How does a kite surf fly?

Are you wondering How does a kite fly? On this page you could easily understand how a kite fly and learn the the kite is very similar the to the wings of an airplane.

Index

I. How Wings Lift the Plane: how an airplane flights

Airplane wings are shaped to make air move faster over the top of the wing.

Lift of an airplane (itesurf) Bernoulli Equation
Lift of an airplane (itesurf) Bernoulli Equation

When air moves faster, the pressure of the air decreases (Bernoulli’s equation simlpified:  P + 1/2 * V^2 / ρ = Constant; P = Pressure, V= Velocity, ρ = Density of fluid) .

On the top of the wing the velocity of the air is more than the velocity in the bottom of the wing. For the Bernouilli’s equatuion that means that the pressure on the top of the wing is less than the pressure on the bottom of the wing.

The difference in pressure creates a force on the wing that lifts the wing up into the air.

forces of flight | forces acting on a airplane
forces of flight | forces acting on a airplane

Forces acting in a airplane during the flight are 4:

  • Lift – upward
  • Drag – backward
  • Weight – downward
  • Thrust – forward.

To have more infos on How an aiplane flights and How to control the Flight of a Plane, look at this NASA link.

II. Differences between an airplane and a kite: How does a kite fly?

Like an airplane wing, a kite can fly due to various forces acting on it.

The main differences are that an airplane has thrust while a kite has line tension and an airplane is balanced by its weight around its Center of Gravity (CoG) while a kite is balanced by its effective tow points (which can be adjusted automatically by the kite or manually by the kiter) and its weight at CoG (center of Gravity).

The forces and torques that act in a kite and how they are acting:

  • Wind Generated Forces
  • Gravity Force
  • Line Tension

To have a better idea about force acting on a kite, read the post Forces and torques on a kite.

III. Kite Design Parameters

The most easy to manipulate and highly visible kite parameters are:

  1. Aspect Ratio (AR)
  2. Airfoil Profile
  3. built-in Angle of Attack (AoA) of the kite
  4. Summary of the Aspect Ratio, Airfoil, AoA parameters.

1. Aspect Ratio

Airplane Wing Geometry Definitions | Aspect ratio, chord Kitesurf
Airplane Wing Geometry Definitions | Aspect ratio, chord Kitesurf

Aspect Ratio (AR) is approximately Span/Chord of the kite or more exactly Span*Span/Area (see more info about a Wing Geometry Definitions at this NASA link).

Since Aspect Ratio determines the shape of the kite it is the most visible kite design parameter that the user will see.

Higher Aspect Ratio kites have less induced drag (upwash and tip vortex effects)  than Lower AR kites of the same characteristics. Induced drag is inverse proportional to AR.  So when stationary at the wind window, a low AR kite can generate the same amount of pull as a higher AR kite (of the same characteristics) but as soon as we need to move the kite for more power (for jumping or underpowered situation), a higher AR kite can accelerate faster therefore get more power sooner than a low AR kite.

As a rule of thumb, a higher AR kite has a larger Power Window (the difference between min power and max power) and a lower AR kite has a smaller Power Window.

Following are the recommended AR ranges:

Kite Type Very Low AR Low AR Moderate AR High AR Very High AR
Foil 2.5- 3 4 5 5.5+
Inflatable / Arc 3- 4 5 6 7+

Note: Inflatable and Arc have spherical shape, a natural stable form, therefore their ARs are normally higher than foil’s.

2. Airfoil Profile

Airfoil has lift but also drag.

A profile with the highest lift when stationary will give the strongest pull when stationary at the wind window (AoA around 5 degrees).

A profile with the highest lift/drag ratio will accelerate faster and will generate strongest pull when flying across the power zone.  A high lift airfoil is sometime labelled a “tractor” airfoil as it will pull like a tractor at the wind window.

A high lift/drag airfoil is labelled a “speed” airfoil as it flies very fast across the power zone and generate tremendous amount of pull while doing so.  A speed airfoil may generate a lot of pull at the wind window but may not be necessary as much as a tractor airfoil.

The following table show the recommended lift and lift/drag ratio ranges:

Very Low Low Moderate High Very High
Lift Coefficient (at AoA = 5) 0.5- 0.7 0.9 1 1.1+ (Tractor)
Lift/Drag 40- 50 70 90 110+ (Speed)

Please note that these Lift/Drag ratios are the calculated ratio and not included Induced Drag.  In reality, the “real world” L/D ratios are reduced by a factor of 6 or 7.

It’s better to use an airfoil design program (such as DesignFoil at http://www.dreesecode.com/) to design, analyze and select the airfoil profile to use for the kite (for kiting purposes, the Reynolds number is around 1,000,000 to 2,000,000). Some kite designers being shy from the complexity of airfoil design and analysis, uses the rule of thumb method of changing the profile thickness/camber  for changing the lift and lift/drag characteristics of a profile.  This methodis not accurate but maybe acceptable for kites.

As a general rule of thumb, increase the profile thickness/camber to increase lift at wind window and decrease a profile thickness/camber to increase the speed of the kite.  The following table show the range of profile thickness/camber used for most kites:

Foil and Arc Inflatable
  • Thin Profile (Speed): 14% or less
  • Moderate Profile: 15%
  • Thick Profile: 16%
  • Thicker Profile: 17%
  • Thickest Profile (Tractor): 18% or more
  • Thin Profile (Speed): 8% – 9%
  • Moderate Profile: 10%
  • Thick Profile: 11%
  • Thicker Profile: 12%
  • Thickest Profile (Tractor): 13% – 14%

3. Built-in AoA

A kite get more lift with a higher Angle of Attacked (AoA) to the wind (more surface projected to the wind and also from 0 to 16 degrees of AoA, the Lift Coefficient of an airfoil normally increase to an optimum value).  Each kite has a “neutral” built-in AoA for the center of the kite and the wing tip when it is at the wind window straight over-head (with front lines and back lines of equal length).

The range of the built-in AoA is normally from 0 to 5 degrees.

Note that the wind-window angle is around 85 degrees such that the in-flight AoA of the center profile at the wind window is the sum of the built-in AoA and 5 degrees (or 90 – 85).  Note that changing the built-in AoA of the kite may also change the wind window angle such that the two will “amplify” each other to have a “double AoA” effect.  E.g., changing the built-in AoA from 2 to 0 may make the wind window angle change from 85 to 86; therefore the in-flight AoA of the kite at wind window is now 4 degrees instead of 7. It is interesting to read Peter Lynn’s Myth 1 and 2 in which he stated that the Lift or pull of the kite at wind window is proportional to the AoA of the kite and the L/D of a kite is inverse proportional to the AoA of a kite (AoA here means AoA within the “dominant AoA” range of 0 to around 20 degrees which is directly influenced by the built-in AoA of the kite).

  • A kite with a lower built-in center AoA has a larger wind window but can over-fly & luff easily and does not pull much at wind window (a Speed kite should have a lower built-in AoA around 0 degrees). These type of kites must have instantaneous AoA control for the kiter to prevent luffing and also for the kiter to “sheet-in” to get more power at wind window if needed.
  • A kite with higher built-in center AoA has a smaller wind window but generate more pull at wind window and hard to luff (a Tractor kite may have higher built-in AoA around 3 to 5 degrees for more pull at wind window)
  • An all-around kite may have a built-in AoA of 2 to  3 degrees.
  • Due to the upwash and the wing vortex phenomena, the built-in wingtip AoA of a kite can be 1 or 2 degrees higher than the center AoA.  The upwash effect reduces the AoA of the wingtip a bit so add 1 or 2 degrees to the wingtip AoA to counter balance that effect.
  • For inflatable and Arc, due to their geometry, the wingtip AoA varies much different than the center AoA and therefore the built-in wingtip AoA can be designed independent from the center AoA and the designer should add 1 or 2 degrees to the desired built-in AoA to counter balance the up-wash and the tip vortex effects.
Very Low AoA Low AoA Moderate AoA High AoA Very High AoA
Range (in degrees) 0- 1 2 – 3 4 5+
Kite Type Racing Speed All-around Wave Tractor (Wake Style)

4. Summary of the Aspect Ratio, Airfoil, AoA parameters

The following tables provide the summary of the AR, Airfoil, AoA parameters:

Low High
AR
  • Small POWER Window
  • Large POWER Window
Lift (at wind window)
  • Weak pull at wind window
  • Strong Pull at Wind Window
Lift/Drag Ratio
  • Slow
  • Fast
Built-in AoA
  • Large WIND Window
  • Small AoA at wind window (less pull)
  • Luff Easily
  • Faster
  • Small WIND Window
  • High AoA at wind window (more pull)
  • Hard to Luff
  • Slower

and their uses in different types of kite:

Kite Type/Wind Light Wind
(6 – 15 Knots)
Moderate Wind
(12 – 27 Knots)
Strong Wind
(27+ Knots)
Sled Kite Size (Foil) 16 m2 (10 m2) & Larger  8 – 16 m2 (5 – 10 m2)  8 m2 (5 m2) & Smaller
School (Stable, Low Lift, Slow) Moderate AR
High Lift
High Lift/Drag
Moderate AoA
Low AR
Low Lift
Moderate – Low Lift/Drag
Low AoA
Very Low AR
Very Low Lift
Very Low Lift/Drag
Moderate – Low AoA
Tractor (Wake Style, Wave, Gusty Wind) Moderate AR
Very High Lift
High Lift/Drag
High AoA
Moderate – Low AR
High Lift
Moderate Lift/Drag
High – Very High AoA
Low AR
Moderate Lift
Low Lift/Drag
Moderate – High AoA
All Around High AR
High Lift
Very High Lift/Drag
High AoA
Moderate AR
Moderate Lift
High – Moderate Lift/Drag
Low AoA
Moderate – Low AR
Low Lift
Moderate – Low Lift/Drag
Moderate AoA
Speed (High Jump, Freestyle) Very High AR
High Lift
Very High Lift/Drag
Moderate – Low AoA
High AR
Moderate Lift
High Lift/Drag
Low – Very Low AoA
Moderate AR
Low Lift
Moderate – Low Lift/Drag
Low AoA

 

Other Kite Design Fundamentals

  • Center profile should be selected for optimum lift and optimum lift/drag ratio (optimum as according to the type of kite requirements specified in the tables above)
  • Wingtip profile should be selected for maximum luff resistance (e.g., reflex profile).
  • For sled kites (Inflatable or Arc in spherical form):
    • A sled kite has similar projected surface of around 63% (2/pi or 2/3.14159) of the flat surface regardless any other parameters of the kite (AR, Tip/Center chord ratio, etc.)
    • If the wingtips are wide enough (effective tow points of the back lines are larger than 80% of center chord), one can reverse relaunch an inflatable or Arc by pulling on the back lines.
    • For LEI (using traditional airfoil), if the wingtip are wide enough and the effective tow point of the front lines is so forward (normally less than 15% of chord) that it reduces the AoA drastically, the kite will not fly on the front lines alone (100% depower)

More Kite Design Info

 

Flat LEI

A Flat LEI has similar structure with a classic LEI except for the following differences:

  • A flatter canopy design (however most still have a deep canopy curve compared to regular foil, to take advantage of the Sled Boosting effect)
  • A bridle system consisting of a simple but somewhat elaborated bridle system for the front lines and a very simple bridle system for the back lines.  The front bridle system has multiple connection points to the leading edge to support the leading (therefore Flat LEIs are also referred to as Support Leading Edge, SLE, kites)

The canopy is more or less equivalent to the center part of the classic LEI canopy (around 3/4 of the classic LEI canopy) and the bridle system is equivalent to the sides of the classic LEI canopy (about 1/4 or 1/8 of the canopy on each side).

Besides for the differences above, a Flat LEI design should be somewhat similar to a classic LEI in theory.  It is then just a matter of properly design the canopy and the towing points via the new bridle systems.

Unfortunately current version of Surfplan does not provide full calculation and analysis of the tow points of the bridle for Flat LEI.  So in the mean time, you have to design a flat LEI with some manual processes. Also, if you are interested in flat LEI kite design, read Bruno’s Flat LEI patent application at http://www.freepatentsonline.com/20050230556.pdf and the Flat LEI section.

 

Airfoil Database

Most kite design or foil design software come with some airfoil database; however should you want more, there are other airfoil databases and one of the most extensive airfoil databases is UIUC Airfoil Coordinates Database.

 

How does a kite surf fly?
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