(SelwayKid) wrote

So what is your understanding of a Dutch Roll? It is interesting to me
the various understandings of this manuever, how it is performed and
its origins.
I've got my own that I was taught by a CFI over 45 years ago and have
taught to virtually every student I've had over the years, including
helicopters.

Well....a Dutch Roll is probably not what you understand it to be,
particularly if you have not flown swept-wing transport aircraft.
The aileron/rudder drill sometimes taught to student pilots is not
a Dutch Roll.

Copied from the following web site:

http://142.26.194.131/aerodynamics1/...ity/Page5.html

Dutch Roll
Many swept wing aircraft suffer a dynamic instability problem known as
Dutch Roll.

Dutch roll happens when the aircraft has relatively strong static
lateral stability (usually due to the swept wings) and somewhat weak
directional stability (relatively.) In a Dutch roll the aircraft begins
to yaw due to a gust or other input. The yaw is slow damping out so the
aircraft begins to roll before the yaw is stopped (due to the increased
speed of the advancing wing and the increased lift due to the swept wing
effect.)

By the time the yaw stops and begins to swing back toward zero slip the
aircraft has developed a considerable roll rate and due to momentum plus
the slip angle the aircraft continues to roll even once the nose has
begun returning to the original slip angle.

Eventually the yaw overshoots the zero slip angle causing the wings to
begin rolling back in the opposite direction.

The whole procedure repeats, sometimes with large motions, sometimes
with just a small churning motion. Like all dynamic stability problems,
Dutch roll is much worse at high altitudes where the air is less dense.

Dutch roll is almost certain to happen in a jet aircraft if the Yaw
dampener is turned off at high altitude. Therefore, the first thing to
check if an aircraft begins to exhibit Dutch roll is that the Yaw
Dampener is on. The pilot should then try to minimize the yawing
oscillations by blocking the rudder pedals (i.e. hold the rudder pedals
in the neutral position.) Next apply aileron (spoiler) control opposite
to the roll. The best technique to use is short jabs of ailerons applied
opposite to the roll. Try to give one quick jab on each cycle (i.e. turn
the wheel toward the rising wing, then return it to neutral.) Finally
accelerate to a higher speed, where directional stability will be
better, or descend into more dense air, for the same reason.

Bob Moore
ATP B-707 B-727
FI Airplanes/Instruments