Publication: Application of aircraft's flight testing techniques to the aerodynamic characterization of power kites
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2021-01
Defense date
2021-04-12
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Abstract
This thesis has developed an experimental methodology for the flight testing and data analysis of
power kites applied to Airborne Wind Energy Systems (AWES). In particular, the Estimation
Before Modeling technique, a well-known method in the aerospace industry for the aerodynamic
characterization of an aircraft using real flight data, has been adapted for tethered aircraft. The
developed methodology has two main building blocks: (i) an experimental setup to record
experimental data during the flight testing, and (ii) a Flight Path Reconstruction algorithm to
estimate the state of the system from the experimental data. From them, the aerodynamic
characteristics of two types of kites were investigated.
The proposed experimental setup was designed to be low cost, portable and easily adaptable to
both, rigid and semi-rigid kites. It is composed of an instrumented kite representative of the ones
used in AWES, an instrumented control bar, a ground computer and a wind station. Whenever it
was possible, commercial off the shelf components have been used, including low cost openhardware
sensors based on the PixHawk platform. However, after the first flight tests were
conducted and the obtained results were discussed, high precision sensors were also included.
The Flight Path Reconstruction (FPR) algorithm for tethered aircraft is based on an Extended Kalman
Filter (EKF). In addition to the standard set of estimated state variables (ie. Euler angles, position
or ground speed), the algorithm also provides the aerodynamic torque and forces upon the kite as
well as the tether tensions and wind velocity vector. The EBM technique, and the FPR algorithm
have been used to identify the aerodynamic characteristics of both, four-line Leading Edge
Inflatable (LEI) kites and two-line Rigid Frame Delta (RFD) kites. Quantitative and qualitative
results have been obtained. Albeit both types of kites exhibited very high AoA during the flight,
some significant differences were found. In particular, the estimated lift coefficient of the LEI
kite showed a behavior identified with a post-stall condition, while the RFD showed a pre-stall
behavior with a lower AoA and a positive relation between the lift coefficient and the kite AoA.
The presented experimental methodology can be of great interest for AWE industry as it helps to
improve modeling of tethered aircraft, leading to more accurate performance figures which may
increase investors interest in the technology. Moreover, flight testing methodologies and
experimental data analysis are of great interest for benchmarking AWES performances,
contributing to de-risk their development process and providing better tools for AWE "best
concept" identification. Finally, as a sub-product of the presented methodology, the FPR
algorithm can be used as a validated state estimator of the tethered aircraft, which is a key
element of a closed loop flight control system.
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Keywords
Airborne wind energy, Kite, Aerodynamic, Flight dynamics, EBM, Estimation Before Modeling