APU Exhaust shapes
 

A tech question but as I am not a professional I ll post it here

BAck in the when the MD80 graced our skies the early ones sported an APU/Exhaust in the conventional cone shape while the newer ones were changed to a boat tail or wedge shape. Boeing went on to use the wedge shape on the T7 whereas the 767 and 747 used the conical version . I assumed that was an aerodynamic advancement to go to the wedge but along comes the 787 and its back to the cone again.

Any one know the reasons/aerodynamics fro the two styles and why the change back?

The MD80 'wedge' fairing was known as the 'screwdriver' and was developed in the mid-1980s. It was an easy retrofit. American Airlines later obtained an STC for their own design of fairing which was cheaper than the official MDC part.

I don't know why Boeing reverted to the conical fairing on the 787 but the 777 has a very noisy APU on the ground on account of a very short jetpipe, due to the shape of the fairing it exhausts to the side.

https://www.youtube.com/watch?v=qc_nFCHnr78

Perhaps the longer axial jetpipe in a conical fairing is quieter.