1. Introduction

2. The experimental setup: from concept to implementation

3. Test Results

3.1. Isolated propeller and T-motor characterization

Before proceeding with the two propeller systems, an isolated propeller thrust characterization has been performed by considering just one active propeller at a time; the obtained results have been also used to implement an XROTOR numerical model.

The noise characterization has been conducted in two different experimental conditions: T-motor and T-motor + propeller run at the same angular velocity. Both tests have been conducted by measuring the thrust generated by the propeller over a 30 $\mathrm{s}$ period starting from the reach of the steady state. Let us notice that from a controller’s logic point of view, the steady state condition was reached when the actual velocity of the propeller corresponded with the target velocity $\pm 60$ rpm. This condition was named the governor lock-ok condition.

3.1.1. Single propeller thrust evaluation

The diameter of the propeller is $D=36\mathrm{c}\mathrm{m}$. The test facility has been arranged to place the propellers at a 1.5D distance from each other and a 1D distance from the supporting horizontal arm. The test setup for the isolated propeller condition is shown in Figure 6. The isolated propeller hypothesis has been fulfilled by considering only one active propeller at the time.

A simple procedure for the acquisition has been set to ensure the appropriate repeatability of the testing campaign. Each acquisition has been carried out for $30\mathrm{s}$ from the lock-ok condition (steady state). To highlight the experimental characteristics of the data set and to provide a comparison with XROTOR numerical results, an error bar diagram has been chosen.

Several information can be derived from the chart in Figure 7: at low rpm, the isolated lower propellers curve and the isolated upper propellers curve exhibit lightly different performances in terms of mean thrust value. This behaviour could have been attributable to some aerodynamic reasons. The two propellers are supposed to be identical from an aerodynamic and geometric point of view. The difference lies in the opposed arrangement of the two propellers. In the upper propeller case, the support is located at the top of the propeller and its wake is free from obstacles. In the lower propeller case, the pillar is located at the bottom of the disk resulting in a disturbance of the wake region. An ulterior motivation is due to the partial ground effect on the upper propeller caused by the lower one. The distance between the two propellers is enough small to generate the ground effect [28,29,30,31]. For some angular velocities, the lower propeller also rotated due to the swirl effect caused by the wake of the upper one [32]. This caused a parachute effect similar to the one characterizing the helicopters during the autorotation phase [33,34,35]. In this condition, the generated ground effect is even larger.

It could also be highlighted that no measurement was possible in a range between [1020-1500] rpm for both the propellers due to a first resonance of the supporting vertical arm; another more serious resonance has been detected in a [1900-2820] rpm range for the upper propeller. This issue is caused by a transversal motion of the centering frame that is triggered by the compression load generated due to the upper propeller motion. In correspondence with this resonance, a rotational motion of the threaded connecting bar leads to a torque applied on the load cell. The load cell’s motion determines a modification of the electronic configuration of the load cell-digitizer interface which results in a zero reference loss. To avoid such an issue, it has been decided to perform the testing up to 1900 rpm, disconnect the system, check for possible non-compliance in the zero-reference value and restart the test. The second resonance range has been avoided accelerating the system straight from 0 up to a 2820 rpm angular velocity: this method has shown good results since, a quality control performed on the zero value and the reference value once the testing has been interrupted, has proven accordance between the pre and post resonance thrust values.

3.1.2. Single propeller noise characterization

The system has been characterized from an acoustic point of view to understand the contribution of both the propeller and the T-motor and to highlight the frequency content for different values of the angular velocity. The $L_{A_{eq}}$ parameter, which has been measured by means of an NTi Audio XL2 sound level meter [36], is shown in Figure 8, Figure 9, Figure 10 and Figure 11. Two configurations have been tested: T-motor (motor running without the propeller) and T-motor + propeller. The microphone was positioned 1.5m away from the propeller’s hub into their rotation plane, perpendicular to the ground. The sampling frequency was set at 51 kHz while the timestep (integration time) of the equivalent level was set at 100 ms.

By observing the charts of the $L_{A_{eq}}$ vs time for both propeller and T-motor + propeller configurations, it is possible to notice that the $L_{A_{eq}}$ measured in by the T-motor + propeller configuration is not always prevalent on the $L_{A_{eq}}$ generated by the T-motor itself. From Figure 9, high peak oscillations are highlightable: this results in distinct tones. It is also possible to emphasize that, at a lower rpm regime, higher periodic oscillation occurs: those are determined by the angular velocity control system which has been reported to be activated with higher frequency.

Figure 12. $L_{A_{eq}}$ spectrogram at 1620 rpm - Tmotor(left), Tmotor + propeller (right).

Figure 12. $L_{A_{eq}}$ spectrogram at 1620 rpm - Tmotor(left), Tmotor + propeller (right).

Figure 13. $L_{A_{eq}}$ spectrogram at 1740 rpm - Tmotor(left), Tmotor + propeller (right).

Figure 13. $L_{A_{eq}}$ spectrogram at 1740 rpm - Tmotor(left), Tmotor + propeller (right).

Figure 14. $L_{A_{eq}}$ spectrogram at 1860 rpm - Tmotor(left), Tmotor + propeller (right).

Figure 14. $L_{A_{eq}}$ spectrogram at 1860 rpm - Tmotor(left), Tmotor + propeller (right).

The test has shown that the T-motor + propeller configuration is prevalent, in terms of $L_{A_{eq}}$ over the T-motor configuration. In Figure 16 it is possible to notice that in correspondence of an angular velocity of 1140 and 1500 rpm, this trend is not fulfilled and the T-motor configuration shows a higher noise level due to distinct tones appearing in the 3^rd octave band of center 1000, 1250 and 1600 Hz. In correspondence with an angular velocity of 2820 rpm, an 8 dB(A) difference of the $L_{A_{eq}}$ can be highlighted. The same difference becomes 6 dB(A) in the case of the afore-mentioned 1140 and 1500 rpm conditions which can be stated as critical in terms of motor functioning.

Figure 15. $L_{A_{eq}}$ spectrogram at 2820 rpm - Tmotor(left), Tmotor + propeller (right).

Figure 15. $L_{A_{eq}}$ spectrogram at 2820 rpm - Tmotor(left), Tmotor + propeller (right).

4. Conclusions and remarks

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