More photos that illustrate how billow contributes to the net geometric anhedral of a swept wing -- views of inverted gliders
This page last updated August 31 2005
Here I've inverted some gliders to tension the flying wires and the sail surfaces, and I've aligned the gliders so that the wing chord is parallel to the ground at the midspan point of each wing. If you look closely you can see a black cross on each leading edge at the mid-span point. (Justification for using the mid-span chord line of each wing as a reference line: the inboard parts of the wing have more area, but the outboard parts of the wing act at a much greater moment-arm from the CG, so a point near or just outboard of the mid-span point might make a good reference point for quantifying anhedral). The first photo is an Airborne Blade with the VG on. In the second photo the VG has been de-tensioned, and the sail billow has increased, and the glider has been re-oriented (by raising the nose as viewed in the glider's reference frame) so that the wing chord is again parallel to the ground at the midspan point of each wing. The glider now has much more droop in the leading edges, as seen by the observer. This is a crude way to demonstrate that the glider has more "net geometric anhedral" with the VG loose, despite the fact that the glider has a standard pulley VG system and the leading edges actually have more droop in relation to the keel when the VG is tight. (Throughout this discussion we're assuming that the flying wires are tight; you can't see any of this geometry when a glider is sitting on its base bar on the ground.) The fundamental point here is that the keel is not a valid reference line for measuring anhedral, and that sail billow contributes greatly to anhedral.
Front view of Airborne Blade (inverted) VG on
Front view of Airborne Blade (inverted) VG off--note the much greater amount of anhedral
Front view of Wills Wing Spectrum (inverted)--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 (inverted) VG off. The flying wires are tight. 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, but 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. The Wills Wing Spectrum has a similar geometry. In a side view of the Airborne Blade with the VG on and the flying wires tight, a sight-line between the outboard tips of the leading edge tubes would passes quite a bit further (at least 6 inches) beneath the line of the keel, but again the keel is a poor reference line--due to the decreased sail billow, the glider actually has less anhedral with the VG on than it does with VG off.
I'm not trying to suggest that orienting a glider with the mid-span chord line parallel to the ground will provide any sort of absolute, quantifiable measurement of the glider's anhedral. In fact the mid-span area is the place where billow creates the most dramatic change in the chord line, so at first glance we might conclude that these photos exaggerate to some extent the way that de-tensioning the VG increases anhedral and tensioning the VG reduces anhedral. On the other hand, these photos do not show the billow created by the load of the pilot's weight, and it seems likely that this load would create more additional billow with the VG loose than with the VG tight.
For a more holistic, 3-dimensional treatment of this geometry see these photos, and the related articles listed on the site map of this website, including "How billow increases the net geometric anhedral of a swept wing--abbreviated version"
For notes on some in-flight observations that support the idea that the glider has more anhedral when the VG is loose and less anhedral when the VG is tight, see the rudder experiments described here and elsewhere on the Aeroexperiments website.
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