Why were jet engines so noisy just a few years ago when compared to modern jet engines of today? Is this a fan blade inovation thing?
I hesitate to answer in the presence of such expert opinion Robert, but the reason is one of the win, win elements of the modern high bypass fan jet. It is well known that Frank Whittle 'invented' the jet engine, but it is not so well known that his genius went far beyond what the then current technology could manage. For example, when he spoke to combustion engineers about burning six hundred gallons of paraffin an hour, in the space of a washing machine, they laughed at him, saying he would be lucky to burn a tenth of that!
Despite that, Whittle knew the engine he was desperately trying to build and run, was not going to be the only way to do it and certainly not the best. Among the recognizably modern innovations Whittle patented were prop-jets, water injection, multi shaft engines and, his most important belief, the
high bypass jet engine. Whittle knew that low volume, high gas velocity jets would mean high speed, but high volume, relatively low gas velocity would mean
power and efficiency. Of course, by the time many of those things were possible, his patents had all expired.
Which is a very long winded way of saying; the enormous volume of relatively cool, slow moving air - although it produces the vast majority of the thrust - from the fan, forms a perfect tube around the very hot, noisy, fast moving gas from the core engine, thereby shielding our ears from it.
Roger.