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The ailerons help the plane to bank left/right by increasing lift to one wing and decreasing in another. Likewise, the rudder also helps in turning the plane by yawing and deflecting the nose right/left. So their performances sound quite similar(i.e. to make a turn)? Then, why should we keep both of them?

Like in a ship or motorboat there is a rudder only and no ailerons.

Are ailerons present for quicker operation and rudder for extra support and stabilization only?

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Short answer:

Rudder and ailerons have different purposes and control rotation about two different axis, but their use is coordinated since a rotation about one axis induces a secondary rotation on the other one.

The rudder use is mostly to prevent unwanted yaw movement for safety and comfort, or to force yaw against the aircraft tendency to keep the aircraft facing the relative wind (e.g. for a cross-wind landing). See Understanding the use of rudder for a more comprehensive explanation.

A turn is performed using the ailerons, but for a non-small turn an adverse yaw opposite to the turn develops, the rudder is used to maintain the desired attitude and what is known as a coordinated turn.

When a yaw damper is available, coordinated turns can be conducted without pilot action on the rudder, as the yaw damper controls the rudder automatically.


Detailed answer

When speed and altitude increase, using the rudder to turn around the yaw axis with the wings horizontal, tends to make the aircraft flies in crab in the same direction rather than to turn it. Turning this way would require a larger rudder area and would generate additional drag. So this is not the way an aircraft is steered in flight.

Banking turns the aircraft

An aircraft actually turns by changing the bank (or roll) angle. When the aircraft spins about this roll axis, the direction of the lift is changed, and can be seen as having two components:

enter image description here

The horizontal component of the lift moves the aircraft away from its original path and a turn starts.

Yawing aligns the aircraft with the wind

Lowering the aileron on a wing makes it more efficient, it generates more lift and raises. But generating more lift also induces more drag.

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This drag in excess tends to slow the aircraft on the upper wing side. This creates an adverse yaw movement in the direction opposite to the one desired.

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The pilot uses the rudder to counter the adverse yaw and keep the fuselage axis facing the airflow, countering slip and skid effects. This is known as performing a coordinated turn.

Ailerons-rudder coordination

The amount of rudder to apply can be determined using (human senses and) different instruments, including the turn coordinator which is a sort of level usually associated with a bank or rate of turn indicator.

enter image description here
(source)

enter image description here
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When executing a coordinated level turn without wind, at sea level, etc, the radius of the turn and the turn rate depend only on the airspeed and the bank angle:

  • TAS 150kt, bank 20°: Radius 1,676m, 360° in 2:16.
  • TAS 300kt, bank 30°: Radius 4,227m, 360° in 3:51.

See this question for additional information.


Ailerons deflection reduced in the turn

Note that when the desired bank angle is reached, the pilot reduces the ailerons deflection. The excess of lift and drag on the upper wing decreases too, as does the adverse yaw. The rudder deflection can be reduced too.

Normally the aircraft should maintain its bank and continue to turn with ailerons in neutral position, due to the upper wing flying faster and generating more lift.

However the aircraft in a rolled attitude actually slips into the turn and the dihedral effect tends to decrease the roll angle of the slipping aircraft (roll stability). So some degree of aileron deflection will be maintained.

The ailerons (and the rudder) are used to return the wings to their horizontal position and stop the turn more accurately.


Yaw damper

In commercial airliners, the yaw control is assisted by one or two yaw damper(s) which function is to increase the yaw stability and prevent the development of the dangerous Dutch roll. Thanks to the yaw dampers, the pilot does not need to use the rudder pedals for coordinated turns.

enter image description here
(source)

Other means of controlling roll/lift

A mean of banking wings directly is required, including when Dutch roll and spin start developing and when flying or rolling with crosswind. But bank control can be obtained by other means than ailerons. While ailerons add lift by changing the camber of the wing,

  • spoilerons combine spoilers and ailerons, and break lift by disturbing the laminar airflow (B-52),

  • rudderons combine rudder and ailerons on flying wings (B-2A),

  • and now bleed air and Coandă effect can bank the aircraft without moving surface (Flaviir project).

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  • $\begingroup$ So, which we use for turning usually, the rudder or the ailerons? $\endgroup$ – Dwiparna Datta Jan 2 '16 at 22:29
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    $\begingroup$ @DwiparnaDatta Both. And you use them both cross controlled for slips and crosswind landings. $\endgroup$ – casey Jan 2 '16 at 23:26
  • $\begingroup$ I'm not sure if it's used in full-scale aviation (a quick search didn't turn up anything), but some RC aircraft do not have ailerons and use the rudder only to turn, relying on the wing's dihedral to then roll the plane. The roll is then what really makes the turn work. $\endgroup$ – Steve Jan 4 '16 at 15:52
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    $\begingroup$ @DwiparnaDatta It's not either or, it's both, unless you are happy being a lousy pilot. $\endgroup$ – KorvinStarmast Oct 6 '16 at 22:05
  • $\begingroup$ @DwiparnaDatta: To turn, you need to roll (bank) the wing, this is done with the ailerons, but as banking is associated with unwanted adverse yaw, you use the rudder to counter the adverse yaw. A coordinated turn is therefore done with the two controls. If you don't want to bother coordinating the turn and can bear some additional drag, e.g. in a small turn, then you can use only the ailerons. The rudder alone will just crab the plane, but it won't really change its direction of flight. $\endgroup$ – mins Sep 30 at 18:30
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I would add one small addition to the adverse yaw issue: some aircraft (notably older airplanes, pre-1950, e.g., and sailplanes) require rudder to effect a turn. They develop so much adverse yaw that they'll simply slip through the air with no heading change. A Taylorcraft L-2 I've flown experienced enough adverse yaw that I had to lead the turn slightly with rudder (rudder first, then aileron).

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