Paraglider Line Shrinkage & Aerodynamic Trim Distortion

The formula

This is the expression the tool evaluates. Every term is named underneath, with the unit it must be supplied in.

How the result is derived

Step by step, from the values you type to the figure on screen.

1. The 9 inputs are read from the form on every keystroke: Front Cascade Length, Rear Cascade Length, Front Line Shrinkage, Rear Line Shrinkage, Mean Chord, Design Angle of Attack, Lift Curve Slope, Design Lift Coefficient and Design Trim Speed.
2. Each value is checked against the accepted range in the input table below. A value outside its range stops the calculation rather than producing a misleading figure — the results blank out and a message appears.
3. The validated values are substituted into the expression above, which resolves Angle of Attack Change together with every supporting figure in one pass — no value is carried over from a previous entry.
4. The supporting outputs — Front Cascade Shortening, Rear Cascade Shortening, Differential, Resulting Angle of Attack, Trim Speed Change and Resulting Trim Speed — come from the same pass, so they always describe the same case as the headline figure.
5. Results are rounded for display only. The full-precision value is used throughout the chain, so reading a rounded intermediate figure back into the tool by hand can shift the last digit.

What each input means

Where to read each value on the floor, the unit it must be in, and the range the tool accepts.

What the tool returns

The headline figure and every supporting value it is built from.

Worked example

The tool loads with this case already solved — the Angle of Attack Change shown above is its answer. Change one value and the difference from this baseline is the sensitivity of the result to that variable.

How to use it

1. Work through the input groups in order — Line Set and Wing. The defaults are a realistic case, so you can change one value at a time and watch what moves.
2. There is no calculate button. Every figure recalculates as you type or drag, which is what makes this usable for a what-if sweep rather than a single answer.
3. Read Angle of Attack Change in the dark results panel — that is the headline figure, expressed in °.
4. Check the supporting rows underneath (Front Cascade Shortening, Rear Cascade Shortening, Differential, Resulting Angle of Attack, Trim Speed Change and Resulting Trim Speed) before acting on the headline — they are where an implausible input usually shows itself first.
5. Reset to defaults returns every field to the reference case, which is the quickest way to check whether a surprising result came from the tool or from an input you had changed earlier.

Where this is used

• Process planning — establishing Angle of Attack Change before a trial is booked, so machine time and material in Ropeway, Cable & Webbing Dynamics are committed against a calculated figure rather than an estimate.
• Costing and quotation — Angle of Attack Change is an input to the cost sheet, and quoting from a worked number rather than a remembered one is what keeps a margin intact.
• Troubleshooting — when the floor result drifts from plan, entering the measured values (starting with Front Cascade Length) shows how much of the gap in Angle of Attack Change each variable explains.
• Teaching and study — the accepted ranges bracket normal Ropeway, Cable & Webbing Dynamics practice, so moving one variable at a time shows the shape of the relationship rather than a single answer.

Assumptions and limits

• A rigid two-point rotation about the mean chord is assumed, which is a considerable simplification of a wing whose profile is held by many cascades at once — real line trim maps are measured line by line against the manufacturer's table, and that measurement is what a trim check consists of. The linear lift slope is only valid well below stall and says nothing about the behaviour that actually matters here, which is how much collapse margin has been lost. Lines can also lengthen rather than shrink, and sheathed and unsheathed lines age differently within the same set. **This is a decision-support estimate only.** Paraglider line trim is airworthiness-critical: have the wing checked and re-trimmed by a qualified service centre against the manufacturer's data.
• Every input is bounded to the range normal practice occupies (Front Cascade Length 2 to 12 m, Rear Cascade Length 2 to 12 m and Front Line Shrinkage -2 to 5 %, and so on for the rest). Those bounds are guard rails against typing errors, not a claim that the formula fails one unit outside them.
• The calculation is deterministic: the same inputs always give the same result. It carries no allowance for machine condition, operator skill, ambient conditions or lot-to-lot material variation unless an input above explicitly represents one.
• Nothing is sent anywhere. The maths runs in your browser, so the numbers you type never leave the page.

Questions people ask