Investigation of the Effect of a Multiple Trailer Wake Model for Descending Flights

The effect of a multiple trailer wake model in a comprehensive rotorcraft analysis is investigated at the US Army Aeroflightdynamics Directorate (AFDD) and German DLR in order to enhance understanding of the current level of airloads correlation and address the effectiveness of the multiple trailers for the rotors in descending flights. The measured data used for this study include the wind tunnel data of the HART-I and HART-II rotors as well as the flight data of the UH-60A rotor. All the rotors are at an advance ratio of 0.15, simulating almost the same descending flight conditions with a shaft angle tilted aft. From the sensitivity study of the lift with various combinations of the trailed vortex filament bundles, te blade root vortex trailer needs to be included in the wake model to obtain a good lift prediction for the rotors in descent. A shift of the tip vortex release point to the inboard would help the M2 cn correlation and this may be due to existence of dual tip vortices in the 1st and 2nd quadrants. The HART and UH-60A databases exhibit a great similarity in the lift distributions over the rotor disk, and the use of a multiple trailer free wake model significantly improves the 3-per-rev lift correlation for the HART-I, HART-II and UH-60A rotors, compared with the rollup wake model. But, a multiple trailer wake model shows a tendency of slight over-prediction of Blade-Vortex-Interaction (BVI) found from the UH-60A airloads correlation. A good prediction of the 3-per-rev lift in the outboard span is important for descending flight, but there exist the equally or more important lift characteristics for accurate prediction in the 1st and 2nd quadrants of the rotor disk from the UH-60A correlation study. The bending moment prediction for the UH-60A rotor is not as good as the lift prediction.