John Cox

Special to USA TODAY

Question: I have heard of a "Dutch Roll" in aircraft. What causes it? Do the passengers notice it?

– 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. The yaw damper senses the yawing motion then applies a small rudder input to counter it. As a result, the ride is smooth and comfortable.

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: I can't understand how a gyroscope, torque and precession were used before fins to stabilize a ship. I understand this is no aviation question, but I really need a kindergarten explanation.

– Caroline Wolter Hall, Keswick, 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 artificial 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 counter acting force can commanded to minimize the movement of the vehicle. This is how jet yaw dampers work, and it is the same principle for gyrostablized fins on ships.

The engineers that figured this out were pretty smart.

John Cox is a retired airline captain with US Airways and runs his own aviation safety consulting company, Safety Operating Systems.



