Future wing Hybrid Laminar Flow Control suction system design and analysis
The work presented is part of the Active Wing Active Flow- Loads & Noise control on next generation wing (AFLoNext) project work package 1.2 which aims to prove the engineering feasibility of the Hybrid Laminar Flow Control (HLFC) technology for drag reduction on a wing by means of large scale wing ground based demonstrators. AFloNext receives funding from the European Community's Seventh Framework Programme FP7/2007-2013, under grant agreement n° 604013. The context of this work is given by the continued development of civil transport aircraft with reduced fuel burn and emissions. The airframer can directly influence this by lowering airframe drag thanks to maintaining laminar flow on a large proportion of wing windswept surfaces. As the cruise Mach number increases beyond Mach 0.70 it becomes increasingly difficult for wing shape alone to maintain a laminar boundary layer due to the increased Reynolds number and wing sweep required to limit compressibility drag. The use of HLFC can alleviate the situation through applying suction ahead of the wing box through the windswept surface stabilising the laminar boundary layer and delaying boundary layer transition. Applying HLFC to the wing leading edge is not a simple undertaking since many constraints are given by load carrying structure, suction skin, high lift / shielding devices, wing ice protection system and HLFC suction system. Each on its own has to fulfil its proper requirements while sometimes conflicting with those of interfering or adjacent structures and systems. The existence of conflicting requirements and integration constraints ask for a trade-off study which is carried out in close collaboration between SONACA, City University London and Airbus Group Innovations. Airbus Group Innovations has the responsibility of defining the sectional shape, suction distribution and suction system. SONACA develops the suction skin concept and the wing ice protection system. City University London studies the Initial feasibility of suction chamber layouts. The paper is split into 4 main sections: Section 1 details the toolsets used to perform the studies described in this paper which include the 2.5D transonic aerofoil solver, integral boundary layer methods including the effects of suction, boundary layer stability analysis methods, methods to determine pressure losses within the HLFC suction system and methods to calculate suction pump power including pump drag. These methods are integrated together into a set of tools able to perform multi point suction optimisation studies used throughout this paper. Section 2 presents analysis undertaken to define a suitable aerofoil pressure distribution philosophy that is compatible with the use case flow conditions (Mach=0.82, Altitude 33,000ft, Sweep=32 deg, Chord = 3.5m). Aerofoil roof top pressure distribution philosophies ranging from conventional turbulent aerofoil (mildly decelerating roof top) to a Natural Laminar Flow (NLF) type (strongly accelerating) had suction distributions optimised for each design point (CL=0.48, 0.55, 0.63) to deliver minimum net drag (viscous, wave & pump drag contributions). Balancing pump power requirements against viscous and wave drag components presented a roof top pressure distribution philosophy / geometry that was best suited to the use case and was taken forward for further analysis. Section 3 details the evolution of the suction chamber layout from the early layouts that were very much aerodynamically optimum. The later layouts incorporated constraints due to the ice protection system and chamber extents necessary to integrate with the chosen SONACA suction skin concept while minimising the aerodynamic impact of these constraints. Section 4 continues to show details of multi point suction optimisation studies where net drag over 3 design points is minimised while observing requirements to have the suction distribution variation for the 3 design points achievable with variation in pump rpm only but with constant metering hole geometry. Included here is a sensitivity study to determine allowable departures from spanwise pressure uniformity within the chambers that gives minimal variation in wing drag. Finally, conclusions are presented whereby the integration of the suction system and pneumatic ice protection system has presented a significant conflict requiring the leading edge chamber to be dual use since it is critical for both controlling cross flow instabilities and ice protection. This complication limits the creativity of the HLFC system; development of a suction skin with an electric ice protection system could open the design space for more creative HLFC suction systems including partially passive suction architectures.