Which type of stitch is commonly used for attaching fabric to the spars?

The zigzag stitch is favored for attaching fabric to the spars because of its ability to provide flexibility and strength in the seams. This type of stitch can accommodate the natural movement of the fabric, which is crucial in aircraft covering applications where materials experience changes due to various stresses, such as temperature fluctuations and aerodynamic forces. The zigzag pattern helps to lock the fabric in place while allowing for some give, which prevents ripping or tearing under strain.

In contrast, a straight stitch is less effective in such applications as it lacks the same degree of flexibility and can lead to fabric tearing along the seam when subjected to stress. A double stitch, while robust in certain sewing contexts, may not provide the same adaptability required for the dynamic environment of an aircraft covering. Similarly, a blind stitch is generally used for finishing hems and might not provide the necessary strength and durability for attaching fabric to spars. Thus, the zigzag stitch presents a balanced approach between durability and the flexibility needed for aircraft fabric applications.