https://www.willswing.com/the-story-of-the-design-and-development-of-the-fusion

Flex wing hang glider technology seems to be poised on the brink
of a paradigm shift. As was the case with batten supported tip chord extension
22 years ago, and enclosed crossbar double surfaces seventeen years ago, the new
semi-cantilevered spar “topless” technology appears to have changed to course of
high performance flex wing design evolution. This is the story of the
development of one such glider.

Flex wing hang glider design is an in-exact and intuitive process – more so than
rigid wing design. The basic aerodynamic principles are the same, but the
geometry of a flex wing is continually changing in response to flight conditions
and as a result there are more variables to deal with. This is one reason why
progress tends to be incremental. What are seen in retrospect as watershed
events in the history of hang gliding design, which are often thought of as
resulting from especially innovative new design ideas, are in reality simply the
points in time during an iterative and incremental design evolution when an
important idea is made so compellingly practical that it changes the course of
equipment development. Many of the design concepts which have revolutionized
flex wing design were failures the first time, or the first several times they
were attempted. The true design challenge is not to come up with the new ideas,
but to make an idea work, or work better. The development of what have come to
be known as “topless” gliders is a good example of this principle. The essential
elements of the semi-cantilevered structure were introduced at least twenty
years ago. Four years ago, they were refined and reintroduced in a few prototype
competition gliders. Two years ago, they were mature enough to justify serial
production by at least two manufacturers. By late 1996, they finally
demonstrated a compelling performance advantage. This year, they are available
from every major manufacturer worldwide. It has taken the collective efforts of
dozens of companies many years to get a very simple idea ready for market.