Jet-Installation Noise Reduction with Permeable Flaps at In-Flight Conditions

An experimental investigation on jet-installation noise reduction with permeable flaps on an aircraft half-model is performed, focusing on the effects on the noise levels as well as on the aerodynamic properties of the model (lift and drag forces). A nozzle with an exit diameter = 113 mm is included for generating a single-stream jet flow in the vicinity of the airframe. Two perforated flaps with different hole distributions are investigated. The first one has a uniform hole distribution with equal hole spacing in both streamwise and spanwise directions, whereas the second one has a gradient permeability with the hole spacing progressively decreasing towards the trailing edge. Aerodynamic force measurements, carried out with a balance, show that the permeable flaps are responsible for a lift reduction in the order of 7%, and a slight drag increase, in the order of 0.5%, with the gradient permeability flap outperforming the uniform one. Flow-field maps indicate that the effect on lift is linked to the formation of side-edge vortices at the spanwise positions corresponding to the discontinuity between solid and porous regions. Acoustic results obtained from phased array measurements show that the source at the flap trailing edge, generated by the interaction with the jet, reduces by 3 dB in amplitude with both permeable flaps, which perform similarly. Therefore, the permeable flaps are able to significantly reduce jet-installation noise for an aircraft configuration in flight conditions, however care must be taken in the design of the permeable structure in order to avoid aerodynamic degradation.