## 4 Data Quality Checks and Experimental Corrections
### Background Noise Correction  
Background noise correction consisted of subtraction of the narrowband spectra of background noise \(L_{BG}\) (i.e., noise when the landing gear model is removed) from the narrowband spectra of landing gear noise \(L_{LG}\) according to Equation (1). If the level difference \(L\) between the total noise and the background noise is less than 3 dB, then the correction can be considered as introducing too-large errors, so that the source level cannot be determined. \[L=10\ \mathrm{log}_{10}\ (10^{L}{}_{LG}/10-10^{L}{}_{BG}/10) \tag{1}\]  
To compare noise spectra, the acoustic signal measured over a time interval of 30 s at a sampling frequency of 51,200 Hz is separated into Hanning-widowed blocks with 66.7% overlap, corresponding to a bandwidth of 3.125 Hz. After Fast Fourier Transform (FFT), the  
Figure 8: Schemes of full-scale landing gear models with different geometric parameters: (a) landing gears with different number and diameters of wheels; (b) landing gears with different main leg heights. final spectrum is averaged over 280 blocks to smooth the curve. All the spectra displayed in this paper show the power spectral density (PSD) in dB/Hz (rel. to 2 10\({}^{-5}\) Pa). Figure 9 shows a comparison of the background noise and landing gear noise for both facilities when the flow velocity is 75 m s\({}^{-1}\); the landing gear model is the four-wheel model with the high main leg (Figure 5b) and microphone position (\(\theta\), \(\varphi\)) = (90\({}^{\circ}\), 20\({}^{\circ}\)). From Figure 9, it can be seen that for the full-scale model the landing gear noise exceeds the background noise by at least 3 dB in the frequency range below 10 kHz, whereas for small-scale models the landing gear noise becomes noticeable over the background noise only for frequencies more than 400 Hz. It is worth noting that a strong tone is present at 840 Hz for the small-scale landing gear model, whereas no such tone is observed for the full-scale model. It is likely that this tone is an Aeolian tone produced by the main leg of the landing gear model; its Strouhal number \(St\) calculated based on the main leg diameter \(d\) = 16.9 mm for frequency \(f\) = 840 Hz and flow speed \(V\) = 75 m s\({}^{-1}\) is \(St\) = \(f\)\(d\)\(/V\) = 0.19, which is close to the expected value of \(St\) = 0.2 for such a tone. The peak is absent from the noise spectra of the full-scale model, because its frequency at the full scale would be -77 Hz, i.e., it is below the low-frequency limit of FL-17 CARDC and is not visible in the spectra. The spectra in Figure 9 are shown for the whole frequency range from 100 Hz to 20 kHz. However, from the perspective of the spectra comparison, only a part of this frequency range is of interest. Indeed, after Strouhal scaling for the scale \(l\) = 10.9 the data from AC-2 TsAGI will be located in the frequency range below 2 kHz, so the higher frequency part of the spectra from FL-17 CARDC cannot be directly compared with AC-2 TsAGI results. Therefore, in the following analysis the highest frequency of FL-17 CARDC data will be limited to 5 kHz. On the other hand, the low-frequency limit of FL-17 CARDC facility is 100 Hz, which implies that only the part of the noise spectra from AC-2 TsAGI with frequencies above 1 kHz can be directly compared with FL-17 CARDC data. As a result, the high level of the background noise at low frequencies in AC-2 TsAGI will not affect the comparison of the spectra with FL-17 CARDC data. The tone at 840 Hz for the small-scale model (Figure 9b) will also be excluded from the full-scale frequency range, after the noise spectra for the small-scale model are scaled to the full scale based on the Strouhal number. Figure 10 shows the parts of the noise spectra from Figure 9 that are used for comparison of small-scale and full-scale noise data.