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What are these things encircled on the horizontal stabilizer on the Diamond DA40? And why are they used from an aerodynamic point of view?

Diamond DA40 Diamond Star

Source: grupooneair.com

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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.

From: Why don't horizontal stabilizers have winglets?

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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.

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  • $\begingroup$ Inspired by what a young Clarence Kelly Johnson did with a new airplane that had large nacelles ahead of the wing. Changing the smaller 3 fin tail of the Lockheed Electra into an end-plated horizontal stabilizer (with 2 vertical fins) greatly improved the stability characteristics. $\endgroup$ Jul 14 at 0:01

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