“More right rudder!”
Who among us hasn’t heard those words, the favorite phrase of flight instructors everywhere?
To better appreciate your instructor’s sentiments, here’s a deep dive into all things rudder. What is the rudder, what does it do, and why is it there? And, for goodness sake, why is my CFI so wound up about it all?
What is an Airplane Rudder?
The rudder is one of the airplane’s three primary flight controls.
For a quick review, movement around each of the three axes of flight has a name, and each type of movement is controlled by its own control surface.
|Axis of Flight||Movement Name||Associated Primary Flight Control|
The rudder is mounted on the trailing edge of the vertical stabilizer. It mimics the rudder of a boat, which is mounted on the rear of a vessel to control its heading, making it turn through the water.
But unlike boats, airplanes operate in three dimensions. So the rudder does not change the airplane’s heading on its own. Instead, airplanes turn with a combination of inputs from all control surfaces.
Pedals in the cockpit control the rudder. Press on the left pedal, and the rear of the rudder is deflected to the left. This makes the nose of the aircraft yaw to the left.
How Does the Rudder Work?
The rudder is mounted to the vertical stabilizer, one of the primary surfaces on the plane that helps it remain directionally stable.
The vertical tail works like the feathers on an arrow–as air flows over it, it encourages the nose to continue traveling straight ahead.
Like the ailerons, flaps, or elevator, the rudder works by changing the angle of attack on the stabilizer. When the rudder is deflected, the trailing edge moves. This results in a force (like lift) that pulls the tail in the opposite direction of the deflection.
This way, you can think of the vertical stabilizer as a wing. Instead of making a lifting force upward (opposite gravity), it makes a horizontal force when the rudder is deflected. When the rudder is centered, there is no horizontal lifting force.
Why Do You Need a Rudder?
As discussed in our article on ailerons, the rudder is a key component in counteracting the adverse yaw created by those flight controls. In a light single-engine plane, this is what the rudder does most of the time.
The rudder is also used to keep the aircraft coordinated. By controlling the yaw of the aircraft independently of the roll, the pilot can exercise complete control as things change on the aircraft.
This enables us to intentionally slip the aircraft, a handy skill to have for crosswind landings. The forward slip to land, another handy maneuver that enables a pilot to approach a runway steeply without flaps, wouldn’t be possible without your trusty rudder.
Single Engine Flying and Left Turning Tendencies
In a single-engine plane, the engine will produce some forces on the plane that we need the rudder to counter. Four different forces make the aircraft want to turn left at certain times. Aircraft designers neutralize these factors so that they are unnoticeable most of the time, but they can’t make them disappear all the time.
So pilots must learn when to apply right rudder to counteract a left-turning tendency. Left-turning tendencies are felt most when the aircraft speed is low, and the power is high. Slow airspeed means that other designed-in corrections will be minimal, and the high power means the engine will make the maximum force.
A single-engine plane’s four turning tendencies are torque, P-factor, spiraling slipstream, and gyroscopic precession.
Torque is a turning force, like the power a mechanic puts into turning a wrench. Engines make torque when they turn the crankshaft and propeller.
When the plane sits on the ground, the torque is countered by the landing gear and tires. But once the plane is flying, the torque must be countered by other things. If it weren’t, the engine would spin the prop one way, and the engine and airplane would spin the other.
If you look at the profile view of a propeller, it is made in the shape of an airfoil, just like the wing. In fact, the prop works like two wings that are swung through the air by the engine.
Just like wings, the prop has chord lines, relative wind, and angles of attack. The prop works just right when the plane is flying level. But if you pitch the nose up, the angle of attack changes between the left side and right side of the aircraft as the prop spins. The ascending blade (left side when viewed by the pilot) has a lower angle of attack, and the descending blade (right side) has a higher angle of attack.
The side with the bigger angle of attack makes more thrust, so the plane experiences more thrust from its right side. This makes the nose of the plane turn left, but only in a climb. When the plane is level, p-factor is neutral. When the nose is pitched down, the opposite happens and the plane wants to turn right.
The prop wash spirals around the plane. On most planes, it hits the empennage on the left-hand side, making the nose of the plane swing left.
Gyroscopic precession is the tendency for forces to be felt differently on spinning objects. A prop is a large object spinning fast, so forces applied to it are felt 90 degrees in the direction of rotation.
In other words, imagine you are flying a taildragger and rolling for takeoff. As you accelerate, the nose is pitched down as the tail comes off the ground. That forward push, applied at the top of the propeller, is felt 90 degrees later–on the right side of the propeller.
The result is a strong nose-left tendency as the plane pitches down. Therefore, taildragger pilots need to be very good with their feet and rudder skills just to get off the ground!
Multiengine Flying and Vmc
In multiengine aircraft, the rudder serves an even more vital purpose. You can use the rudder to counter the effects of differential thrust, which occurs when one side of the aircraft is making more power than the other. For example, in the event of an engine failure in a twin-engine airplane, the rudder is the plane’s primary tool for controlling its heading.
Since it’s so important to multiengine flying, the rudder deserves a little more attention in these planes. A few factors that don’t apply to single-engine planes need to be considered.
When an engine failure occurs, the plane will roll and yaw toward the dead engine. The pilot flying’s first task is to counter those forces with the rudder and ailerons to keep the plane level and flying on course.
The problem is that the rudder can only apply a limited force. The amount of authority the rudder has to counter the pull of the remaining engine(s) depends on the size of the rudder and the plane’s airspeed. So once the rudder is at its stop, the only way to make it more powerful is to fly faster.
To this end, multiengine pilots must know that the plane may be uncontrollable with one engine at full power below a certain airspeed. That speed is called the minimum controllable airspeed, or Vmc.
Vmc is published on the airspeed indicator–that’s how important it is. It appears as a red line, usually on the lower end of the scale but higher than the stall speeds.
The Vmc line on the airspeed indicator is not always accurate. In multiengine flying, you’ll learn all the factors to determine why it might be different, along with all those techniques you can use to your advantage to control the aircraft when flying on one engine.
To learn more about the rudder and other flight controls, see the FAA’s Pilot’s Handbook of Aeronautical Knowledge, Chapter 6: Flight Controls.
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