gyroscopic precession
Gyroscopic precession is a phenomenon occurring in rotating bodies in
which an applied force is manifested 90 degrees later in the direction of
rotation from where the force was applied. Although precession is not a
dominant force in rotary-wing aerodynamics, it must be reckoned with
because turning rotor systems exhibit some of the characteristics of a
gyro. This diagram shows how precession affects the rotor disk when force
is applied at a given point:
A downward force applied to the
disk at point A results in a downward change in disk attitude at point B. And
upward force applied at Point C results in an upward change in disk attitude at
point D.
Forces applied to a spinning
rotor disk by control input or by wind gusts will react as follows:
"table at bottom of page 2-44"
This behaviour explains some of
the fundamental effects occurring during various helicopter manoeuvres. For
example, the helicopter behaves differently when rolling into a right turn than
when rolling into a left turn. During roll into a left turn, the pilot will have
to correct for a nose down tendency in order to maintain altitude. This
correction is required because precession causes a nose down tendency and
because the tilted disk produces less vertical lift to counteract gravity.
Conversely, during a roll into a right turn, precession will cause a nose up
tendency while the tilted disk will produce less vertical lift. Pilot input
required to maintain altitude is significantly different during a right turn
than during a left turn, because gyroscopic precession acts in opposite
directions for each.
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