The new ALTTA HLF fin design

Kurzfassung

Wind tunnel and flight tests have clearly demonstrated the potential of hybrid laminar flow control (HLFC) to reduce 2d-airfoil drag on a modern transport aircraft by about 50% leading to a total drag reduction of about 10% to 15%. However, these tests also revealed that much of HLF technology's advantage might be obliterated by the weight of additional structures and systems. Consequently, one central objective of the EC funded programme ALTTA is the simplification of the suction system for an Airbus A320HLFC fin. The simplest internal structure for an HLF system would have only one suction chamber, the leading edge box itself. Unfortunately, the usage of combination with a constant-porosity suction surface results in a very ineffective suction speed distribution. From the aerodynamic point of view the porous surface has to be supported by stringers arranged in spanwise direction. Then the risk of inducing turbulent by nonporous parts of the surface is very small. Lift forces, meanwhile, have to be lead to the front spar by ribs. A very efficient structural solution is a second sheet between stringers and ribs. The outer porous surface, the supporting stringers and the inner sheet form a double sheet surface with a relatively high number of chambers. This structure also allows for a very efficient control the suction distribution. Assuming that the porosity of the suction surface is constant in chord and span direction, the local mass flow between two stringers can be adjusted by metering orifices in the inner sheet, making the whole leading edge box a single suction duct. Active computer control of the mass flow in the suction pipes through butterfly valves and corresponding sensor equipment have also contributed to the high complexity of suction systems in the past. In the present design such elements will not be used. The mass flow-controlling elements are the porous outer surface, the inner sheet with orifices and a suitable suction pump assumed to be located within the leading box. The system is designed to be fully self adapting for a certain range of Mach numbers, flight levels and yaw angles. The paper describes the assumptions and equations to calculate suction speed, mass flow, pressure and pressure losses of such a system. It is shown that this system allows very efficient suction distribution. Reversed turbulence inducing suction velocity (local outflow) can be avoided for a range of yaw angles of +/-3°. The transition location on the fin is determined by linear stability calculations. Based on general flight conditions, surface geometry, surface pressure distribution and the suction speed distribution the stability analysis of the laminar boundary layer is performed by means of the local stability code COAST3. The transition location is determined using the stability results in combination with the so-called 2-N-factor criterion. The calculation procedure described above is applied on a set of design and off-design flight conditions. It includes Mach numbers from 0.76 to 0.80, flight levels between 23000 and 39000ft, yaw angles of +/-4°, ruder deflections of 2° and a variation of the suction surface porosity. With increasing Mach number the extent of laminar flow moves slightly downstream. The influence of the flight level is negligible. Varying yaw angle induced by the pressure distribution has a strong influence on the development of laminar flow. The essential result is that up to +/-3° of yaw angle the boundary layer is kept laminar. At larger yaw angles local outflow causes transition of the laminar boundary layer. The extent of laminar flow varies between nearly 40% on the outer half of the fin and about 30% on the lower part. The simplified layout of the suction system fulfils the design objectives agreed by the ALTTA partners.