Project TETA

Test and Evaluation of tethered airfoils

"THIS WOULD REVOLUTIONIZE THE DEVELOMENT PROCESS AND THE WAY WE JUDGE KITES IN GENERAL. A DESIGNER´S DREAM COME TRUE!"

'Imagine if you could compare kite models against each other based on figures, similar to a car-magazine test. Parameters such as steering pressure, lift, drag, depower-ability, turning speed and more could be compared by looking at objective data. This would revolutionize the development process and the way we judge kites in general. A designer’s dream come true!’ says Ralf Grösel, Duotone kite designer. Back in 2013 Jan Hummel approached Ralf with exactly that idea – project TETA (Test and Evaluation of Tethered Airfoils), operated by the University of Berlin. It was the primary subject for Jan’s PhD thesis to figure out how to create valid, testable, repeatable data to establish an objective way to measure various performance attributes of a kite.

Dietmar Göhlich, professor at the department of Methods for Product Development and Mechatronics at the Technical University of Berlin explains: ‘The problem is that compared to rigid wings, the physics of flexible membranes such as kites is very complex. The general knowledge about flexible wings is limited due to the deformation of the kite under aerodynamic load.’
The solution to this problem is the world’s first test bench, TETA, which was specifically developed for validating flexible wings. It is essentially a trailer towed behind a van, bristling with scientific apparatus, from which a kite can be flown automatically or manually.

Duotone Kiteboarding Dice 2019 Project Teta True Magazine

Jan explains, ‘we drive the test bench behind a van on a military runway in calm wind conditions to reduce the influence of gusts. This allows us to simulate different windspeeds. Downscaling is not appropriate for flexible wings; and since lines and control bars must be taken into effect, unscaled wind-tunnel tests are not suitable because of their size.’ Driving under cruise control with a full-length lineset creates ‘reproducible, controlled conditions which allow us to develop a systematic understanding of how kites perform under realistic conditions. This enables us to compare various designs.’
Inside the test bench there is a pilot and a generic bar attached to wires which are fed into winches and servos. Attached to the test bench there are cameras and a weather station. The kite flies from the bench at the end of a standard kite bar which is controlled and monitored by separate servos.

Says Jan, ‘To perform repeatable automated maneuvers, repeatable steering inputs are of course necessary, the steering inputs are implemented by two servo motors and can be controlled by the pilot via fly-by wire or from the control unit itself or from both in combination’.

‘The entire setup leads to an objective validation of kite-related parameters; for example, depower-ability, steering pressure, flight angle (upwind-ability), turning speed, turning radius, backstall behaviors, power to front lines, power to back lines, and more … From a designer’s point of view, all aspects which are relevant to kite design are captured with this unique test bench’ says Ralf. ‘The opportunities are great for future developments. Kite design has reached a point where tiny changes in geometry have a huge impact on the overall flight parameters.
The test bench allows me to dive into aspects of kite design which haven’t been accessible before. This will automatically lead to new innovations or more specific kite models as separation will be more obvious from the first development steps onwards. As kitesurfing is becoming an Olympic discipline soon, the carpet has been rolled out for this field of wings also’.

Duotone Kiteboarding Click Bar 2019 TRUE Magazine Project TETA

Jan mentions another aspect. ‘In the long-run there are implications for computer assisted design … If you don’t have valid data, you are not able to feed complex simulations like FEM (Finite Element Method) and CFD (Computational Fluid Dynamic). Only with correct information of how the flexible wing will react to different windspeeds and rider input will a simulation software be able to provide realistic results. It might still be years away, but the information we are gathering from the test bench will lead to great leaps in our ability to model flight characteristics of a kite on a computer before we even build the first prototype.’

‘I would wish to see that this test bench becomes a standard within the kite industry. Magazines could take this opportunity into consideration to validate different kites from different brands,’ says Ralf. ‘For me this project showcases the capabilities of advanced testing technology, and future developments will surely benefit from it. Especially with the next generation of fully automated tests, which are currently under development. These tests will be precise and accurate as nothing else ever before.’