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attempts to change direction. The keel effect only
works when the floats are moving through the water. If
the current is moving the seaplane, there may be little
or no motion relative to the water, even though the
seaplane is moving relative to the shore. Using wind,
current, and thrust to track the desired course requires
careful planning and a thorough understanding of the
various forces at work.
With the engine shut down, most flying boats sail
backward and toward whichever side the nose is
pointed, regardless of wind velocity, because the hull
does not provide as much keel effect as floats in proportion
to the side area of the seaplane above the
waterline. To sail directly backward in a flying boat,
release the controls and let the wind steer. Sailing is
an essential part of seaplane operation. Since each
type of seaplane has its own peculiarities, practice
sailing until thoroughly familiar with that particular
type. Practice in large bodies of water such as lakes
or bays, but sufficiently close to a prominent object in
order to evaluate performance.
Before taxiing into a confined area, carefully evaluate
the effects of the wind and current, otherwise the seaplane
may be driven into obstructions. With a seaplane
of average size and power at idle, a water current of 5
knots can offset a wind velocity of 25 knots in the
opposite direction. This means that a 5 knot current
will carry the seaplane against a 25 knot wind.
Differential power can be used to aid steering in multiengine
seaplanes.
PORPOISING
Porpoising is a rhythmic pitching motion caused by
dynamic instability in forces along the float bottoms
while on the step. An incorrect planing attitude sets off
a cyclic oscillation that steadily increases in amplitude
unless the proper pitch attitude is reestablished. [Figure
4-13]
A seaplane travels smoothly across the water on the
step only if the floats or hull remain within a moderately
tolerant range of pitch angles. If the nose is held
too low during planing, water pressure in the form of a
small crest or wall builds up under the bows of the
floats. Eventually, the crest becomes large enough that
the fronts of the floats ride up over the crest, pitching
the bows upward. As the step passes over the crest, the
floats tip forward abruptly, digging the bows a little
deeper into the water. This builds a new crest in front
of the floats, resulting in another oscillation. Each
oscillation becomes increasingly severe, and if not corrected,
will cause the seaplane to nose into the water,
resulting in extensive damage or possible capsizing. A
second type of porpoising can occur if the nose is held
too high while on the step. Porpoising can also cause a
premature lift-off with an extremely high angle of
attack, which can result in a stall and a subsequent
nose-down drop into the water. Porpoising occurs during
the takeoff run if the planing angle is not properly
controlled with elevator pressure just after passing
through the “hump” speed. The pitching created when
the seaplane encounters a swell system while on the
step can also initiate porpoising. Usually, porpoising
does not start until the seaplane has passed a degree or
two beyond the acceptable planing angle range, and
Right
Aileron Up
Left Rudder
Left Aileron
Down
Direction of Motion
with Engine Idling
Direction of Motion
with Power Just
Balancing Wind
Direction of Motion
with Enough Power
to Overcome Wind
Direction of Motion
with Power Off
Figure 4-12. By balancing wind force and engine thrust, it is
possible to sail sideways or diagonally forward. Of course,
reversing the control positions from those illustrated permits
the pilot to sail to the opposite side.
Figure 4-13. Porpoising increases in amplitude if not corrected promptly.
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4-10
does not cease until after the seaplane has passed out of
the critical range by a degree or two.
If porpoising occurs due to a nose-low planing attitude,
stop it by applying timely back pressure on the elevator
control to prevent the bows of the floats from digging
into the water. The back pressure must be applied and
maintained until porpoising stops. If porpoising does not
stop by the time the second oscillation occurs, reduce the
power to idle and hold the elevator control back firmly
so the seaplane settles onto the water with no further
instability. Never try to “chase” the oscillations, as this
usually makes them worse and results in an accident.
　
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