Dutch roll
Encyclopedia
Dutch roll is a type of aircraft
motion, consisting of an out-of-phase
combination of "tail-wagging" and rocking from side to side. This yaw-roll coupling is one of the basic flight dynamic
modes (others include phugoid
, short period, and spiral divergence). This motion is normally well damped in most light aircraft, though some aircraft with well-damped Dutch roll modes can experience a degradation in damping
, as airspeed
decreases and altitude
increases. Dutch roll stability can be artificially increased by the installation of a yaw damper
. Wings placed well above the center of mass, sweepback (swept wing
s) and dihedral wings tend to increase the roll restoring force, and therefore increase the Dutch roll tendencies; this is why high-winged aircraft often are slightly anhedral
, and transport-category swept-wing aircraft are equipped with yaw dampers.
In aircraft design, Dutch roll results from relatively weaker positive directional stability
as opposed to positive lateral stability. When an aircraft rolls around the longitudinal axis, a sideslip
is introduced into the relative wind
in the direction of the rolling motion. Strong lateral stability begins to restore the aircraft to level flight. At the same time, somewhat weaker directional stability attempts to correct the sideslip by aligning the aircraft with the perceived relative wind. Since directional stability is weaker than lateral stability for the particular aircraft, the restoring yaw motion lags significantly behind the restoring roll motion. As such, the aircraft passes through level flight as the yawing motion is continuing in the direction of the original roll. At that point, the sideslip is introduced in the opposite direction and the process is reversed.
The Dutch roll mode can be excited by any use of aileron
or rudder
, but for flight test purposes it is usually excited with a rudder singlet (short, sharp motions of the rudder to a specified angle, and then back to the centered position) or doublet (a pair of such motions in opposite directions). Some larger aircraft are better excited with aileron inputs. Periods can range from a few seconds for light aircraft
to a minute or more for airliner
s.
Dutch roll is also the name (considered by professionals to be a misnomer) given to a coordination maneuver
generally taught to student pilots to help them improve their "stick-and-rudder" technique. The aircraft is alternately rolled as much as 60 degrees left and right while rudder
is applied to keep the nose of the aircraft pointed at a fixed point.
This coordination technique is better referred to as "rolling on a heading", wherein the aircraft is rolled in such a way as to maintain an accurate heading without the nose moving from side-to-side (or yawing). The yaw motion is induced through the use of ailerons alone due to aileron drag, wherein the lifting wing (aileron down) is doing more work than the descending wing (aileron up) and therefore creates more drag, forcing the lifting wing back, yawing the aircraft toward it. This yawing effect produced by rolling motion is known as adverse yaw
. This has to be countered precisely by application of rudder in the same direction as the aileron control (left stick, left rudder - right stick, right rudder). This is known as synchronised controls when done properly, and is difficult to learn and apply well. Knowing the correct amount of rudder to apply with aileron is different for each aircraft.
The origin of the name Dutch roll is uncertain. However, it is likely that this term, describing a lateral asymmetric motion of an airplane, was borrowed from a reference to similar-appearing motion in ice skating. In 1916, aeronautical engineer Jerome C. Hunsaker published the following quote:
“Dutch roll – the third element in the [lateral] motion [of an airplane] is a yawing to the right and left, combined with rolling. The motion is oscillatory of period for 5 to 12 seconds, which may or may not be damped. The analogy to ‘Dutch Roll’ or ‘Outer Edge’ in ice skating is obvious.”
In 1916, Dutch Roll was the term used for skating repetitively to right and left (by analogy to the motion described for the aircraft) on the outer edge of one’s skates. By 1916, the term had been imported from skating to aeronautical engineering, perhaps by Hunsaker himself. 1916 was only five years after G.H. Bryan did the first mathematics of lateral motion of aircraft in 1911.
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
motion, consisting of an out-of-phase
Phase (waves)
Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
combination of "tail-wagging" and rocking from side to side. This yaw-roll coupling is one of the basic flight dynamic
Flight dynamics
Flight dynamics is the science of air vehicle orientation and control in three dimensions. The three critical flight dynamics parameters are the angles of rotation in three dimensions about the vehicle's center of mass, known as pitch, roll and yaw .Aerospace engineers develop control systems for...
modes (others include phugoid
Phugoid
A phugoid or fugoid is an aircraft motion where the vehicle pitches up and climbs, and then pitches down and descends, accompanied by speeding up and slowing down as it goes "uphill" and "downhill." This is one of the basic flight dynamics modes of an aircraft , and a classic example of a positive...
, short period, and spiral divergence). This motion is normally well damped in most light aircraft, though some aircraft with well-damped Dutch roll modes can experience a degradation in damping
Damping
In physics, damping is any effect that tends to reduce the amplitude of oscillations in an oscillatory system, particularly the harmonic oscillator.In mechanics, friction is one such damping effect...
, as airspeed
Airspeed
Airspeed is the speed of an aircraft relative to the air. Among the common conventions for qualifying airspeed are: indicated airspeed , calibrated airspeed , true airspeed , equivalent airspeed and density airspeed....
decreases and altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...
increases. Dutch roll stability can be artificially increased by the installation of a yaw damper
Yaw damper
A yaw damper is a device used on many aircraft to damp the rolling and yawing oscillations due to Dutch roll mode. It involves yaw rate sensors and a processor that provides a signal to an actuator connected to the rudder...
. Wings placed well above the center of mass, sweepback (swept wing
Swept wing
A swept wing is a wing planform favored for high subsonic jet speeds first investigated by Germany during the Second World War. Since the introduction of the MiG-15 and North American F-86 which demonstrated a decisive superiority over the slower first generation of straight-wing jet fighters...
s) and dihedral wings tend to increase the roll restoring force, and therefore increase the Dutch roll tendencies; this is why high-winged aircraft often are slightly anhedral
Anhedral
* Anhedral angle, the downward angle from horizontal of the wings or tailplane of a fixed-wing aircraft* Anhedral , a rock texture without crystal faces or cross-section shape in thin section...
, and transport-category swept-wing aircraft are equipped with yaw dampers.
In aircraft design, Dutch roll results from relatively weaker positive directional stability
Directional stability
Directional stability is stability of a moving body or vehicle about an axis which is perpendicular to its direction of motion. Stability of a vehicle concerns itself with the tendency of a vehicle to return to its original direction in relation to the oncoming medium when disturbed away from...
as opposed to positive lateral stability. When an aircraft rolls around the longitudinal axis, a sideslip
Slip (aerodynamic)
A slip is an aerodynamic state where an aircraft is moving somewhat sideways as well as forward relative to the oncoming airflow. In other words, for a conventional aircraft, the nose will not be pointing directly into the relative wind .A slip is also a piloting maneuver where the pilot...
is introduced into the relative wind
Relative wind
In aeronautics, the relative wind is the direction of movement of the atmosphere relative to an aircraft or an airfoil. It is opposite to the direction of movement of the aircraft or airfoil relative to the atmosphere...
in the direction of the rolling motion. Strong lateral stability begins to restore the aircraft to level flight. At the same time, somewhat weaker directional stability attempts to correct the sideslip by aligning the aircraft with the perceived relative wind. Since directional stability is weaker than lateral stability for the particular aircraft, the restoring yaw motion lags significantly behind the restoring roll motion. As such, the aircraft passes through level flight as the yawing motion is continuing in the direction of the original roll. At that point, the sideslip is introduced in the opposite direction and the process is reversed.
The Dutch roll mode can be excited by any use of aileron
Aileron
Ailerons are hinged flight control surfaces attached to the trailing edge of the wing of a fixed-wing aircraft. The ailerons are used to control the aircraft in roll, which results in a change in heading due to the tilting of the lift vector...
or rudder
Rudder
A rudder is a device used to steer a ship, boat, submarine, hovercraft, aircraft or other conveyance that moves through a medium . On an aircraft the rudder is used primarily to counter adverse yaw and p-factor and is not the primary control used to turn the airplane...
, but for flight test purposes it is usually excited with a rudder singlet (short, sharp motions of the rudder to a specified angle, and then back to the centered position) or doublet (a pair of such motions in opposite directions). Some larger aircraft are better excited with aileron inputs. Periods can range from a few seconds for light aircraft
Light-sport Aircraft
A Light-sport aircraft, also known as light sport aircraft or LSA, is a small aircraft that is simple to fly and which meets certain regulations set by a National aviation authority restricting weight and performance...
to a minute or more for airliner
Airliner
An airliner is a large fixed-wing aircraft for transporting passengers and cargo. Such aircraft are operated by airlines. Although the definition of an airliner can vary from country to country, an airliner is typically defined as an aircraft intended for carrying multiple passengers in commercial...
s.
Dutch roll is also the name (considered by professionals to be a misnomer) given to a coordination maneuver
Aerobatic maneuver
Aerobatic maneuvers are flight paths putting aircraft in unusual attitudes, in air shows, dog fights or competition aerobatics. Aerobatics can be performed by a single aircraft or in formation with several others...
generally taught to student pilots to help them improve their "stick-and-rudder" technique. The aircraft is alternately rolled as much as 60 degrees left and right while rudder
Rudder
A rudder is a device used to steer a ship, boat, submarine, hovercraft, aircraft or other conveyance that moves through a medium . On an aircraft the rudder is used primarily to counter adverse yaw and p-factor and is not the primary control used to turn the airplane...
is applied to keep the nose of the aircraft pointed at a fixed point.
This coordination technique is better referred to as "rolling on a heading", wherein the aircraft is rolled in such a way as to maintain an accurate heading without the nose moving from side-to-side (or yawing). The yaw motion is induced through the use of ailerons alone due to aileron drag, wherein the lifting wing (aileron down) is doing more work than the descending wing (aileron up) and therefore creates more drag, forcing the lifting wing back, yawing the aircraft toward it. This yawing effect produced by rolling motion is known as adverse yaw
Adverse yaw
Adverse yaw is a yaw moment on an aircraft which results from an aileron deflection and a roll rate, such as when entering or exiting a turn. It is called "adverse" because it acts opposite to the yaw moment needed to execute the desired turn. Adverse yaw has three mechanisms, listed below in...
. This has to be countered precisely by application of rudder in the same direction as the aileron control (left stick, left rudder - right stick, right rudder). This is known as synchronised controls when done properly, and is difficult to learn and apply well. Knowing the correct amount of rudder to apply with aileron is different for each aircraft.
The origin of the name Dutch roll is uncertain. However, it is likely that this term, describing a lateral asymmetric motion of an airplane, was borrowed from a reference to similar-appearing motion in ice skating. In 1916, aeronautical engineer Jerome C. Hunsaker published the following quote:
“Dutch roll – the third element in the [lateral] motion [of an airplane] is a yawing to the right and left, combined with rolling. The motion is oscillatory of period for 5 to 12 seconds, which may or may not be damped. The analogy to ‘Dutch Roll’ or ‘Outer Edge’ in ice skating is obvious.”
In 1916, Dutch Roll was the term used for skating repetitively to right and left (by analogy to the motion described for the aircraft) on the outer edge of one’s skates. By 1916, the term had been imported from skating to aeronautical engineering, perhaps by Hunsaker himself. 1916 was only five years after G.H. Bryan did the first mathematics of lateral motion of aircraft in 1911.