Definition
Two weave styles of fiberglass fabric used in composite aircraft construction. Bi-directional cloth has fibers running in two directions (typically at 90 degrees to each other), giving the finished part strength along both axes. Uni-directional cloth has the bulk of its fibers running in a single direction, giving maximum strength along that one axis but little across it. Both are layered with resin to form composite structures such as wings, fuselages, and control surfaces.
Plain English
Two types of woven glass fabric used to build aircraft parts. Bi-directional cloth is strong in two directions because its threads cross each other. Uni-directional cloth is strong in one direction because nearly all its threads run the same way. Builders pick the type based on which direction the part needs to be strongest.
Context Anchor
Seen in discussions of composite aircraft construction, especially when describing how fiberglass layers are arranged in an airframe part.
Derivation
Bi-' means two and 'uni-' means one (both from Latin). 'Directional' refers to the direction the fibers run. So the names simply describe whether the fabric carries strength in two directions or one.
Why Pilots Care
Selecting the correct cloth orientation determines whether a repair or component can safely carry expected flight loads without adding unnecessary weight.
Analogy
Think of bi-directional cloth like a woven basket -- strong when pulled either way. Uni-directional cloth is more like a bundle of drinking straws taped side by side -- very stiff lengthwise, but weak across.
Intuition Check
Do not read “cloth” as ordinary fabric used for covering or decoration here. In this context, fiberglass cloth is a strength-carrying material built into a composite aircraft part.
Example Sentence 1
The wing spar cap was built up using uni-directional fiberglass cloth so the fibers run along the length of the spar, where bending loads are highest.
Example Sentence 2
Bi-directional fiberglass cloth was used on the fuselage skin because loads could come from several angles during flight.