Characterisation and Reduction of Aircraft Landing Gear Noise

Abstract

Aircraft noise has become an important concern in aviation design as it has a negative impact on the health of people living in the vicinity of airports. The total noise of an aircraft is generated by the engines and by the airframe as a result of unsteady flow over the aircraft structure, in particular high-lift devices, such as flaps and slats, and landing gears. While at take-off the airframe contribution to the overall noise is small and it is the engine (jet and fan) noise that dominates, at approach the airframe and fan noise are most significant, particularly during landing approach, when the engines are operating at low thrust. Therefore, reducing airframe noise is critical to the reduction of environmental noise for exposed communities. A lot of work has been done in order to study the noise generated from both nose and main landing gears. In particular, numerous experimental studies in open- and closed-jet wind tunnels, analysing landing gear flow and associated sound fields, have been reported. With some exceptions, the majority of experimental airframe noise research has been performed using small-scale models, with much of the work presented at 0.25 scale. This leads to great difficulty for full-scale noise predictions, due to insufficient detail in the geometrical modeling. Thus, despite these efforts, a complete understanding of the flow physics and noise generation mechanisms of landing gear flows is still lacking. The presence of some detailed parts of the landing gear, such as the torque link, doors and bay cavity, is often simplified or omitted, in both numerical and experimental studies. Add-on technologies such as perforated fairings and meshes are currently being evaluated as potential low noise technologies, which will result in even more detail to be modelled, with the dynamics of such additions needing to be evaluated. For this reason, further study into these flow mechanisms and the associated noise generation are required on complete geometries, including such components, and ideally at full-scale. This study considers all the geometrical details of a full-scale nose landing gear and a half-scale main landing gear. In particular, this thesis presents experimental and numerical results from the Euro- pean Clean Sky-funded ALLEGRA (Advanced Low Noise Landing (Main and Nose) Gear for Regional Aircraft) project. This project was developed in order to assess low noise technologies applied to a full-scale nose landing gear model and a half-scale main landing gear model of a regional aircraft. One of the significant contributions of ALLEGRA is that a full representation of the landing gear detail and associated structures (e.g. wheel bay cavity, bay doors, belly fuselage and hydraulic dressings) have been included and addressed at a realistic scale. The main objective of this study is to identify the principle aerodynamic noise sources generated by the nose and main landing gear in approach conditions and characterise these noise sources in the frequency domain. A numerical and experimental study of the wheel-bay cavity noise is also performed, in order to assess the contribution of this noise to the overall landing gear noise. It is also of interest to evaluate the achievements of some of the low noise treatments applied and to study the effect of these technologies on the main noise sources. Finally, the thesis compares the experimental results achieved in this work with those in literature, both numerical and experimental, to increase the understanding of the noise source mechanisms of landing gears, so that optimised configurations can be proposed for the future.