Scarselli G.

In this paper the airframe noise of civil transport aircraft is numerically evaluated and compared with measurements taken in some european airports with the target of separating the airframe contribution from other noise contributions effective on a typical aircraft. The attention has been focused on airframe noise since it sets a lower limit below whichever reduction of noise generated by engine have no significant effect on the overall noise level perceived by an observer and due to the aircraft flyover. The intensity of airframe noise depends on the aircraft configuration: during the cruise the aircraft exhibit an aerodynamically ‘clean’ configuration that produces less noise than the configuration assumed by the aircraft during landing and take-off, that usually is referred as ‘dirty’ one. The configuration with slats extended, flaps down and undercarriage lowered is more noisy than the clean configuration. In this paper the results of numerical simulations performed on common operating aircraft are presented: these simulations allow the contribution breakdown and, therefore, the classification of the most noisy airframe components for the different approach and take-off configurations. The following computation procedure has been implemented: at first noise emitted by each airframe components has been modeled, then the combination of these sources has been performed by summation of the different noise levels: a computer program has been developed for obtaining the free field noise levels emitted by an aircraft during a flyover. These emissions have been then corrected taking in account the atmospheric absorption and the ground reflection: in this way on one hand the airframe noise contribution is numerically obtained; on other hand the noise contributions different from airframe have been numerically evaluated by the taken measurements. An examination of the current technologies available for the reduction of the noise emission of slats, flaps and undercarriage has been performed.