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Seaplane Skiplane Flying Menu > Performance >Flight
Characteristics Of Seaplanes With High Thrust Lines
Many of the most common flying boat designs
have the engine and propeller mounted well above the airframe’s
CG. This results in some unique handling characteristics. The
piloting techniques necessary to fly these airplanes safely
are not intuitive and must be learned. Any pilot transitioning
to such an airplane is strongly urged to obtain additional training
specific to that model of seaplane.
Designing a seaplane with the engine and propeller
high above the water offers some important advantages. The propeller
is out of the spray during takeoffs and landings, and more of
the fuselage volume can be used for passengers and cargo. The
pilot usually sits well forward of the wing, and enjoys an excellent
view in almost every direction.
Pilots who fly typical light twins are familiar
with what happens when one engine is producing power and the
other is not. The airplane tends to yaw toward the dead engine.
This happens because the thrust line is located some distance
from the airplane’s CG. In some respects, this situation
is similar to the single-engine seaplane with a high thrust
line, except that the seaplane flies on one engine all the time.
When power is applied, the thrust tends to pitch the nose down,
and as power is reduced, the nose tends to rise. [Figure 5-4]
This is exactly the opposite of what most pilots are accustomed
to. In typical airplanes, including most floatplanes, applying
power raises the nose and initiates a climb.
Naturally the magnitude of these pitch forces
is proportional to how quickly power is applied or reduced.
The most extreme pitch force logically results
from a sudden engine failure, when the full thrust of the engine
and its associated downward pitching force are suddenly removed.
Forward thrust is replaced by the drag of a windmilling propeller,
which adds a new upward pitching force. Since the seaplane is
already trimmed with a considerable elevator force to counteract
the downward pitch force of the engine, the nose pitches up
abruptly. If this scenario occurs just after takeoff, when the
engine has been producing maximum power, airspeed is low, and
there is little altitude, the pilot must react instantly to
overpower the upward pitching forces and push the nose down
to avoid a stall.
The reversal of typical pitch forces also comes
into play if porpoising should begin during a takeoff. As discussed
in Chapter 4, Seaplane Operations - Preflight and Takeoffs,
porpoising usually occurs when the planing angle is held too
low by the pilot, forcing the front portion of the floats to
drag until a wave builds up and travels back along the float.
The
same thing can happen with the hull of a flying boat, and the
nose-down force of a high thrust line can make porpoising more
likely. Once porpoising develops, the standard solution is to
reduce power and let the airplane settle back into the water.
But if power is reduced too quickly in a seaplane with a high-mounted
engine, the sudden upward pitching force can combine with the
porpoising to throw the seaplane into the air with inadequate
airspeed for flight, decreasing thrust, and inadequate altitude
for recovery.
Depending on how far the engine is from the
airplane’s CG, the mass of the engine can have detrimental
effects on roll stability. Some seaplanes have the engine mounted
within the upper fuselage, while oth-
Figure 5-4. Pitching
forces in seaplanes with a high thrust line.
ers have engines mounted on a pylon well above
the main fuselage. If it is far from the CG, the engine can
act like a weight at the end of a lever, and once started in
motion it tends to continue in motion. Imagine balancing a hammer
upright with the handle on the palm of the hand. [Figure 5-5]
Figure 5-5. Roll instability
with a high-mounted engine.
Finally, seaplanes with high-mounted engines
may have unusual spin characteristics and recovery techniques.
These factors reinforce the point that pilots need to obtain
thorough training from a qualified instructor in order to operate
this type of seaplane safely.
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