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Why don't rudders stall at 30 deg deflection, as they don't have downwash to reduce local AOA like flaps and horizontal tails do?

or do they?

I understand flaps and horizontal tails don't stall at 30 deg deflection as the downwash from the wing reduces the local Angle of Attack, so flaps and horizontal tails aren't seeing an angle of attack of 30 deg even though they are deflected by 30 deg.

However, there is no "vertical downwash" from the wing to help the rudder. Why don't rudders stall at 30 deg deflection when most classic NACA airfoils stall at about 15-20 deg?

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  • $\begingroup$ It's just a wing sticking up, so there is a "sidewash" no? Same thing pretty much. $\endgroup$ – John K Aug 28 at 0:10
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    $\begingroup$ @Fred - Are you sure they don't? $\endgroup$ – quiet flyer Aug 28 at 0:27
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    $\begingroup$ Are you talking about an all-moving vertical tail? Otherwise I don't see what AOA has to do with rudder deflection. $\endgroup$ – Jimmy Aug 28 at 5:49
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    $\begingroup$ Why do you expect a trailing edge surface deflection to be comparable to the deflection of the entire airfoil? $\endgroup$ – AEhere supports Monica Aug 28 at 5:50
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    $\begingroup$ @RobertDiGiovanni not my point; a $30^o$ deflection of an aileron will not increase the $\alpha$ of the entire airfoil by anywhere near $30^o$, more like $15^o$ if the aileron takes about half the chord (for small angles, etc). $\endgroup$ – AEhere supports Monica Aug 28 at 7:29
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At an extreme deflection (like 30º) a rudder is probably stalled. But in that condition, lift does not entirely disappear, and drag increases a lot. The resultant of the two forces (the 'aerodynamic force') pushes the tail sideways, yawing the airplane. Hence, that stall doesn't matter much...

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an old uni book about stability & control

Deflect the rudder relative to the vertical tail and there will be sideslip $\beta$. In the situation in the drawing: deflect the rudder to the left, and it will be in line with $\beta$ but will have a deflection relative to the vertical tail.

So you may deflect the rudder 30° relative to the vertical tail, have for instance a 12° side slip angle as a result, and have an 18° angle relative to the free stream.

Note that the drawing depicts induced side slip flow at the vertical tail as well.

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    $\begingroup$ +1 for showing (wing tip like) airflow created by the FUSELAGE AOA to the free stream. But notice the plane must be yawed for this to occur. The greatest danger of rudder stall is the initial, sharp application. Even stalled, it will still yaw the plane. But another plane (like the Stearman) with a delta-like Vstab/rudder, will "snap" faster, giving it an advantage. $\endgroup$ – Robert DiGiovanni Aug 28 at 15:06

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