Originally Posted by
tdracer
On the original pure turbojet and low bypass turbofan jets, the engine noise so dominated the noise generation that nobody paid much attention to the aerodynamic noise generation. But as the engines have gotten progressively quieter, the airframe contribution has become increasingly important. Years ago Boeing did a noise test of the 747 (I forget which model but is was long enough ago that I suspect it was a -200). They flew over the microphone array at max takeoff power, then repeated the tests - same speed, altitude, etc. but with the engines at idle. It only made 3 db difference - basically meaning that at takeoff power the airframe contribution to the noise was roughly equal to the engines
On the new, quieter aircraft (787, 747-8, A350, etc) the noise engineers spend as much time working things like the flaps and landing gear as they do the engines.
I m not surprised. The engines ususally radiate noise behind the aircraft.The noise intensity with hen engines on might make only 3dB difference, the duration is however longer. But for the certification tests the duration was not relevant.
One frequently forget, that landings harms more people than starts. Landing is flat and aircrafts follow the ILS as on rails. The starts are steeper and the routes diverge soon.
And yes some trivial improvements like castellated bolts on the wheels make a significant difference on the landing noise.