  We study experimentally a four-winged flapping flyer with chord-wise flexible
wings in a self-propelled setup. For a given physical configuration of the
flyer (i.e. fixed distance between the forewing and hindwing pairs and fixed
wing flexibility), we explore the kinematic parameter space constituted by the
flapping frequency and the forewing-hindwing phase lag. Cruising speed and
consumed electric power measurements are performed for each point in the
$(f,\varphi)$ parameter space and allow us to discuss the problem of
performance and efficiency in four-winged flapping flight. We show that
different phase-lags are needed for the system to be optimised for fastest
flight or lowest energy consumption. A conjecture of the underlying mechanism
is proposed in terms of the coupled dynamics of the forewing-hindwing phase lag
and the deformation kinematics of the flexible wings.
