A numerical model that estimates the parameters of a small, unmanned coaxial rotor helicopter with a takeoff mass of 11.9 kg was developed. The modeled coaxial rotor helicopter did not have any additional damping to its dynamics such as a stabilizer bar or external gyro, which are common in commercial hobby helicopter models. Therefore, any sinusoidal actuator input from manual flight led to a large deviation of attitude rates, resulting in uncontrollable situations. A feedback attitude controller was included during test flights and parameter estimation to address this problem. Time domain experimental data were used to help estimate and validate the parameters of the helicopter. Attitude responses predicted by the model show extremely small discrepancies with experimental data. Therefore, the developed numerical model can be used for coaxial rotor helicopter controller design and other simulation purposes.

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This paper presents the work being carried out in order to deduce hover performance of a small-scale single rotor blade as a preliminary study of a small coaxial rotor helicopter development. As an initial research, a test stand capable of measuring thrust and torque of a small-scale rotor blade in hover state was constructed and fabricated. The test stand consists of three parts; a rotating device, a load measuring sensor and a data acquisition system. Thrust and torque were measured with varying collective pitch angle at fixed RPM. Through this research, hover performance tests were conducted for a small-scale single rotor blade operating in low Reynolds numbers (Re ≈ 3 × 10 5 ). The rotor blades investigated in this paper have maximum FM values varying from 0.59 to 0.65, which are low relative to modern full-scale helicopters. From these differences in FM between a small and a full-scale helicopter, the induced power factor is determined as varying from 1.35 to 1.42. Through this study, tests of hover performance were conducted for a single small-scale rotor blade, as well as verifying the test stand itself for the acquisition of hover performance.

When the speed of a coaxial rotor helicopter in forward flight increases, the wake skew angle of the rotor increases and consequently the position of the vena contracta of the upper rotor with respect to the lower rotor changes. Considering ambient air and the effect of the upper rotor, this study proposes a nonuniform inflow model for the lower rotor of a coaxial rotor helicopter in forward flight. The total required power of the coaxial rotor system was compared against Dingeldein's experimental data, and the results of the proposed model were well matched. A plant model was also developed from first principles for flight simulation, unknown parameter estimation and control analysis. The coaxial rotor helicopter used for this study was manufactured for surveillance and reconnaissance and does not have any stabilizer bar. Therefore, a feedback controller was included during flight test and parameter estimation to overcome unstable situations. Predicted responses of parameter estimation and validation show good agreement with experimental data. Therefore, the methodology described in this paper can be used to develop numerical plant model, study non-uniform inflow model, conduct performance analysis and parameter estimation of coaxial rotor as well as other rotorcrafts in forward flight.