Recently, someone asked me if it made sense to add turning vanes to a radiused elbow. That’s one of those questions where there is no exactly a right answer that applies for every situation. The impact of the turning vanes has a lot to do with both the geometry of the fitting and the velocity of the air moving through it. To gain some insight into this, lets consider a 12” square elbow with a 12” centerline radius (i.e. the centerline radius is equal to the duct width) like the one pictured below.