Dutch Roll

What is Dutch roll?

Dutch roll is an oscillatory instability associated with swept-wing jet aircraft. It is the combination of yawing and rolling motions. When the aircraft yaws, it will develop into a roll. The yaw itself is not too significant, but the roll is much more noticeable and unstable. This is so because the aircraft suffers from a continuous reversing rolling action.

What causes Dutch roll?

Swept wings. Dutch roll occurs when a yaw is induced either by a natural disturbance or by a commanded or an uncommanded yaw input on a swept-wing aircraft. This causes the outer wing to travel faster and to become more straight on to the relative airflow (in effect, decreasing the sweep angle of the wing and increasing its aspect ratio). Both these phenomena will create more lift. At the same time, the inner wing will travel slower and, in effect, becomes more swept relative to the airflow, and both these phenomena will reduce its lift. Therefore, a marked bank occurs to the point where the outer, upward-moving wing stalls and loses all lift, and therefore the wing drops, causing a yaw to the stalled wing and thus leading to the sequence being repeated in the opposite direction. This sequence will continue and produce the oscillatory instability around the longitudinal axis we know as Dutch roll. Pitch fluctuations only occur with an extreme degree of Dutch roll.

What is the recovery technique from Dutch roll?

For a pilot to recover an aircraft suffering from Dutch roll, he or she would apply opposite aileron to the direction of the roll, assuming that the yaw dampers are not serviceable.

Although the root cause of Dutch roll is the yawing motion, application of a correcting rudder input by the pilot normally would worsen the situation. This is so because the yawing motion in the oscillatory cycle happens extremely quickly, and the pilot’s reaction would not be quick enough to catch the yaw, which already has developed into a roll and dissipated. Therefore, a rudder input to correct the initial yaw (which has since dissipated) would in fact aggravate the roll effect further into a sideslip. Aileron control therefore is employed because the roll cycle is of sufficient duration to allow the pilot to apply the correct opposite aileron control. A severe Dutch roll may require two or three aileron inputs to dampen the oscillation gradually.

What prevents Dutch roll?

Yaw dampers prevent Dutch roll on swept-wing aircraft. A basic reason for the Dutch-rolling tendency of an aircraft (apart from the wing sweep, of course) is the lack of effective fin and rudder area to stop it. The smaller fin and rudder area is a design compromise that makes the aircraft spirally stable to a degree. Therefore, the effectiveness of the fin area must be increased in some other way to prevent Dutch roll. This is achieved with yaw dampers.

 

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