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Cross Braced Information
Cross Braced Information - By NZ Aerosports
Background
In the mid 1990's, we decided to get serious about developing an all out, high performance canopy that would represent a significant step up from anything else available. Although the market for these canopies is relatively small, developing a high performance canopy was both a passion for us and an opportunity to prove ourselves in the most difficult and highest profile market in the sport.
The new canopy was definitely going to be ZP and Elliptical - but after extensive prototyping with conventional construction, we realized that nothing came close to the improved performance we had achieved years previously with a rectangular ZP Cross Brace Tri-cell. Having given up on this design due to construction, bulk and opening problems, technological changes around 1993 enabled us to reconsider it.
In early 1994 we installed a computerized plotter/cutter and had software customized to calculate the shapes. We were then able to begin computer modeling and building our first Zero P, Elliptical Cross Brace Tri-cell; the ICARUS EXTreme. It flew brilliantly straight off the computer but was far from useable. A further year of testing and development followed, before this design produced a product suitable for market release.
Because a Cross Braced Tri-Cell has a smooth cell structure, we were able to enclose the nose of the airfoil, giving an almost complete wing. This significantly reduced cell mouth drag (which accounts for a large proportion of ram air canopy drag).
In January 1997, we completed testing and released our latest Cross Brace Tri-cell - The ICARUS EXTreme-FX. The name stands for Elliptical Cross Braced(X) Tri-cell (EXTreme - EXTreme-FX).
Compared with the ICARUS EXTreme, the ICARUS EXTreme-FX delivers even smoother openings, an even stronger flare, and a slightly smaller pack volume. Most importantly, the EXTreme-FX will now give you turf surfing performance that is far superior to anything you will get from a conventional canopy.
What is a Cross Brace Tri-cell?
Cross Brace Tri-cell means the cell is divided into three chambers instead of two, with the chambers diagonally braced to force each cell back into shape. Viewed from the front, a canopy normally has a zigzag appearance. The cells are deformed due to a lack of internal support, with only a free floating, non load bearing rib between them. But if you look at the front of a Cross Brace Tri-cell, both the upper and lower surfaces appear smooth.
Where did the idea come from?
PD originated the idea with a parachute called the Excaliber, in the late eighties. It was a rectangular, F-111 Cross Brace Tri-cell, and in its day it was awesome. The Excaliber out performed anything else available at the time. It was eventually superseded by Zero-P parachutes, which out performed the Excaliber using only conventional construction.
What are the advantages?
There is less drag because there are less lines. In effect the canopy is a 7 cell not a 9 cell, yet is almost an 11 cell in shape. The canopy is more rigid in flight. Due to the triangulation of the cell structure the cells are 'locked' into position rather than being free floating and able to breathe.The canopy surfaces are less distorted. A ram air canopy inflates to form a wing; ideally one that is smooth and straight like an airplane wing. But rather than being a rigid structure, a canopy wing is made from an inflating membrane with surfaces that distort; significantly reducing their effectiveness. There are 3 types of distortion which occur:
- Spanwise distortion (bulge and zig zag)
- Dynamic distortion (on landing)
- Spanwise consistency (wing tip shape)
See 'Distortion' for a further explanation.
Disadvantages of this canopy design
There are a couple of unavoidable trade-offs with this design, and jumpers went through the same issues when ZP canopies first appeared. Pack volume and price will undoubtedly put a few people off.
1.Pack volume - A regular 9 cell canopy consists of 40 different panels. The EXTreme-FX consists of 53 different panels and an increased amount of material is required to support the cell structure. Consequently a 104 sq. ft. cross brace tri-cell will pack up about the same as a 125 sq. ft. regular zero-p canopy - an approximate 20% increase in pack volume. You will undoubtedly go down in canopy size but you probably will not want to go down 20% to get the equivalent pack volume (unless you were intending a reduction anyway). So you will probably end up with a bigger rig than with another canopy.
2.Price - Material and construction time involved is significantly increased when manufacturing a cross brace tri-cell. The EXTreme-FX has more fabric and takes us twice as long to manufacture as a conventional canopy. As there are only 8 line groups the loading is a little higher on each line attachment point so EXTreme-FX reinforcing is sewn throughout the entire parachute.
Distortion
Spanwise Distortion
When a ram air canopy inflates, air pressure causes the canopy surfaces to bulge between the ribs. This both effects the airfoil shape and draws the ribs closer together reducing the span of the canopy (and therefore the surface area). The more ribs we have, the less distortion and shrinkage occurs, but bulk and line drag increase. On a conventional canopy, not only do the cells bulge they also zigzag up and down between load bearing and non-load bearing ribs, further distorting the canopy and further reducing its span/area.
To quantify this, bulge distortion alone reduces a 9-cell canopy area by 9% and zig zag distortion by a further 4%. If you are jumping a PIA measured 100sf canopy, you are actually flying with 87sf of wing area above your head.
With the EXTreme-FX, you still get the bulge distortion (reduced slightly through 21 chambers instead of 18) but zig zag distortion is eliminated completely. On a 100sf EXTreme-FX, bulge distortion will reduce your area by a mere 8%. So you still have 92sf of wing above your head (compared to 87sf); 5% more lifting area and no extra drag - less in fact.
Dynamic Distortion
When you look at a photo of a conventional canopy on full drive the zig zag appearance is obvious, but on a landing photo, the zig zag appears much more pronounced - and it is! During flare, your canopy is both slowing down and pulling more load, which reduces the supporting pressure within the canopy and pulls it further out of shape. In fact, during flare, zig zag distortion will increase a further 7-9%, to around 12%. Add bulge distortion and a 100sf canopy is now providing 79sf of lifting area on landing.
If you look at a photo of a canopy at the end of its flare, even at this point there is no zig zag distortion at all. You are landing with 92sf of lifting area, compared with 79sf; a massive difference of 16.4%. Our figures come from measurements taken of models inside a wind tunnel and have been proven in practice through building very useable small canopies - down to 37sq ft for the JVX. Can you imagine landing a 46 sq ft conventional canopy?
But why not just buy a 16% larger conventional canopy? It will still land softly, pack down smaller and cost less! And you would be right; except for one thing. This larger canopy would also have 16.4% more drag, causing you to fly slower, turn slower and swoop in for landing slower than the EXTreme-FX, as it is this airspeed that you are using to produce your landing lift. And the EXTreme-FX is a lot more fun…
Spanwise Consistency
On an airplane wing, the airfoil usually gets proportionally thinner towards the wing tips to help reduce induced wingtip drag. On an elliptical canopy the cells are usually the same width right across the canopy. At the wing tips the canopy is shorter, meaning the cell is proportionally wider and will therefore bulge proportionally more - producing a proportionally deeper airfoil at the tips. This is exactly what we do not want. Often designers have dealt with this distortion by adding extra non load bearing ribs into the end cells.
On the EXTreme-FX, we have gone one step further; keeping every cell on the canopy at an equal aspect ratio. If you compare the cells in the center with the wing tips, they are narrower and the airfoil depth remains totally consistent over the span of the canopy.
In summary, the EXTreme-FX will totally eliminate both zig zag and dynamic distortion, marginally reduce bulge distortion and will deliver spanwise consistency to reduce wingtip drag.
Extra Performance
As with previous steps forward in parachute design, the extra performance is realised by being able to reduce the size of the canopy. When jumping a similar size canopy you actually lower your decent rate and therefore fly, and turn, slower. Although this technically represents an improvement in performance, it was not what we were trying to achieve.
Reducing the canopy area by, say, 10% over your conventional elliptical ZP canopy gives a good benchmark for comparison. With a 10% reduction in area you would find:
- Descent rate is comparable
- Forward speed is greater
- Turn speeds are therefore faster
- The canopy feels rigid in flight
- The range of control is greater (the canopy also flies better at slow speeds)
Extra Lift
This all adds up to more lift. On a lightly loaded canopy the extra performance is not that noticeable, but if the canopy is 'heavily loaded' it becomes very noticeable. This is the same for most performance steps; for example, the difference between an F-111 230 sq ft and ZP 230 sq ft canopy is not that much but you can imagine the difference when a 95 sq ft F-111 canopy is compared with a 95 sq ft ZP.
By 'heavily loaded' we do not just mean a big person under a small canopy. When a canopy is being flown hard and the person under it is pulling a lot of G's, their weight in the harness is greatly increased. The small increments in performance start to add up.
Conventional elliptical canopy landing performance starts to drop off at a steady flight wing loading above about 1.7 PSF (pounds per square foot). Highly loaded conventional canopies are more radical and still easily useable but not as efficient. This is due in part to the parasitic drag of the jumper, and other non lift producing objects. As a canopy reduces in size, this drag becomes a larger proportion of the flying unit, as they do not reduce relative to the canopy's size reduction. The performance drop on a conventional canopy is also due to the effects of dynamic distortion.
With the EXTreme-FX, this landing performance drop off does not start occurring until a steady flight wing loading of around 2.00 PSF is reached (in testing we have taken them to 4.6 PSF). During a landing maneuver, you may be pulling 1.5G's, and around 1.2 G's at the beginning of your surf. This is where you realize the extra performance.
Going to a smaller canopy has a compounding effect - smaller canopy = greater maneuverability = greater airspeed = more G's = canopy maintains performance = canopy responds = can go smaller etc....until you reach a performance drop off.
We are not suggesting you load your canopy to 2.00 PSF, but simply demonstrating that the 'performance envelope' carries further and becomes more noticeable. If you are wanting to jump a canopy of 1.2 PSF or below, performance gain will not be as marked. You would be better off sticking to conventional canopies, and we make them too. But if you jump a canopy above 1.4 PSF, you will notice a considerable improvement in performance.
Openings
The biggest hurdle to designing a zero-p cross brace tri-cell has always been the openings. We have had a lot of trouble with them - narrowly avoiding maiming ourselves under the first one we built in 1993. We have spent a lot of time getting them to the stage we have them now.
The EXTreme-FX openings are mainly controlled by the nose configuration and are very slow and progressive. As soon as the canopy leaves the bag it starts to inflate immediately and slowly. You will know things are happening immediately and can monitor the opening as it blossoms into a full canopy (rather than screaming earthward with a streamer at line stretch wondering what is going on). Openings take a long time, but use relatively little height as the majority of the opening sequence is waiting for inflation to finish rather than inflation to start.
Most elliptical canopies have a high proportion of off heading openings and the EXTreme-FX is no exception. By slowing down the opening sequence, we have eliminated the steep dive that commonly occurs with conventional ellipticals immediately after opening. With the EXTreme-FX, off heading openings are quite tame as the canopy is either streaming (not flying at all), or is growing and has stopped flicking around. This means the canopy has time to settle before it tries to fly. Off heading openings on the EXTreme-FX are therefore not a big problem - causing less malfunctions and leaving your nerves intact.
Another reason we made the openings this slow, is because zero-p canopies get occasional rogue openings (sometimes attributed to line dump). By slowing down the entire opening sequence, those rogue openings become acceptable; no longer a 'killer'.
Overall, the openings are better than under any other ellipticals we have jumped, although not as nice as those achieved under a 500 jump F-111 7 cell. The state of the art has not reached that stage yet - for any high performance elliptical canopies.
Flying Characteristics
The disadvantage of the small recovery arc of many modern canopies, is that you need to 'hook lower' to get the full force of the canopy to carry you down to the ground on full drive. In contrast, the EXTreme-FX has been designed with a large recovery arc. You can hook higher, get plenty of speed up from your hook, then maintain it longer on full drive, until ready to flare. You will have plenty of time to make fine adjustments as you get closer to the ground. Also, the larger the recovery arc, the higher you can do your hook, and the bigger the final height difference will be between full brakes and full drive. Your hook height will need to be higher but does not have to be judged as accurately, or as quickly, to remain safe.
When changing from one canopy to another, it is worth getting a serious feel for the flying characteristics of the new canopy before committing yourself to any landing maneuver.
- Front risering is very heavy, and also smooth and stable with no bucking
- Canopy stability and pressurization is not a problem
- Toggle pressure is no different to other ellipticals
- Overturning is more pronounced than on other ellipticals. Overturning means the canopy keeps turning after you have finished your turn. You can control it by finishing the turn early (the best technique), or by stopping the turn with the opposite toggle. It usually takes people a dozen jumps before they have a 'feel' for it and are adjusting automatically to the amount of overturn
- The flare is very powerful, but may feel quite late. The "powerband" is deeper into the toggle stroke than on most other canopies
- The size of your canopy's recovery arc is greatly affected by wing loading
Manufacture
We computer cut all our canopies with a CAD/CAM (computer aided design/computer aided manufacture) system. We use no patterns or templates and program each order individually into our computer system to customize colors, size and options.
Every model of canopy is available in any size you wish. Our cutter marks, labels, then cuts and seals each panel, as well as calculating and generating line lengths.
As you can imagine, in an elliptical canopy there are many different shaped panels, rather than the same shapes being repeated. With the EXTreme-FX this is even more pronounced, as many more panels must be cut and placed to an individual shape and location in order to form this canopy structure. To do this by hand would be totally impractical (generating the prototype shapes took us months of CAD work on a customized canopy design program).
Each panel is cut to an accuracy of 0.2 millimeters. A high degree of accuracy is required as some of the angles in the cross brace are so acute that if positioned incorrectly, the whole parachute would not be shaped correctly. This would defeat the whole purpose of the cross braces pulling the canopy into true.
On all our canopies, we use a doubled patch at every line attachment point to eliminate lower surface damage.
