These are photos of Tecnam P2006T rudder trailing edge. Why it has such flat plate mounted at the end of the rudder?
What seems be the case here is that a truncated trailing edge has a flat plate riveted on (called a double Gurney flap). This places the plate perpendicular to airflow (more or less). This, in turn, has the effect of increasing air pressure on whichever surface of the rudder is deflected into the airstream, giving a similar effect to increasing rudder area - but without the added weight of extending the trailing edge of the rudder.
Since weight in the tail is generally bad - because of the moment arm from the aircraft center of mass, a kilo added to the tail requires five kilos or more in the nose to return to correct balance -- this is probably done mainly as a weight saving on an aircraft that (as a twin) needs good rudder authority. As a bonus, it reduces the amount of deflection required for a given yaw force, which makes trim inputs more effective.
It's a double Gurney flap, but I'm not sure how it helps, here.
The Eurocopter AS355 TwinStar helicopter uses a double Gurney flap that projects from both surfaces of the vertical stabilizer. This is used to correct a problem with lift reversal in thick airfoil sections at low angles of attack. The double gurney flap reduces the control input required to make the transition from hover to forward flight.
In the context of aircraft, it's misleading to call this nonmoving surface a flap, of course. But the term and the concept came from "hey, Billy Joe, let's rivet this on and see how she runs" motorsports, a culture that wasn't fussy about nomenclature.
1$\begingroup$ Well done! Some information in this pdf. $\endgroup$– minsJun 20, 2021 at 21:21
$\begingroup$ The Tecnam P2006 is a twin, so they may have decided to have a bit more rudder authority for one engine out flying. $\endgroup$ Jun 21, 2021 at 1:16
$\begingroup$ First, since it doesn't move, it's a bit silly to call this a flap. Second, thick trailing edges on control surfaces improve centering, so there is a steeper hinge moment gradient near zero deflection. A moveable T-shaped strip was tried out by Doetsch on the Gloster Meteor in the early Fifties, predating the work of Gurney. By making the strip moveable (shifting between only left and only right) the rudder trim was markedly improved. $\endgroup$ Jun 21, 2021 at 3:16
1$\begingroup$ the gimmick seems to be to have the strip as wide as the turbulent boundary to avoid significant increase in drag. As flaps improve lift coefficient, the term is accurate. Putting one right at the end would improve yaw stability, and, as said, improve both low (rudder) AOA response and high AOA authority. $\endgroup$ Jun 21, 2021 at 7:06
1$\begingroup$ Now I wonder when the the race car drivers will start using (fore and aft) rudders to help them turn. Gurney actually made it as an improved spoiler. $\endgroup$ Jun 21, 2021 at 7:15
Here is another consideration.
At the aft end of an aerodynamic body, we taper the body so the streamlines in the air flow over it do not separate, because of the flow separation drag penalty.
But in doing so, we increase the length of the body's tail end, which increases wetted area drag.
There then comes a point where truncating the fuselage profile (so as to minimize wetted area drag) is balanced by the creation of separation drag i.e., a "crossover point" exists where we can truncate the profile and not pay a penalty.
Note that several years ago, VW built a prototype ultra-high-mileage car with a teardrop shape- but with a blunt, chopped-off tail. The designers had discovered that extending the tail taper (making the vehicle longer) did not reduce drag enough to be worthwhile, and so they determined where that crossover point was and chopped off everything aft of it.
1$\begingroup$ Yes but if you look at the photos, they don't simply show truncation. They show that a plate is attached at the trailing edge of the rudder to increase the width of the trailing edge of the rudder. Most clearly visible in the lower photo. $\endgroup$ Jun 21, 2021 at 13:43
$\begingroup$ @quietflyer Actually, look again at the photos. It looks like there's a truncated trailing edge with a plate riveted on perpendicular to the direction of flight. $\endgroup$ Jun 21, 2021 at 16:49
$\begingroup$ Yes, that's what I see too, maybe I didn't describe it clearly enough. $\endgroup$ Jun 21, 2021 at 17:18
$\begingroup$ This truncation is called a Kammback after its 1930's inventor. But it doesn't apply to this question. $\endgroup$ Jun 21, 2021 at 17:32