The calculator
Runs in your browser. Inches or millimetres at the top. Fractions such as 3/4 work.
How the geometry works
The FAA Airframe Handbook (FAA-H-8083-31, Chapter 7): “Some manufacturers give a taper ratio, such as 1:40, and others prescribe a taper distance like a 1-inch overlap for each existing ply of the face sheet.” Either way the result is a step per ply: the ratio times the cured ply thickness, or the overlap directly. At 1:40 with 0.0075 in plies, each ply steps out 0.300 in.
The taper on each side is the step times the number of plies. Across the repair: top ply = hole + 2 × plies × step. Repair ply k, counted from the bottom, covers the opening at the top of the parent ply it replaces: hole + 2 × k × step.
For solid laminates the Handbook reports scarf “thickness-to-length ratios of 1:10 to 1:70”, and asks for the exterior finish to be removed for an area at least 1 inch larger than the border of the taper. Both appear in the result.
Worked example
A skin of 8 plies, 0.0075 in cured each, damage cleaned up to a 2 in hole, taper 1:40 from the SRM.
- Step per ply = 40 × 0.0075 = 0.300 in.
- Taper each side = 8 × 0.300 = 2.400 in.
- Top ply = 2 + 2 × 8 × 0.300 = 6.800 in. Ply 1 = 2 + 2 × 0.300 = 2.600 in.
- Remove finish to 6.800 + 2 × 1 = 8.800 in.
Eight plies from Ø 2.600 to Ø 6.800 in, 0.600 in apart, and a sanded area close to 9 in across: for a 2 in hole.
Limits — when NOT to use this
- The taper ratio is a repair decision. It comes from the SRM or the engineering behind the repair. This page does not suggest one.
- Extra plies and orientation. Many repairs add a larger outer ply or specify ply orientations and stacking. Lay up to the repair drawing; this gives only the step geometry.
- Cured, not dry, thickness. Use the cured ply thickness from the material data. Dry fabric thickness overstates it.
- Honeycomb sandwich repairs taper each face sheet separately and replace core; see the SRM.
Questions people actually ask
Why does a small hole need such a big repair?
Because the taper is shallow on purpose: a gentle slope keeps the load path close to the parent's and the bond line in shear. At 1:40, every 0.01 in of thickness costs 0.40 in of taper on each side.
Does the drawing show real thickness?
No. The horizontal scale is true; the thickness is exaggerated so each ply is visible.
Sources
- FAA-H-8083-31, AMT Handbook — Airframe, Volume 1, Chapter 7, Repair of Composite Structures (Honeycomb Sandwich and Solid Laminates) — taper ratio “such as 1:40” or “a 1-inch overlap for each existing ply”; finish removed at least 1 inch beyond the taper; scarf thickness-to-length ratios of 1:10 to 1:70 for solid laminates.