Condensed version of a DHV article by Michael Nesler | Click here for his detailed original
Plastic rods in paragliders have become ubiquitous. Is it hype or do they help as much as they seem to? DHV pilot Michael Nesler sheds some light.
There are different types of rods and they can indeed produce several benefits with less weight and lower cost than prior stiffening methods with expensive mylar and Dacron. Thicker rods are stiffer which can improve glider behavior and performance when used in the right areas. Types are Nylon (PA6.6), PVC (cheapest), ABS, Fiberglas, Carbon, and Titanium. They all keep the sailcloth tight, cells open for better inflation, improve airfoil shape, and can hasten recovery after deflation. All good.
Mr Nesler observed that rods reduced the chances of cravat in a pair of prototypes that were otherwise identical: the other had Mylar/Dacron reinforcements. Rods were safer in this case.
There’s always a downside
Nearly all paragliders made in the last 20 years use similar leading edge reinforcements. It provides a good balance of performance, safety and way. Plastic rods allow A-lines to be placed a bit farther back from the leading edge while still holding the profile and has one more benefit. With the A lines back more, it’s easier to make three, or even two-line gliders and better airfoil shapes. A downside is that once a glider enters front or asymmetric stall, reactions can be more violent and unpredictable than conventional designs. Plus, gliders with plastic rods are difficult to test in a way that predicts what happens in real turbulence.
Pushing Limits with Plastic Rods
Performance improves but inflation suffers if A-lines are positioned too far aft of the leading edge. Stiffer, longer rods lets this point be moved quite a ways. Doing this on the top and bottom increases performance even more. But collapses get more dicey. The profile can fold at the rod ends which causes a sharp corner with no camber. That delays or prevents recovery while the folded part becomes a barn door to the wind, causing violent pitching, rolling and yawing. More aft: more bad.
Rod reinforcements are sometimes also put under the B- and C-line attachments to better distribute loading, further allowing fewer lines. Performance improves–more at higher speeds–but possibly at some increased peril.
Not surprisingly, manufacturers and pilots of these craft defend their new gliders, citing better performance and stability, arguments that can’t be simply dismissed. Even high aspect ratio gliders, when flown by experienced pilots flying actively, hardly ever suffer more than minor collapses. But that’s due to pilot skill and the enormous brake effectiveness which even more on higher aspect ratio gliders. But fly passively and these gliders will bite. In EN/LTF tests of 75% collapses on high performance gliders there are frequently spectacular crashes (into water, presumably).
Stiffening rods have pros and cons with more pros than cons. When they’re used to push performance boundaries too far, though, risks increase:
- Unpredictable collapse recovery behavior.
- Possible severe performance and safety degradation should rods become bent or permanently deformed.
- Increase danger of deep stalls in aging gliders if porosity at the leading edge allows the rod to move from its designed position.
- Increased packing complexity.
- Some loss of “easiness“ in piloting.