Looking at anhedral with VG off and VG on

Looking at anhedral in flex-wing hang gliders: VG off versus VG on

Steve Seibel
www.aeroexperiments.org

This page is still under construction!
September 16, 2006 edition

 

Front view of Airborne Blade hang glider, VG on. The glider is inverted so that gravity places a load on the sail and tightens the flying wires. The glider is positioned so that the mid-span chord line is horizontal and parallel to the camera's line of sight. This gives us a view of the wing's overall anhedral geometry that has nothing to do with the height of the wingtips in relation to the line of the keel tube. Note that there is less anhedral than when the VG is off.

Front view of Airborne Blade hang glider, VG off. The glider is inverted so that gravity places a load on the sail and tightens the flying wires. The glider is positioned so that the mid-span chord line is horizontal and parallel to the camera's line of sight. This gives us a view of the wing's overall anhedral geometry that has nothing to do with the height of the wingtips in relation to the line of the keel tube. Note that there is more anhedral than when the VG is on. Note also that the keel tube looks longer than when the VG was on--due to the greater billow, we've had to rotate the glider in a nose-up manner to keep the mid-span chord line horizontal as we loosened the VG, so we are seeing the keel tube in a manner that is less edge-on. This change in pitch attitude can be felt in actual flight--when we fly at trim, the bar moves forward as we loosen the VG and the bar moves aft as we tighten the VG.

Neither of the above photos is meant to be a perfect illustration of the glider's anhedral in a true aerodynamic sense--the idea of keeping the mid-span chord line horizontal is somewhat arbitrary--but less arbitrary than measuring the height of the wingtips in relation to the line defined by the keel tube.

Front view of Wills Wing Spectrum. The glider is inverted so that gravity places a load on the sail and tightens the flying wires. The glider is positioned so that the mid-span chord line is horizontal and parallel to the camera's line of sight. This gives us a view of the wing's overall anhedral geometry that has nothing to do with the height of the wingtips in relation to the line of the keel tube. Note that the anhedral in this photo appears roughly similar to what we see in the photo of the Airborne Blade, VG off.

Side view of Airborne Blade hang glider, VG on. The glider is inverted so that gravity places a load in the sail and tightens the flying wires. The glider is positioned so that the mid-span chord line is horizontal. Note that a sight-line between the outboard tips of the leading edge tubes passes just under the keel, appearing at first glance to suggest that the glider has only a slight amount of anhedral. This is not really the case--the keel is a poor reference line and due to sail billow, the glider has a significant amount of anhedral, especially in the outboard parts of the wings. We can see this easily in this photo by noting that we can see the top surface of the tip area of the near wing and the undersurface of the tip area of the far wing. With VG on, the outboard tips of the leading edge tubes would ride a few inches lower in relation to the line of the keel tube, but there would be much less billow in the sail, and the glider would therefore have much less anhedral in the outboard parts of the wings.

(More photos to be added, including oblique photos of flex-wing hang gliders to compare the sail shape with VG on and VG off, with the sail loaded by the wind and the lower side wires tight...)

 

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