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Intentional slips are
used to dissipate altitude without increasing airspeed
and/or to adjust airplane ground track during a crosswind.
Intentional slips are especially useful in forced landings
and in situations where obstacles must be cleared during
approaches to confined areas. A slip can also be used
as an emergency means of rapidly reducing airspeed in
situations where wing flaps are inoperative or not installed.
A slip occurs when the bank angle of an airplane is
too steep for the existing rate of turn. (Unintentional
slips are most often the result of uncoordinated rudder/aileron
application.)
A slip is a combination of forward movement and sideward
(with respect to the longitudinal axis of the airplane)
movement, the lateral axis being inclined and the sideward
movement being toward the low end of this axis (low
wing). An airplane in a slip is in fact flying sideways,
which results in a change in the direction that the
relative wind strikes the airplane. Slips are characterized
by a marked increase in drag and corresponding decrease
in airplane climb, cruise, and glide performance. It
is the increase in drag, however, that makes it possible
for an airplane in a slip to descend rapidly without
an increase in airspeed.
Most airplanes exhibit the characteristic of positive
static directional stability and, therefore, have a
natural tendency to compensate for slipping. An intentional
slip, therefore, requires deliberate cross-controlling
ailerons and rudder throughout the maneuver.
A “sideslip” is entered by lowering a wing
and applying just enough opposite rudder to prevent
a turn. In a sideslip, the airplane’s longitudinal
axis remains parallel to the original flightpath, but
the airplane no longer flies straight ahead. Instead,
the horizontal component of wing lift forces the airplane
also to move somewhat sideways toward the low wing.
The amount of slip, and therefore the rate of sideward
movement, is determined by the bank angle. The steeper
the bank is, the greater the degree of slip. As bank
angle is increased additional opposite rudder is required
to prevent turning. Sideslips are frequently used when
landing with a crosswind to keep the aircraft aligned
with the runway centerline while stopping any drift
left or right of the centerline.
A “forward slip” is one in which the airplane’s
direction of motion continues the same as before the
slip was begun. Assuming the airplane is originally
in straight flight, the wing on the side toward which
the slip is to be made should be lowered by use of the
ailerons. Simultaneously, the airplane’s nose must
be yawed in the opposite direction by applying opposite
rudder so that the airplane’s longitudinal axis
is at an angle to its original flightpath. The degree
to which the nose is yawed in the opposite direction
from the bank should be such that the original ground
track is maintained. In a forward slip, the amount of
slip, and therefore the sink rate, is determined by
the bank angle. The steeper the bank is, the steeper
the descent.
In most light airplanes, the steepness of a slip is
limited by the amount of rudder travel available. In
both sideslips and forward slips, the point may be reached
where full rudder is required to maintain heading even
though the ailerons are capable of further steepening
the bank angle. This is the practical slip limit because
any additional bank would cause the airplane to turn
even though full opposite rudder is being applied. If
there is a need to descend more rapidly, even though
the practical slip limit has been reached, lowering
the nose not only increases the sink rate but also increases
airspeed. The increase in airspeed increases rudder
effectiveness permitting a steeper slip. Conversely,
when the nose is raised, rudder effectiveness decreases
and the bank angle must be reduced.
Discontinuing a slip is accomplished by leveling the
wings and simultaneously releasing the rudder pressure
while readjusting the pitch attitude to the normal glide
attitude. If the pressure on the rudder is released
abruptly, the nose swings too quickly into line and
the airplane tends to acquire excess speed. Because
of the location of the pitot tube and static vents,
airspeed indicators in some airplanes may have considerable
error when the airplane is in a slip. The pilot must
be aware of this possibility and recognize a properly
performed slip by the attitude of the airplane, the
sound of the airflow, and the feel of the flight controls.
Unlike skids, however, if an airplane in a slip is made
to stall, it displays very little of the yawing tendency
that causes a skidding stall to develop into a spin.
The airplane in a slip may do little more than tend
to roll into a wings level attitude. In fact, in some
airplanes stall characteristics may even be improved.
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