###### Abstract  
In this paper, the dynamic balance of a rigid variable speed rotor is tested and analyzed. The vibration acceleration is measured by vibration analyzer instrumentations. The rotor adjustment parameters of counterweight, pitch, and trailing-edge flap are considered. The amplitude and phase of the 1 \(\Omega\) vibration acceleration are analyzed through an all-phase fast Fourier transform. The experiments are conducted using two rigid rotors with the same geometry. The accelerations of the fuselage in the \(x,~{}y\), and \(z\) directions are measured. Through a waterfall diagram of the auto-power spectrum, it is found that the imbalance of counterweight, pitch, and trailing-edge flap causes an obvious increase in 1 \(\Omega\) and 2 \(\Omega\) acceleration. The hub counterweight mainly causes the lateral and longitudinal vibration in the disc plane, and the aerodynamic factors such as pitch and trailing-edge flap mainly affect the vertical vibration. In order to achieve dynamic balance for variable speed rotors, the counterweight adjustment should be given the highest priority for the vibration in the disc plane, and the pitch and trailing-edge flap adjustment should be given the highest priority for the vertical vibration. The results obtained from this experiment may be helpful toward reasonable designs of variable speed rotor helicopters. variable speed; scale-model rigid rotor; acceleration; rotor dynamic balance +
Footnote †: journal: Applied Sciences  
[PERSON] and [PERSON] [10] established a mapping relationship between blade parameters and vibration in the frequency domain and applied it to control high-order vibration. At the same time, the optimal parameter is searched by an optimization algorithm to reduce the fundamental vibration. [PERSON] [11; 12; 13; 14; 15; 16; 17; 18] published several papers that report on the effect of rotor diagnosis in different ways. His research involves many different types of rotor faults, such as beam crack, composite material delamination, lag damper failure, and pitch-rod damage, including rotor mass imbalance caused by weight loss and aerodynamic imbalance caused by blade pitch or blade trailing-edge flap. The research objectives include various parameters such as natural frequencies of the blade, six-force-factor of the hub, tip displacement, etc. [PERSON] et al. [19; 20] used the finite element method to model the rotor system of an S-60 helicopter, and used the relationship between rotor adjustment parameters and fuselage vibration for digital simulation. At the same time, combined with flight test data, a neural network relationship model of the rotor system was established. [PERSON] et al. [21; 22] established a rotor fuselage model. Based on the assumption that each blade in the rotor model is independent of each other, they analyzed the effect of rotor adjustment parameters on the vibration of the fuselage with a frequency of 1 \(\Omega\) (\(\Omega\) is the rotor speed). With the increase in flight speed, the vibration increases continuously. Based on the research of rotor imbalance test, [PERSON] et al. [23; 24] proposed an optimization adjustment of rotor dynamic balance to reduce the helicopter vibration level and improve the adjustment efficiency of rotor dynamic balance. [PERSON] et al. [25] reviewed and summarized vibration-based techniques for helicopter transmission diagnostics. [PERSON] et al. [26] presented an efficient and comprehensive vibration analysis method for a helicopter rotor-fuselage coupling system. [PERSON] et al. [27] alleviated numerically both the rotor and airframe vibrations of a lift-offset compound helicopter using two different active vibration control techniques. [PERSON] et al. [28] studied to reduce helicopter 1/rev vibration using an extendable trailing-edge plate. [PERSON] et al. [29] investigated the actively controlled flap (ACF) to reduce the rotor dynamic load by using a three-blade helicopter rotor. [PERSON] et al.