## 1 Introduction  
Landing gears are an important source of aircraft noise at approach and landing, which has motivated a significant amount of research on their noise [1, 2, 3]. A number of the studies have been concerned with comparing acoustic results for landing gears of different scales and geometric fidelity. A classic work on the topic was published by [PERSON] and [PERSON] [4], who compared the noise of small-scale models of landing gears with the results of flight tests. The small-scale landing gears had a generic geometry, which was different from the landing gears of the aircraft (Jetstar, BAC 111, HS 125, and VC 10) in flight tests. Nevertheless, the comparison was generally favorable, although there were discrepancies in the normalized noise spectra for four-wheel landing gears. A possible explanation of these discrepancies was proposed by [PERSON] [1], who noticed that the noise spectra for the flight tests with four-wheel landing gears were considered in [4] for the gear bay doors closed, while the small-scale tests were carried out with an open wheel well. If the small-scale data are compared with the flight tests with open doors, the normalized noise spectra demonstrate a much better agreement. Later, [PERSON] et al., performed a study [5] where a small-scale (4.6%) model of a DC-10 wing with a simplified main landing gear was investigated. The small-scale tests showed that the landing gear had a negligible effect on the airframe noise, which contradicted the flight tests where the effect of landing gear on measured noise was significant. Continuation of the work for a 6.3% wing model with simplified and detailed main landing gears of a B777 led to the important observation [6] that for higher frequencies the noise of the detailed landing gear model is higher than for the simplified one. This effect was then demonstrated for simplified and detailed 26% models of isolated main landing gears of the B777 [7], as well as main landing gears on the 26% wing model [8, 9]. A comparison for the 6.3% high-fidelity main landing gear model with B777 flight tests was provided in [10, 11], demonstrating a rather good agreement, despite significant scatter in the flight test data. The scatter often appears in flight tests results due to the uncontrolled environment of the experiment; therefore, it is important to perform full-scale experiments in the controlled environment of large anechoic facilities. Indeed, a new era of experimental research has begun with the acoustic measurements of full-scale landing gears in anechoic conditions. In [12; 13] such tests were performed for A320 and A340 landing gears, with a view on developing noise reduction technologies. A study on the full-scale B737 main landing gear with modifications and simplifications is provided in [14]; this work compares high-fidelity and low-fidelity full-scale landing gear models. A similar work was recently performed under the ALLEGRA project [15] for a full-scale nose landing gear model and in [16] for a 16.7% nose landing gear model of B77. A comparison of full-scale and smaller-scale (25%) main landing gear models of A340 is reported in [17; 18], where both the appearance of tones and broadband noise spectra were considered. The spectra for landing gear models were found to be in a good agreement, except for higher frequencies, which was explained by simplifications of fine details for the 25% model. Noise results for an 18% wing model with a main landing gear, a full-scale model landing gear and flight tests are compared in [19]. Full-scale tests and flight tests give rise to similar noise spectra, whereas 18% model tests turned out to provide lower noise levels. Novel tests with the 18% model are reported in [20], where the agreement with full-scale and flight tests was improved, although discrepancies both in low-frequency and high-frequency parts of noise spectra were still observed. These small-scale landing gear models had slightly different geometry than full-scale landing gear and did not reproduce fine elements of the full-scale model.