Canard Drag
Standard methods for parasite drag prediction work quite well for canards:
The tricky part of drag prediction is induced drag which includes most of
what is generally termed "trim drag". (A part of the trim drag
will also appear as profile drag.)
Given the lift on each of the surfaces, the induced drag may be computed
from the expressions derived earlier for multiple lifting surfaces.
e is the overall span efficiency and is given by:
if the surfaces are individually elliptically-loaded.
By twisting the wings in just the right way the induced drag can be reduced
from this value somewhat (see Ref. 5) but the preceding equations (which
assume an elliptical distribution of lift over each of the surfaces) provide
a good practical estimate. This is especially true when structural design
is considered: the distribution of wing lift for minimum drag with fixed
span carries a large fraction of the total lift on the outboard wing sections,
leading to larger bending moments and greater weights. When drag is minimized
at fixed weight, the optimal loading is more nearly elliptic.
Results for a particular set of cases is shown below.
Unrelated to canard system drag, the maximum CL of these systems based on
total area is shown below. These results are very sensitive to many of
the assumed parameters and are included here only as representative results,
not general conclusions.
