What is a ‘Dutch roll?’

Question: What causes a “Dutch roll” in aircraft?

– Chris Hawkins, Tallahassee, Florida

Answer: Dutch roll is a natural aerodynamic phenomenon in swept-wing aircraft. It is caused by the design having slightly weaker directional stability than lateral stability. The result is the tail of the airplane seeming to “wag” or move left and right with slight up and down motion. In today’s airplanes, this tendency is reduced in the design and with the installation of a yaw damper.

Q: Is there an instrument that indicates the degree of yaw? I seem to remember years ago some aircraft had a yaw string on a white painted center line on a black background so that you could determine if the aircraft was in a yaw position.

– Richard Sancibrian, Merced, California

A: Yes, in older airplanes, there is a needle and ball instrument. The ball shows yaw. In more modern airplanes, there is an indicator in the Primary Flight Display under the bank indicator that shows yaw. Gliders still use a string, as do some aerobatic airplanes.

Q: How was a gyroscope used before fins to stabilize a ship? – Caroline Wolter Hall, Ontario A: A gyroscope remains rigid in space regardless of orientation. While in that rigid position, the orientation of the vehicle does not matter. This means in an airplane, a gyroscope can be used to show a pilot an artificial horizon even when he or she cannot see it. It is possible to take electrical signals from the relationship of the vehicle and the gyroscope and calculate direction and rate of movement. Once that is determined, a counteracting force can commanded to minimize the movement of the vehicle.

John Cox is a retired airline captain with US Airways.