Takeoff/approach noise for a model counterrotation propeller with a forward-swept upstream rotor

Abstract

A scale model of a counterrotating propeller with forward-swept blades in the forward rotor and aftswept blades in the aft rotor (designated F39/A31) has been tested in the NASA Lewis 9-by 15-Ft Anechoic Wind Tunnel. This paper presents aeroacottstic results at a takeoff/approach condition of Mach 0.20. Laser Doppler velocimeter results taken in a plane between the two rotors are also included to quantify the interaction flow field. The intention of the forward-swept design is to reduce the magnitude of the forward rotor tip vortex and/or wakes which impinge on the aft rotor, thus lowering the interaction tone levels. A reference model propeller (designated F31/A3I), having aft-swept blades in both rotors, was also tested. Aeroelastic performance of the F39/A31 propeller was disappointing. The forward rotor tip region tended to untwist toward higher effective blade angles under load. The forward rotor also exhibited steady state blade flutter at speeds and loadings well below the design condition. The noise results, based on sideline acoustic data, show that the interaction tone levels were up to 8 dB higher with the forward-swept design compared to those for the reference propeller at similar operating conditions, with these tone level differences extending down to lower propeller speeds where flutter did not occur. These acoustic results are for a poorlyperforming forward-swept propeller. It is quite possible that a properly-designed forward-swept propeller

Investigation of a Pylons Effect on the Character of Counter-Rotating Open Rotor Noise Using Beamforming Technology

“…The signals of the various microphones were individually processed, providing information as to the directivity and the spectral content of the signals [2]. Woodward et al conducted measurements in order to study many parameters, including the effects of angleof-attack, reduced rotor diameters, advanced blade designs, and the inclusion of a pylon and fuselage in the model [2][3][4][5][6][7]. Around the same time, the analytical investigations of research groups, such as Parry and Crighton, applied asymptotic approximation techniques combined with linear array measurements of the unsteady loading on the blades in order to predict the noise production mechanisms of CROR [8].…”

Initial Investigation of the Acoustics of a Counter Rotating Open Rotor Model With Historical Baseline Blades in a Low Speed Wind Tunnel

“…Turbines within the drive rig are fed by a 450 lb/in. 2 air source, which turns the rotor blades. The drive rig is also mounted on a turntable, which provides angle of attack (AOA) testing capability.…”

Tone and Broadband Noise Separation from Acoustic Data of a Scale-Model Counter-Rotating Open Rotor

“…Some of the low frequency noise was the empty tunnel background noise while other portions of it are due to the wake from the rotors flowing over the ORPR strut. [15] Methods for accounting for the background noise of the facility have not been included in the tone/broadband results presented in this report. The spectra were computed using a sample size of 3 million and a Fourier transform of length 2 14 A table of the reading numbers and a few relevant parameters are shown in Table 2.…”