The proper use of the term "Dutch Roll" applies to a characteristic of
the lateral stability of an aircraft, inherent in all aircraft but
thoroughly investigated during the test flight and certification stage.
The manoeuvre can be seen by simply letting go of the stick in trimmed
level flight and yawing the aircraft. Ideally one would provide one yaw
impulse and then let the aircraft return itself to steady flight while
observing the dynamics of the recovery (how many oscillations, damping
etc.) but it is possible to keep the manoeuvre going by alternating the
impulse on the rudder pedals with each change in yaw direction.

Note that I have only mentioned the rudder above. This manoeuvre does
not involve aileron input. In fact a pilot could get himself into
trouble by trying to correct with aileron input and could produce
amplified pilot induced oscillations.

The dynamics are as follows:
With the first rudder input comes a yaw impulse. This causes the
opposite wing (starboard wing if port rudder input) to swing forward -
gaining a few knots (and lift) - and the other wing to swing back -
losing a few knots. This is why the aircraft seems to roll during the
yaw. If the pilot keeps alternating the rudder inputs this will cause an
oscillating yaw condition. Looking at the wing tip you would see it
describing an ellipse on the horizon. The way to stop this motion is not
to fight it. Simply ceasing control input will rapidly stabilise the
aircraft, remember that aircraft are designed to be laterally stable (in
most cases), loosing part of the tailplane or rudder could make this a
problem. Trying to fight the motion with control inputs will only
prolong the motion at best, and amplify it at worst. Your natural
instinct to correct in a particular direction with aileron is working
against you because at the point that you feel you should correct is the
point where the factors influencing the motion are changing direction
and you are now only adding to them. If a pilot got caught out he could
stall one wing tip while overcorrecting and end up in a spiral or even
spin. Unlikely but possible.

Incidentally this manoeuvre is best experienced in the tail of the
aircraft. The pilots are usually near the wing junction and as such are
on the centre line of a helix that the the passengers in the rear of the
fuselage are following.


Dennis O'Connor wrote:
Read, Stick And Rudder... Wolfgang Langewiesche

Denny

Matthew Jurotich" wrote in

When doing this correctly are the rudder and ailerons co-ordinated

or opposite? Why?