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Effect of adding fillets to wing?

Old 09-28-2009, 04:10 AM
Larry G
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Default Effect of adding fillets to wing?

I am rebuilding a badly smashed GWS Formosa I. Part of piecing it back together was gluing the wing to the fuselage, where before it was removable. I was thinking of making fillets (maybe not the right term?) on the leading and trailing edge of the wing where it joins the fuselage, kinda like a Mustang has on the front, and maybe quite long and swoopy on the trailing edge. What aerodynamic effect could this have?
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Old 09-28-2009, 07:27 AM
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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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Old 09-28-2009, 08:48 AM
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On the plus side - they can look cool
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Old 10-19-2009, 02:42 AM
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Originally Posted by Larry G View Post
... I was thinking of making fillets ... on the leading and trailing edge of the wing where it joins the fuselage, .... What aerodynamic effect could this have?

The following applicables excerpts are from Wainfan, Barnaby; 199509; Here's more on high- vs low-wing configurations; Kitplanes; pp50-53:
  • Even on a well designed, clean airplane, interference drag is rarely less than 5% of the total parasite drag.
  • There are several ways that aerodynamic interfernce can cause a drag increase.
    • The first is by increasing the local velocity of the airflow over the surface. The higher the speed of the air scrubbing over a piece of the skin of the airplane, the higher the skin friction drag of that skin. If the flow over two parts of the airplane combines in a way that increases the airspeed over a portion of the skin that would not otherwise experience high local airspeeds, the drag of the airplane will be increased.
    • The second way interference can cause a drag increase is if the interaction of the airflows over the two components of the airplane causes flow separation. On an otherwise clean configuration, even a relatively small area of separated flow can cause a large increase in drag. Areas where two components both pull away from the airflow at once are likely candidates for this type of problem. In general, if one component is decreasing in cross section, the one it is joined to should be increasing cross section at that point.
    • A third source of interference drag is the formation of vortices that are shed into the airflow. These vortices are much like wingtip vortices, and they take energy away from the airplane and cause drag.
  • A low-wing airplane with a rounded fuselage cross section and/or dihedral has two problems in the wing/fuselage intersection corner. First, the included angle is relatively small, and second, the air flowing through the channel is moving fast, having been accelerated by the wing.
  • Fillets can be used to clean up this problem on low-wing airplanes. By radiusing the corner with a fillet, two things are accomplished.
    • First, the wetted area in the area of the corner is reduced.
    • Secondly, the air will not have to rub against both surfaces forming the corner and will produce less skin friction drag.
  • The addition of the fillet will improve the flow in the corner area and reduce the drag of the airplane.
I find his explanations to be most understandable.
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