Was it Spider-Man or Elvis who complained about not getting enough power despite having quality lines? Don’t expect to get the best out of your kite either if its connections aren’t optimized! Lines and bridles naturally evolve as the Dyneema shrinks, but they can also be intentionally modified for comfort or performance reasons. Let’s delve into these complex concepts of trim, angle of attack, backstall, and trim adjustment. By Vincent Chanderot with Donatien Roger in Glassy Vol. 1
The Dyneema (UHMWPE) fiber used for lines and bridles is known for its tendency to shrink, which can lead to a left-right asymmetry and a change in trim (front-to-back). While the front lines move very little due to the significant and constant load they bear, the rear lines, even those made of high-end pre-stretched Dyneema, will contract as they get thicker, which can dramatically alter the wing’s angle of attack.
AN UNSUITABLE SETUP?
Materials : Rear lines contract relative to the front lines over time, as the depower rope stretches.
Line Change: Switching from thick lines to thin lines reduces drag and brings the kite flying slightly closer to the wind window edge, making it feel less sheeted in.
Intentional Modification to Suit a Riding Style or Rider Size (used kites): Some riders shorten or lengthen the rear lines to improve bar feel or to adjust for arm length.. Some schools may lengthen them, as may riders with shorter arms using waist harnesses.
Development with a specific approach: Designers can choose a factory-setting that will better suit certain practices or riding styles than others, reflecting their signature in design. Donatien also acknowledges that the bridle settings, mostly for atypical sizes, can’t sometimes be as meticulously fine-tuned as the most popular sizes, given that a collection rich in models and sizes can involve setting nearly 150 kites in different conditions… so some might receive a more average setting.
TRIM FOR SPEED?
Pulling the trim-setting on the front lines allows for some correction of the trim that would be affected by retracting the rear lines, but that’s not its primary function. Its main purpose is to give the kite the correct angle of attack based on the conditions and the rider’s needs.
Trimming = the angle of attack decreases, in other words, the angle between the kite’s profile and the wind is reduced. The profile, almost aligned with the airflow, deflects it very little. This transmits less energy but gives the kite a higher airspeed. It will move further upwind to maintain its position. Is a kite optimized when trimmed or un-trimmed? For Nicolas Caillou (Zeeko, PLKB), a profile is obviously designed for a given angle of attack. It will then be the designer’s skill to define its position on the trim and adapt it empirically to other positions. Dom Zimmermann at Ozone says : « I don’t design kites for a given trim position, but for the entire intended range of angle of attack. So, theoretically, from 0° to 5-8° depending on the model and size…«
Un-trim: by easing the trim setting, the angle of attack increases, and you feel more pull in the kite… up to a certain point (the point at which the airflow will stall). Un-trimming doesn’t make you go faster, it just makes it pull harder! It can be advantageous to un-trim (for a powerful kite) for waterstarts, but then progressively trim (for a fast kite) to reach your top speed and the best upwind performance. Downwind, un-trimming allows you to move the kite further back in the wind window and increase drag for better downwind performance. With double pulley bars adjustable to the centimeter, racers constantly adjust the trim, just like sailors who adjust the mainsail traveler.
Note: we are talking here about trim on the fronts. With ‘displayers’ on the backs (click bar, etc.), the action is reversed.
FINDING THE STALL LIMIT SETTING
If you use your bar on multiple kites, adjust the kites themselves rather than the lines or bar’s pre-lines. It’s simpler and will remain compatible with all kites, provided, of course, that kite’s bridles are correctly adjusted and the rear lines are stretched. Avoid fiddling with the bridles, we’ll adjust instead the Dyneema connectors with extensions (or pigtails). Preferably adjust the rear lines to avoid altering the wind window.
- Two people are needed to launch, land the kite safely, and make the adjustments.
- An unobstructed area with wind at the lower end of its wind range.
If the wind is too light, the kite will tend to stall earlier. Be prepared in case it falls in the middle of the wind window!
– Position the kite at the zenith, completely release the trim (on the front lines), sheet in fully, and observe what happens:
We’re just looking for the setting at which the kite begins to stall. There’s no need to make the kite fall from the sky. If you see it stalls, raise your arms to get it flying again!
– Perform successive tests, gradually adjusting the trim length.
A- As long as the kite starts stalling when fully sheeted in: the kite is oversheeted. Pull the front trim until it stops stalling. Then measure the length of line used by the trim and transfer this measurement to the back lines using an extension (see C). In wakestyle, some riders prefer a kite that’s a few centimeters oversheeted. However, if the kite stalls when unhooked, it’s oversheeted.
B- As long as the kite doesn’t stall: the kite is undersheeted. We’ll shorten the back lines in 2cm increments using the knots closest to the kite (or creating them). If that’s not possible, we’ll add a 10cm extension to the front lines and shorten the trim until the kite stalls. We’ll keep the shortened extension on the front lines, adjusted to the trimmed length (e.g., 10-6 = 4cm).
If you’re only using the bar for only one kite, you can use the knots on the bar’s back pre-lines.
C- When the kite stalls, fully sheeted in: you’re not « limit of stalling » since you can backstall it. Lengthen the back lines by 2cm, and you should be at the limit. In reality, you’ll be « on the limit of stalling » when stationary on the beach, at the edge of the wind window. But on the water, in dynamic conditions, the apparent wind shifts and the kite will be more sheeted in. For Jules Chollet and Sky Solbach, developers at F-one and Duotone, if this setting is good on land, it will work well at sea.
Trim -> low incidence -> less power -> more speed
De-trim -> high incidence -> more power -> less speed
GLIDE
A kite, whether a foilkite or an inflatable one, looks a bit like a paraglider, whose angle of attack can be varied with a speed bar (accelerator) operated by foot. By pushing the bar out, we reduce the angle of attack and speed up the wing by several knots, but it will cover a shorter distance. Without using the accelerator, the lift of the wing is at its maximum and we will cover a longer distance, but more slowly. We pilots talk about « maximum glide » for wings, a term also used in our nautical sports, with or without a foil. In big air, this is exactly what hang time corresponds to: staying in the air longer by easing the trim to its maximum… which will also make the kiteloop less violent.
FOILKITES, HYBRIDS AND SINGLE-SKINS
In snowkiting, we appreciate being able to stall the wing to flop it down solo, but this does not require an oversheeted setting, since it is done by pulling the rear pre-lines. A foilkite can go over your head in gusty wind, front-stall, and fall. Their performance in the wind allows them to advance further toward the edge of the window than tubekites, and to flirt with angles of attack that can cause frontal closures (leading-edge collapses). To avoid this phenomenon, it would theoretically be necessary to relax the trim — which increases the angle of attack and prevents the wing from moving forward too much — and/or to temporize by braking when the wing surges, but that is not, in practice, feasible in the high wind range. This is where « reflex profiles » prove their worth. They are less efficient and less manageable, since a whole flap spontaneously rises on the trailing edge, but they are more stable at very low angles of attack and cushion sudden surges. Pulling the bar at this point cancels the reflex effect, because you’re pulling on the trailing edge instead of letting it rise. Even in the case of a strong surge, a sharp braking maneuver (a « temporization ») remains essential. However, if, to increase bar pressure, you adjust the rear lines too short — to the point of constantly deflecting the trailing edge — this will prevent the autopilot from engaging!
Donatien Roger leads a universal-kite bars company called Kite Attitude. Vincent Chanderot is editor in chief, rider and pilot.
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