Bat tail on tailless: how big?

[deh6]

Do you have any references for how big the “bat tail” on a swept-wing tailless plane should be?

The On-The-Wing CD has an article showing that Horten thought the bat-tail length is best if it makes the ¼-chord line form a 140 deg angle at the center of the wing. Someplace on the CD a comment is made that the bat tail helps for a thermaling design. What I don’t find is anything that indicates the best width of the bat-tail (maybe none is best!?). Carried to the extreme the bat tail turns the wing into a diamond with the tips clipped—intuitively that looks rather low performance.

[Ollie]

The bat tail is supposed to reduce the drag associated with the 25% chord line meeting at a sharp angle. The wider you make the bat tail the more effective it will be at preventing the extra drag but the more problems it will cause by reducing the benefits of sweep back. This is a classical design problem of trying to find a compromise between conflicting objectives. The higher the maximum lift coefficient the bigger the width of the bat tail. The higher the aspect ratio, the wider it can be. The greater the sweep angle, the more bat tail needed. Many successful F3B designs from Germany use low maximum lift coefficient airfoils and get away without any bat tail. I suggest that you make the bat tail of thin pywood and make it removable so that you can experiment with various sizes to see if you notice any benefits.

[deh6]

Thanks Ollie. The removable bat tail is an interesting idea. Hmm...I wonder if it could be made retractable...

[Ollie]

Another way to prevent the 25% chord lines from meeting at a sharp angle is to have the leading and trailing edges of the wing center sweep around from one side to the other in a gentle arc. this is the method used by the Swiss, full scale, SB-13 and several other designs.