Comparison of the Acoustic Characteristics of Large-Scale Models of Several Propulsive-Lift Concepts

Abstract

Wind-tunnel acoustic investigations were performed to determine the acoustic characteristics and the effect of forward speed on these characteristics of four propulsive lift concepts: the over-the-wing externally-blown jet flap (OTW), the under-the-wing externally-blown jet flap (UTW), the internally-blown jet flap (IBF), and the augmentor wing (AW). The data in this paper represent the basic noise generated by the powered-lift system without acoustic treatment, assuming all other noise sources, such as the turbofan compressor noise, have been suppressed. Under these conditions, when scaled to a 45,500 kg (100,000 Ib) aircraft, the OTW concept exhibited the lowest perceived noise levels because of dominant low-frequency noise and wing shielding of the highfrequency noise. The AW was the loudest configuration because of dominant high-frequency noise created by the high jet velocities and small nozzle dimensions. All four configurations emitted noise 10 to 15 PNdb higher than the noise goal of 95 PNdb at 153 m (500 ft). The AW is the only powered-lift concept which has shown the capability of acoustic suppression to this level. The effect of forward speed did not approach that expected from the relative velocity increments investigated. The dominant low-frequency noise of the OTW and UTW was reduced 2 db by an 80 knot free-stream velocity. The dominant high-frequency noise of the IBF and AW was unaffected by forward speed. Nomenclature OASPL = overall sound pressure level, db PNL = perceived noise level, PNdb SPL = sound pressure level, db referred to 2 x 10'5 N/m2 (0.0002 microbar) Vj = aver age isentropic jet exhaust velocity, m/sec (fps) V^ = freestream velocity, knots (fps) a = angle of attack with respect to wing chord plane, deg df = flap deflection with respect to wing chord plane, deg