Elevator
Down spring
Pivot points
Bell crank
Figure 6-12. When the aerodynamic efficiency of the horizontal tail
surface is inadequate due to an aft CG condition, an elevator down
spring may be used to supply a mechanical load to lower the nose.
Antiservo tab
Balance weight
Stabilator
Pivot point
Figure 6-13. The stabilator is a one-piece horizontal tail surface
that pivots up and down about a central hinge point.
Figure 6-14. The Piaggio P180 includes a variable-sweep canard
design that provides longitudinal stability about the lateral axis.
airflow over the empennage. This, coupled with the reduced
landing speed, makes the elevator less effective.
As this discussion demonstrates, pilots must understand and
follow proper loading procedures, particularly with regard
to the CG position. More information on aircraft loading,
as well as weight and balance, is included in Chapter 10,
Weight and Balance.
Stabilator
As mentioned in Chapter 3, Aircraft Structure, a stabilator is
essentially a one-piece horizontal stabilizer that pivots from
a central hinge point. When the control column is pulled
back, it raises the stabilator’s trailing edge, pulling the nose
of the aircraft. Pushing the control column forward lowers
the trailing edge of the stabilator and pitches the nose of the
aircraft down.
Because stabilators pivot around a central hinge point, they
are extremely sensitive to control inputs and aerodynamic
loads. Antiservo tabs are incorporated on the trailing edge to
decrease sensitivity. They deflect in the same direction as the
stabilator. This results in an increase in the force required to
move the stabilator, thus making it less prone to pilot-induced
overcontrolling. In addition, a balance weight is usually
incorporated in front of the main spar. The balance weight
may project into the empennage or may be incorporated on
the forward portion of the stabilator tips. [Figure 6-13]
Canard
The canard design utilizes the concept of two lifting surfaces.
The canard functions as a horizontal stabilizer located in front
of the main wings. In effect, the canard is an airfoil similar to
the horizontal surface on a conventional aft-tail design. The
difference is that the canard actually creates lift and holds
the nose up, as opposed to the aft-tail design which exerts
downward force on the tail to prevent the nose from rotating
downward. [Figure 6-14]
The canard design dates back to the pioneer days of aviation.
Most notably, it was used on the Wright Flyer. Recently, the
canard configuration has regained popularity and is appearing
on newer aircraft. Canard designs include two types–one with
a horizontal surface of about the same size as a normal aft-tail
design, and the other with a surface of the same approximate
size and airfoil of the aft-mounted wing known as a tandem
wing configuration. Theoretically, the canard is considered
more efficient because using the horizontal surface to help
lift the weight of the aircraft should result in less drag for a
given amount of lift.
Aerodynamic force
Left rudder
CG
Left rudder forward
Yaw
Figure 6-15. The effect of left rudder pressure.
Figure 6-16. Beechcraft Bonanza V35.
Rudder
The rudder controls movement of the aircraft about its vertical
axis. This motion is called yaw. Like the other primary control
surfaces, the rudder is a movable surface hinged to a fixed
surface in this case, to the vertical stabilizer or fin. The rudder
is controlled by the left and right rudder pedals.
When the rudder is deflected into the airflow, a horizontal
force is exerted in the opposite direction. [Figure 6-15] By
pushing the left pedal, the rudder moves left. This alters the
airflow around the vertical stabilizer/rudder and creates a
sideward lift that moves the tail to the right and yaws the nose
of the airplane to the left. Rudder effectiveness increases with
speed; therefore, large deflections at low speeds and small
deflections at high speeds may be required to provide the
desired reaction. In propeller-driven aircraft, any slipstream
flowing over the rudder increases its effectiveness.
V-Tail
The V-tail design utilizes two slanted tail surfaces to perform
the same functions as the surfaces of a conventional elevator
and rudder configuration. The fixed surfaces act as both
horizontal and vertical stabilizers. [Figure 6-16]
The movable surfaces, which are usually called ruddervators,
are connected through a special linkage that allows the control
wheel to move both surfaces simultaneously. On the other
hand, displacement of the rudder pedals moves the surfaces
differentially, thereby providing directional control.
When both rudder and elevator controls are moved by the
pilot, a control mixing mechanism moves each surface the
appropriate amount. The control system for the V-tail is more
complex than the control system for a conventional tail. In
addition, the V-tail design is more susceptible to Dutch roll
tendencies than a conventional tail, and total reduction in
drag is minimal.
Secondary Flight Controls
Secondary flight control systems may consist of wing flaps,
leading edge devices, spoilers, and trim systems.
Flaps
Flaps are the most common high-lift devices used on aircraft.
These surfaces, which are attached to the trailing edge of
the wing, increase both lift and induced drag for any given
AOA. Flaps allow a compromise between high cruising
speed and low landing speed because they may be extended
when needed and retracted into the wing’s structure when not
needed. There are four common types of flaps: plain, split,
slotted, and Fowler flaps. [Figure 6-17]
The plain flap is the simplest of the four types. It increases
the airfoil camber, resulting in a significant increase in the
coefficient of lift (CL) at a given AOA. At the same time, it
greatly increases drag and moves the center of pressure (CP)
aft on the airfoil, resulting in a nose-down pitching moment.
The split flap is deflected from the lower surface of the airfoil
and produces a slightly greater increase in lift than the plain
flap. More drag is created because of the turbulent air pattern
produced behind the airfoil. When fully extended, both plain
and split flaps produce high drag with little additional lift.
The most popular flap on aircraft today is the slotted flap.
Variations of this design are used for small aircraft, as well
as for large ones. Slotted flaps increase the lift coefficient
significantly more than plain or split flaps. On small aircraft,
the hinge is located below the lower surface of the flap, and