Propeller rotational movement plays a crucial role in determining the motion characteristics of drones and presents potential enhancements for diverse applications, such as enhancing navigation stability and autopilot control, besides drone identification and localization. Previous studies have demonstrated that radio wave-based propeller rotation speed sensing improves the precision required for stable flight and ensures reliable navigation. However, these studies have primarily evaluated radio wave performance when the drone is in a stationary state, lacking assessment in dynamic situations. In this paper, we present a case study of flying drones and specifically compare two sensors operating at frequency bands of 5GHz radio frequency (RF) waves and 77GHz millimeter-waves (mmWave) radar to sense the propeller rotation speed at different distances from the drone. Comprehensive flight arena experiments are conducted to compare the performance of both approaches using a commercial drone. The results demonstrate that both RF and mmWave have highly accurate propeller speed measurements throughout the whole flying speed range (up to 16000rpm which was the maximum for the drone used in experiments). However, we also show that the mmWave radar outperforms the 5GHz RF approach in terms of the sensing distance reaching up to 8m, rather than 1m maximum observed for the 5GHz RF. The longer sensing distance of the mmWave approach has the potential to extend the coverage area while preserving the high-resolution requirement for various drone applications, including intruder detection, identification, and localization.