Cross-Braced Canopies

Q:

 

What is the purpose of a cross-braced canopy design, and should I consider buying one?

 

A:
The structure that shapes the top and bottom surfaces of a conventional canopy consists of vertical, airfoil-shaped ribs running the length of the interior. The outside ribs of each cell are the “loaded ribs,” since the suspension lines attach at several points along the bottom. A third, vertical, “unloaded” rib runs down the center of each cell.

Performance Designs introduced cross-bracing with its Excalibur canopy around 1987. Between each of the two loaded ribs, designers added two unloaded ribs instead of one, plus two diagonal cross-bracing ribs running the length of the cell from the bottom of each of the loaded ribs to the top of the unloaded ribs. The Excalibur still had a rectangular planform (shape when viewed from above) and was built from F-111, low-porosity fabric. It landed great but met with limited success due to hard openings and larger pack volume. PD eventually pulled it from the market to focus on developing a conventional canopy made from zero-porosity fabric.

ASKARIGGER201110-1A conventional canopy. Photo by Niklas Daniel.
ASKARIGGER201110-2A cross-braced canopy. Photo by Niklas Daniel.

New Zealand manufacturer NZ Aerosports returned in 1995 with the Icarus EXT-reme, a cross-braced design that used zero-porosity fabric and featured a tapered (sometimes called “elliptical”) planform. NZ Aerosports’ website (nzaerosports.com) contains a lot of detailed design philosophy regarding cross-braced canopies and useful information for anyone considering buying one.

In a nutshell, the upside of cross-bracing is in the speed and landing flare. The main reason people buy them is to swoop. The downside is added pack volume, about 20 percent greater, and the attention required to pilot one. PD, speaking about its very successful Velocity cross-braced model, remarks accurately on its website, “From opening to landing, this is a very capable but very demanding canopy.”

The internal structure of a cross-braced canopy makes better use of the flight surface, keeping it flatter and extending its effective wingspan. Cross-bracing also reduces the ballooning of the fabric between the cells inherent in any inflatable wing, and it makes the wing more rigid. The flatter top-to-bottom profile reduces drag, so the wing can cut through the air more easily and go faster for the same square footage, as well as maintain speed induced from the dive following a turn. Maneuvers, including stalls, are more abrupt.

Cross-braced canopies require a lot more technique to fly safely. They take a long time to open and get under control, covering a lot of ground in the process. The pilot has to be on top of the game from start to finish and think well ahead. Considering the poor track record skydivers have had lately with collisions, any apprehension about opening nearby and flying into the same landing area with cross-braced canopies is easy to understand. It’s a lot of trust to place in someone else.

NZ Aerosports softly dismisses the benefits of cross-bracing at lighter wing loadings, saying that on “a canopy of 1.2 PSF [pounds per square foot] or below, performance gain will not be as marked... but if you jump a canopy above 1.4 PSF, you will notice a considerable improvement in performance.” So if you’re considering upping your canopy performance, getting really good on a smaller conventional wing would make more sense than going right to a cross-braced design. It’s rare to find a cross-braced canopy much larger than 120 square feet, so the intended market for cross-braced canopies is pretty clear.

The performance differences between conventional and cross-braced canopies have narrowed somewhat over the last five years, so jumpers can choose from a variety of exciting designs without incurring the liabilities of a cross-braced model. Although USPA does not make specific recommendations to those who are considering flying a cross-braced canopy, it’s not unreasonable to say that they should be reserved for specialists making 500 or more jumps per year and who focus their training and jumps on canopy piloting, rather than freefall activities.

Jumping a cross-braced canopy takes you to the extreme edge of an already extreme sport. It’s a serious decision with potential consequences for not just you, but others on the DZ.

—Kevin Gibson | D-6943
Master Rigger
Orange, Virginia

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