A Bio-Inspired Flapping Wing Rotor of Variant Frequency Driven by Ultrasonic Motor

Abstract

By combining the flapping and rotary motion, a bio-inspired flapping wing rotor (FWR) is a unique kinematics of motion. It can produce a significantly greater aerodynamic lift and efficiency than mimicking the insect wings in a vertical take-off and landing (VTOL). To produce the same lift, the FWR’s flapping frequency, twist angle, and self-propelling rotational speed is significantly smaller than the insect-like flapping wings and rotors. Like its opponents, however, the effect of variant flapping frequency (VFF) of a FWR, during a flapping cycle on its aerodynamic characteristics and efficiency, remains to be evaluated. A FWR model is built to carry out experimental work. To be able to vary the flapping frequency rapidly during a stroke, an ultrasonic motor (USM) is used to drive the FWR. Experiment and numerical simulation using computational fluid dynamics (CFD) are performed in a VFF range versus the usual constant flapping frequency (CFF) cases. The measured lifting forces agree very well with the CFD results. Flapping frequency in an up-stroke is smaller than a down-stroke, and the negative lift and inertia forces can be reduced significantly. The average lift of the FWR where the motion in VFF is greater than the CFF, in the same input motor power or equivalent flapping frequency. In other words, the required power for a VFF case to produce a specified lift is less than a CFF case. For this FWR model, the optimal installation angle of the wings for high lift and efficiency is found to be 30° and the Strouhal number of the VFF cases is between 0.3–0.36.

Aerodynamic Performance of a Flyable Flapping Wing Rotor With Passive Pitching Angle Variation

“…On the other hand, under the same input voltage, the FWR of PPAV in Cases 2 share the highest aerodynamic efficiency (over twice than that in Cases 1, Cases 3 and Cases 4). According to the previous studies [14], [25], optimal average twisting angle of the FWR should be 30° (as given in both Cases 2 and Cases 4) and thus the poor performance of Cases 4 is mainly due to its larger weight. The results demonstrated that a pair of lighter wings (Wing A) would have higher efficiency.…”

“…A measurement system was built to capture the kinematics of motion and measure the corresponding lift produced by the FWR. Assuming the FWR average rotary speed is constant [23]- [25], a trace line diagram of the FWR trailing edge during a flapping motion is illustrated in Figure 4. Wherein, the green line stands for the trace line of the wing with pitching angle variation while the red line for the wing of constant pitching angle (30°).…”

“…Wherein, the green line stands for the trace line of the wing with pitching angle variation while the red line for the wing of constant pitching angle (30°). The reason for setting the pitching angle as 30° is that the FWR can produce the maximum average lift at this constant angle [25]. It is feasible to measure the average rotary speed through calculating the average revolutions per second (rps) of the FWR rotary motion captured by video frames.…”

“…The test data were filtered by the 2nd order low-pass Chebyshev algorithm in the LabVIEW code and the low-pass cutoff frequency was set as 5 times the flapping frequency of the FWR model [26], [27]. The effectiveness of the force measurement system was validated in the previous study [25].…”

“…However, according to the experiment test, the FWR structure can sustain 10 V input voltage during its free flight test when no external force constraint was applied to the FWR. Also, the average twisting angles in Cases 1, Cases 2, Cases 3 and Cases 4 are all set to be 30° since the flapping wing rotor share the highest lift efficiency under this average twisting angle when the average twisting angle is a constant value during the flapping process [14], [25]. According to our previous experiment and theoretical results [14], [25], it is because the 30° average twisting angle would make the Strouhal number of the flapping wing rotor approach 0.3 that help the flapping wing rotor get the optimal lift efficiency.…”