Brendows From Norway, joined Apr 2006, 1020 posts, RR: 4 Reply 2, posted (7 years 4 hours ago) and read 4671 times:
Quoting RichardPrice (Reply 1): Properly designed, they can aid in the mixing of the air from the engine as it exits into the outside airflow. This can reduce drag.
I believe that these chevrons actually increase drag a little bit. And they reduce the available thrust, that was at least what Airbus said in a news article when they were asked why they didn't have chevrons on one of the former versions of the A350. The main purpose with these chevrons is to reduce engine noise, by mixing the bypass/core airflow and bypass air with the air passing around the engine.
The engines on the 787 and 748 will be equipped with chevrons, but I haven't seen any pictures from Airbus where the A350 has chevrons.
In 2001, Boeing and RR went flight testing on the B777, and tried a number of novel noise technologies. One of the most promising was the serrated hot / cold nozzles. I guess AI have done similat testing / analysis and arrived at the same conclusion.
Chevrons are a design balance between benefits of mixing (noise reduction) and cost in terms of drag and fuel burn. In QTD2 actuating chevrons were studied with good success. In this version the chevrons are in the stream at takeoff (for noise reduction at high power) then out of the stream at cruise (for best fuel burn at low power). Very good idea that will probably make their way on to some airplane in the future.