## 5 Discussion  
Let us consider the normalized noise spectra for the two facilities for different parameters. Figure 14 shows the spectra for the same landing gear model and flow speed, but for different azimuthal angles \(\theta\). The effect of flow speed is demonstrated in Figure 15 for the same landing gear model. Again, a good agreement is observed.  
Figure 14: Comparison of the normalized spectra of the four-wheel landing gear model with the high main leg. Flow speed is 75 m s\({}^{-1}\). Microphone is at (**a**) (\(\theta\), \(\varphi\)) = (70\({}^{\circ}\), 20\({}^{\circ}\)), (**b**) (\(\theta\), \(\varphi\)) = (110\({}^{\circ}\), 20\({}^{\circ}\)).  
Figure 13: Comparison of the normalized spectra of the four-wheel landing gear model with the high main leg. Flow speed is 75 m s\({}^{-1}\). Microphone is at (\(\theta\), \(\varphi\)) = (90\({}^{\circ}\), 20\({}^{\circ}\)).  
Finally, Figure 16 illustrates the effect of landing gear model geometry, providing results for the six-wheel model with high and medium main legs. The spectra collapse very well, with the possible exception of a hump, which is present in the FL-17 CARDC spectra at the frequency \(\sim\)1250 Hz. Localization of the noise sources for the frequency is shown in Figure 17 and demonstrates that the hump is likely to be related to the support structure between the wheels, so that the noise generation mechanism can be similar to that considered in [22] for two-wheel landing gear models. Investigation of wheels of different diameters in [22] leads to a conclusion that the tonal noise can be generated by a laminar flow around the support structure between the wheels and is more likely to appear for wheels with smaller diameters, which is exactly the case for the six-wheel landing gear model (Figure 8a).  
Figure 16: Comparison of the normalized spectra of the six-wheel landing gear model. Microphone is at (0, \(\varphi\)) = (90\({}^{\circ}\), 20\({}^{\circ}\)). Flow speed is 75 m s\({}^{-1}\). The main leg is (**a**) high, (**b**) medium.  
Figure 15: Comparison of the normalized spectra of the four-wheel landing gear model with the high main leg. Microphone is at (0, \(\varphi\)) = (90\({}^{\circ}\), 20\({}^{\circ}\)). Flow speed is (**a**) 34 m s\({}^{-1}\), (**b**) 55 m s\({}^{-1}\).  
In fact, it seems that while the results of small-scale tests agree reasonably well with the results of large-scale tests in terms of broadband noise of landing gear models, the small-scale tests fail to predict the tonal noise that appeared for some landing gear configurations in FL-17 CARDC. An example of this situation is shown in Figure 18 for the two-wheel landing gear model.  
It can be seen that the normalized spectra from AC-2 TsAGI tests do not exhibit the peaks that are present for FL-17 CARDC data.  
The inability of small-scale tests to reproduce tonal noise of the full-scale models is expected. Indeed, the tonal noise is known to strongly depend on the parameters of flow and model geometry [17]. In particular, the Reynolds number effect plays a prominent role in noise generation for both Aeolian tones and cavity tones. The results of the present study corroborate this conclusion that small-scale tests are ill-suited for tonal noise investigation.  
At the same time, a good agreement of noise spectra for broadband noise of the small-scale and large-scale landing gear models implies that the small-scale tests can be used to predict and reduce broadband noise of large-scale landing gear models.  
Figure 17: Localization of noise sources for the six-wheel landing gear model with high main leg. Flow speed is 75 m s\({}^{-1}\); frequency is 1250 Hz.  
Figure 18: Comparison of the normalized spectra of the two-wheel landing gear model with high main leg. Microphone is at (\(\theta\), \(\varphi\)) = (90\({}^{\circ}\), 20\({}^{\circ}\)). Flow speed is (**a**) 75 m s\({}^{-1}\), (**b**) 34 m s\({}^{-1}\).