from Ralf Groesel, kite designer:
Tube vs Softkite - an epic battle?
Ladies and gentlemen Today we will have a close look at an ongoing battle that’s been present in the kitesurfing industry since the beginning. A battle between two very different concepts, a classic David vs Goliath situation which still hasn’t lost its intensity. Even after 20 years of development, controversy still exists around the application range. The forums are full of lovers and haters of each concept, and arguments and discussions are often driven by passion, rather than facts.
Based on my paragliding knowledge and background, in 2001 I started the Soft-kite brand FLYSURFER, with a large amount of enthusiasm and the conviction that Softkites were to be the future of the sport.
How wrong I was…..
Now, having worked as a Tube-kite designer for many years, it’s safe to say i’ve seen both sides. So let’s have an analytical look at both concepts, focusing on the main design aspects in the first stages, before we move on to explore the resulting differences of the in-flight characteristics.
As Tube-kites vary a lot depending on their application range - wave, freeride, wakestyle, light wind, race - we will look at concepts used for the biggest market segment, the “freeride” sector. For comparison, we will take an intermediate to race Soft-kite, as this is the biggest market for self inflating kites.
Design - THE PROFILE
The l biggest overall difference between the two concepts are their profile geometries. The inflatable Tube-kite profile is quite a unique airfoil concept which combines a round, inflated nose with a single skin topsail. As the inflated nose distributes structural load through the kite frame, the LE has to have a certain diameter which is usually no smaller than 7%. The profile height itself can vary a lot, but on average it is about the same height in % as the LE diameter. As a Tube-kite profile only has a top sail the profile height is measured from the centre chord towards the highest point of the profile. I do have to mention this point, as a Soft-kite profile is at least twice as big due to both a top and bottom saila top and a bottom sail.
A Soft-kite inflates by itself during the initial launch. The internal pressure, created by the airstream, is very low in comparison to a pumped LE and therefore has to be supported by a decent amount of bridles to keep its shape. To insure a Soft-kite doesn't nosedive (front-stall), the profile is designed in a self-stabilizing S-Shape.
Taking a quick glance at the performance attributes, a Tube-kite profile creates more lift than a Soft-kite profile in stationary flight conditions, but the drag is higher in low angles of attack. To increase the profile lift in a Soft-kite, a flap has to be deployed by banding down the TE and simultaneously increasing the profiles angle of attack. The lift of a Tube-kite profile can only be increased by the angle of attack once the bar is sheeted in.
The picture below shows the differences in lift and drag at 20° angle of attack. The Tubekite profile creates less drag and more lift.
Design - THE OUTLINE
The 2 dimensional outlines of both concepts vary mostly in the LE curve and aspect ratio. Most of the wave and freeride Tube-kites have a strongly curved LE with a moderate aspect ratio. A race Soft-kite has a way higher aspect ratio, and the LE and TE are designed in almost symmetrical oval shapes.
Design - THE RADIUS
The differences in radius are very obvious,. A Tube-kite has a round to medium-round arc, while a Soft-kite is designed to be as flat as possible in order to increase the projected surface area. The Soft-kite features bridles which lend support to the flat arc. On the other hand, the Tube-kite doesn’t necessarily have to be supported by bridles. The classic C-Shape 4 line kite from the old days has a very round arc and therefore the entire load from the canopy could be supported through the inflated Leading Edge. As the sport has evolved over the years, we’ve seen the so called Hybrid Tube-kites become the more popular kite for freeriders. These Kites have a medium-curved arc with very few bridles needed to support the structural stiffness.
Flight characteristics - THE HANDLING
Now we’ve looked at the basic differences in design, let’s look at the in-flight characteristics. The handling on a Tube-kite is highly related to the canopy radius (arc). The smaller the radius, the tighter the kite can turn around the tip.. But the turning radius itself is not the most important aspect about the handling. It’s more the “delay,” or let’s say a fast response to the turning input induced by the steering lines. By increasing the tip area on a Tube-kite with a swept back LE in addition to back pigtails positioned further down towards the trailing edge, the response time of steering inputs can be adjusted relatively easy. By tweaking these four aspects, the handling can be adjusted to target requirements. Soft-kites, on the other hand, do require much more attention to detail when it comes to handling attributes. A Soft-kite has a higher radius and higher aspect ratio, which automatically results in a wider turning radius. But again, it’s not the turning radius which defines the handling, it’s the response time to steering inputs that have to be as quick as possible.
Now this is where the trouble starts. A Soft-kite has a much thinner line along which to operate perfectly within an aerodynamic window between stall points. To steer, one side of the kite has to increase the angle of attack to slow down, while the other side has to be speeded up by decreasing the angle of attack. Additionally, on some models a brake flap is deployed which helps to steer faster and reduces delay. All these changes on the wing are performed by a “line-mixer”, which contains pulleys and controls profile and flap angles. To find the balance between speed and precise handling with good feedback on the bar is technically much more difficult to achieve on a Soft-kite. The inflated Tube-kite frame however, makes the design process much more controllable and allows for handling aspects and bar feedback without compromising the overall performance too much. As a result, the handling can be adjusted to various settings just by moving the back pigtail positions on the kite tip, a fast and easy way for the kiter to change the handling and performance attributes of the kite to suit their personal needs.
Flight characteristics - POP AND LIFT / PERFORMANCE
The kitesurfing DNA is all about jumping, so let’s look at this topic together.
Lift in kitesurfing defines the ability to jump high. A Tube-kite profile has the advantage here over the Soft-kite and here’s why. As we all know, a kite has to be steered from horizontal flight as fast as possible up above our head in order to get lifted. Within this motion sequence, the kite flies through various angles creating both drag and lift. The extraordinary profile geometry of a round Leading Edge with a single skin topsail, has the unique capacity to speed up throughout this sequence as the profile has more ‘pitch’ moment than a double skin Soft-kite profile. This ‘pitch’ moment forces the profile to rotate forward, which helps to retain more kinetic energy, up to the point where the bar is sheeted in to generate lift. A Soft-kite profile however, does not have such a high ‘pitch’ moment and therefore has to be designed to fly as fast as possible through this window.This is mostly achieved by high aspects outlines that ensure the depower range of the bar is transferred into a low angle of attack on the kite. As a Tube-kite gains a lot of structural stiffness from the frame, the kite can be heavily loaded before the release from the water. This helps to jump higher in strong wind conditions, while the jumping performance in lighter winds - up 12 knots - are very similar on both concepts.
Generating pop is different to generating lift. Pop is needed if you are going for horizontal jumps like a Raley. Softand Tube-kites are able to create a similar amount of pop, but due to the rounder arc of Tube-kites, combined with the fact that they are less supported with bridles, they work like a spring once you load them up. After the pop, the kite has to have slack for unhooked moves. C-Tube-kites are specially designed to create this slack while Soft-kites are generally developed for upwind performance.
Which brings us directly to the next question, what is performance? Well, that’s a very good question. As the sport has been developing in several niches, there is no general meaning of performance any more. A race or foil kite has totally different performance attributes when compared to a wakestyle kite which is specifically developed to create pop and slack. So where does the Softkite shine. As the profile creates less drag in medium to low angles of attack, the ideal application is race as the kite has to fly deep into the edge of the wind window. Additionally,, a Soft-kite has benefits in super light wind conditions as the weight of the kite material is less in comparison to a Tube-kite design.
Flight characteristics - THE WEIGHT
A Softkite can reach a material weight of 0,2kg/m² where a normal Tube-kite weighs around 0,35kg/m².
Does weight change the flight characteristics?
Yes it does. But just because a Soft-kite is lighter in design, it does not necessarily mean that it turns faster. First, we have to understand that a Softkite is filled with air. The 10m² model I am using here for comparison, has a profile height of 18% and a total air-volume of 1,864m³. When you steer a kite, the entire mass,- including the air-volume has to be moved. In our case the weight of the 10m² Softkite increases to 0,441kg/m², which is twice as much as the material weight only and around about the same weight as an inflated 10m² Tube-kite.
It’s very often forgotten, that air itself has a mass from 1,292kg/m³ at 0°C and that this weight has to be added once the kite changes direction.
Flight characteristics - WATER RELAUNCH
Knowing that we have to take the entire mass into consideration, let’s look at the water relaunch. A Tube-kite has an entirely different behaviour as the inflated struts ensure the kite never sinks, even when it gets washed by a wave. In order to guarantee that a Tube-kite has a fast and effortless relaunch from the water, it has to be designed in a DELTA shape, while the tip area should be flexible enough to catch air once a steering line has been pulled. The Soft-kite is mainly relaunched by pulling on its steering lines which activates the brake lines on the trailing edge. A flap is deployed which allows the kite to fly backwards if the kite is sitting face down on the water's edge. As the internal pressure of a Soft-kite is not high and the seams are not sealed, time is an important factor. Generally, the water relaunch on a Soft-kite is very easy and fast, as long there is no water inside the glider. As Soft-kites are often used in very light wind conditions, the kiter must react fast enough to avoid any water draining into the glider, as this will delay and complicate the release. Taking everything into consideration, a Soft-kite is not easier to restart than a Tube-kite, while a Tube-kite has the advantage of being a permanent floating device.
Flight characteristics - SAFETY
Safety is essential to our sport and without a safe product, kiting would never have become a mainstream sport. Here, both concepts have developed their own techniques to insure that the kite can be released and landed safely, without the user being pulled or dragged. The standard way for Tube-kites is by using the “single-front-line-safety” which allows the kite to flag out on one side. A Soft-kite safety system can be operated by pulling the steering lines, or flagging out the kite on one side.
Both concepts do have pro and cons under various conditions but generally speaking, they are safe and intuitive to handle as the release system on the bar is nearly identical on all kite brands.
Let’s talk about one essential topic within this comparison, the price. A Soft-kite is more complex than a Tube-kite in terms of its design and requires more material and labour work. The 10m² Softkite sample we are looking at, has an overall material consumption from 35,24m² for top, bottom, ribs, V-Tapes an H-Stripes. Additionally, 117m of lines are running into the bridle system. As shown in the diagram below, the Tube-kite has an overall consumption of 19,67m² for the canopy, LE, struts and bladder material. As the Soft-kite design has to be as light as possible to compensate the captured air mass the material has to be light. However, light material is expensive which leads to an overall higher production price. As a result, the prices of a Soft-kite with similar constructions details as shown in my sample, costs around 1370,-€ net retail. A 10m² Tubekite around 1100,-€ net. This is a delta from 270,-€ net. If you go for a 3 kite quiver, you end up with approximate 800,-€ price difference.
As you have seen, both concepts vary a lot but it’s the Tube-kite that stands victorious.
From a designers point of view, it shines in every aspect and its simplicity in design is the key to controlling its complexity. Tube-kites rule most of the sport related requirements, although a high aspect race Soft-kite does have performance advantages when it comes to its ability to run upwind in ultra light wind conditions.
Soft-kites have an approximate market share of 4%, meaning that 96% of all kites sold worldwide are Tube-kites. This figure basically validates the success of the concept. But YES, even though the production cost is very high, Soft-kites do have an right of existence for niche applications, especially in light wind race conditions on a foil board.. Without being too sarcastic, you have to be an believer to think that Soft-kites are the future of the entire kite market with all its facets.