

too long or front lines are too short.