A Dutch roll – it sounds delicious, but do not start licking your lips and drooling over the thought of some new scrumptious pastry delight just yet. Rather than describing a dessert, the Dutch roll is the name given to a series of aircraft motions that in most cases pilots enter into unintentionally.
Today we will touch on everything you need to know about a Dutch roll including what it is, how it got its unique name, and how to get out of a Dutch roll should you inadvertently find yourself in one.
What is a Dutch Roll?
The FAA describes the Dutch roll as “lateral oscillatory motions.” This means that when an aircraft is in a Dutch roll it is moving around two axes at the same. It is rolling and yawing in opposite directions, with each rolling motion causing another yawing motion and each yawing motion precipitating another rolling motion. If no further control inputs are given, each set of motions will lessen until eventually the aircraft stabilizes. An unplanned Dutch roll happens more commonly at higher altitudes and with a swept-wing aircraft configuration.
Before getting into the details of a Dutch roll, we must first review and understand a bit about aircraft stability. Stability reflects the aircraft’s ability to return to a straight and level flight state following a disturbance and it is the driving factor behind the Dutch roll.
The fewer oscillations the aircraft goes through, and the faster the aircraft returns to straight and level flight, the higher degree of stability it is said to have. If an aircraft is too stable, it will be difficult and sluggish to turn. If is not stable enough, it will also be hard to fly as it will easily be thrown off course. Designers must find a happy medium for the stability of each aircraft they create based on the performance and handling characteristics they want it to have.
Aircraft have varying degrees of stability across their three axes which control roll, pitch, and yaw. On a swept-wing aircraft, the roll stability is higher than the yaw stability meaning that the plane will return to wings-level configuration faster if it rolls than if it yaws.
A Dutch roll is precipitated by a roll to either the right or left. As the aircraft rolls, its lift vector tilts in the same direction as the roll. In a swept wing aircraft, this means that the relative wind is hitting the lower wing’s leading edge more straight on than it is the higher wing. The lower wing thus generates more lift which starts to roll the aircraft back towards wings level.
A side-effect of the extra lift is the lift-induced drag which pulls the nose of the plane in the direction of the initial roll. If the plane started by rolling right, the drag that accompanied the increased right wing lift, now pulls the nose right creating a right yawing motion.
With the nose yawed to the right, the left side of the vertical stabilizer on the tail of the aircraft is receiving more airflow than the right. It generates lift towards the right and the nose yaws back toward the left.
In theory, that would seem to be the end of it, however as we mentioned, on a normal swept wing aircraft, the yaw stability is weaker than the roll stability, so before the vertical stabilizer can negate the right yaw, the aircraft has rolled past level and is now in a left roll which causes the whole process of alternating roll and yaw to repeat again, this time to the opposite side. Left unchecked, the oscillations continue with each set getting less extreme as the aircraft slowly regains stability and returns to straight and level flight.
The stability characteristics of the aircraft will affect the handling and recovery from a Dutch roll. Some aircraft have strong directional stability and usually go through the entire Dutch roll sequence quickly with minimal yaw and roll overshoots. The downside is that this directional stability comes with weak dihedral effect which means spiral instability. The opposite is true of aircraft with weak directional stability. Those planes usually have good spiral stability, but a very prolonged Dutch roll with numerous overshoots.
Why is it called a Dutch Roll?
In the early 20th century when swept wing aircraft were first introduced and the sequence of oscillations which we now call the Dutch roll were initially observed, there was no name for the new phenomenon. The rolling motion of the aircraft was reminiscent of Dutch ice skaters leaning from side to side as they skated along the canals, and so the oscillating motions were christened the Dutch roll.
How do you get out of a Dutch Roll?
In defining airworthiness standards for dynamic stability of Part 25 aircraft, the FAA states that, “Any combined lateral-directional oscillations (“Dutch roll”) occurring between 1.13 VSR and maximum allowable speed appropriate to the configuration of the airplane must be positively damped with controls free, and must be controllable with normal use of the primary controls without requiring exceptional pilot skill.”
Per FAA standards, a modern swept wing aircraft equipped with a functional yaw dampener will simply fly itself out of the Dutch roll if the pilot does not add additional control inputs. Aircraft without a yaw dampener or aircraft with an inoperative yaw dampener will need to be flown out of the Dutch roll manually or the pilot can simply wait for the roll to subside on its own.
To manually negate a Dutch roll, the pilot can use rudder inputs to supplement the yaw stability and negate the alternating oscillations, thus returning the aircraft to straight and level flight more quickly than if it were left to self-stabilize.
For a detailed and easy to follow description of Dutch roll, watch Mentour Pilot’s What is a “Dutch Roll”?! video.