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Plastic rods, yes or no?

from Michael Nesler

Almost all new paragliders designs now use plastic rods to support the leading edge of the glider. Manufacturers promise us better stability, increased performance and reduced weight through their use, but plastic rods can also be used to push performance limits and reduce production costs.

Not all plastic rods are the same!
Plastic rods are available in several different materials: Nylon (PA6.6), PVC, ABS, Fiberglas, Carbon and Titanium. The cheapest are made from PVC or Nylon, and here it is profitable for manufacturers to replace expensive Mylar and Dacron in production with them. A pleasant side effect is also the reduction in glider weight. Rod stiffness depends on diameter and materials used. Through careful choice, the „old“ support provided by classical Mylar/Dacron constructions can be replaced by using plastic rods without influencing flying characteristics or the safety of the glider. The result is a glider that is a little lighter, cheaper to manufacture and which can be presented as a new and innovative product. Using stiffer rods changes the way in which a glider flys and reacts to turbulence: cell sailcloth is kept taught like a drumskin and the profile leading edge becomes progressively more rigid. When the glider is spread out at the start the cells are held open – this makes inflating the glider on launch easier, the glider also recovers faster after collapsing under turbulence, and performance is increased through the improved aerodynamic form.
 
We noted during the development of the  Nikita³-Acro glider, that the use of plastic rods markedly reduced the chances of a cravat, when compared with Mylar/Dacron reinforcements. Two prototypes of the glider were built, identical apart from the reinforcements and tests clearly indicated that plastic rods were safer. A possible explanation for this behavior is, that gliders recover from collapses not primarily through internal pressure transfer over cell cross ports, but through the lift produced by the upper sail at the folding point. Gliders which don't have cell crossports open only a little slower than those that do. At the folding point, the profile camber increases which leads to decreased air-flow pressure over the wingtop. This in turn helps to „suck“ the collapsed cells up, and a chain reaction occurs until the wing is completely reinflated. Gliders with plastic rod reinforced leading edges seem to make this procedure happen faster – the rods help maintain an aerodynamic structure better than Mylar, which may fold internally and not create an aerodynamic camber profile.

To conclude, it can be said that the use of plastic rods instead of Mylar or Dacron reinforcements has positive benefits for glider weight, performance and recovery after turbulence.

The bad side of plastic rods
Over the last 20 years of paraglider development the size of leading edge reinforcements have become roughly standardised. There is hardly a glider on the market using reinforcements significantly larger than the average size. Optimal reinforcement size is the product of many years trial and error experimenting, and offers a balanced mixture of performance, safety and weight. On a paraglider, the position of the A-line suspension points is an important design parameter. The further back A-lines are placed, the better a glider tolerates turbulent airflow and the better it will dampen out
abrupt load changes. On the other hand, A-line suspension points position cannot be placed too far to the rear, as at some stage the support for the leading edge will be insufficient to maintain the profile during flight.
Using plastic rods it is possible to place the A-line suspension points further to the rear than before. This is beneficial for performance. Thanks to the reduced distance to the next suspension point, it is now possible to build gliders with only 3 or even 2 line at-tachment planes. In addition, higher performing profiles can be used, and their stability problems can be compensated for (at least under normal flight conditions) through careful choice of attachment point, rod stiffness and internal pressure. Profiles with a lot of camber towards the rear of the lower sail are typical for these new designs. They are very tolerant to changes in angle of attack, perform better and present the pilot with (apparently!) more stability. This apparent stability has its limits – once the glider enters a front- or asymentric-stall, then dynamics are increased and reactions become more violent and unpredictable when compared with conventional designs. Another problem is that gliders with plastic rods are difficult to test – inducing collapses requires more force and is hardly comparable with turbulence met in practice. Even the use of folding-lines or other tricks supplied by manufacturers is not really a solution.

 


Glider designs pushed to the limit with plastic rods

On the lower sail the suspension points for the A-lines must be placed such that the glider can still be launched – too far to the rear and the glider will only inflate in strong winds. Generally the A-line suspension points are chosen such that the glider can still be reasonably started. Depending on the stiffness of the rods used, the position can be placed quite a long way back from the leading edge and requires appropriately long rods built in to the lower sail. If corresponding rods are built into the top sail then the total stiffness of the leading edge becomes effectively doubled and the sailcloth is spanned taught like a drumskin. Pushing the design to its limits, it becomes possible to build a glider where the front quarter or even third of the profile is about as stiff as that of an old-timer hangglider, it's just that hanggliders don't collapse!
Long plastic rods in the upper- and lower-sail are problematic should the glider suffer a collapse. A collapse can fold the profile at the end of the rods and cause a sharp corner to form with no camber. This then delays or prevents recovery, and the collapsed parts of the glider do not deflate readily, they hang like a garage door in the wind and create massive resistance leading to violent pitching, rolling and yawing. The further back A-lines are placed, the more challenging a gliders behaviour becomes in the event of a collapse.

Rod technology can be followed further – placing rod reinforcements under the B- and C-line suspension points to better distribute loading, then fewer lines are required in total. Here there are two possibilities – either straight rods can be sewn into connecting cell hems on the upper sail, or arches are attached to the cell walls to load the lower sail. Together with the suspension point in the middle of the arch, there are then 3 load points distributed across the lower sail.

The question is, do we really need more and more performance to be happy and satisfied when in the air? In my opinion, performance is only important when a direct comparison is present, such as in competitions or when comparing points in an online contest. Pilots going cross-country, looking for fun on their local site, risking out-landings without overtaxing their abilities do not need that little extra performance. In particular because the performance gains are only marked when flying accelerated, and hobby pilots tend to avoid doing this except in emergency situations – sadly often close to the ground or a ridge.

In reply, many manufacturers and enthusiastic plastic rod glider pilots will defend the performance and greater stability of their new gliders. These arguments cannot be simply dismissed, high aspect ratio gliders in the hands of experienced pilots flying actively hardly ever suffer collapses, and when, then the collapses are mostly only small. This is due to the pilots themselves and the experience they have, and due to the enormous leverage the brakes have on the glider to correct the effects of turbulence. The higher the aspect ratio of a glider, the better the brake leverage, which in turn means that the brake travel distance can be reduced and also the corresponding reaction times required to achieve the same effect.
It's still better not to forget, that flying passively or not concentrating will still result in these gliders collapsing. It's no secret that most large (EN/LTF tests 75%) collapses on high performance gliders lead to more-or-less spectacular crashes. Statistics here are only relatively positive, because pilots try to prevent such collapses from happening in the first place, mostly with success.

Conclusions
Plastic rods have both advantages and disadvantages. When used responsibly, they provide clear benefits for pilots. If on the other hand, they are purely used to improve performance in glider design then they introduce a whole set of new dangers:
-    Unpredictable recovery behaviour after a collapse. Argumenting that "they hardly ever collapse" is not sensible if the answer to "and if they do?" is an uncontrollable crash.
-    Possible severe performance and safety degredation, should rods become bent or permanently deformed.
-    Increase danger of deep stalls when gliders age: porosity at the leading edge causes it to no longer remain in its designed position.
-    Difficult to pack, bulky for airline transport.
-    Loss of the “easiness“ of paragliding.