What is a rudder or fin stall, how will the aircraft behave in that situation, and how should the pilot recognise it? What is the recovery procedure? And what is rudder reversal?


Right side slip from left rudder, rudder stalls, what happens?

This is comparable to a wing stalling in that it is caused by the rudder/vertical stabilizer exceeding a critical angle of attack. There for, it has passed its maximum deflection potential. The result will be the fuselage slip will reduce until the rudder/vertical stabilizer assembly unstalls. Just like with the wing and elevator, reducing rudder deflection will also unstall it.

Notice designers have learned 3 techniques to help prevent this: 1. Lower aspect ratio vertical stabilizer. Just like wings, these stall at higher AOA. 2. Swept or delta shaped leading edge. Many 1940s era Vstabs were fantasticly ahead of their time had their planforms been made into wings. These shapes will stall at higher AOA, allowing greater rudder deflection. 3. The all moving rudder. Making the entire assembly a rudder (as on the SR-71 Blackbird) and making them large enough will help over come the directional stabilizing effect of the rear fuselage when turning.

Rudder stalling would most likely involve some fairly aggressive control application, which may exceed the structural strength of the vertical stabilizer. Beyond fighters, there may not be much benefit in that type of manuever.

  1. When the side slip is high, fin/rudder stall can happen. This is similar to stall of wing, but in a different plane i.e. for vertical stabilizer. When angle of attack (for vertical fin it is called side slip or Beta) of the vertical stabilizer is high in terms of relative airflow, the lift characteristics of vertical stabilizer can also behave same as wing at high angle of attack. Beyond a certain angle, airflow separation takes place and vertical stabilizer may not generate requisite force ( similar to lift for wing) and ultimately vertical stabilizer would stall.

  2. Aircraft’s directional stability in general would be lost in this condition. How aircraft would do so, would be too particular to aircraft depending on its specific properties like inertial and aerodynamic characteristics etc.

  3. Pilot would recognize by preemptive high side slip angles if he has indication for the same in the cockpit. Excessive Side-forces may be felt by the pilot.

  4. How to recover ? Exactly don’t know but an educated guess, reduce side slip angles by turning into wind, may be using ailerons, there by get out of stall... guesswork, please correct me if Iam wrong.

  5. A large rudder deflection input in one direction followed immediately by a rudder deflection input in the opposite direction is called rudder reversal.

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    $\begingroup$ Related: aircraft designers can prevent (or at least postpone) this by adding vortex generators upstream of control surfaces. As the name suggest, these generate vortices, which better 'stick' to the surface. You can see that for example on the Boeing 727. $\endgroup$ – Bram Jan 24 '19 at 13:19
  • $\begingroup$ I was also kinda hoping to get an answer in what direction would the aircraft turn if stall occurs on vertical stabiliser. Lets say we are in a right sideslip (by giving left rudder), if the rudder stalls, would the aircraft start to rotate anti-clockwise and perhaps then even enter a spin or it would turn in a clockwise direction and basically stabilise itself? $\endgroup$ – Darjan Jan 24 '19 at 16:17
  • $\begingroup$ That depends on the stability characteristics of the aircraft. Most aircraft are designed to be stable, but especially newer generation fighter jets might be designed unstable, as that makes them more agile. These craft need their flight computers to maintain stable flight. The principle is the same as with regular wing stall: if a wing stalls, most aircraft will descend nose down, reducing the angle of attack and allowing the wing to pick up lift again. $\endgroup$ – Bram Jan 24 '19 at 16:56
  • $\begingroup$ This answer could be improved by noting that there is are two other meanings to "rudder reversal" which may be more suited to the context of this answer-- although, granted, a series of reversing rudder inputs sometimes does in fact allow the pilot to work with the aircraft's yaw rotational inertia and "pump up" a yaw oscillation that eventually reaches a larger slip angle than the pilot could achieve by simply deflecting the rudder and holding it. (Think of a kid on a swing "pumping up" a large motion, versus simply moving their legs to full-deflected and holding them there.) (Ctd) $\endgroup$ – quiet flyer May 22 '20 at 0:27
  • $\begingroup$ (Ctd) So this could in fact contribute to tail stall. Anyway, the other two other meanings are 1) malfunction of the control system, typically due to failure of a hydraulic component, that moves the rudder in the wrong direction in relation to the deflection of the pedals. (You'll see lots of hits for this on google in relation to Bng 737.) And 2), if you are in a left-rudder sideslip (ball to right, yaw string to left) but you are having to hold pressure on the right rudder pedal to stop the rudder from moving FUTHER left, you might call this "rudder reversal" as well. This can be (ctd) $\endgroup$ – quiet flyer May 22 '20 at 0:35

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