## 3 Results  
First, a series of measurements were carried out with the drone switched off. These measurements serve as a basis for comparison. If the same trend is visible in a similar form later in flight, then the influence of motor vibrations and other external influences during flight is negligible. The result can be seen in Figure 9. Each removed screw results in a significant increase in the damage indicator.  
For simulating flight conditions, the drone hovered above the table in the laboratory. As it was fixed to the table for safety reasons, the flight control system could not change the position of the drone as desired in order to remain balanced. This repeatedly led to heavy touchdowns and impacts on the table, causing the drone to shake. These impacts are comparable to the effect of outdoor turbulences, but many times stronger. It can be assumed that no similarly strong effects are to be expected during a real flight.  
Figure 10 shows the raw signals in the time domain together with the signals resulting from the FFT in the frequency domain. The 100 ramps of a measurement were averaged. Semi-transparent areas show the minimum and maximum variances of the ramps from their mean value. It should be noted that the variances are very small even in flight. The setup is very precise and has high repeatability despite external influences. Only at low frequencies in the frequency domain is a larger variance visible, which is why the low frequencies are ignored by a range gating when calculating the DI.  
Figure 11 shows the DI during hovering with the same clear trend as when the drone was on the ground. For a systematic evaluation, each condition was repeated three times. In the figure, the baseline state has only two bars, because one measurement serves as a reference for all other measurements and is therefore zero. The drone was temporarily on ground between the measurements. The three DI values for each condition differ only minimally from each other and agree sufficiently well. Only the last measurement in condition _Dam. 5_ showed an outlier. After carrying out the three measurements of _Dam. 5_, one battery of the drone had to be recharged. After that, an additional measurement was taken to ensure continuity. The DI of this fourth measurement is slightly higher than the DIs of the first three measurements. Apparently, the DIs are sensitive to the available power of the motors. However, reloading does not mask the effect of the next damage step. In a real-world application, changes in engine power during flight would have to be taken into account. The same is probably true for all dynamic changes during flight. The extent to which these factors influence the DI in a significant way still needs to be researched.  
Figure 9: Development of the DI for the drone on the ground. A clear trend can be observed as the number of removed screws increases.  
Both the results on the ground and in hovering flight also show the influence of the size of the removed screws. The size of the screws starts with a diameter of 2 mm and increases with _Dam._ 2 to 4 mm, with _Dam._ 4 to 6 mm, and with _Dam._ 5 to 8 mm. The steps from _Dam._ 4 to _Dam._ 5 and _Dam._ 6 to _Dam._ 6 are visible in the measurements on the ground in Figure 9. In the measurements during hovering flight, the step from _Dam._ 3 to _Dam._ 4 can also be seen in Figure 11.