Author: Peter Slade
Line trimming is necessary for all paraglider pilots looking to maximise performance, and in
avoiding the inherent degradation of performance, handling and safety over time. There are
various line materials in use each of which have their own characteristics. The length of the
lines change depending on the material, loading, temperature, humidity, dust ingress and UV
exposure.
Some of the benefits of trimming are;
1. Correcting the profile and trim of the glider so that it flies as it was intended in relation
to handling and speed.
2. For safety. A better handling glider which handles turbulence, has better pitch
stability, turns, collapse recovery and normal stall speed.
3. It is necessary in order to be competitive at high level competition.
4. To intentionally increase the intended speed of the glider by changing the relative
trim*
5. Required to comply with certification. In order to compete in PWC and Cat 1 glider
must pass line length verification tests.
6. The psychological benefit of knowing that your glider is at its optimum.
*Many pilots at the top level change the relative trim for the purpose of increasing the
intended speed of the glider.
The downsides of trimming the glider fast are;
1. Collapses at higher speed requiring more pilot expertise.
2. Reduced climb performance.
CIVL states that a glider trimmed slow by greater than 10mm is out of trim and ´´potentially
unsafe´´ This view is shared by PWC, manufacturers and test pilots. My ICEPEAK 8 was out
of trim by more than 10mm in 3 cascades with 20hr flight time.
Here is an excerpt for CAT 1 line length verification from CIVL.
11.3.1 Relative line length verification 1: Angle of attack test
The purpose of this verification is to detect deliberate changes to the canopy’s angle of attack with the intention of
increasing the paraglider’s performance. Additionally, this verification also allows pilots to verify whether their
wing’s line set complies with certification. Verification is done by executing the following test procedure:
1. The length from the carabiner end of the riser to the sail is measured at each line attachment point.
2. For each line group (groups based on lower main lines: A1, A2, A3; B1, B2, B3 etc) the average line length is
calculated.
3. For each line group the difference between the average A line length and the B line length is calculated. (A1 –
B1, etc). The wing passes if the difference between average A line length and average B line length matches the
value specified in the manual with tolerance +2cm/2cm.
4. The glider fails if it is trimmed fast >2cm. If the glider is trimmed slow >+10mm the pilot should be warned
that the glider is out of trim and potentially unsafe.
11.3.2 Relative line length verification 2: Camber test
This verification only applies to gliders with 3 or more lines chordwise. (see §4.17). The purpose of this
verification is to detect deliberate changes to the canopy’s camber (the arching of the profile in chordwise
direction) with the intention of increasing the paraglider’s performance. The tests performed in 11.3.1 are
repeated for each of the B and C line groups (groups based on the lower main lines: B1, B2, B3; C1, C2, C3 etc)
to confirm that aswell as the AB differences, the difference between B and C groups matches the values
specified in the manual +2cm/2cm.
11.3.3 Absolute line length verification: 3. Arc test
The purpose of this verification is to detect deliberate changes to the canopy’s spanwise arc with the intention of
increasing the paraglider’s performance. Verification is done by executing the following test procedure:
For each line attached to the tested paraglider’s canopy, calculate the difference between the nominal overall
length given in the paraglider’s user’s manual, and the actual measured overall length. The glider fails if both
lines of three or more symmetric line pairs differ from the manual nominal line length, with tolerance +5cm/5cm.
These are the PWC Competition Rules for the 2015 season. Refer to appendix E2
What do we need to check line lengths and trim?
1. A laser measurer.
I purchased the Leica disto D110 on ebay for $155 delivered. It has Bluetooth which is
worthwhile for remote recording of values. The laser allows us to get repetitive
measurements with 1mm accuracy.
2. A line tensioning system.
A good system is smooth, calibrated accurately with good repetitive accuracy. The normal
setup is to attach one riser to the system which is at the same distance as the target on a
backing The system has a pulley and a weight between 3-5 daN ( 3-5.089kg)
The system below is the best commercial system I have seen online and has the advantage
of the laser connected permanently to a rail. The recording system also appears to be highly
automated. Fast and efficient.
http://www.highadventure.ch/en/railcheckinnovativeslaserleinenmesssystemen.html
The system below has the riser attached to a sliding kitchen draw rail.The table appears
fixed to the wall. And there is a pulley and line passing down through the table.
3. Excel template for storing and interpreting measurements.
This is a challenge to set up the first time but it is possible to share templates.
You will need to have the measurements provided in the manual for LINES + RISER +
MAILLON mm
Example provided below:
If we take the lines as 4 rows as above, I measure all the As, half the Cs in the 1st and 2nd
cascade, and all the Cs in the 3rd cascade.
I measure to roughly the beginning of each tab, which is the material that attaches upper
lines to the sail. We may improve the accuracy and speed by using a mount on the laser
measurer so that the tab runs up to it.
When setting up an excel template include:
1. Factory Measurements
2. Your Measurements
3. Difference between 1. and 2.
4. Relative trim for each line.
5. Average relative trim on each cascade.
6. What trimming has been put in, and what was change in mm (important for future
reference)
7. Difference in relative trim between left and right for each cascade.
It is worth noting that the measurements change depending on the temperature and humidity
and the amount of weight used. So it may be best to complete measurements at one time
and indoor preferably. Despite 5daN being the weight force used by PWC for line
verification, it seems to lead to more stretching which causes the values to change during
measurement. Using 3kg doesn’t seem to cause these varying measurements as much.
How to interpret the values?
1. Look for anomalies within different lines on the same cascade. Consider placing
loops high in the cascade for changes as small as 5mm, with the help/advise of a
professional.Try to make the cascade as uniform as possible. This is important as
loops on maillons will not correct discrepancies across same cascade.
2. In order to correct relative trim place either half loops, full loop or 2 half loops on
maillon.
These changes cause the following length changes on the icepeak 8. The amount of change
depends on thickness of the line and maillon.
Maillon A 1st 2nd cascade FULL LOOP15mm - 17mm
Maillon A 1st 2nd cascade HALF LOOP 9mm - 11mm
Maillon A 3rd cascade HALF LOOP 15mm - 16mm
Maillon B 1st 2nd cascade HALF LOOP 8mm - 11mm
Mallion B 3rd cascade. HALF LOOP 7mm - 8mm
Here are some examples of different loops on maillons. Initially there was some concern that
it may reduce the breaking strength of lines. It has been shown by one manufacturer not to
do so and it is standard practice at top level comps.
Some other options for correcting trim
1. Different sized maillons
2. Soft links
3. Releasing factory placed loop in B lines(if present) 12mm change approx.
Many thanks to Ivan who has helped me with trimming and sharing his knowledge.
Martin Havel has also kindly offered to measure gliders at the CHGC for a small fee.
Author: Peter Slade