Why Birds and Airplanes Need No Vertical Tail

Abstract

The question of why airplanes need a vertical tail whereas birds seem to do very well without one has been longstanding. Some have suggested that it is a matter of scale; that the rationale for a vertical tail disappears when coming down the scale from the regime of "full" scale aircraft to bird/ small scale flight regime. This hypothesis has now been debunked. This paper shows that the lateral-directional modal dynamics and stability criteria remain unchanged over the relevant range of flight Reynolds numbers. It conclusively establishes that a vertical tail is not essential for directional or lateral stability at any scale from that of bird flight to "full" scale airplane flight.