Canard Stability and Trim
The performance of a canard design depends strongly on the amount of
lift that the canard must carry. This is set by stability and trim requirements.
It is first necessary to determine the position of the c.g. and the relative
loads carried by the wing and the canard. The ratio of lift carried by
the canard to that carried by the wing:
depends on several parameters listed below.
Stability:
If moving the c.g. back by sm * cref makes the airplane neutrally stable
then:
Trim:
Summary:
The value of the static margin, sm, should be large enough to provide acceptable
handling qualities at the most aft center of gravity position. This may
require an analysis of the aircraft dynamics at various flight conditions.
Since the destabilizing effect of the fuselage is not included explicitly
here the value of sm used in the above expressions should be increased
appropriately.
Lift Curve Slopes
A difficulty with the above equations is that a, the ratio of canard to
wing lift curve slopes must be calculated. If the wings did not interfere
with each other one could use the approximate relations:
However, the surfaces produce upwash and downwash on each other so that
the effective lift curve slope is changed. Unless the canard and wing are
very close together, the major effect is that of the canard on the wing.
The canard produces downwash on the inner part of the wing and upwash outboard
of the canard tip vortices. The net effect, though, is a reduction in wing
lift which can be estimated roughly by the following formula which is based
on the Hayes Reverse Flow Theorem (see Ref. 3):
where kappa is a correction which is applied if the canard is very close
to the wing or does not lie in the plane of the wing. kappa should be computed
from a 2-surface lifting line or lifting surface program.