عنوان مقاله [English]
Payload placement on quad-rotors (QR) is a subject of considerable importance during its flight and maneuvering phases. In this sense, any changes in the payload position, especially along the vertical axis, could significantly affect the QR center of gravity (CG) position and in turn its performance dramatically. The current experimental study investigates the effect of payload vertical positioning on performance and stability characteristics of a typical QR. The latter is accomplished in two stages. first the lowest QR center of gravity, along its vertical axis, for which a safe flight will be feasible is analytically determined via the development of a mechanical system. In this respect, it is shown that having a payload beyond this point would jeopardize the QR flyability. Secondly, the QR is test flown for more than eighty times with different payload positions, whose recorded flight data (RFD) are utilized for QR frequency domain analysis. The RFDs are used to identify the QR longitudinal as well as lateral modes. In addition, the mode changes trend against the center of gravity location has led to determination of the CG position at which, instability occurs. The experimental results show that as the QR CG moves up along its vertical axis, its dynamics modes move towards the origin on the real axis taking the QR closer to borderline dynamic instability. Moreover, the longitudinal as well as lateral modes damping behavior with respect to CG have been extracted that in turn can lead to CG based techniques for QR damping control. In addition, in order to generalize the present results to be useful for other QRs, the QR parameters have been nondimensionalized and an effective nondimensional parameter through which results can be extended to other QRs is determined. The newly introduced nondimensional parameter is used against another set of data, extracted from a different QR for verification and comparative purposes. Finally, to account for uncertainties and stochastic noise in the measures RFD each experimental stage is repeated four times and the results mean behavior are reported as well.