24/03/2019
Aerodynamic Design
The elfin represents the logical next step forward into a new era of soaring and recreational aviation, based on recent and continuing advances in the development of electric propulsion systems. To optimize our application of this new propulsion technology, however, we need to provide the best possible airframe. Here are some of the concepts underlying our approach to this new design:
A modern sailplane is a complex combination of various disciplines, but all of these must be based on rigorous aerodynamic design optimized for modern high-performance soaring. To advance the state of the art, we must begin with a thorough consideration of basic principles. In the case of elfin, this has meant performing extensive CFD (Computational Fluid Dynamics) analysis of the basic aircraft geometry using VS-Aero software.
The results have been impressive. Please refer to the fuselage overview below.
The extensive laminar flow over the wetted surface is of special benefit for the side-by-side cockpit arrangement.
Special emphasis was put on the wing-fuselage junction, long recognized as a source of intersection drag with significant potential for reduction, and now, after spending months including consultations with the program designers, this has been achieved.
Near the fuselage, the ideal elliptical lift distribution of sailplane wings is disturbed, increasing the induced drag. Therefore, elfin was designed from the outset with a high-wing configuration. This configuration is, in general, aerodynamically superior to the mid-wing configuration often used in sailplane design. With a high-wing configuration, the lift reduction due to the fuselage is minimized.
Furthermore, a positive twist was applied to the wing root section of elfin, to compensate for the loss of lift in the fuselage region, resulting in a nearly elliptical lift distribution and minimizing induced drag.
The flow at a wing-fuselage junction is generally turbulent. Thus, in addition to the wing root twist, we designed a new turbulent root airfoil, especially for elfin. It is optimized to reduce friction and pressure drag in the root area. Several iterations of the root twist and of the airfoil were performed to ensure a separation-free root junction. In addition, adequate fillets between wing and fuselage of the elfin prevent the so-called diffuser effect. Without proper fillets the intersection of wing and fuselage forms a nozzle resulting in a pressure peak on the lower side of the wing just behind its stagnation point, followed by a steep adverse pressure gradient which triggers separation.
Needless to say, elfin also features all the other qualities of high-performance sailplane design:
• Five different airfoils are used in the wing to maximize the performance of the elfin. All are derived from the main airfoil, optimized at each spanwise section for the specific chord length and Reynolds number.
• The airfoils feature a well-balanced and steady lift coefficient slope, without a premature lift plateau or local maxima as is characteristic for some other airfoils.
• The shape of the elfin fuselage was optimized to ensure laminar flow over the greatest possible area in order to minimize drag.
• The fuselage is tilted nose down compared to the wing in order to match the incoming streamlines.
• The fuselage contraction ratio is well adjusted to reduce the wetted surface area and avoid separation.
• Winglets were optimized to ensure maximum cross-country performance.
