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Kites for Future - Flying Wind Turbine

A flying wing drone that can periodically pull a tether + a generator on the ground that acts like a rowing machine = Flying Wind Turbine

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An autonomous low-cost ground gen, fixed wing, crosswind airborne wind energy generator

https://en.wikipedia.org/wiki/Crosswind_kite_power

How:

Play with the flight simulation at:
https://www.kitesforfuture.de/simulation17/kite_simulation.html
The kite flies figure eights like a sports kite while pulling the tether. The spool on the ground thus spins and drives a generator. More wind -> faster kite -> more line tension and the spool spins faster -> more output power (increase is cubic with the wind speed, linear with the wing area). During the reel-in phase the kite exerts almost no force on the tether -> power needed to reel-in is very small.

Why:

No huge tower, no heavy foundation -> this flying wind turbine is far cheaper to build than a comparable conventional wind turbine.

Publishing our work as a hackaday project was inspired by
https://hackaday.io/project/159049-portable-kite-t

Kites for Future in action:


From Thomas Haas' Phd thesis we borrow the following picture:

Many projects around the world have tried to implement an autonomous airborne wind turbine and have proven in recent years that it works great.
For an overview of airborne wind energy systems, see:

Kitemill in Norway share their power production measurements, which look quite amazing:

However so far, none of these projects have an online shop selling the system. Except for https://kitewinder.fr/ but their system does not launch and land autonomously and sweeps a rather small area. So we had to make our own.

Our design goals, partly going well ahead of many of the existing projects, were:

- As simple as possible, i.e. if possible no GPS, only one tether, ground station and kite communication kept to an absolute minimum, as few control surfaces and propellers as possible

- Easy to transport, i.e. flying wing if possible

- Fully autonomous to enable service-free operation for months

- Easy and cheap to manufacture at scale, using readily available parts

- Efficient and profitable even at very small scale (2m wingspan)

Advantages of our design that we discovered during testing:

- Scalable to very large size due to the possibility to have many tether attachment points along the wing spar that join at a common knot to the single tether going to the ground station

- Really fast precision landing as a glider not needing the propellers

- Really fast launch straight up (10 seconds)

- Can use conventional folding propellers

- Can use a wing without dihedral

- Can turn with either rudder or ailerons.

- Figure Eight autopilot extremely simple

- No wind sensors neccessary

tailsitterkite.pdf

LaTeX-style detailled description of the working principle

Adobe Portable Document Format - 124.73 kB - 11/07/2022 at 13:30

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  • Kite Connectivity

    Benjamin01/11/2023 at 14:44 0 comments

    Our most recent addition of functionality:

    1. Configuring PID variables, target height, etc. to tune autopilot behaviour (locally via WiFi)

    2. Initiating launch and landing from home, viewing the current status (via Internet)

    Here is how it's implemented:

  • GUI for configuring autopilot

    Benjamin12/14/2022 at 14:41 0 comments

    We spent a considerable amount of time tuning PID constants in the field. The process involved connecting the autopilot to the PC via a USB cable, editing C code, compiling and flashing the whole autopilot code for every adjustment of a PID variable.

    We want anybody to be able to build this kite in any size and configure the autopilot accordingly without having to edit C code.

    And we want to make life easier for ourselves so we can draw any smartphone to change the autopilot configuration.

    So now we can do exactly that. While the kite is on the ground, we can also test sensors and motors via Wifi using the GUI below.

    Best part: Adjusting the PID constants should work during flight too.

    So from now on, no programmer is needed to continue on this project. If you want to copy our kite power station, we're happy to help to make it as easy as possible.

View all 2 project logs

  • 1
    Kite

    The kite is simply a wing. It has no dihedral. All our tests with diredral resulted in an aerodynamic pendulum in the launch/hover phase. When the kite moves left-right it is forced to roll due to the dihedral, which is undesired. Instead of dihedral we use two or more line attachment points as depicted in the technical drawing above. This gives good roll stability.

    Two drone propellers are attached to the front of the wing. Their airflow flows over an elevon each. The propellers are attached near the wing tips to enable strong differential thrust yaw control. We use folding propellers. They turn in opposite directions (using a custom 3d-printed folding mechanism attached to originally non-folding drone propellers to avoid an undesired torque, but this might turn out to be negligible and thus unnecessary.

    There is also an airbrake to enable slow but steep landings.

    A large stabilizer is very important because the kite will otherwise simply not turn when flying crosswind. A rudder actually also works great for steering during crosswind flight, as you can see here:

    Attached to the main wing spar extending in negative z-direction are the line attachment points. They are below the centre of gravity and the centre of pressure to ensure longitudinal stability. They should not be too far below to not interfere with pitch control while launching. They can be anywhere along the wing spar in any number, but there have to be at least two and those have to be far enough apart from the centre to provide roll stability. From those wing attachment points all lines meet at a common knot. The distance from this knot to the wing influences the roll stability and must be chosen accordingly. The aim of this design is to achieve a stiff roll stability but a gentle longitudinal stability that can be overpowered by the elevon forces.

    How to build:

    Until now we built nearly all our prototypes by folding 6mm Depron sheets around a carbon fibre wing spar (e.g. a 13mm hollow tube with 45° fibre orientation will do the job). Glue used is Uhu Por. The outside can be laminated with package tape:

    Now we started to use our hot-wire foam cutter (https://rckeith.co.uk/) with the software tool from https://www.diyrcwings.com/app/

    The result looks like this:

    Kite Electronics:

    You need:

    - 2 Motors with propellers (We use Sunnysky 4006 740 or 380Kv variant with 15 inch props)

    - 2 ESCs (We use the Spedix IS30)

    - an ESP32 Devkit-C v4 with an external wire antenna (the really simple ones do the job, but the PCB antenna is not sufficient)

    - 4S ca. 800mAh LiPo battery

    - 3 Servo motors (MG90S works fine, but beware of the fakes with plastic gears).

    - BMP280 breakout board (this must be calibrated, because the BMP reacts to temperature, will add instructions soon, otherwise ask me. Also BMP doesn't like brightness differences, so must be covered in black tape or similar)

    - custom PCB including an MPU6050 (the accelerometers for the three coordinate axes must be calibrated by a constant offset, brokking.net knows how, I will add instructions soon, otherwise ask me) and buck converters, see

    https://oshwlab.com/benjamin.kutschan/kitepcb1_copy

    for the pcb design.

    - Code from

    https://github.com/KitesForFuture/kite/tree/figure-eight-2

    (Alternatively you could use any basic drone flight controller and somehow make our code work on that. which would be awesome!!!)

    The MPU6050 should be positioned close to the centre of gravity of the kite.

    Combined with a BMP280 at the ground station you could determine the height difference precisely even in changing overall air pressure due to weather.

  • 2
    Groundstation

    The groundstations main part is an electric motor (used as a motor and of course as a generator). We have chosen a very commonly available 3-phase BLDC brushless motor. It has a stator with 3 phases and a rotor with alternating permanent magnets.

    Attached to the shaft of the motor is a spool. The line goes from the spool to a mechanism that redirects the line in any direction. This mechanism can be as simple as a screw eyelet or more sophisticated using ball bearings and pulleys.

    There is an arm driven by a servo motor that pushes the line sideways to ensure it is being spooled across the whole length of the spool. The line redirection is positioned such that without the spooling arm the line reaches the spool orthogonally on one end of the spool. This way the spooling arm only needs to push the line in one direction and can move out of the way when reeling out. The other direction is passively pulled by the position of the line redirection mechanism. You can also use any other spooling mechanism.

    The whole groundstation assembly is being held in the ground by a large tent peg or a ground screw or similar. The closer the line redirection mechanism is to the ground the less leverage and thus the easier it is to secure the assembly to the ground.



    Groundstation Electronics:

    You need:

    - ESP32 Devkit-C v4 with a short external antenna

    - beefy 3-phase 50-90Kv BLDC (We use Alien Power System C80100 50Kv)

    - VESC 6 75V with ON/OFF Switch (https://vesc-project.com/)

    - 16S LiIon battery with >1kWh capacity, e.g. 20Ah.

    - 1 Servo motor

    - 1 Switch

    - either wiring and a LM2596HV board or our custom PCB:

    https://oshwlab.com/benjamin.kutschan/groundstation-vesc6-esp32

    - The source code is available at https://github.com/KitesForFuture/groundstation

    You could also consider a cheaper slightly lower voltage setup: 90Kv motor, FSESC 6.x (we haven't tested this yet), 12S-battery.

  • 3
    Understanding the Algorithms

    Flight Modes:

    There are three flight modes: Launching the kite, flying figure eights (synonymous: Eight mode) to generate electricity, and Descending (synonymous: landing) towards the ground station. The latter can be final, meaning that the descent will be until the ground station is being hit by the kite.

    First the system is in launch mode until the kite line has reached a certain length.

    If this length is not being reached fast enough, it can be deduced that the wind is not strong enough and the system goes into final landing mode.

    Otherwise the system goes in to Eight mode. When a certain line length has been reached while in Eight mode, the system switches to descent mode.

    Either the Descent mode is final, in which case the kite will land at the ground station. Or the Descent mode will switch to Eight mode once the line length is below a certain threshold.

    The flight mode is being decided by the groundstation, which counts the line length using built-in VESC functionality.

    Line Angle:

    The angle of the kite line above the horizon can be calculated once its length and the height of the kite are known. The length of the kite line is being sent via Wifi from groundstation to the kite. The air pressure is being measured on the kite and at the groundstation. The latter is sent to the kite for comparison and height calculation on the kite. The line angle is being calculated on the kite.

    Launch:

    While in launch mode the groundstation keeps the line tension at a minimum.

    The kites propellers are spinning and the kite hovers with its pointing up, slightly to the back to ensure the line being reeled out even in low or no wind.

    The algorithms for stabilizing this kind of tailsitter plane are well known and have been implemented in numerous toy airplanes.

    It is however important that the kite line angle does not become too steep as in this situation the bridle would inhibit the differential thrust yaw control. The kite would lean sideways and ultimately crash.

    To control the line angle, the height at which the line has its desired angle (height = linelength/2 for a 30 degrees angle) is being calculated and controlled by the propeller thrust.

    This simple control is sufficient to make the kite launch in a predefined angle to the horizon in the direction of the wind. In good winds this launch takes our prototype to 50 metres of height in about 10 seconds.

    Transition from Launch to Figure Eights:

    To transition from Launch mode to Eight mode the propellers stay turned on for a small amount of time while the line tension is being increased by the ground station. This forces the kite into a stable gliding state.

    Figure Eights:

    While in Eight mode the groundstation keeps the line tension high, ideally at the optimum of maximum power output but first of all large enough to enable a stable flight. It can happen that the line is being reeled in to keep the line tension up, while the kite is still flying figure eights. This can happen in low wind scenarios in the curves of the figure eight.

    While flying figure eights the angle α between horizon and kite line needs to be controlled. Seen from the groundstation the eight looks as in Figure 5. We view an eight as two curves and a somewhat horizontal flight in between.

    During the horizontal part the nose of the kite can be controlled to point slightly up or down depending on whether the kite line angle is too large or too small. A P-controller sets this nose angle β depending on the deviation from the line angle α.
    During a turn this nose angle is changed at a predefined rate (which we call turning speed).

    The angle β is held by a common PD-Controller using the aileron functionality of the elevons. Also a rudder could be used and we found this to also work very well.

    Descent:
    The kite descends in the same orientation a normal aircraft would when landing.

    During the descent phase the line tension is kept just high enough to keep it straight by rolling the kite slightly in case of flying too far left or right.

    Steering the kite left or right is being done by the aileron functionality of the elevons. When the kite threatens to fly to far left the line tension and the bridle cause it to roll. This roll is being sensed by the orientation sensor and used as an input to a P-Controller affecting the elevons. A D-term was not neccessary during our test flights, possibly because the wing produces enough drag in roll direction.

    The line angle α Figure 5 needs to be controlled by the elevator functionality of the elevons. The desired height is calculated for example as linelength*0.2 to achieve a descent with 20% steepness. Then the desired dive angle of the kite is being set by a P-Controller relative to the height error. The actual dive angle is then controlled by a PD-Controller relative to the error between the actual and the desired dive angle.

    Transition from Descent to Figure Eights:

    To transition from Descent to Eight mode the desired dive angle is being set to a positive number to make the nose point up. Then Eight mode can begin from a well defined orientation and laminar air flow.

View all 3 instructions

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Discussions

luneart wrote 11/15/2022 at 17:57 point

Hi, 

Beside the obvious "so what's your figure for generated energy" question, I was wondering if the propellers could be used for small energy generation while in 8-flight mode? Because as you evoke the months-long flights, I think about the on-board battery powering the flight control and probably safety lights in this perspective. 

Kudos for the project and realisation! 

  Are you sure? yes | no

Benjamin wrote 11/17/2022 at 09:39 point

Hi,

we have measured peak power of 100W with the old wobbly foam board. Theoretically more than 1kW continuous power should be possible with this 2m wingspan size. In the simulation (second link in the description) the output power is displayed. It depends quadratically on the speed of the figure eight flight. A more precisely cut wing will fly much faster.

So far we have concentrated on getting the autonomous launch, landing and eights working. Now we work on reliability. Then optimization of Energy output.

The onboard battery lasts about a day. It is common to use a small onboard propeller to recharge it during flight. Would be awesome of course if one could use the launch propellers that are already there. Maybe as easy as releasing the ESC brake?

  Are you sure? yes | no

laurin wrote 11/11/2022 at 09:49 point

whats the reason for propellers instead of maneuvering with lines on the left and right edge?

  Are you sure? yes | no

Benjamin wrote 11/11/2022 at 11:02 point

Hi Laurin,

that's a very good question. Other projects do it with lines, like Enerkite or Skysails.

We use the propellers only for launching during the first 10 seconds. This way we don't need to build a crane arm fo launching. When the line length goes from 0 to 100 metres, i.e. during launch, the propellers work like a quadcopter, pulling the kite up. When the line is long enough and the kite can be lifted by the wind alone, the ailerons are used to steer.

We do it this way, because the ailerons are there anyway because they are needed for the propeller launch to divert the propeller air flow and pitch or roll the wing. Also all the sensors are inside the kite, so we wanted the controls to be on the kite too to avoid wifi communication.

  Are you sure? yes | no

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