force and yawing moment are shown i

force and yawing moment are shown in Figs. 14 and 15, respectively.
A thrust is produced at all three frequencies. In Fig. 14,
positive values of force indicate thrust and negative values indicate
drag. Clearly, the device has produced more thrust than drag.
The net values of the moment can be made nonzero and acting
toward port or starboard by operating the flaps differentially. The
data in Fig. 15 have been used to design control laws for maneuvering
~Bandyopadhyay et al. @3#!. Observe the 90 deg phase difference
between axial force and yawing movement.
Yawing moment is at a maximum or a minimum when the flap
is at its extremity and axial force is at a maximum or a minimum
when the flap is at the mid-point of its travel.
3.3.2 Thrust Coefficient and Efficiency. The distributions of
the coefficient of axial force with Strouhal number are shown in
Fig. 16. The dual flapping foil data are compared with the twodimensional
theories of Lighthill @9# and Chopra @24# and the
two-dimensional measurements of Triantafyllou et al. @16# and
Isshiki and Murakami @25#. Our measurements are also compared
with the discrete forced vortex momentum model of Bandyopadhyay
@26#. Here, the mass of water affected per unit span of
flap, as contained in the discrete vortices formed at the salient
edges of the flaps, is taken to be