The NPWC Falken is a sparless single skin kite which is flown directly from kite lines, using no bridle. View the NPWC Falken video to see that it is a simple design which flies well. The NPWC Falken design was created by myself and Randy Falkenberg.
Contents:
This is part one of a two part article on the NPWC Falken. In this part, we go over the Falken design and describe the history of this type of kite. The second part contains the Falken plans and dives into construction details.
Design
The NPWC family of kites has two main design characteristics:
- Three panels: two wing panels and one center panel.
- Wing panel leading edge to trailing edge is a 90° angle.
These basic design choices make a simple base for further experimentation. For example:
- Alter the curve of the wing panel, including depth and curve distribution.
- Alter the length of the leading edge or the trailing edge of the wing panel.
- Alter the same parameters of the center panel.
In other words, basic scientific exploration: hold some parameters constant to observe the effects of a specific change.
From a mathematical perspective, each design variable which can be adjusted adds a new dimension to the design space. Even with the base NPWC design parameters fixed, there is still a huge multidimensional landscape to explore. Our walk through these dimensions was more instinctive than anything else, so there is almost a guarantee that we have missed a higher performance peak in our wanderings and simply found a local high point.
To reduce the complexity of exploration, we found it useful to collect individual parameters into groups which are adjusted en masse to achieve a specific aim. This may sound contrary to “hold some parameters constant to observe the effects of a specific change”, but here we are reducing the number of dimensions by enforcing a linear relationship between variables.
The translation of this gobbledygook and hand waving? For example, the leading edge length vs. trailing edge length ratio (LE/TE) is used to “set the tension” of the front and rear lines. LE/TE is a group of parameters: wing panel leading edge length, center panel leading edge length, wing panel trailing edge length, and center panel trailing edge length. Fine tuning of the individual parameters happens only after the entire group is adjusted with their respective lengths held constant.
Keeping the original NPWC design characteristics, we applied two refinements in the Falken design:
- Trapezoidal center panel. Increases the TE length for the desired LE/TE value, and separates the rear line attachment points for better flying characteristics.
- Notched panel corners. Distributes line forces into the canopy (minimizing wrinkles), and contributes to the three dimensional shaping of the kite.
The “print and tape model” from the third page of the plans pdf found on the Falken construction page shows both the original NPWC characteristics and the Falken refinements.
The iterative development process we used was messy, but cumulated with the clean panel layout seen here.
History
The year 2003 was pivotal for single skin kite design. Work was well underway on three separate continents to develop something most people thought impossible: a useful single skin kite with no bridles. To appreciate how big a leap this was, consider that the state of the art briddleless single skin design at the time was something called a playsail, an ‘almost kite’ that has more drag than lift and can only be flown with a wide separation of lines.
Over here in Europe, Terence “Tel” Kearey had developed a minimally bridled ESP Mod 2, a design with a shaped canopy based on the classic NPW5. The first flight of his ESP Mod 3, an entirely bridleless design, was in 2003. Today, the top images for google:"single skin kite" originate from Tel's web page — long gone from the internet, but thankfully saved at archive.org.
The most famous effort, which resulted in a patent (filed in 2003) and the commercial offering of the Outleader kite, was lead by Dave Culp and Dean Jordan in the United States. I can highly recommend reading the untranscribed version of Dave's patent, it is amazing.
Their stated goal was to “find a racing rule-legal configuration that would significantly increase performance.” I believe this was written after side by side testing showed that these kites outclassed normal spinnakers in many conditions. Dave's firm was actually approached by Oracle BMW Racing by a more interesting motivation: test the limits of an IACC rule which effectively means that a finish is recorded when any part of the ship (including the sails!) crosses the finish line. In this context, the name ‘Outleader’ makes more sense: a rule legal spinnaker power kite that can lead out in front of the yacht to cross the finish line first.
Less well known is team New Zealand kite effort. The billionaire yacht racing world is not big, so it was inevitable that the Kiwis got wind of the American effort to develop this new technology despite the opaque veil of secrecy covering their research efforts. The Kiwis leveraged their home field kite experts, Peter Lynn and family, to come up with an answer to the American's technological leap.
The story ends with a whimper as neither syndicate uses their kite, and (coincidentally?) neither win the cup. In summary: an IACC rule anomaly kicks off a wave of bridleless single skin kite design across the globe. Had these kites been actually used in the 2003 America's cup races, we would certainly have seen a more well developed single skin kite landscape today.
I was late to the party in 2005 with my first NPWC design. The only contribution here was an SLE (supported leading edge) type control line setup, and relatively easy construction. I held off on exploring a kite more similar to the Outleader because of the patent in that space.
I began thinking about these brilliant kites again when I learned that Dave's patent had lapsed. “A shame,” I thought to myself, “the single skin kite world would look quite different today if kites had been used in the America's cup.” But, dreams remain dreams until you put ink to paper and thread into ripstop.
The dream began actualization in January of 2015, when I was contacted by a high energy individual and kite enthusiast, Randy Falkenberg, who is intrigued by this style of kite as much as I am. A partnership evolved where Randy did most of the work and I watched videos and contributed some ideas. After hundreds emails and more than 20 prototypes, we arrived at the NPWC Falken.
Evaluation
Here I show some pictures of the Falcon and comment on how the design decisions and parameters affect the final shape and performance characteristics of the kite.
No fancy picture presentation software — built-in browser functionality does all we need: click on a picture to show a bigger version, use your browser “back” button to return to the article. Depending on your browser, the “big” version of the picture may be scaled down to fit in the browser window and you will have to do something special (like click on it again) to show the picture at actual resolution.
Pilot View
The very first picture on this page is a view over the shoulder of my daughter, Dana, while flying the Falken. Yes, this is the same “little girl” sitting in the co-pilot and photographer tent in the original NPWC article. The years fly by and she is really the co-pilot now! Remember to carpe diem while we still have diems to carpe.
This is one aspect of the Falken I really like: the shape is aesthetically pleasing. Note how the width of the kite is defined not by the wing panel tip, but by the trailing edge of the wing panel flaring outward.
One might expect the projected shape of this design to be round. The shape is actually more oval because of the tension of the rear lines on the canopy. For a bridled kite, SLE and STE (supported trailing edge) configurations have the nice property of making the canopy wing curve flatter. The wide rear line connection points impart some STE effect without using bridles.
There is a price for the rear line connection points. All rear line pressure is directed into the canopy at a single point. This causes the stress wrinkles you see in the bottom portion of the canopy. More about this in the next section.
Rear View of Canopy
If you listen closely, you can hear the fabric speak to you. By this, I do not mean the lovely rustling sound crisp ripstop makes when being handled, sewn, or blown by the wind. I mean close observation of the fabric and pondering causality will yield insight to the stress on, and behavior of, a kite.
A short version is: look at the wrinkles.
Sparless single skin kites speak the loudest to me. Their voice is unmuffled by spars holding skin taunt, or pressured chambers making surfaces smooth. I may have got my start in kite design by emulating other designs, but now most input comes from listening to kites themselves.
Try it yourself! Don't read any more text, but take a look at the four pictures in this section and “listen” to what the kite is saying. I will wait at the next paragraph and join you after your “kite whisperer” insight.
Did the rear view pictures talk to you? To me they say that the next version of this kite should adjust the angle of the large notches at the trailing edge of the main panel. If you click on the picture to enlarge it, you can see how the main canopy wrinkle is aligned just under the sew notch line. For maximum effect, the notch and major stress wrinkle should be in alignment.
The rear line connection points are one of the most difficult (and therefore interesting) points in this design: a large point force being directed into the kite at a very broad location.
Ideally, the force from the flying lines would only be transferred into the reinforced hem band and only from there be transferred into the canopy fabric, thus distributing the force as broadly and evenly as possible.
At the wing tips, the job is easier because the hem band is only 45° out of alignment from the direction of the line force. A little notch on the wing tip and the force path is shorter to the hem bands and longer if traveling a straight line. Because of the large force involved, the resulting wing tip does have stress wrinkles, but they are many small ones radiating across the tip instead of one large wrinkle concentrated in the middle of the tip.
At the trailing edge connection point, the challenge is much greater: the hem band is 90° out of alignment with the force entering the system. The situation seems impossible!
The Outleader answer for this design challenge is to “embrace the wrinkle”. The shape of the canopy at the trailing edge connection point is designed so that an internal ridge is formed to carry the load into the canopy. This works well because there is a single trailing edge connection point in the middle of the canopy — the natural angle of the ridge is in alignment with the airflow exiting the canopy.
However, with Falken trailing edge connection points offset from the center, the natural stress wrinkle angle cuts across the flow of air exiting the canopy. Emulating the Outleader solution and creating an artificial ridge in line with the air flow, but not in line with the natural stress wrinkle would require a complicated series of notches along the wing and center panel connection. This solution goes against the otherwise simple design.
Another option is to use small ribs at the trailing edge connection point as I believe was done with the “cata” version JC's 3-DY model (can't verify as the 3-DY is currently off the internet). The problem here is that the angle to the flying line is so acute (see pictures of rear line angle in relation to the canopy in the profile view section) that the rib has very little depth in which to disperse the force.
The Falken solution is to use the same technique seen in the wingtip: a notch in line with the stress forces to disperse them more radially. This works well enough, but could benefit from a couple of more development iterations to smooth this area out.
Top View of Canopy
The first picture in this section is a view of the front of the kite diving toward us. From this angle, the Falken appears quite muscular. I attribute this to the weights used to hold the fabric in place during its construction. I will use heavier weights next build to see if this effect can be enhanced.
Taking a close look at the stress fold on the nose, we see a notch mis-alignment similar to what we saw with the center panel trailing edge notch in the previous section. Next iteration: rotate the center panel leading edge notch into the stress fold we see here.
The second picture in this section shows the Falken heading back up into the sky. Both this and the previous picture shows the wrinkles that appear at the wing to center panel connection. These wrinkles are much less pronounced than the original NPWC models because the curve of the wing panel to center panel is more mild.
A relatively mild wing panel to center panel curve is possible because the center panel notches serve dual purposes:
- Reduce stress wrinkles by redirecting force vectors.
- Create a curved three dimensional shape from a flat panel.
The second job is leveraged here: part of the canopy curve normally imparted by the wing panel to center panel curve is carried out by the notch! Thus, less wing panel to center panel curve is required in the design. Synergistic features make a designers heart pitter-patter with joy!
The next two pictures tell much the same story as in the rear view section, just from a “back of the canopy” perspective. TLDR: rotate the notch into the stress fold.
Positive note: the rear section of the wing panel to center panel curve is so mild that almost no wrinkles appear.
As the majestic Falken turns back to rise up into the blue sky, we shift our attention to the flying profile.
Profile View
The profile view shows something unexpected. For the lift drag ratio achieved (see the performance section), I thought the top canopy to be more horizontally oriented, i.e. having a lower angle of attack.
Instead, the photo evidence shows a very high angle of attack.
When at apogee, the trailing edge of the wing and center panels are roughly parallel to the plane of the ground, which makes the leading edge of the wing panel perpendicular to the ground.
This gives an interesting visual effect of the kite “sitting up” while the flying lines attempt to pull it down.
The potential for learning is greatest when something unexpected happens. My mental model of how this kite functions must be updated to incorporate this result. A better mental model of how things work means an increased ability to create successful new designs. A useful lesson in how to live life.
Performance
View the NPWC Falken video to get an idea of how this kite flies.
Here we see my co-pilot, Dana, bringing the Falken up to zenith.
This shows a flight angle indicative of a 1.4 lift to drag ratio. I think the actual value is a little higher, but I don't want to claim anything I can't show on film.
What matters in kite traction is the amount of pull available at the edge of the window, and what kind of upwind progress can be made. I will leave it up to Randy, or one of you, to evaluate this and compare with other kites.
The lack of bridles should not be underestimated. You can't snag or hopelessly tangle something which does not exist!
Summary
The NPWC Falken is a sparless bridleless single skin kite which is easy to build and performs well. This style of kite has an interesting history. The history is not complete, and there are many fascinating avenues of exploration for kite whisperers in this design space.
Have fun!
Back to the NPWC page.
Forward to the Falken Plans page.
