Dutch roll is an aircraft motion that is identified by a combination of a continuous back-and-forth rolling and yawing motion. In general, a Dutch roll is considered to be dynamically stable, meaning that the oscillations tend to decrease in amplitude. Most large airliners have a yaw damper installed that can artificially increase stability. Aircraft that have wings placed above the center of mass, dihedral wings and swept wings tend to increase the roll restoring force, therefore increase the tendency of the aircraf to initiate a Dutch roll. This is the reason why high-winged aircraft are often slightly anhedral, and swept wing aircraft rely on the operation of the yaw damper.
In aircraft design, relatively weaker positive directional stability as opposed to positive lateral stability can result in a Dutch roll. Rolling the aircraft around the longitudinal axis means that a sideslip is introduced into the relative wind in the direction of the rolling motion. What happens is that the somewhat weaker directional stability attempts to correct the sideslip by aligning the aircraft with the perceived relative wind. Because directional stability is weaker than lateral stability for the aircraft in particular, the restoring yaw motion lags significantly behind the restoring roll motion. The aircraf then passess through level flight as the yawing motion is continuing in the direction of the original roll. At this point, a sideslip is introduced in the opposite direction and the process is reversed.
Once the aircraft is in Dutch roll mode, this effect can be excited by any use of rudder or aileron. Periods can range from some seconds for light aircraft to several minutes for large airliners.
The question remains where the name Dutch Roll originates from. Unfortunately this is unknown. However, there are two popular theories:
- It comes from the original design of Dutch ships that had rounded bottoms for a lower draft. The rounded bottom in combination with a lack of a good keel area resulted in a tendency to roll more than other ships of that time that were of a more conventional design.
- Then name is based upon the movement that Dutch skaters make when skating on ice.
As said, these are theories that have no solid sources provided with them.