Flexible strip with pressure sensors applied to a rigid wing in the TU Delft wind tunnel.
Bryan Franca MSc Graduate Delft University of Technology Faculty of Aerospace Engineering
Traverse 5 3831 RW Leusden
The Netherlands bryan.franca@gmail.com
lr.tudelft.nl
A Tool for Aerodynamic Analysis of Flexible Kites
Bryan Franca, Roland Schmehl
Faculty of Aerospace Engineering, Delft University of Technology Studies have shown that the power harvesting efficiency
of a pumping kite system is determined mainly by two performance characteristics of the kite. For a given size of the wing the cycle power output increases on the one hand with the lift-to-drag ratio [1] and and on the other hand with the available de-power range [2]. In search of an improved kite design, this study aims at identifying shortcomings of current design procedures, with a spe-cial focus on the aerodynamics. These procedures are mostly derived from the kitesurf industry where thorough aerodynamic analysis is generally not integrated system-atically in the iterative development cycles. As a result, little is known of the kite while in-flight, such as shape, angle of attack, and pressure distribution. Knowledge of these parameters is essential for the quantification and identification of design improvements. These values are furthermore crucial for the validation of numerical mod-els. In order to provide a solution for the lack of exper-imental data of in-flight kites, a system for in-situ mea-surement of the surface pressure distribution was devel-oped and validated.
The developed system is based on Micro-Electro-Mechanical-System (MEMS) barometric pressure sensors, the LPS25H by STMicroelectronics [3]. A set of sensors are mounted on a flexible circuit board printed on PEN (Polyethylene naphthalate) which is completed with a single board computer to control and store
measure-ments. The barometric nature of the LPS25H sensors in addition to the necessity of the dynamic pressure in the computation of the pressure coefficient led to the development and integration of a pitot-static unit. All components were individually tested to evaluate their reliability and accuracy.
The validation of the system was performed in an open jet low-speed wind tunnel mounted on a rigid wing sec-tion featuring integrated pressure tabs. With the addisec-tion of improvements to reduce the system roughness, the re-sults showed a minimum pressure coefficient overesti-mate of 5% compared to the pressure tabs at 0.4 chord. The sensors between 0.15 and 0.65 chord showed simi-lar response with errors varying between -5% and +10%, while at the leading edge and trailing edge of the profile larger errors were observed. The latter is explained by the larger relative thickness of the pressure strip compared to the airfoil thickness in those regions.
The system still requires further research before it can be fully deployed, but the initial results are promising. References:
[1] Loyd M. L.: Crosswind Kite Power. Journal of Energy, Vol. 4, No. 3, pp. 106–111 (1980)
[2] Noom M. N.: Theoretical Analysis of Mechanical Power Genera-tion by Pumping Cycle Kite Power Systems. M.Sc. Thesis, Delft Uni-versity of Technology (2013)
[3] LPS25H sensor datasheet. http://www.st.com/web/catalog/ sense_power/FM89/SC1316/PF255230. Accessed 1 June 2015
