Aileron deflected up
Aileron deflected down
Differential aileron
Figure 6-6. Differential ailerons.
Figure 5-4. Frise-type ailerons.
Lowered
Neutral
Raised
Drag
Figure 6-7. Frise-type ailerons.
effectively maneuver the aircraft. As a result, the increase
in aileron deflection causes an increase in adverse yaw. The
yaw is especially evident in aircraft with long wing spans.
Application of the rudder is used to counteract adverse
yaw. The amount of rudder control required is greatest at
low airspeeds, high angles of attack, and with large aileron
deflections. Like all control surfaces at lower airspeeds,
the vertical stabilizer/rudder becomes less effective and
magnifies the control problems associated with adverse yaw.
All turns are coordinated by use of ailerons, rudder, and
elevator. Applying aileron pressure is necessary to place
the aircraft in the desired angle of bank, while simultaneous
application of rudder pressure is necessary to counteract the
resultant adverse yaw. Additionally, because more lift is
required during a turn than during straight-and-level flight,
the angle of attack (AOA) must be increased by applying
elevator back pressure. The steeper the turn, the more elevator
back pressure that is needed.
As the desired angle of bank is established, aileron and
rudder pressures should be relaxed. This stops the angle of
bank from increasing, because the aileron and rudder control
surfaces are in a neutral and streamlined position. Elevator
back pressure should be held constant to maintain altitude.
The roll-out from a turn is similar to the roll-in, except the
flight controls are applied in the opposite direction. The
aileron and rudder are applied in the direction of the roll-out
or toward the high wing. As the angle of bank decreases,
the elevator back pressure should be relaxed as necessary
to maintain altitude.
In an attempt to reduce the effects of adverse yaw,
manufacturers have engineered four systems: differential
ailerons, frise-type ailerons, coupled ailerons and rudder,
and flaperons.
Differential Ailerons
With differential ailerons, one aileron is raised a greater
distance than the other aileron and is lowered for a given
movement of the control wheel or control stick. This produces
an increase in drag on the descending wing. The greater drag
results from deflecting the up aileron on the descending wing
to a greater angle than the down aileron on the rising wing.
While adverse yaw is reduced, it is not eliminated completely.
[Figure 6-6]
Frise-Type Ailerons
With a frise-type aileron, when pressure is applied to the
control wheel, or control stick, the aileron that is being raised
pivots on an offset hinge. This projects the leading edge of
the aileron into the airflow and creates drag. It helps equalize
the drag created by the lowered aileron on the opposite wing
and reduces adverse yaw. [Figure 6-7]
The frise-type aileron also forms a slot so air flows smoothly
over the lowered aileron, making it more effective at high
angles of attack. Frise-type ailerons may also be designed
to function differentially. Like the differential aileron, the
frise-type aileron does not eliminate adverse yaw entirely.
Coordinated rudder application is still needed when ailerons
are applied.
Coupled Ailerons and Rudder
Coupled ailerons and rudder are linked controls. This is
accomplished with rudder-aileron interconnect springs, which
help correct for aileron drag by automatically deflecting
the rudder at the same time the ailerons are deflected. For
Rudder/Aileron interconnecting springs
Rudder deflects with ailerons
Figure 6-8. Coupled ailerons and rudder.
Figure 6-9. Flaperons on a Skystar Kitfox MK 7.
Nose up
Tail down
Control column
aft
Up elevator
Downward
aerodynamic force
CG
Figure 6-10. The elevator is the primary control for changing the
pitch attitude of an aircraft.
example, when the control wheel, or control stick, is moved
to produce a left roll, the interconnect cable and spring pulls
forward on the left rudder pedal just enough to prevent the
nose of the aircraft from yawing to the right. The force applied
to the rudder by the springs can be overridden if it becomes
necessary to slip the aircraft. [Figure 6-8]
Flaperons
Flaperons combine both aspects of flaps and ailerons. In
addition to controlling the bank angle of an aircraft like
conventional ailerons, flaperons can be lowered together
to function much the same as a dedicated set of flaps. The
pilot retains separate controls for ailerons and flaps. A mixer
is used to combine the separate pilot inputs into this single
set of control surfaces called flaperons. Many designs that
incorporate flaperons mount the control surfaces away from
the wing to provide undisturbed airflow at high angles of
attack and/or low airspeeds. [Figure 6-9]
Elevator
The elevator controls pitch about the lateral axis. Like the
ailerons on small aircraft, the elevator is connected to the
control column in the flight deck by a series of mechanical
linkages. Aft movement of the control column deflects
the trailing edge of the elevator surface up. This is usually
referred to as the up-elevator position. [Figure 6-10]
The up-elevator position decreases the camber of the elevator
and creates a downward aerodynamic force, which is greater
than the normal tail-down force that exists in straight-and-
level flight. The overall effect causes the tail of the aircraft
to move down and the nose to pitch up. The pitching moment
occurs about the center of gravity (CG). The strength of the
pitching moment is determined by the distance between
the CG and the horizontal tail surface, as well as by the
aerodynamic effectiveness of the horizontal tail surface.
Moving the control column forward has the opposite effect.
In this case, elevator camber increases, creating more lift
(less tail-down force) on the horizontal stabilizer/elevator.
This moves the tail upward and pitches the nose down. Again,
the pitching moment occurs about the CG.
As mentioned earlier, stability, power, thrustline, and the
position of the horizontal tail surfaces on the empennage
are factors in elevator effectiveness controlling pitch. For
CG
Figure 6-11. Aircraft with a T-tail design at a high AOA and an aft CG.
example, the horizontal tail surfaces may be attached near
the lower part of the vertical stabilizer, at the midpoint, or
at the high point, as in the T-tail design.
T-Tail
In a T-tail configuration, the elevator is above most of the
effects of downwash from the propeller, as well as airflow
around the fuselage and/or wings during normal flight
conditions. Operation of the elevators in this undisturbed air
allows control movements that are consistent throughout most
flight regimes. T-tail designs have become popular on many
light and large aircraft, especially those with aft fuselage-
mounted engines because the T-tail configuration removes
the tail from the exhaust blast of the engines. Seaplanes and
amphibians often have T-tails in order to keep the horizontal
surfaces as far from the water as possible. An additional
benefit is reduced noise and vibration inside the aircraft.
In comparison with conventional-tail aircraft, the elevator on a
T-tail aircraft must be moved a greater distance to raise the nose
a given amount when traveling at slow speeds. This is because
the conventional-tail aircraft has the downwash from the
propeller pushing down on the tail to assist in raising the nose.
Aircraft controls are rigged so that an increase in control force
is required to increase control travel. The forces required to
raise the nose of a T-tail aircraft are greater than the forces
required to raise the nose of a conventional-tail aircraft.
Longitudinal stability of a trimmed aircraft is the same for
both types of configuration, but the pilot must be aware that
the required control forces are greater at slow speeds during
takeoffs, landings, or stalls than for similar size aircraft
equipped with conventional tails.
T-tail aircraft also require additional design considerations
to counter the problem of flutter. Since the weight of the
horizontal surfaces is at the top of the vertical stabilizer, the
moment arm created causes high loads on the vertical stabilizer
that can result in flutter. Engineers must compensate for this by
increasing the design stiffness of the vertical stabilizer, usually
resulting in a weight penalty over conventional tail designs.
When flying at a very high AOA with a low airspeed and
an aft CG, the T-tail aircraft may be more susceptible to a
deep stall. In this condition, the wake of the wing impinges
on the tail surface and renders it almost ineffective. The
wing, if fully stalled, allows its airflow to separate right after
the leading edge. The wide wake of decelerated, turbulent
air blankets the horizontal tail and hence its effectiveness
diminished significantly. In these circumstances, elevator or
stabilator control is reduced (or perhaps eliminated) making
it difficult to recover from the stall. It should be noted that an
aft CG is often a contributing factor in these incidents, since
similar recovery problems are also found with conventional
tail aircraft with an aft CG. [Figure 6-11] Deep stalls can
occur on any aircraft but are more likely to occur on aircraft
with “T” tails as a high AOA may be more likely to place
the wings separated airflow into the path of the horizontal
surface of the tail. Additionally, the distance between the
wings and the tail, the position of the engines (such as being
mounted on the tail) may increase the susceptibility of deep
stall events. Therefore a deep stall may be more prevalent
on transport versus general aviation aircraft.
Since flight at a high AOA with a low airspeed and an aft
CG position can be dangerous, many aircraft have systems to
compensate for this situation. The systems range from control
stops to elevator down springs. On transport category jets, stick
pushers are commonly used. An elevator down spring assists in
lowering the nose of the aircraft to prevent a stall caused by the
aft CG position. The stall occurs because the properly trimmed
airplane is flying with the elevator in a trailing edge down
position, forcing the tail up and the nose down. In this unstable
condition, if the aircraft encounters turbulence and slows down
further, the trim tab no longer positions the elevator in the nose-
down position. The elevator then streamlines, and the nose of
the aircraft pitches upward, possibly resulting in a stall.
The elevator down spring produces a mechanical load on the
elevator, causing it to move toward the nose-down position if not
otherwise balanced. The elevator trim tab balances the elevator
down spring to position the elevator in a trimmed position.
When the trim tab becomes ineffective, the down spring drives
the elevator to a nose-down position. The nose of the aircraft
lowers, speed builds up, and a stall is prevented. [Figure 6-12]
The elevator must also have sufficient authority to hold the
nose of the aircraft up during the roundout for a landing. In
this case, a forward CG may cause a problem. During the
landing flare, power is usually reduced, which decreases the