## 3 Results
### IWT Tests
#### 3.2.3. IWT Test Results  
In the following, the results related to the IWT tests performed on the coated and uncoated test article are reported. Test 1-2 T = -5 \({}^{\circ}\)C; v = 70 m/s; H = 3000 m; MVD = 20 \(\mu\)m; LWC = 0.3 g/m\({}^{3}\); \(t\) = 140 s--Baseline  
Tests 1 and 2, performed at \(-\)5 \({}^{\circ}\)C with a velocity of 70 m/s near the cruise velocity of the M28 PZL vehicle and an LWC of 0.3 g/m\({}^{3}\), represent the base-line tests for the uncoated and coated test article, respectively. Test 1 has been performed on the uncoated surface (Figure 9a-d) and test 2 on the coated one (see results in Figure 9e-h). The pictures taken after the two tests highlighted that in these conditions and for the selected exposure time, i.e., 140 s, the amount of the ice accreted on both uncoated and coated surfaces is poor and concentrated at the impingement area, whereas both inner and outer sides of the nacelle lip-skin were free from ice. Average thickness values of 1.78 mm and 1.16 mm were measured at the uncoated and coated leading edge, respectively. The accreted ice seems similar to dry rim ice [27; 28; 29]. Nevertheless, beyond the dense and compact ice, an area with isolated ice droplets appears, as schematically illustrated in Figure 3c, especially for the coated surface. Here, the isolated ice droplets are greater in quantity and size (Figure 9g) than those observed on the uncoated surface. Moreover, they have the same spherical shape as that observed for isolated droplets of ice frozen in static conditions (Figure 6). Test 3-4: T = -5 \({}^{\circ}\)C; v = 70 m/s; H = 3000 m; MVD = 20 \(\mu\)m; LWC = 1 g/m\({}^{3}\); \(t\) = 140 s--LWC Effect  
In order to study the effect of the LWC, tests 3 and 4 have been performed at an LWC equal to 1 g/m\({}^{3}\). Pictures taken after test 3 of the uncoated test article are shown in Figure 10a-c, whereas pictures of test 4 carried out on the coated test article are displayed in Figure 10d-f. By comparing the pictures in Figure 10 with those in Figure 9 (baseline test), it is highlighted that the effect of increasing the LWC corresponds to an increased amount of accreted ice, which was observed mainly in correspondence to the impingement area and alongside the internal part of the nacelle lip. Here, the ice has accreted as frozen rivulets instead of isolated ice droplets, as observed at the lower LWC. This is due to the larger amount of liquid water as the impacted water mass would only freeze partially, while the remaining water would coalesce into rivulets and run back over the downstream surface. Instead, the ice accreted in a spherical-like shape can be observed only at the impingement line, before the rivulets, on both the uncoated and coated surfaces. They seem to make a separation between the dense ice below them and the rivulets beyond the stagnation line. The formation of this spherical-like ice accumulated at the stagnation line along with the isolated rivulets downstream are typical of glazed ice accretion [27]. The average thickness of ice measured on uncoated and coated surfaces were, respectively, 1.75 mm and 1.74 mm. Measurements of the impingement length of the accreted ice, Lei, reported in Figure 11 reveal that no differences can be observed between the uncoated \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Test ID** & **Coating** & **T [\({}^{\circ}\)C]** & **v [m/s]** & **Liquid Water Content [g/m\({}^{3}\)]** & **Exposure Time [s]** & **Flight Path [km]** \\ \hline
1 & & \(-\)5 & 70 & 0.3 & 140 & 9.8 \\
3 & & \(-\)5 & 70 & 1 & 140 & 9.8 \\
5 & UNCOATED & \(-\)12 & 70 & 0.3 & 140 & 9.8 \\
7 & & \(-\)12 & 70 & 0.3 & 420 & 29.4 \\
9 & & \(-\)5 & 95 & 0.3 & 140 & 13.3 \\ \hline
2 & & \(-\)5 & 70 & 0.3 & 140 & 9.8 \\
4 & & \(-\)5 & 70 & 1 & 140 & 9.8 \\
6 & COATED & \(-\)12 & 70 & 0.3 & 140 & 9.8 \\
8 & & \(-\)12 & 70 & 0.3 & 420 & 29.4 \\
10 & & \(-\)5 & 95 & 0.3 & 140 & 13.3 \\
11 & & \(-\)5 & 95 & 0.3 & 140 & 13.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: IWT test conditions. and coated surfaces.