To answer my question – This is interesting (to some, perhaps); http://aaac.larc.nasa.gov/tsab/cfdlarc/aiaa-dpw/Workshop3/AIAA.2005-4730.copyrighted.pdf

Shows a fairing (FX-1) devised in 2006, according to the paper, to reduce the ‘included’ angle. This gave significant drag reduction.. but what that fairing is in fact doing is near-straightening the root (note the rear view, Fig.9). Not completely though. There was still a little bit of flow separation in the corner, so a fairing extending beyond the trailing edge (FX-3) then improved the situation a little further.. Talk about re-inventing the wheel.

Edit – actually a touch unfair – that fairing is a ‘bulge’ intended to keep to flow lines. But I wonder whether they would be divergent in the first place if that line was properly straight. But surely it can’t be as simple as that.. can it? Graham?

The older theory was that the angle was critical, so if you went less than 90 you hit interference problems. This was because low-wing, tubular fuselaged aircraft needed a fairing to ease the angle out that would otherwise exist. It was noted that the L-10 Electra didn’t need a fairing, and a generation of designs ( Hurricane, Defiant, Hellcat) went along with the idea that this was because the flat sides avoided an acute angle, and the angle was the point. They all ended up needing various sizes and forms of fairing anyway. The 1938 NACA paper in question refined this slightly but crucially in terms of what I was referring to, placing the emphasis on a straight join in plan form arrived at, in this case, by local alteration of section profile to give flat sides

This subtlety was passed over generally until the very late forties. And consequently everything continued to need at least a little fairing, be it Mustang, Ta152 or even first-generation Sabre. However, if you look at the WW, there is NO fairing ( ignore the plastic kits), and a completely straight-in-plan longitudinal wing/fuselage junction aft of the mainspar.

That is what I am talking about, not the general shape of a fuselage or the angle to the wing. And interestingly it’s in fact at an acute angle on the WW – 88 degrees – indicating that the designer knew it wasn’t so much about that angle as the root form.

A modern low-wing airliner’s wing root has growths and bulges that make that join straighter in plan. Very conventional now to straighten that line. Not in 1938.

Sorry I am probably not using the precise words an aerodynamicist such as yourself would. I hope you can stop laughing long enough to give me a chance, nevertheless. I have been looking at this in some depth.

So the next production design that I can find to have the locally flat section over the wing root, progressing back to a curved section aft of the trailing edge as a deliberate design feature, instead of any kind of fairing, was the F-86D.. a redesign of the earlier curved/faired F-86A fuselage. This puts the Whirlwind 10 years ahead of the thinking here. Interestingly, it might have been the 22 year old John Frost rather than Petter who did this. Certainly someone at Westland was keeping abreast of the absolute latest coming out of NACA in the US. The nacelles follow a theoretical shape described in a 1937 research note, while the report into the theoretical potential of locally flat sides (and straight junctions) rather than fairings on low wing designs appeared in early 38.

Does anyone have any more on this, especially John Frost – beyond the wiki article?