What are these things encircled on the horizontal stabilizer on the Diamond DA40? And why are they used from an aerodynamic point of view?
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As explained in a related post:
The canted tips of the original DA42 are working like an anhedral and create a compensating positive rolling moment for the vertical tail's negative sideslip-induced rolling moment. Ideally, the whole horizontal tail would have anhedral, but it was preferred to keep the elevator hinge line straight.
Aside from the eloquent discussion of reducing "negative side-slip induced rolling moments" (putting larger spats on the wheels would do that too), delving into the lost art of horizontal stabilizer design has yielded an interesting thought:
Under normal flight conditions the tail usually exerts a downforce to keep the center of pressure aligned (longitudinally) with the center of gravity.
However, in situations of excessive positive AoA, such as stall/spin events, tail upforce is required to help the nose down. Putting anhedralled endplates on the tail is a rather ingenious way of making the tail "bigger" (greater coefficient of lift) when saving upforce is needed, while making the tail "smaller" (lower drag coefficient-vortex mitigation) in a normal flight regime where downforce is required.