## 4 Results
### Helicopter Main Rotor Results
#### 4.2.1 Azimuthal Variation of Loads and Deflections at 12.3 m/s Flight Speed  
In this low speed case, comparisons of predictions and measurements are made for blade loads and elastic displacements and the results are depicted as azimuthal variations. 0\({}^{\circ}\) azimuth corresponds to the blade pointing towards the tail boom, as shown in Figure 11 in Appendix A.  
Notable differences both in the mean value, as well as in the amplitude of the \(1/rev\) variation are observed in the lead-lag bending moment (Figure 36). Load variations due to high frequency harmonics are well captured by the AL simulations. The LL model predicts a smoother variation of loads at higher frequencies, while the predicted signal is slightly shifted with respect to AL predictions and measurements. In the lead-lag deflections (Figure 37), predictions lead with respect to measurements. It is noted that positive deflection is in the lag direction. This is common in all three flight speeds and is mainly attributed to the technique used to measure blade deflections in the experimental campaign. The Stereo Pattern Recognition (SPR) that was used for the first time in the HART II and HeliNOVI projects, was repeatedly reported to show a consistent and unexplained 2-4 cm (\(\approx\)1%) translational offset in the lag direction with respect to numerous and of varying fidelity computational method results [62; 63]. In [32], the flexibility of the drive train system and the hub, which are regarded as infinitely stiff in our simulations, are considered as an extra source of discrepancies between measurements and predictions. The \(1/rev\) amplitude of the variation is well predicted by the computational models, however high frequency oscillations seen in measurements seem to be almost completely damped in both simulations.  
Good agreement is observed in the mean value of the flapwise bending moment (Figure 38). The \(1/rev\) amplitude is fairly predicted. High frequency variations seen in the measurements (mainly \(2/rev\) and \(3/rev\)) are effectively reproduced by the simulations, with a small under-prediction of the respective amplitudes in LL results. Furthermore, a minor phase difference is seen in AL predictions which appears to be more pronounced in the LL predictions. The \(1/rev\) flapwise deflection amplitude (Figure 39) is clearly under-predicted by the two computations, which however agree well with each other. The high frequency variations seen in measurements are again effectively predicted.  
Regarding twisting moment (Figure 40), a small difference is seen in the predicted mean value. A clear \(3/rev\) variation is seen in measurements, which is slightly under-predicted by the simulations. The mean value of the torsion angle is well predicted  
Figure 35: Rotor power requirement comparison. Simulation predictions vs experimental measurements.  
by the models (Figure 41), however with almost zero \(1/rev\) amplitude. Although the measured signal exhibits a dominant \(3/rev\) variation, a small \(1/rev\) component is also noted. The \(3/rev\) variation is well captured by the simulations, while a small phase shift is observed. The high frequency ripple in the LL simulation is related to higher harmonics or some higher frequency mode due to the BVI phenomena. This does not appear in the AL results due to the increased numerical diffusion of the AL analysis associated with the coarser far wake grid.  
Figure 36: Forward flight at 12.3 m/s, case 851. Lead-lag moment at \(r/R=0.03\).  
Figure 37: Forward flight at 12.3 m/s, case 983. Lead-lag deflection at \(r/R=0.99\).  
Figure 38: Forward flight at 12.3 m/s, case 851. Flapwise moment at \(r/R=0.1465\).  
Figure 39: Forward flight at 12.3 m/s, case 983. Flapwise deflection at \(r/R=0.99\).