Which type of external patch has edges of the patch and skin overlap?

The type of external patch where the edges of the patch and the skin overlap is known as a lap patch. In this repair technique, the patch is laid over the damaged area of the skin, and the edges are folded over the underlying structure, creating an overlapping effect. This design enhances the strength of the repair by distributing loads over a larger surface area and helps to prevent stress concentrations that could lead to further failure.

Lap patches are particularly beneficial in aircraft applications because they provide a reliable means of reinforcing the structure while ensuring a streamlined surface. The overlapping nature of the patch allows for better bonding and can be more aesthetically pleasing, leading to improved aerodynamic qualities.

In contrast, other types of patches, such as a scab patch, do not overlap in the same way. A scab patch is applied directly over the damaged area without the edges overlapping significantly with the surrounding skin. A corner patch is specifically designed to reinforce corners or areas where two surfaces meet, typically not involving overlap with the skin in the same manner as a lap patch. A through patch usually refers to a type of repair that extends through both sides of the structure, which also differs from the overlap characteristic of a lap patch.