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Aerofoil Design

Aerofoil design is the key to the overall performance of a sailplane, so the design process of the JS1 Revelation started with development of the main wing aerofoil. Attie Jonker together with an NWU final year student had conducted wind tunnel tests on one of Attie’s aerofoil designs. Combining the wind tunnel data together with new technology and research on techniques that might help climbing performance, then after making hundreds of iterations made with the XFOIL program, we ended up with the JJB44 main wing aerofoil.

Johan Bosman, JS Chief Aerodynamicist, explains:
aero3Enthusiastically we send the coordinates to Loek Boermans at TU Delft for a check up. His comments weren’t all bad, but he made some significant recommendations. The aerofoil had very low profile drag, but would lack low drag at high lift coefficients due to laminar flow that extended too far aft on the upper surface. Using the aerofoil at low Reynolds numbers would also be detrimental to overall performance.

After several hundred iterations later working with the aerofoil and considering Loek’s recommendations, we have ended up with the T12 aerofoil used in the JS1 Revelation today. By careful design we have managed to maintain very low profile drag, but also enlarge the laminar drag bucket for low drag at high lift coefficients as well. To optimize climbing in turbulent thermals the T12 does not have the typical flat top Cl-Alpha curve at high lift coefficients.

T12 characteristics:

  • Maximum thickness/chord ratio 12.7%
  • 14% camber changing flap
  • Low drag with extensive regions of laminar flow
  • Laminar to turbulent transition on the lower surface occurs at 93% where artificial transition is applied for negative flap settings
  • Transition on the upper surface occurs at approximately 65% for a 0° flap setting and 2° angle of attack
  • The top surface is smooth at 13.5° with almost 70% laminar flow
  • Fluent 2D calculation (with new transition models) of T12 aerofoil, showing turbulent kinetic energy as the flow moves over the aerofoil. The long laminar flow region on the lower surface is clearly visible.

Although aerodynamically optimised, there are structural challenges using such a thin aerofoil. At the time the T12 aerofoil was the thinnest main aerofoil used on modern sailplanes. However the decision was made to use the thin aerofoil, consistent with the JS motto of “no aerodynamic compromise”. The aerodynamic performance gain is worth more than the manufacturing cost and structural complications.

The aerodynamic design of the wing root is very challenging as this is turbulent flow, rather than the laminar flow over most of the wing. We developed a new root aerofoil in conjunction with the wing/fuselage junction design, which reduces separation problems at the trailing edge and optimised overall drag.

Six different aerofoils are used in the wing for maximizing the performance of the glider. All are derived from the main T12 aerofoil, optimised at each spanwise station for the specific chord length and Reynolds number. The wingtip aerofoil is designed with an ample lift reserve to help handling characteristics and avoid any tendency for wing drop.