Paraglider lines can be more than 7m long and less than 1mm in diameter. After 10's of hours in the sun, with cyclic loading, moisture etc. their length can change by ~1%. This may sound small, but it has a large effect on the "trim" of the glider. This project proposes a 2 point system that gives a precise way to measure the relative lengths of each line on a paraglider.
Compared to other line measurement kits, here are some Pros and Cons.
Pros:
Does not require expensive and bulky linear slides
Does not require careful setup to make sure everything is lined up and perpendicular
Target is automatically pulled perpendicular when the lines are pulled taught
Small/light enough to travel with.
Similar results to other Laser trim methods
Cons:
Line sag/riser weight can cause a small angle error
Target is smaller to minimize angle error
Takes more time to aim at this smaller target
Risers can flip over which requires walking between the wing and the risers to flip the risers right-side-up again
Some notes about some possible reasons why lines shrink (non have been officially proven/disproven):
Lines usually shrink, not stretch. Think of a new piece of clothing after its first wash.
If all of the lines shrink by 100mm, then the trim would still be okay since the relative lengths would still be the same. But each of the lines may shrink different than other lines.
I think cycling loading, twisting and other mechanical things have a large effect (I did a test where I placed a new, 2700mm-long, sheathed, dyneema line in a washing machine and there was no effect, but after twisting the lines up they shrank by 20mm)
Possible reasons why different lines shrink different amounts:
Since 1% of 7m is 70mm and 1% of 6m is 60mm. There can be 10-20mm difference in shrink depending on the overall length of the lines
Since the the rear lines on a glider are typically less loaded than the front lines, they will typically shorten MORE. This leads to a "slowing down" of the glider. (2cm of length is about 10-20% of the throw of the speedbar!)
Parts and Tools Required to make this kit:
1. Laser Holder
3d Printed Laser Holder
(I am embarrassed that its taken me so long to come out with this guide that this laser is no longer available. There are many like it on Amazon, but I have not tested any of them out yet, they would likely need to be attached to the holder with some other method like double-sided tape since none of them have the 1/4-20" mounting threads)
1/4-20 FHCS to mount laser Obsolete Since the Laser is Discontinued
Assembly step A. Inspect line guide and make sure it is free of debris that could get in the way of a repeatable measurement
Assembly step B. Attach laser to laser holder.
2. Laser Target
3d Printed Target
5mm or 4mm accessory cord https://www.amazon.com/BlueWater-Ropes-5mm-Accessory-Cord/dp/B0011W4LH4/ (or any paracord https://www.amazon.com/dp/B09TRD7JZ9/ref=twister_B09V19X484 )
Formable Stainless steel wire 28 gauge (or 2 high quality paper clips)
Wire cutters for cutting wire/paper clips
Needle nose pliers for handling wires
Carabiner: Aluminum Mallion Rapide, D-070-ZN, https://atlanticriggingsupply.com/products/pe7mm9zidequ?_pos=1&_sid=5f5e00285&_ss=r (or a carabiner if you need a larger attachment point)
Assembly Step A. Tie an overhand or figure-of-eight knot on a bite in the accessory cord. Make sure to tighten the overhand knot as much as possible. Using two carabiners to get a first grip on the cord can help.
Assembly Step B. Feed the accessory cord thru the target. The knot should be on the flat side of the target and should start 25-75mm (1-3inch) beyond the target.
Assembly Step C. Cut off a 3" segment of wire. For the next steps it can help if the ends are sharp. So cut the wire at a 45 deg angle so that its ends are sharp. Be careful! Wear Safety glasses and make the cut out of sight so that the loose end cannot fly away.
Assembly Step D. Fold the wire in half. Poke each end thru one of the pair of small holes in the target. Continue pushing so that they also pierce through the accessory cord. Note, if the 3d print was messy or if the wire gauge is too large, a drill can be used to expand the holes in the target.
Assembly Step E. Use Pliers to twist the two metal wires together. Clip off the extra wire.
Assembly Step E. Repeat the wire cutting, inserting, and twisting steps for the second placement on the target. This time however, make sure the rope is taut!
Assembly Step F. Check that the cord is well secured enough by pulling the each end of the tail and making sure that the cord does not slide relative to the target. The cord should move less than 2mm, which is about the length of one weave of the accessory cord.
Assembly Step G. To finish up the target tie an overhand knot on a bight on the rear end of the cord as well. This one does not need to be tightened like the other knot.
Assembly Step H. ****Attach the carabiner to the target end of the laser target.
3. Constant force spring block
3d Printed Spring Holder (Two Pieces, Top and Bottom)
QTY 2 Constant Force Spring, 5lbf : McMaster 9293K337
QTY 2 self tapping screws : McMaster 94629A310 (Or a similar wood screw will likely work!)
High strength string - 3mm Dyneema works well
QTY 4 6x22x7mm "608" skate bearings (any will do, even cheap ones like: https://www.amazon.com/dp/B07R7PR72H )
Assembly Step A. Slide 2 bearings onto each of the posts of the Bracket
Assembly Step B. Slide Both Constant Force springs onto the bearings. Girth hitch a loop of string onto the holes.
Assembly Step C.Thread a string through the block on the bracket. This will become the mounting point for the constant force spring block.
Assembly Step D. Use self tapping screws to attach the top hat to the bracket.
Assembly Step E. Attach the mounting end to a stable object and pull and the cable from the spring end. Confirm everything runs smoothly.
4. Simple Constant force alternative: 5kg weight and Pulley
Ronstan Pulley
String
A sandbag with 5kg of sand... (like those used for weighing down camera tripods)
(Or 2x 1.75L growlers full of water)
Assembly Step A. Tie a string to the bracket of the pulley. Feed accessory cord through the pulley. One side of the accessory cord will be tied to the 5kg load. The other will be tied to the back end of the target:
Putting everything together
First find an area big enough to lay out the full length of the glider
The canopy side of the glider can be left in a rosette, but it will be easier to stay organized and find the next line to measure if the space to work in is bigger.
Keep each riser separated from each other.
Mount the constant force block to a spot on the wall at approximately stomach height.
Attach the moving end of the constant force block to the back end of the laser target.
Attach the Left Riser to the carabiner on the end of the laser target.
Walk over to the canopy and find the inner most A cascade.
Slide the line into the groove on the laser holder until the girth hitch on the canopy tab reaches the laser holder
pull backwards on the laser holder so that the constant force is engaged.
Be careful, don't pull too far as the spring can come unwound after 2 feet of pull.
Hit the measure button on the laser. aim the laser at the target. When the laser is visible on the target select the measure button again and record the measurement 6.523m = 6523mm (note, you may need to set the units in the laser ruler so that it is in meters or millimeters)
Test your repeatability. Without releasing the load, try measuring the same line 3+ times. Record the range of measurements. It is normal for the measurements to vary by 2-5mm especially for lines that may relax after being loaded. If there are any larger variations, however, take the time to figure out what might be causing the variation.
Test your reproducibility. Release the load and remeasure the same line 3+ times. Record the range of measurements. Confirm that the range of measurements make sense.
Test the target variability. Without releasing the load, measure the same line 3+ times, but make sure to aim at different areas of the target. We want to make sure that no matter where on the target we aim to, we get a measurement that is within our uncertainty allowance of 2-5mm.
Repeatability and reproducibility is everything for line trimming! So if these tests seem good, then proceed to measuring the other lines on the glider!
The trim sheet
Open up https://docs.google.com/spreadsheets/d/1GpzM81scyKkIJCpFvK0DdlKnRfCVztLzXDeeEX-875o/edit?usp=sharing
Make a copy onto your own google drive and customize it based on your own wing.
You'll need to get information for the mfg for the trim check numbers.
Sometimes these are on their website, other times you will need to send an email to the mfg for the numbers.
If the wing was certified by DHV or Air Turquoise, the certifying agency will also make trim check measurements. It is worth noting these are of the specific wing that was certified - not the nominal lengths the mfg uses. There are ups and downs of using the DHV numbers vs. the MFG numbers.
Follow instructions on the first sheet of the trim template.
Start taking measurements!
The chart is intended to be filled out Top-to-bottom and column by column. So, once you finish column A, start moving to the right.
Using Everything
After making the measurements, you get to the second half of the trim sheet. Columns N+
The goal here is to make adjustments to a whole cascade of lines in such a way that it makes the set of lines more trimmed overall.
Note, if the A's and B' are too long compared to the C's, you may need "add a loop" to the A's and B's. This will effectively shorten the A's and B's Since the lines are relatively long compared to the adjustments, the overall length is not as critical as the relative length of each line.
Making adjustments is an art where the goal is to balance left-to-right as well as fore-to-aft. Talk to a friend to get suggestions about what to adjust.
After making adjustments remeasure to make sure you had the desired effect! Taking it to kite to see if you can feel the difference :)
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