  Unmanned aerial vehicles (UAVs) have steadily gained attention to overcome
the harsh propagation loss and blockage issue of millimeter-wave communication.
However, UAV communication systems suffer from energy consumption, which limits
the flying time of UAVs. In this paper, we propose several UAV energy
consumption minimization techniques through the aid of multiple intelligent
reflecting surfaces (IRSs). In specific, we introduce a tractable model to
effectively capture the characteristics of multiple IRSs and multiple user
equipments (UEs). Then, we derive a closed form expression for the UE
achievable rate, resulting in tractable optimization problems. Accordingly, we
effectively solve the optimization problems by adopting the successive convex
approximation technique. To compensate for the high complexity of the
optimization problems, we propose a low complexity algorithm that has marginal
performance loss. In the numerical results, we show that the proposed
algorithms can save UAV energy consumption significantly compared to the
benchmark with no IRSs, justifying that exploiting the IRSs is indeed favorable
to UAV energy consumption minimization.
