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Fig. 3. Force balance diagrams for gliding and computation of angle of attack and
camber. (A) Steady (non-accelerating) glides. The resultant aerodynamic force
is oriented vertically and is equal to mass x acceleration due to
gravity. The angle between the resultant aerodynamic force vector and the lift
vector is equal to the glide angle, so lift and drag can be computed as the
magnitude of the resultant aerodynamic force x the cosine and sine,
respectively, of the glide angle. The lift-to-drag ratio is equal to the
cotangent of the glide angle, and is therefore inversely proportional to it.
(B) Non-steady (accelerating) glides. Horizontal accelerations indicate that
the resultant aerodynamic force is inclined with respect to the vertical and
vertical accelerations indicate that the magnitude of the vertical component
of the resultant aerodynamic force is not equal to mass times acceleration due
to gravity. More complicated computations of lift and drag are required (see
text) and there is no necessary relationship between lift-to-drag ratio and
glide angle. Angle of attack is the angle between a line connecting the wrist
and ankle and the direction of the whole body velocity. See text for details
on calculations. Camber is computed as the perpendicular distance from the
patagium marker to a line connecting the wrist and ankle. The 3D angle between
the chord line and the line connecting the wrist and the patagium marker is
computed as a reference. Camber height is estimated as the distance from the
wrist to the patagium marker times the sine of the reference angle. V,
velocity vector; V', direction of velocity vector; R,
resultant aerodynamic force vector; M, mass of glider;
g, acceleration due to gravity; L, lift; D,
drag; 
, glide angle; 
, reference angle between drag and the
resultant aerodynamic force; 
, angle of attack; h, camber
height; ap, anterior patagium distance (between wrist and patagium
markers); 
, reference angle between chord line and anterior patagium
line.
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