how much less drag with a symmetrical fin

[sailplaneguy]

Does anyone know how much less drag there is with a symmetrical fin/rudder airfoil than there is with one that has flat sides? I notice all full size airplanes have an airfoil shape in the fin and rudder, and most model sailplanes don't. Is it worth the effort to build one in to the model?

[Ollie]

Roughly speaking, the vertical tail will produce about 4 or 5% of the total drag of the sailplane. At zero angle of attack the flat plate will have just a bit more drag than a thin symmetrical airfoil. At four degrees angle of attack, the flat plate will have, perhaps twice the drag of the symmetrical airfoil. The area of the horizontal tail will be roughly twice the area of the vertical tail so, it is about twice as important a drag source as the vertical tail. That's assuming the flat plate has a rounded nose and the trailing edge tapers to a knife edge. If the flat plate is squared off at the leading and trailing edges, then the drag will be much higher and depend on the thickness of the flat plate.

So the question boils down to a percent or two difference in glide angle during straight flight and about twice that much during gentle maneuvers.

Only you can value the trade between your effort and the plane's performance. Some competitors are willing to spent $1000 for a 10% edge in performance.

[Sparky Paul]

I like to think a symmetrical surface gives a bit less drag, and more precise control than a flat plate.
When I want both, I use symmetrical.

[markdrela]

If the "slab" tail airfoil is really crude with simply a rounded leading edge, the effect on drag may or may not be significant. It depends mainly on the Reynolds number. At low Reynolds numbers there won't be much effect, but at larger Reynolds numbers the round LE will trip the flow very early, and considerably increase the skin friction.

As an example, Xfoil predicts that a DLG slab tail has a 10-20% drag penalty at glide speeds, and a 100% drag penalty at launch speeds. These are relative to an ideal fully-laminar tail. Needless to say, I go to considerable effort to accurately shape my DLG tail airfoils.