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Aircraft are designed to remain stable regardless of prevailing wind and air forces. However, stability issues like a dutch roll can occur during the flight.
Now, imagine you're seated on an airplane, and it begins to oscillate from side to side. Chances are, the aircraft is experiencing a dutch roll.
To counteract the effects of a dutch roll, aircraft manufacturers design modern airplanes with a Yaw Damper. This functions by sending exact information to the aircraft's rudder to prevent a dutch roll. In modern airplanes, the yaw damper can automatically cancel out a dutch roll.
When the yaw damper is not inoperable for whatever reasons, the pilot can also reduce the effect of a dutch roll by manually sending control inputs to the rudder. When the pilot sends too many inputs or the dutch roll is not properly handled, then it can result in an accident.
From the standpoint and expertise of an aviation pilot, this article gives an in-depth look at what a dutch roll is all about and the built-in systems designed to counteract it.
What is a Dutch Roll?
At first glance the name looks like something out of your local baker's shop, but there's absolutely nothing delicious about a dutch roll. It is an oscillatory movement of a flying aircraft caused by a combination of rolling and yawing movements. This can be seen as the swaying motion of the airplane.
It is a natural phenomenon mostly triggered unintentionally by weather forces or mistakenly by the pilot. However, more often than not, an aircraft will recover naturally from a dutch roll.
Why is it Called a Dutch Roll?
If you're a fan of ice skating, you're well aware of a skater's occasional rhythmical flowing movement. At the time, the Dutch were some of the best ice skaters at the Olympics.
As humans, it's commonplace to attribute a new phenomenon to something familiar or easily recognized. So it didn’t take much for aeronautical engineers to adopt the term to explain this oscillatory asymmetrical movement of aircraft in the air.
Understanding The Basics of an Aircraft’s Aerodynamics
To understand the cause of a dutch roll, you have to begin with the basics of aerodynamics—control, and stability. When in flight, an airplane moves in three unique axes. These axes include:
- Pitch axis: This is the axis in which the airplane makes an upward and downward movement.
- Roll axis: In this axis, an airplane can rotate anywhere from a few degrees to a full 360° like in fighter jets.
- Yaw axis: In this axis, the airplane can move sideways in a yaw pattern. The rudder controls it.
The combination of the movement in all three axes ensures the airplane moves smoothly in the intended direction.
Next, we have the aircraft’s stability to contend with. Airplanes are susceptible to weather disturbances at any time during a flight. This causes them to move slightly out of their flight axis.
In aerodynamics, stability is the ability of an aircraft to return to its flight axis after such disturbance. However, the degree of an airplane's stability varies on each axis.
In other words, the stability reaction on its pitch axis differs from that on its roll or yaw axis. So, this brings us finally to the cause of a dutch roll.
What Causes a Dutch Roll?
Dutch roll occurs at high altitudes and is more prevalent in aircraft with swept wings. Although mostly caused naturally, they can also be triggered when a pilot (during manual control) accidentally hits the yoke in the wrong direction.
For instance, whatever the trigger, the aircraft begins to roll slightly to the right on its roll axis. If there's no input to correct this, the aircraft stays in this position and rolls slowly to its right.
Consequently, the aircraft’s yaw axis is affected as the aircraft is seen to move slightly to its right too. At this point, relative airflow begins to come from the right because the right-wing is slightly dipped while the left wing is also slightly raised.
This position will cause the right-wing to experience more lift for swept-winged aircraft. This is the stability function coming to play. It's trying to return the aircraft to its initial roll axis.
Aerodynamically, as the right-wing tries to garner more lift, it will also experience a relative drag. This causes the aircraft to yaw further to the right. So, while the aircraft may have achieved stability on its roll axis, it is yet to achieve it on its yaw axis.
To counteract this yaw movement, the rudder mechanism is triggered and causes the yaw stability to take effect. This pulls the aircraft to the left. Theoretically, this is an attempt to return the aircraft to its initial yaw axis.
The roll stability is generally greater than the yaw stability for obvious reasons. The row stability uses the aircraft’s wings, while the yaw stability works with the tail fins.
The aircraft will likely roll slightly beyond its initial axis with a more significant lift generated on the right wing. As the roll stability attempts to counter this on the left-wing, the aircraft seems to oscillate gradually on its axis.
The same applies to its yaw stability. Since the yaw stability is lesser than the roll stability, the yaw reacts slower, causing the aircraft to tilt to the left.
Both the roll and yaw stability will ensure the aircraft returns to its axis. Over time, the degree of oscillation reduces.
With the lift and counterbalancing, the aircraft sways and oscillates on a given axis. This resulting movement is known as the Dutch Roll.
How to Recover From a Dutch Roll?
As stated earlier, a modern aircraft will naturally recover and return to its axis, but this is because it has a Yaw Damper built in. A yaw damper is simply a computer explicitly designed to feel and notice changes in the aircraft’s roll and yaw movements.
The yaw damper takes all of this information and sends it to the rudder on the vertical tail wing of the aircraft . The idea is to ensure the rudder gets the necessary input at the right time to quickly begin the yaw stability. This is how a yaw damper works in larger airplanes like the Boeing.
On smaller and older airplanes, the pilot may need to manually key in the input to activate the rudder before making a turn. Again, the input is appropriately timed to ensure the yaw damper kicks in to allow a smooth maneuver.
Can The Yaw Damper Fail?
The yaw damper is part of an aircraft’s hydraulic system. This means it is also susceptible to failure. However, the yaw damper is not necessarily regarded as a piece of crucial equipment on an aircraft.
This means an aircraft can still be effectively flown without it functioning. At worst, the flight may feel slightly more uncomfortable than you are generally accustomed to.
A secondary yaw damper is available that works outside the hydraulic system. When an aircraft’s hydraulic system fails, there's always a standby hydraulic system to help the pilot regain control. The secondary yaw damper is to help reduce the difficulty in control as the pilot uses the secondary hydraulic system.
Is a Dutch Roll Dangerous?
With a better understanding of the causes of a dutch roll, it's safe to admit that a dutch roll is not dangerous. This is because a dutch roll will eventually cancel out with the help of a yaw damper.
Any well-constructed airplane flying at a high altitude is exposed to the effect of a dutch roll. Left alone, the aircraft will recover all by itself.
In more recent times, on March 5th, 2005, Air Transat Flight 961 suffered structural failure to its rudder. This caused an unexpected dutch roll mid-air. Luckily, the pilot was able to safely land the aircraft without injuries or accidents.
However, several recorded accidents in aviation history have been caused by the dutch roll. After detailed investigations, it has been shown that this was a result of improper inputs fed to the rudder system by the pilots.
This was a major reason the yaw damper was designed. Unfortunately, since the human balance system cannot notice and detect when the dutch roll has started, inputs will often be sent to the rudder a little too late.
If we rely on our senses to know when to send inputs to the rudder, the late input may worsen the already occurring dutch roll.
So to reiterate, a dutch roll is not dangerous in itself. It becomes dangerous when it is not properly handled by over-inputting information to the tail wing rudders.