HELICOPTER MODEL SCALE RESULTS OF BLADE-VORTEX INTERACTION IMPULSIVE NOISE AS AFFECTED BY TIP MODIFICATION

An experimental investigation of the effect of tip-shape modification on blade-vortex interaction induced helicopter blade-slap noise has been conducted. The general rotor model system (GRMS) with a 3.148-m (10.33-ft) diameter, four-bladed fully articulated rotor was installed in the Langley Research Center V/STOL tunnel. Tests were conducted over a range of simulated flight and descent velocities which have been shown to produce blade slap. Aerodynamic performance parameters of the rotor system were monitored to ensure properly matched flight conditions among the tip shapes. The tunnel was operated in the open-throat configuration with treatment to improve the acoustic characteristics of the test chamber. Four promising tips (based on previous investigations) were used (ogee, subwing, 60° swept tapered, and end plate) along with a standard square tip as a baseline configuration. This investigation provided a detailed acoustic evaluation on the same rotor system of the relative applicability of the various tip configurations for blade-slap noise reduction. The microphone data were also used to compute the location in the rotor disk where the blade-vortex interaction noise was generated. These were compared with probable locations calculated by an existing rotor wake analysis model, thus vortex age and orientation could be proposed.