## 4 Repair Technology
The pieces of old bonding ply remain on the rubber after the separation of the metal shield from the rubber. There are two ways to remove them: mechanically or chemically. The mechanical attempts with sandpaper caused damage to the rubber. Therefore, the chemical method was applied. According to the manual [12] for the cleaning of the rubber on the leading edge, gasoline or isopropyl alcohol can be used. The tests showed that gasoline was not effective for the removal of the old bonding ply residue, so isopropyl alcohol was chosen. Cleaning was conducted by wiping the rubber surface with a cotton cloth slightly dampened in isopropyl alcohol. Isopropyl alcohol evaporates quickly after cleaning, so it does not pose a further threat to the rubber and to the new joint. The development of the surface preparation method was laboratory-tested using the water-break test method, using distilled water in accordance with [29; 30; 31]. During the repair of an actual main rotor blade, this test will not be possible because it would significantly complicate the bonding process--it is difficult to dry the water in the slots, as well as check that the drying was conducted correctly. It was assumed that surface preparation and cleaning must be performed according to the developed method. This guarantees a properly prepared surface for bonding. The surfaces have to be dried by a heat gun (Figure 7d) after chemical cleaning (shield with acetone and rubber with isopropyl alcohol). Although the fact that the solvents used evaporated easily, drying requires quite a long time to remove them from porous structures--sandblasted metal and rubber. Based on experience, 10 min at 35 \({}^{\circ}\)C was assumed to be sufficient. After the cooling of the surfaces, the primer is applied by a brush (Figure 7e). The primer is cured in ambient conditions for 8 h [32; 33]. Contrary to appearance, the application of adhesive paste is a complicated process. The paste applied to the target surfaces should be as shown in Figure 7f, with the thickest layer being about 3 mm. In this way, when the elements are pressed together, the excess adhesive paste will flow out, leaving an even layer between them. Applying the paste in the same thickness on the entire surface (without the characteristic bulges) or leaving a layer containing air bubbles will cause the adhesive to be uneven when pressed, or air bubbles will form in the center (Figure 8). The shield is composed of a titanium alloy characterized by a high modulus of elasticity. Therefore, clamping the shield during bonding requires significant force evenly distributed over the surface of the shield (Figure 7g). In order to achieve this, an assembly tool was designed and manufactured to press the shield properly against the edge, allowing the blade profile to be reproduced (Figure 9). The composite back parts of the assembly tool (Figure 9a) were installed on the trailing edge of the blade first. Next, the bonded shield was compressed to the leading edge using the composite front part of the assembly tool by hand after applying adhesive paste to the bonded surfaces of the leading edge under repair. Then, the front part of the assembly tool was joined to the back part of the assembly tool using a fastener (M6). Therefore, the shield was pressed to the leading edge. In order to additionally pressed the shield to the top and bottom of the leading edge, bar clamps were installed as the last step. The excess adhesive paste squeezed out was removed by a wooden spatula. Figure 8: Fracture surfaces of bonding layers: (**a**) Bonding ply with voids—wet adhesive paste was not properly applied; (**b**) Bonding ply without voids—wet adhesive paste was correctly applied. Figure 9: Bonding process: (**a**) Assembly tool—scheme; (**b**) Assembly tool—front; (**c**) Assembly tool—top/back; (**d**) Leading edge after repair. The last step was the curing process--hardening (Figure 7g) and post-curing process--improvement properties (Figure 7h).