In aviation, the turn and slip indicator and the turn coordinator variant are
essentially two aircraft flight instruments in one device. One indicates
the rate of turn, or the rate of change in the aircraft's heading, the other
part indicates whether the aircraft is in coordinated flight, showing the slip
or skid of the turn. The slip indicator is actually an inclinometer that at rest
displays the angle of the aircraft's lateral axis with respect to horizontal,
and in motion displays this angle as modified by the acceleration of the
aircraft. Name
The turn and slip indicator can be referred to as the turn and bank
indicator, although the instrument does not respond directly to bank angle.
Neither does the turn coordinator, but it does respond to roll rate, which
enables it to respond more quickly to the start of a turn.
Operation = Turn indicator=
The turn indicator is a gyroscopic instrument that works on the principle
of precession. The gyro is mounted in a gimbal. The gyro's rotational axis is
in-line with the lateral axis of the aircraft, while the gimbal has limited
freedom around the longitudinal axis of the aircraft.
As the aircraft yaws, a torque force is applied to the gyro around the vertical
axis, due to aircraft yaw, which causes gyro precession around the roll axis.
The gyro spins on an axis that is 90 degrees relative to the direction of the
applied yaw torque force. The gyro and gimbal rotate with limited freedom
against a calibrated spring. The torque force against the spring reaches an
equilibrium and the angle that the gimbal and gyro become positioned is
directly connected to the display needle, thereby indicating the rate of
turn. In the turn coordinator, the gyro is canted 30 degrees from the horizontal
so it responds to roll as well as yaw. The display contains hash marks for the
pilot's reference during a turn. When the needle is lined up with a hash mark,
the aircraft is performing a "standard rate turn" which is defined as three
degrees per second, known in some countries as "rate one." This translates
to two minutes per 360 degrees of turn. Indicators are marked as to their
sensitivity, with "2 min turn" for those whose hash marks correspond to a
standard rate or two-minute turn, and "4 min turn" for those, used in faster
aircraft, that show a half standard rate or four-minute turn. The supersonic
Concorde jet aircraft and many military jets are examples of aircraft that use 4
min. turn indicators. The hash marks are sometimes called "dog houses," because
of their distinct shape on various makes of turn indicators. Under instrument
flight rules, using these figures allows a pilot to perform timed turns in order
to conform with the required air traffic patterns. For a change of heading of 90
degrees, a turn lasting 30 seconds would be required to perform a standard rate
or "rate one" turn. = Inclinometer=
Coordinated flight indication is obtained by using an inclinometer, which
is recognized as the "ball in a tube." An inclinometer contains a ball sealed
inside a curved glass tube, which also contains a liquid to act as a damping
medium. The original form of the indicator is in effect a spirit level
with the tube curved in the opposite direction and a bubble replacing the
ball. In some early aircraft the indicator was merely a pendulum with a
dashpot for damping. The ball gives an indication of whether the aircraft is
slipping, skidding or in balanced flight. The ball's movement is caused by
the force of gravity and the aircraft's centripetal acceleration. When the ball
is centered in the middle of the tube, the aircraft is said to be in
coordinated flight. If the ball is on the inside of a turn, the aircraft is
slipping. And finally, when the ball is on the outside of the turn, the aircraft
is skidding. A simple alternative to the balance
indicator is a yaw string, which allows the pilot to simply view the string's
movements as rudimentary indication of aircraft balance.
= Turn coordinator= The turn coordinator is a further
development of the turn and slip indicator with the major difference
being the display and the axis upon which the gimbal is mounted. The display
is that of a miniature airplane as seen from behind. This looks similar to that
of an attitude indicator. "NO PITCH INFORMATION" is usually written on the
instrument to avoid confusion regarding the aircraft's pitch, which can be
obtained from the artificial horizon instrument.
In contrast to the T/S, the TC's gimbal is pitched up 30 degrees from the
lateral axis. This causes the instrument to respond to roll as well as yaw. This
allows the instrument to display a change more quickly as it will react to
the change in roll before the aircraft has even begun to yaw. Although this
instrument reacts to changes in the aircraft's roll, it does not display the
roll attitude. The turn coordinator may be used as a
performance instrument when the attitude indicator has failed. This is called
"partial panel" operations. It can be unnecessarily difficult or even
impossible if the pilot does not understand that the instrument is
showing roll rates as well as turn rates. The usefulness is also impaired
if the internal dashpot is worn out. In the latter case, the instrument is
underdamped and in turbulence will indicate large full-scale deflections to
the left and right, all of which are actually roll rate responses. In this
condition it may not be possible for the pilot to maintain control of the
aircraft in partial-panel operations in instrument meteorological conditions.
For this and other reasons, many highly experienced pilots prefer the "older"
turn and bank indicator design. Practical implications
Slipping and skidding within a turn is sometimes referred to as a sloppy turn,
due to the perceptive discomfort it can cause to the pilot and passengers. When
the aircraft is in a balanced turn, passengers experience gravity directly
in line with their seat. With a well balanced turn, passengers may not even
realize the aircraft is turning unless they are viewing objects outside the
aircraft. While aircraft slipping and skidding are often undesired in a usual
turn that maintains altitude, slipping of the aircraft can be used for
practical purposes. Intentionally putting an aircraft into a slip is used
as a forward slip and a sideslip. These slips are performed by applying opposite
inputs of the aileron and rudder controls. A forward slip allows a pilot
to quickly drop altitude without gaining unnecessary speed, while a sideslip is
one method utilized to perform a crosswind landing.
See also Flight dynamics
ROT Slip
References Advisory Circular AC 61-23C, Pilot's
Handbook of Aeronautical Knowledge, U.S. Federal Aviation Administration, Revised
1997. FAA-H-8083-15 Instrument Flying
Handbook, U.S. Federal Aviation Administration,
Operation demonstration of a Turn & slip indicator. The indicator cover is
removed and gyro is displayed as it operates.