Definition
The side of a vacuum-bagged composite layup that faces the vacuum bag during cure, as opposed to the tool side (which faces the mold or tooling surface). The bag side typically has a rougher, less dimensionally precise finish because it conforms to the bag, breather, and bleeder materials rather than to a smooth mold surface.
Plain English
When a composite part is being made, it sits on a smooth mold on one side and is covered by a plastic bag on the other side while it cures. The side that was under the bag is called the bag side, and it usually comes out a little rougher than the side that was against the mold.
Context Anchor
Seen in composite aircraft construction and repair, especially when identifying which surface of a part was against the mold and which was under the vacuum bag.
Derivation
Named directly for the vacuum bag used in the layup process. The term simply identifies which face of the cured part was on the bag side of the layup during cure.
Why Pilots Care
Pilots and owners dealing with composite repairs should know that the bag side is normally the rougher, less cosmetic surface. It affects how repairs are oriented and where filler or finish work may be needed.
Analogy
If you pressed wet paper into a bowl and covered it with plastic wrap, the bowl side would come out smooth and shaped by the bowl; the plastic-wrap side would be the bag side.
Intuition Check
Bag side does not mean the side of a storage bag or luggage. In this context, it means the side of a composite part that touched the vacuum bag during making or repair.
Example Sentence 1
The technician oriented the repair so the tool side faced outward and the bag side stayed against the internal structure.
Example Sentence 2
The bag side received an extra coat of dope to match the tension on the outer surface.