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Figure 5-3. Vertical surfaces added to the tail help restore
directional stability.
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5-3
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-
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.
Figure 5-4. Pitching forces in seaplanes with a high thrust line.
Increasing Thrust
Decreasing Thrust
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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]
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.
MULTIENGINE SEAPLANES
A rating to fly single-engine seaplanes does not entitle
a pilot to fly seaplanes with two or more engines. The
addition of a multiengine sea rating to a pilot
certificate requires considerable additional training.
　
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