Main- and tail-rotor on the helicopter are the dominant noise sources for the external noise, which are characterized by a very characteristic and unmistakable sound. The sound of a flying helicopter therefore differs significantly from that of a propeller or jet aircraft.
Depending on the flight conditions, such as cruise, descent or faster cruising, different rotor noise generation mechanism with different directional characteristics may occur. As shown in Figure 1, extremely different flow conditions prevail on a rotor blade of the main rotor during each revolution, which in turn produce different noise characteristics. Rotor noise consists of the rotor rotational noise, the "Thickness noise" (by volume displacement) and the "Loading noise" (by lift and drag forces). The thickness noise is only important for the low frequency range when impulse-like noise phenomena, such as "high-speed impulsive (HSI) noise" and "blade vortex interaction (BVI) noise" do not occur.
HSI noise is produced during fast forward flight, when at rotor advancing blade side (where rotation and forward flight speed adding up) supersonic areas occur with compression shock. Compression shocks generate a impulse-like sound signal. It has its maximum intensity in the plane of the rotor and is directed to the front. Any measures that lead to a reduction of the sonic area on the rotor blades contribute to a reduction of the HSI noise.
BVI noise is the result of an interaction between the blade tip vortices and the subsequent rotor blades. BVI noise occurs mainly in the moderate descent flight with a flight slop angle between 3° and 9 °, which occurs during the landing approach. In this flight condition the vortexes generated at the blade tip are sucked into the rotor plane and interact with the following rotor blades. The strong BVI noise can also occur in curving flight. In addition, Blade-vortex interactions (BVI) also play a crucial role in the vibrational excitation of the rotor and thus also for the level of vibration in the helicopter airframe.
BVI noise has fundamentally different characteristics of acoustic directivity compared to HSI noise. The maximum intensity of the BVI noise is radiated in the flight direction forward and opposite to the flight direction downwards at an angle of about 45 ° - 60 ° to the plane of the rotor. To reduce the BVI noise there is, physically, a number of ways, for example, (1) to reduce the vortex strength of the local vortex which interacts downstream with a rotor blade, (2) to minimize the intensity of the very interaction process and (3), to increase the distance the vortex passes the blade, i.e. the “miss distance”. Another possibility to reduce BVI noise is to let the pilot fly during the landing approach with for example, low (<3 °) or large slop angle (> 9 °) to avoid flying in BVI conditions (see also article on low-noise flight procedures).
With the aid of numerical simulation, aeroacoustic phenomena of the main and tail rotor can be simulated. Various methods have been developed to calculate the rotor noise, known are among others:
Since the CAA-method is still too expensive and requires very long computation times that can last for many days for 3-dimensional rotor simulations, integral formulations are in general used for helicopter rotor noise prediction. For the prediction of main and tail rotor noise, the DLR Institute of Aerodynamics and Flow Technology develops and uses APSIM (Acoustic Prediction System Based on Integral Method) which is based on the acoustic analogy by Ffowcs Williams and Hawkings. The method requires as input the unsteady pressure distributions on the blades, which can be determined experimentally or numerically. As an efficient numerical method for the calculation of the unsteady pressures on the blade surfaces, the unsteady panel method UPM (Unsteady Panel Method) is developed at DLR. UPM can accurately predict blade tip vortexes and BVI with a "3D Free-Wake" model.
Figure 2 (top): Simulation on the development of main/tail rotor wakes for Bo 105 helicopter
Figure 3 (bottom):
Noise directivity underneath BO 105 model helicopter in wind tunnel
left: Simulation with UPM-APSIM
(without taking into account fuselage noise shielding)
Figure 3 shows the result of an acoustic simulation for a wind tunnel test using a model of the BO 105 helicopter and the comparison to the measured data. The simulation results were calculated using the "Tool chain" UPM-APSIM. The noise footprint was obtained below the helicopter model for a fast cruise flight at a speed of 220 km/h. The positions of the main rotor and the tail rotor as well as the direction of flow are indicated in both figures. The big difference between the measured and simulated noise contour in the area in front of the helicopter (blue area in the experimental data) is due to the absence of the helicopter fuselage in the simulation. The fuselage has a noise shielding effect which is not considered in the calculations.