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Steamlined Downtubes

Low Drag Streamlined Tubing For All Hang Gliders

The Seedwings' Sensor “Freestream Tubes” since 1995

*Maintains the glide performance of the Sensor

*Adds glide performance to other hang gliders

Designing the lowest drag tubing shape with a comfortable secure grip was not easy. It took 34 separate iterations using Panda on a Mackintosh and interior design using AutoCAD. The resulting shape has a truncated trailing edge for drag reduction similar to that of automobile rear body design. Modern aircraft also have square trailing edges for drag reduced and improved wing performance because of the same phenomenon. The effect of a square tailing edge produces two counter rotating vortexes in a triangular shape filling in the absent trailing edge structure and allowing the free stream air to come together smoothly behind the object. I call it the “Phantom trailing edge effect”.

The design goal was to maintain attached flow down to 50,000 Reynolds number (Re) corresponding to near stall speed of a hang glider. For low Re a 40% (1" thick) airfoil has an excessively steep sloping and stalled aft section to the airfoil accounting for most of the drag. Early in the computer study it became difficult to maintain 100% attached flow at low Re with a finer or thinner trailing edge, so a 20% truncated blunt trailing edge design was successfully tested. Smoke studies over the airfoil visibly show two counter rotating vortexes perfectly filling in the absent trailing edge triangle accounting for the very low drag.

Tests were conducted at the University of Stuttgart wind tunnel in cooperation with Steve Pearson from Willswing verifying the Seedwings Sensor Freestream tubes are the lowest drag of any 1" (25 mm) thick streamlined tube. The tests also show that the older style tubes like Finsterwalder, the Attack tube and the Willswing type I streamlined tubes are 3 to 5 times higher drag than the Sensor's Freestream tubes.

Here is a link to that test study:

 http://www.willswing.com/Articles/windtunnel_results2.pdf


The Phantom trailing edge effect.


A turbulator bump placed at 8% cord aft of the stagnation point but still far forward proved best.



Airfoil shape before the trailing edge was truncated (blunted) at 80% of original cord length.

Fineness Ratio: 2.5, Alloy: 6063 T6 aluminum anodized
Outside dimensions: 25mm x 62.5mm (.995” x 2.455”)
Inside dimensions:  .875” diameter sleeve and plug end fittings
Available for any hang glider or use upon request.


Sensor's should use 8 degrees toe-in angle which is .070" off-set from perpendicular.


Courtesy of: Kevin Caldwell, re-print from Hang Gliding Mag. Feb. 89.

 

Gliders with more CB rake require more streamlined tube toe-in angle.