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Ref: International Council of the Aeronautical Sciences
In spring of 2006, Airbus decided to stop the development of the initial A350, which was based on the A330 in order to offer the customers an all-new aircraft with even better performance, e.g. higher cruise Mach number and aerodynamic efficiency (L/D).
The new clean-sheet design was named A350 XWB, where Extra Wide Body (XWB) referred to the modified fuselage cross-section.
The new design to provide a 25% reduction in fuel efficiency compared to its then current long range competitors. This was achieved with a new wing layout with more wing sweep allowing for a cruise Mach-number of 0.85. The highly tapered inboard loaded wing is optimized for aerodynamic cruise efficiency and low structural weight.
It has been designed in a fully integrated process including aerodynamics, loads and structures. A new high bypass ratio engine is contributing to the low fuel burn, as well as the low weight of the airframe, which was achieved by applying modern materials like Carbon Fiber Reinforced Plastic (CFRP).
The A350 XWB-900 is the baseline layout of a family concept, which adopted a stretched (-1000) and a shrinked (-800) versions.
The -900 accommodates 315 passengers in a typical two class layout, providing a range of up to 7750 nautical miles. The maximum take-off weight is 268 tonnes.
Airbus focused on multidisciplinary design of unconventional high-lift devices, which enabled the aircraft to achieve
• an outstanding climb performance during take-off, including hot and high conditions, leading to a low drag requirement
• a low approach speed (CAT D, Vappr ≈145kts) for safe approaches, supported by the large wing area leading to a moderate maximum lift (CLmax) requirement
• good handling qualities (A/C attitude in approach, pitch up characteristics, roll capabilities, etc.)
• favorable wake vortex characteristics
• low airframe noise to reduce impact on airport communities
The wing leading edge is equipped with a Droop Nose Device (DND) inboard and slats outboard. At the trailing edge two Adaptive Drooped Hinge Flaps (ADHF) are installed, covered by a droop panel and seven spoilers. Two ailerons are located outboard of the flaps.
The main purpose of leading edge devices is to protect the high-lift wing at high angle of attack (α) against too early flow separation, meaning to shift the αmax and subsequently CLmax to higher values.
By deploying a Droop Nose Device (DND), the local angle of attack is reduced far enough, to delay the stall.
The DND in combination with the engine installation leads to favorable stalling mechanism, where the inboard wing stalls, while the flow over the outboard wing shall still be attached and the roll control surfaces are not yet affected by the stall.