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What do you know about yaw control and more precisely about the result caused by the rudder (R on picture)?

Below the right pedal of the airplane has been depressed, and the rudder creates a rotation around the yaw axis and generates an aerodynamic result (R on picture) to the left on the vertical tail. Can you develop how this result is create?

aerodynamic result

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    $\begingroup$ I modified your question to fix some grammar errors. However, I still don't understand what you're asking... $\endgroup$
    – JZYL
    Apr 10, 2020 at 18:09
  • $\begingroup$ why when the rudder is shifted to the right, the plane is deviated in the same direction. what is the effect of the rudder's direction? a change in the air flow? $\endgroup$
    – L'aviateur
    Apr 10, 2020 at 18:17
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    $\begingroup$ The irony is not lost in this thread. But seriously, what is the question? If you press the right rudder pedal, the rudder will deflect to the right (starboard) side of the plane. The tail will yaw (clockwise) around to the left (port) side of the Center of Gravity. The nose will subsequently yaw (clockwise) around to the right side of the CoG. If you press the left rudder pedal, the plane will yaw counter-clockwise. The degree of yaw will be related to the force of the relative wind, the amount of rudder deflection, and the length of the moment arm of the rudder from the CoG. $\endgroup$
    – Dean F.
    Apr 10, 2020 at 18:21
  • $\begingroup$ Are you sure that when you depressed the right pedal the aircraft turn to the opposite direction? $\endgroup$
    – L'aviateur
    Apr 10, 2020 at 18:28
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    $\begingroup$ You should must a precise question in the title so that it is easier to navigate through the website without opening each question and because this is a Q&A website. $\endgroup$
    – Manu H
    Apr 10, 2020 at 19:45

4 Answers 4

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When the rudder is deflected, it creates a "lift" on the vertical tail due to additional camber. This "lift", when viewed from the airplane perspective, is a side force opposite to the direction of the rudder deflection. Since the centre of pressure of this side force is aft of the centre of gravity, it generates a net yaw moment on the airplane.

For example, a right rudder deflection generates a left side force, which generates a nose right yawing moment, pulling the aircraft nose right.

Since the side force is relatively small in the big scheme of things, the airplane motion is essentially unchanged. As soon as the airplane nose begins to yaw, the airplane begins to have finite sideslip against the incoming airflow. This sideslip generates a side force that opposes the rudder force (from an increased airflow incidence against the vertical tail), as well as a yawing moment that opposes the yaw moment from rudder (i.e. a restoring moment).

For example, a right rudder generates nose right yaw motion, which creates a sideslip that produces a nose left yaw moment.

For a finite rudder deflection, the airplane begins to accumulate sideslip, until such a point that the restoring moment balances out the rudder moment.

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  • $\begingroup$ Ok so On an aircraft the rudder is used primarily to counter adverse yaw but not to turn the airplane. A rudder operates by redirecting the fluid past the hull or fuselage, thus imparting a turning or yawing motion to the craft. $\endgroup$
    – L'aviateur
    Apr 10, 2020 at 18:45
  • $\begingroup$ Is it correct ? Yaw is the result of high air pressure?i.pinimg.com/originals/12/7c/f8/… $\endgroup$
    – L'aviateur
    Apr 10, 2020 at 18:48
  • $\begingroup$ @L'aviateur I prefer to say that rudder causes a lower pressure on other side, sucking the vertical stabilizer. But for intuitive thinking, sure. Rudder has many roles other than to counter adverse yaw during coordinated turn. But yes, rudder is rarely used to initiate course change. $\endgroup$
    – JZYL
    Apr 10, 2020 at 18:51
  • $\begingroup$ Thanks you for your awnser $\endgroup$
    – L'aviateur
    Apr 10, 2020 at 18:53
  • $\begingroup$ @L'aviateur - FYI, the rudder’s primary purpose is to indeed counter adverse yaw. It will also yaw the nose of the aircraft independent of adverse yaw. Doing this will increase the airspeed of the wing on the outside of the turn created by the yawing motion and decrease the airspeed of the wing on the inside of the turn. This airspeed difference will create more lift on the outside wing than on the inside wing, banking the aircraft. This bank will turn the plane without creating adverse yaw. A great tool during slow flight or to affect minute course corrections. $\endgroup$
    – Dean F.
    Apr 10, 2020 at 18:56
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If you press the right rudder pedal, the rudder will deflect to the right (starboard) side of the plane. This action will deflect the relative wind towards the right-side of the airplane much the same as if you were to turn the entire vertical stabilizer counter-clockwise. This deflection to the right generates a force acting towards the left.

The tail will yaw (clockwise) around to the left (port) side of the Center of Gravity. The nose will subsequently yaw (clockwise) around to the right side of the CoG. This will be due to the force imparted to the rudder/vertical stabilizer assembly by the angle of attack of the relative wind.

If you press the left rudder pedal, the plane will yaw counter-clockwise.

The degree of yaw will be related to the force of the relative wind, the amount of rudder deflection, and the length of the moment arm of the rudder from the CoG. The faster the relative wind is, the more force will be imparted on the empennage. The more the rudder is deflected, the more force is imparted on the empennage. The denser the air, the more force is imparted on the empennage. The greater the distance from the center of gravity to the vertical stabilizer, the more the force imparted on the empennage will create movement through the greater torque (moment) on the aircraft around the y-axis located at the CoG.

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  • $\begingroup$ Thanks to the awnser but why "The tail will yaw (clockwise) around to the left (port)" ?because of the strength of the flux against the rudder (at right if it's deviated to the right)? $\endgroup$
    – L'aviateur
    Apr 10, 2020 at 18:39
  • $\begingroup$ @L'aviateur - The vertical Stabilizer is like a wing mounted in an up and down orientation. The rudder is like an aileron or flap. If you deflect the air moving past that vertical wing to the right, it will creat force towards the left. It will also create lift towards the left. Get in a car driving at 100 kmh. Stick your hand out of the window. Turning your hand thumb-side (leading edge) up will make your hand rise. Now, stick your hand out of the sunroof. Turning your hand thumb-side (trailing edge in this case) right will move your hand to the left. $\endgroup$
    – Dean F.
    Apr 10, 2020 at 18:48
  • $\begingroup$ Ok thank you for you awnser $\endgroup$
    – L'aviateur
    Apr 10, 2020 at 18:52
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The rudder, being a vertical variable camber wing, applies a left lift force when displaced right. The left lift force yaws the plane right and also applies a force trying to slew the plane to the left. The yaw results in the relative wind striking the side of the fuselage, creating a certain amount of lift to the right, depending on how effective an airfoil the fuselage is (some more than others). The lifting force applied to the fuselage is added to by the offset thrust line due to the yaw, also providing a lateral force to the right.

The result is the side lift of the fuselage plus the offset thrust line is more than the left side force of the rudder, and the plane slips sideways to the right, and since it's going forward as it does so it turns, although inefficiently.

enter image description here

It's easier to visualize if you think of an aerobatic airplane in an airshow that flies past you flying at a 90 degree bank, or flying "knife edge". The wings are completely unloaded and are doing nothing except acting like big fins. The offset thrust line of the high powered engine, and the lift generated by the plane's fuselage, is enough to hold up the entire weight of the plane, even though the rudder at the back is pushing "down" like an elevator on a flying wing.

Roll the Extra aerobatic airplane back to level flight and do the same thing, and you get a skid-turn to the right from the rudder pushing the tail to the left.

enter image description here

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  • $\begingroup$ This good answer could be perhaps be even more improved by noting that the rudder acts at a long moment-arm from the CG and thus only a relatively small force A is needed to generate the torque that maintains the sideslip angle against whatever torques are opposing it. B is the main thing (or only thing?) that is opposing the sideslip, and includes a contribution from the fixed vertical fin, but the overall moment-arm for B is much smaller than for A, so B must be much larger than A for the torques to balance out. $\endgroup$
    – quiet flyer
    Apr 11, 2020 at 19:05
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A very simple explanation here. Turning the rudder to the right is initially very similar to turning the rear tires of a forklift to the same direction.

It will make the plane tail pull left and creates the yaw.

Of course soon the similarity ends because of the interplay of fuselage with the wind and the fact that rudder forces want to roll the plan counterclockwise around its length.

But that image should give an intuitive motivation.

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