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Pilots must learn and practice the correct pitch attitudes
for takeoff, planing, and landing for each type of seaplane
until there is no doubt as to the proper angles for
the various maneuvers. The upper and lower limits of
these pitch angles are established by the design of the
seaplane; however, changing the seaplane’s gross
weight, wing flap position, or center of gravity location
also changes these limits. Increased weight increases
the displacement of the floats or hull and raises the
lower limit considerably. Extending the wing flaps frequently
trims the seaplane to the lower limit at lower
speeds, and may lower the upper limit at high speeds. A
forward center of gravity increases the possibility of
high angle porpoising, especially during landing.
SKIPPING
Skipping is a form of instability that may occur when
landing at excessive speed with the nose at too high a
pitch angle. This nose-up attitude places the seaplane at
the upper trim limit of stability and causes the seaplane
to enter a cyclic oscillation when touching the water,
which results in the seaplane skipping across the surface.
This action is similar to skipping flat stones across
the water. Skipping can also occur by crossing a boat
wake while taxiing on the step or during a takeoff.
Sometimes the new seaplane pilot confuses a skip with
a porpoise, but the pilot’s body sensations can quickly
distinguish between the two. Askip gives the body vertical
“G” forces, similar to bouncing a landplane.
Porpoising is a rocking chair type forward and aft
motion feeling.
To correct for skipping, first increase back pressure on the
elevator control and add sufficient power to prevent the
floats from contacting the water. Then establish the proper
pitch attitude and reduce the power gradually to allow the
seaplane to settle gently onto the water. Skipping
oscillations do not tend to increase in amplitude, as in
porpoising, but they do subject the floats and airframe
to unnecessary pounding and can lead to porpoising.
TAKEOFFS
A seaplane takeoff may be divided into four distinct
phases: (1) The displacement phase, (2) the hump or
plowing phase, (3) the planing or on the step phase, and
(4) the lift-off.
The displacement phase should be familiar from the
taxiing discussion. During idle taxi, the displacement
of water supports nearly all of the seaplane’s weight.
The weight of the seaplane forces the floats down into
the water until a volume that weighs exactly as much
as the seaplane has been displaced. The surface area of
the float below the waterline is called the wetted area,
and it varies depending on the seaplane’s weight. An
empty seaplane has less wetted area than when it is
fully loaded. Wetted area is a major factor in the creation
of drag as the seaplane moves through the water.
As power is applied, the floats move faster through the
water. The water resists this motion, creating drag. The
forward portion of the float is shaped to transform the
horizontal movement through the water into an upward
lifting force by diverting the water downward.
Newton’s Third Law of Motion states that for every
action, there is an equal and opposite reaction, and in
this case, pushing water downward results in an
upward force known as hydrodynamic lift.
In the plowing phase, hydrodynamic lift begins pushing
up the front of the floats, raising the seaplane’s nose
and moving the center of buoyancy aft. This, combined
with the downward pressure on the tail generated by
holding the elevator control all the way back, forces
the rear part of the floats deeper into the water. This
creates more wetted area and consequently more drag,
and explains why the seaplane accelerates so slowly
during this part of the takeoff.
This resistance typically reaches its peak just before
the floats are placed into a planing attitude. Figure 4-14
shows a graph of the drag forces at work during a seaplane
takeoff run. The area of greatest resistance is
referred to as the hump because of the shape of the
water drag curve. During the plowing phase, the
increasing water speed generates more and more
hydrodynamic lift. With more of the weight supported
by hydrodynamic lift, proportionately less is supported
by displacement and the floats are able to rise in the
water. As they do, there is less wetted area to cause
drag, which allows more acceleration, which in turn
increases hydrodynamic lift. There is a limit to how far
this cycle can go, however, because as speed builds, so
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