<p>Forces estimated from wake measurements.</p>

Vertical blue lines indicate the timing of peak vertical force, and vertical red lines indicate the timing of minimum vertical force. The minimum vertical force coincides with the lower thrust peak (minimum of negative thrust, encircled in red).

The peak of the main thrust is delayed compared to the timing of the peak vertical force, as indicated by the blue arrows. Flight speed 9 m/s. <b>(b)</b> The multiple of the wingbeat frequency for the fft analysis of frequency from the thrust data across flight speed suggests dual thrust peaks during the wingbeat. The blue data show the dominant frequency of the thrust signal divided by the dominant frequency of the vertical force signal (i.e., wingbeat frequency).