Next generation wings for long range aircraft: hybrid laminar flow control technology drivers

Abstract

Due to the societal need for mobility of more and more people, the need to reduce CO2 emissions in civil aviation is very high and led to Europe’s effort labeled “Green Deal”. New aircraft with (hybrid) electric & distributed propulsion for short & mid-range are the focus of actual research. In contrast, long-haul aircraft will continue to have jet engines in the future. Therefore, a rather classical two wing mounted engine configuration, but with significantly boosted performance and reduced emissions footprint is very likely. Beside the use of synthetic fuels and more efficient engines, drag reduction for transonic aircraft (approx. Mach 0.85) has to be obtained. Hybrid laminar flow control (HLFC) technology offers a remarkable drag reduction potential. Aerodynamic efficiency of hybrid laminar flow control has been demonstrated in several wind tunnel and flight tests from the physics point of view. The major drawbacks preventing the application of this technology are the complexity and the high production & operating cost associated. Fighting these major drawbacks is the objective of the HLFC projects under the CleanSky2 umbrella. In this article, overviews of the important technology elements developed by DLR for HLFC purposes are presented. The following technology bricks are described:  Tubeless suction system  Inductive heating based wing ice protection system (WIPS)  (resulting) Aircraft performance assessment By using distributed compressors and consequent structural integration, an almost tubeless suction system can be realized. Hence, large vacuum tubes can be avoided and the space allocation for the leading edge is relaxed. In order to reach such a tubeless suction system, the structural integration and the development of a lightweight compressor with suitable mass flow are addressed. The integration of a WIPS in an HLFC wing is very challenging. Classical bleed air WIPS consumes a lot of energy and requires tubing; electro-thermal WIPS with heating stripes induces a lot of blockage to the perforated outer skin. A promising alternative solution is the inductive heating, which can induces heat in the titanium skin contactless. The maturity of inductive WIPS is low, but the opportunities seen for the HLFC technology outweigh their risks, which is why this technology was chosen. In order to enable laminar flow, high surface tolerances have to be ensured for the laminar flow part of the wing (beyond the leading edge). In order to ease the operability of a HLFC wing, a design with a removable outer skin for the active part performing suction is developed. In case of contamination or damage (e.g. by insects or hail) the perforated outer titanium skin can be replaced individually. It is therefore not necessary to replace the complete wing leading edge during operation of the aircraft. To constantly feed the design process, the overall aircraft assessment is used to identify the correct balance between the figures of merit (drag reduction, mass and power off-take) and the cost of the envisaged design including materials, manufacturing and assembly. At the moment, the project is right before Technology Readiness Level (TRL) 3 where all relevant sub-technologies are down selected and an overall wing concept is established. The preliminary assessment reveals a 5.5% drag reduction. The overall concept will be now matured by analysis of small scale demonstrators focusing on key technical challenges to verify the initial performance gain level.