Conference article

The aim of this investigation is to assess if the use of Natural Laminar Flow airfoils alone improves the efficiency of a wing in comparison to their NACA equivalents. Two wings have been defined, one with a general NACA-developed NLF airfoil for a low-speed design and one with an optimized NLF airfoil for high-speed applications. These wings are then compared against the same design with their 4-digit NACA equivalent airfoils. To compare performance, the lift-to-drag ratio (Efficiency) of the wings has been considered. A typical cruise altitude of 4900m and a cruise speed range from M0.1 to M0.3 (in steps of 0.05) was analysed for the low-speed wing, while an altitude of 11000m and a Mach sweep between M0.3 and M0.85 (in steps of 0.1 up to M0.5 and 0.05 after that) was considered for the high-speed wing. A final analysis was performed to assess the effect of the addition of a sweepback angle in the wing, from straight leading edge to a 10-degree-sweepback. All cases have been modelled using the Lattice Boltzmann software XFlow. This software was selected because the turbulence is solved by introducing the corresponding terms in the balance equations, while traditional RANS software would require the user to select a method to model this effect. For the low-speed wing, it was observed that the efficiency of the laminar wing is slightly decreasing with speed (up to 17% between M0.1 and M0.3) but is increasingly higher than in the NACA wing (from 5% at M0.1 to 16% at M0.3). This means that the aim of the laminar airfoil is met, and for a cruise speed between 100 and 300km/h lower drag is produced and therefore lower thrust (and fuel consumption) is required. In the case of the high-speed application with a traditional configuration, it was found that the laminar flow wing had lower efficiencies when compared to its NACA equivalent. The expectations are not fulfilled, so a motivation was searched and, analysing the results, it was observed that the ratio of the lateral forces to lift had a direct relation to the efficiency: when this ratio is increased, the efficiency is decreased, and vice-versa. This means that, in order to reach optimal efficiencies in high-speed wings, the lateral force needs to be restrained, but this force is caused by cross-flow in the wing, which in turn is induced by the sweepback angle. It was observed that, in fact, in the case of laminar wings, not applying a sweepback to the leading edge is optimal and duplicates the efficiency with respect to adding any angle. Moreover, this is the only case observed where the efficiency of the laminar wing is higher than its NACA equivalent.

environmentally friendly technology, laminar flow, wing performance, aerodynamic efficiency, low-speed aerodynamics, high-speed aerodynamics