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27-10-2006 > Windtech design technology secrets
Alvaro Valdes, chief designer at Windtech, reveals his paraglider design secrets and opinions in a conversation with Spain's Vuelo Libre Parapente magazine.
Translation: Gabriel Canada / Carlo Borsattino.

What are your main technical contributions to paragliding, and what was your greatest step forwards in paraglider design?

Windtech's highest priority has always been our customers' safety, from the beginner to the expert and competition pilot. I think this our main contribution to paraglider design - a constant evolution in all parameters up to our current new generation of paraglider aerofoil profiles.

Our very latest aerodynamic profile, used on the Tempest, Kali and Combat, has extremely good behaviour characteristics. Into it we have implemented our own unique SAS (Safety Active System) technology, a double layer of cloth along the leading edge with special 'floating' reinforcements which improves the wing's resistance to collapses.

Throughout our history we have made many innovations, but probably the most spectacular design was with the Kronos. This had an inner structure with 100 cells and X-diagonal ribs, a state-of-the-art creation admired by all designers… and impossible to repeat due to its incredible complexity!

What is your view about following the design path of increasing aspect ratio, and where is the limit for this?

The secret of paragliding's great success is its pure simplicity - of use, transportation, storage, etc. The price for this simplicity - the paraglider's lack of rigid structures - has a big impact on performance and safety. We will never achieve the performance of gliders that have a rigid structure. However, since the beginning of the sport designers have sought to increase the aspect ratio, especially with entry-level paragliders. A high aspect ratio wing without any rigid structure by definition brings with it many potential problems. Although every glider designer has to take the aspect ratio into consideration we should not get obsessed with always increasing it, although we will continue to work on ways to find solutions to the problems.

What do you think of 'winglets' on the upper surface (Advance) or on the wingtips (Bionic)?

All designers know that winglets are superfluous for a glider flying at 35km/h, and that without them the glider will fly better. However as a marketing idea it seems to bring good results in terms of sales. Regarding the Bionic, I have a good relationship with its designer and I think it is a great idea. It's original, pretty and flies well.

And the 'reflex' profiles used in some paramotor wings?

For paramotoring in particular you have to concentrate on the safety side of things, even more so than you do in other flying activities. This is what paramotor pilots are looking for, especially as most are more relaxed about their approach to flight, with less experience of flying in turbulent conditions. The incorrectly-named “reflex” profiles (all profiles raise the trailing edge when accelerated) are those with the stagnation point moved forward and the lower surface made deeper. This makes them fly faster but the lift generated is significantly reduced. They have good collapse resistance but are more difficult to take off with.

The problem with so-called reflex profiles comes when they do collapse, or receive a negative angle of attack. Because of the design of their lower surface the response is quite violent, much more so than for a normal profile which may collapse more easily but is much easier to control when it does. This is a vicious circle and can be a very dangerous one. The pilot wishes to fly faster in windier, more turbulent conditions and nearer the limits of dangerous negative angles of attack. This is a dead-end street for paramotor wing design, since most paramotor pilots fly only for fun and want to enjoy nice easy flying. Added to this, since the amount of lift generated by these profiles is less you need to rev the engine more to climb and even to maintain height. The net effect is like flying a normal profile paramotor wing but two sizes smaller - which will be much easier to control if a collapse does occur.

Why are the cell openings such an important part of the design?

The position and shape of the leading edge cell openings is an important parameter which paraglider manufacturers and designers have been working on since the very beginning of paraglider design. They have most effect during launch, accelerated flight and recovery from unstable flight situations. Put simply, the larger and more forward the cell openings are the better behaviour you will get for these three conditions, but this will also degrade the aerodynamic efficiency of the leading edge.

The rigidity of the cell openings is also very important, more so than their shape since during steady flight there is only airflow through them in cases of pitching or collapsing. The division of the laminar airflow between the upper and lower surfaces is well ahead of them. As always the important thing is the balance of all parameters. It's not always possible to make simple statements as a good solution for one problem might not be so for another

What about the latest fashion of using CFD (Computational Fluid Dynamics) in paraglider design? How much computer and how much flight test there are in your designs?

Software has an important role to play and we have developed our own exclusive programs and methods of working for solving some problems with software which before we could only solve with live flight tests. Nevertheless there is nothing more real than actually flying. We are surrounded by infinite variables (you have to remember that paragliders experience continuous deformations in all three axes) and software is just one of the many tools in a designer's toolbox. Flight tests and modifications can be expensive, but the experience you gain from these are invaluable and the conclusions are firmer - especially the negative ones!

How would you describe your new high performance competition wing?

The Combat is the result of all our experience and knowledge to date, with this new wing we have taken the balance and trim of our new profile somewhat further. Our goal is the same as always: To be in top 2 or 3 for performance and without doubt number one for safety in it's category. I firmly believe we have achieved this. The Combat has excellent launch characteristics, handles very nicely indeed and can turn as flatly as the pilot desires (giving a superior climb rate). It's special double leading edge (S.A.S) ensures a great safety margin when using the speed system or crossing a turbulent area.

Collapses on the Combat are not overtly traumatic, surging forwards only slightly with hardly any turn at all, and recovery with only minor pilot input is practically instantaneous. Even with more than half of the wing collapsed you can still easily pilot the wing using opposite brake and weight shift, and be able to steer away from the collapsed side. Cravats are not an issue with this wing.

However, above all of this, what is most important in any wing is the 'feeling' and feedback it gives you, without any negative aspects which could instil fear, anxiety or loss of concentration, and that it handles with absolute fluidity, leaving you free to concentrate fully on your flight. This was our goal when we were designing the Combat.

Since the Kinetik, Windtech are now very well known amongst paramotor pilots. How do you see the evolution of wings made specifically for powered paragliding?

Some paramotor wing designers have sacrificed launch and safety characteristics to go a little bit faster. This is not our philosophy. Take-off and landing are generally the most critical moments when flying any type of aircraft, and especially so when paramotoring - when you add 35kg of extra weight on your back you'd better think very carefully about it! You don't want to get a 70% wing collapse low down with the thrust of an engine behind you and the trimmers fully released, without enough height to recover and return to safe trim. As always, our customers' safety  is our highest priority - we must strive to prevent accidents occurring whenever we can!

How do you see the EN certification? Will you use it, continue with DHV or use both?

Whatever the pilots want! We hope on behalf of the pilot community that the certification matter will unify and that the French and German factions will abandon their political war and their fight for power.

What is the difference between your new 1-2 glider, the Kali, and it's predecessor, the Pulsar?

The Kali belongs to the new generation of profiles, developed as a response to changes made to the certification processes with less surging forwards, milder reactions to any kind of instability, tucking without turns – a pilot’s guardian angel!

Without forgetting the pilot sector which this wing was aimed at (intermediate) we did not want to compromise on performance. Performance wise the Kali is somewhat superior to the Pulsar, and in terms of accessibility it is also greatly improved. We can say that the spectrum of pilots for which the Kali is suitable has grown considerably, on the lower end due to its ease and simplicity, and on the higher end because it's improved performance.

The Kali was designed with new software, with a very innovative inner rib layout. It has a thicker profile, especially toward the wingtips. The vault, planform and sizing are all new. In the end it has nothing at all to do with the Pulsar. It is a totally new generation of paraglider.

I advise all low airtime pilots, and more experienced ones, to try out the Kali's great quality for themselves. They will not be disappointed!

How long is the useful live of a paraglider? When should we stop flying it?

It is very hard to be precise about the average life of a paraglider – actually, it's almost impossible. I know gliders which are ten years old and still in very good shape, and others less than four years old which are in a terrible state. You can't compare wings used regularly by a professional competition pilot with those of the weekend recreational pilot, or ones laid out on volcanic ash in the Canaries with those used on lush green Alpine slopes.

Contrary to popular thinking, paraglider cloth suffers more from humidity than from UV radiation. During an average take-off, flight and landing our glider may, for example, be exposed to the sun (a corrosive agent for the fabric, in particular causing colour fade) for a period of three hours. By folding and packing away the glider this exposure ceases, but if we pack it away with any degree of humidity this damaging exposure can last days, weeks or months, seriously damaging the cloth's essential coating and making it become porous.

We advise at least one thorough annual check-up in a recognised workshop. They should check the tenacity and porosity of the cloth, resistance and stability of the line set, state of the stitching, risers, etc. We advise against the continued use of any paraglider once materials have lost 35% or more of their qualities or the cloth has lost 80% or more of its impermeability. However the stability of line lengths is far more important than porosity in affecting a paraglider's flight behaviour and line length changes should be almost nil.

Could there be much difference between one production unit and another, or between the original model and a production one? What quality control processes do you employ?

There should be no differences. We always work within a margin of just a few mm for the lines and even less for the fabric. Our quality control processes have been positively certified under European standards.

The fabric for the construction of our paragliders are cut in carefully controlled groups, as are the lines. This means that each paraglider is a perfect clone of the others. Once all of the necessary layers to make each paraglider have been grouped together, a control file sheet accompanies them from start to finish of the construction process in which we detail all of the processes and the personnel involved, time schedules, serial numbers of the cloth rolls used (for each roll used), and the same again for the lines.

During this process four thorough visual inspections are completed by two experienced personnel and a computer checks the lines against the specifications, and finally a test-inflation on the field. By the end we have spent over three hours inspecting and checking every single paraglider.

Some of your manufacturing is done outside your Asturias headquarters - in Andalusia, Portugal and Sri Lanka. How much is done in Spain and how much abroad?

Currently, our production runs for 12 months of the year in Spain and three months in Asia.

With the Kronos (1997), Syncro (2002) and Tempest (2005) you have designed three highly sophisticated developments as regards diagonal V-rib structures, leading edge and secondary ribs respectively. Could you explain a little about what was behind these ideas?

The Kronos was the first (and only?) paraglider with V-ribs on two levels. Its construction was a challenge: 100 cells, 576 patterns, 400 V-ribs and few lines, with a relatively low aspect ratio compared to it’s high performance. It was a commercial success, and we also certified it AFNOR Performance with micro-lines. From the Kronos we learned not to limit our ideas for construction.

One of the most dangerous flying situations is encountering turbulent air whilst using the speed system, and this is something we concentrated on with the Syncro. After experimenting with and adjusting in all possible ways the trim of the wing and torsion of the wing-tips, the next thing was to achieve a more rigid leading edge. We attached load-bearing webbing tape along the bottom panels at the cell openings, and stitched a Dacron reinforcement from there to the upper surface in the centre of each cell. With this we achieved a more rigid definition of the leading edge and the glider was safer in turbulence, especially when accelerated.

The Tempest uses a new profile generation. It has little in common with our previous models except the idea of increasing the rigidity of the leading edge. We evolved the system further by making a double leading edge (SAS, above) with one leading edge inside the other. In the main (external) one we stitch the main reinforcements and the ribs, and in the secondary (inner) one we stitch the floating leading edge reinforcements. With this unique construction method no external stitching is visible whatsoever, and the load is spread across a wide region of the upper and lower surfaces, and the ribs. The Tempest's new profile and its other design features have made it possible to get DHV2 certification - a great success considering its amazing performance.

What are the developments planned for this year, and the next in Windtech's line up?

We are replacing each model approximately every three years, whereas at one time it was every year. This is because it is becoming harder and harder to improve on current designs. We would like to achieve a five-year cycle, which would mean that we were near to achieving perfection!

First published in Vuelo Libre Parapente magazine, October 2006


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