Day 2 (Friday 1 August 2014)

Day 2 of the OSTIV Congress began with a presentation by Jan Himisch Germany. Jan discussed the experience of using a design stream for winglets that uses an optimization algorithm. The underlying performance calculations are based on a potential flow method that captures any viscous effects with profile data and other corrections. For the optimization process, Jan stressed the importance of well-defined target functions. During the design process several winglet parameter were manipulated, for example height, twist distribution, cant angle, and sweep. Jan discussed the impact of the different parameters on the average cross-country speed for various weather models. He also introduced flight-test results of two different winglet designs for the SB 14. Theoretical and experimental results show similar trends in drag differences of the two configurations, but disagreement in the absolute drag values.

The second speaker of the day was Loek Boermans of the Netherlands. He discussed the purpose of turbulators and the different kind that are commonly used, zig-zag tape and pneumatic turbulators. Both devices are used to avoid laminar separation by causing transition from laminar to turbulent flow, which in general reduces profile drag. Loek also discussed how to size the height of zig-zag tape and the subsequent drag penalties of zig-zag tape and pneumatic turbulators.

After the coffee break Andreas Reh of Darmstadt, Germany presented his research on the impact of turbulence on airfoil performance. Two turbulence levels affect the aerodynamics of laminar airfoils: moderately large scale turbulence with respect to chord length that result in the aircraft to be upset and smaller scale turbulence that can cause early transition. Andreas´ focus was on the moderately large scale turbulence that changes the pressure distributions and pressure gradients. One of the consequences is the change in transition behaviour. Unsteady and quasi-unsteady analyses show promising results, although some discrepancies do exist.

The next presentation was related to the previous one. Milan Marejka talked about measurements of atmospheric turbulences using a glider and hotwire anemometry. The measurements were done in thermals and wave lift at various altitudes. The turbulence levels were of similar magnitude with the ones measured in wave being slightly greater. The measured longitudinal turbulence levels ranged from approximately 0.25% to 0.75%, which can possibly impact the transition behaviour due to Tollmien-Schlichting instabilities.

The last presenter of the day was Louis van der Walt from South Africa. Louis did a review of common practises related to the fatigue life of sailplanes. Certification requirements of sailplanes demand adequate fatigue behaviour, whereas manufacturers need the ability to assess the actual fatigue state. For certification purposes full-size fatigue tests are generally accepted as well as static load tests to elevated load values. Full-size fatigue testing is very time intensive and elevated load tests provide little insight in failure behaviour. An alternative is an accelerated load test that uses a mix of load spectrum and static test. Also equivalent load levels are considered. Louis proposed a test using a beam and a simple step load, equivalent load, as well as a load spectrum test.