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Old 09-28-2009, 07:27 AM
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Join Date: Mar 2009
Location: Aberdeen, Scotland
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Where two surfaces join you get a type of drag called interference drag. The airflows on the (in this case) fuselage and wing interfere which can cause flow seperation especially from the wing upper rear surface and hence lad to increased drag.

The fillet is to reduce this interference drag.. You will usually find the largest fillet on the rear upper surface of the wing which is where seperation is most likely.

While the theory is sound the actual gains are modest. At model scale you would be highly unlikely to gain any measurable benifit from a fillet. In fact if you were not careful it would be easy to add enough weight to have an overall negative effect.

Hope this helps
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