Highlights from a discussion on hang gliding dot org. Steve Morris
writes:

Here's a reasonable treatment of drag build up for hang gliders:

http://www.willswing.com/articles/hang-glider-design-and-performance.pdf

Be sure to check out the parasite drag build up on page 9. According to this
paper, a T2's drag at best L/D consists of the following buildup:

Parasite (wires, pilot + harness, airfoiled tubes) =2 ft^2

Profile (2-D airfoil drag) 1.94 ft^2

Induced 4.32 ft^2

The minimum induced drag (elliptic loading) for this wing's aspect ratio is 4.0
sq-ft equivalent drag area, only 0.32 sq-ft lower than the design estimate. This
means that optimal twist at best L/D would move the polar from 14:1 to 14.5:1.

Reduced twist has a greater impact on the high speed portion of the polar than
at best L/D, but as has been pointed out, this happens automatically on flex
wings. Since there is only less than 0.5 ft^2 of drag area to be reduced through
optimal twist at best L/D it looks like the biggest gains are from cleaning up
the pilot and reducing airfoil profile drag.

Big reductions in airfoil profile drag require laminar flow design which
is difficult to achieve in a non-molded wing design. The pilot/harness system is
the next place to look for big performance gains.

If you take a glider like the Swift and compare it to the T2 here's what the
drag numbers look like for best L/D. The Swift has a clean pilot fairing and a
30% laminar flow wing.

T2

Parasite = 2 ft^2

Profile = 1.94 ft^2

Induced = 4.32 ft^2

Swift

Parasite = 0.70 ft^2

Profile = 1.0 ft^2

Induced = 2.5 ft^2

It seems unlikely that flex wings will ever break 20:1 L/D without cleaning up
the pilot.

Read more:
http://www.hanggliding.org/viewtopic.php?t=32431&postdays=0&postorder=asc&start=20#ixzz3RDUP4s4S