Flight Control Surfaces

Jose Damian Peñaranda Ramirez - IET1552

The directional control of a fixed-wing aircraft

takes place around the lateral, longitudinal, and
vertical axes by means of flight control
surfaces designed to create movement about
these axes.
These control devices are hinged or movable
surfaces through which the attitude of an aircraft
is controlled during takeoff, flight, and landing

They are usually divided into two major

groups:
Primary or main flight control
surfaces

Secondary or auxiliary control

surfaces.
Primary Flight Control Surfaces
Primary control surfaces construction:
Aluminum alloys
Composite materials
 There is a critical need for primary control
surfaces to be balanced so they do not vibrate or
flutter in the wind.
Ailerons
Ailerons are the primary flight control surfaces
that move the aircraft about the longitudinal
axis.
In other words, movement of the ailerons in flight
causes the aircraft to roll.

Ailerons are usually located on the outboard

trailing edge of each of the wings.
They are built into the wing and are calculated
as part of the wing’s surface area.
 The pilot’s request for aileron movement and roll
are transmitted from the cockpit to the actual
control surface in a variety of ways depending on
the aircraft.
A system of control cables and pulleys, push-
pull tubes, hydraulics, electric, or a
combination of these can be employed.
 Simple light aircraft usually do not have
hydraulic or electric fly-by-wire aileron control.
These are found on heavy and high-performance
aircraft.

Large aircraft and some high performance aircraft

may also have a second set of ailerons located
inboard on the trailing edge of the wings.
Elevator
The elevator is the primary flight control surface
that moves the aircraft around the horizontal or
lateral axis.
This causes the nose of the aircraft to pitch up or
down.

The elevator is hinged to the trailing edge of the

horizontal stabilizer and typically spans most or
all of its width.
It is controlled in the cockpit by pushing or
pulling the control yoke forward or aft.
Light aircraft use a system of control cables and
pulleys or push pull tubes to transfer cockpit
inputs to the movement of the elevator.
High performance and large aircraft typically
employ more complex systems.
Hydraulic power is commonly used to move the
elevator on these aircraft.

On aircraft equipped with fly-by-wire controls, a

combination of electrical and hydraulic power is
used.
Rudder
The rudder is the primary control surface that
causes an aircraft to yaw or move about the
vertical axis.
This provides directional control and thus points
the nose of the aircraft in the direction desired.
Most aircraft have a single rudder hinged to the
trailing edge of the vertical stabilizer.
It is controlled by a pair of foot-operated rudder
pedals in the cockpit.
When the right pedal is pushed forward, it
deflects the rudder to the right which moves the
nose of the aircraft to the right.

The left pedal is rigged to simultaneously move

aft.
When the left pedal is pushed forward, the nose
of the aircraft moves to the left.

As with the other primary flight controls, the

transfer of the movement of the cockpit controls to
the rudder varies with the complexity of the
aircraft.
 Many aircraft incorporate the directional
movement of the nose or tail wheel into the
rudder control system for ground operation.
This allows the operator to steer the aircraft with
the rudder pedals during taxi when the airspeed
is not high enough for the control surfaces to be
effective.

Steering rod from

rudder pedal
 Dual Purpose Flight Control Surfaces
The ailerons, elevators, and rudder are
considered conventional primary control surfaces.
However, some aircraft are designed with a
control surface that may serve a dual purpose.

For example:
Elevons perform the combined functions of the
ailerons and the elevator
 A movable horizontal tail section, called a
stabilator, is a control surface that combines the
action of both the horizontal stabilizer and the
elevator.
Basically, a stabilator is a horizontal stabilizer that
can also be rotated about the horizontal axis to
affect the pitch of the aircraft.
A ruddervator combines the action of the rudder
and elevator.
This is possible on aircraft with V–tail
empennages where the traditional horizontal and
vertical stabilizers do not exist. Instead, two
stabilizers angle upward and outward from the aft
fuselage in a “V” configuration.
Each contains a movable ruddervator built into
the trailing edge

Movement of the ruddervators can alter the movement of

the aircraft around the horizontal and/or vertical axis.
Cirrus Vision SF50
Flaperons are ailerons which can also act as
flaps.
 Secondary
or
Auxiliary Control Surfaces
There are several secondary or auxiliary flight
control surfaces.
 Flaps
Flaps are found on most aircraft.
They are usually inboard on the wings’ trailing
edges adjacent to the fuselage. Leading edge
flaps are also common.

They extend forward and down from the inboard

wing leading edge. The flaps are lowered to
increase the camber of the wings and provide
greater lift and control at slow speeds.

They enable landing at slower speeds and

shorten the amount of runway required for takeoff
and landing.
The amount that the flaps extend and the angle
they form with the wing can be selected from the
cockpit.
Typically, flaps can extend up to 45–50°
 Flaps are usually constructed of materials and
with techniques used on the other airfoils and
control surfaces of a particular aircraft.
Aluminum skin and structure flaps are the norm
on light aircraft.
Heavy and high-performance aircraft flaps may
also be aluminum, but the use of composite
structures is also common.
 Flower Flap

An enhanced version of the fowler flap is a set of

flaps that actually contains more than one
aerodynamic surface.
a triple slotted flap. In this configuration, the flap
consists of a fore flap, a mid flap, and an aft
flap. When deployed, each flap section slides aft
on tracks as it lowers.
Heavy aircraft often have leading edge flaps that
are used in conjunction with the trailing edge
flaps.
They can be made of machined magnesium or
can have an aluminum or composite structure.
 While they are not installed or operate
independently, their use with trailing edge flaps
can greatly increase wing camber and lift.
When stowed, leading edge flaps retract into the
leading edge of the wing
Krueger
flap,
recognizable
by its at
mid-section.

Slats
Another leading-edge device which extends wing
camber is a slat.
Slats can be operated independently of the flaps
with their own switch in the cockpit.
Slats not only extend out of the leading edge of
the wing increasing camber and lift, but most
often, when fully deployed leave a slot between
their trailing edges and the leading edge of the
wing.
This increases the angle of attack at which the
wing will maintain its laminar air flow, resulting in
the ability to fly the aircraft slower and still
maintain control.
 Slats

Leading edge flaps

and slats.

Speed Brakes
A spoiler is a device found on the upper surface
of many heavy and high-performance aircraft.
It is stowed flush to the wing’s upper surface.
When deployed, it raises up into the airstream
and disrupts the laminar air flow of the wing, thus
reducing lift.
Speedbrakes fall into two categories:
Those that are deployed at controlled angles
during flight (flight spoilers) to increase descent
rate or control roll, and those that are fully
deployed immediately on landing (ground
spoilers) to greatly reduce lift ("lift dumpers") and
increase drag.
In modern fly-by-wire aircraft, the same set of
control surfaces serve both functions.
 On the wing where the aileron is moved up, the
spoilers also raise thus amplifying the reduction of
lift on that wing.
On the wing with downward aileron deflection, the
spoilers remain stowed.
As the speed of the aircraft increases, the ailerons
become more effective and the spoiler
interconnect disengages.

Spoilers are unique in that they may also be fully

deployed on both wings to act as speed brakes.
The reduced lift and increased drag can quickly
reduce the speed of the aircraft in flight.
Dedicated speed brake panels similar to flight
spoilers in construction can also be found on the
upper surface of the wings of heavy and high
performance aircraft.
 The speed brake control in the cockpit can
deploy all spoiler and speed brake surfaces fully
when operated.
Often, these surfaces are also rigged to deploy
on the ground automatically when engine thrust
reversers are activated.

Tabs
The force of the air against a control surface
during the high speed of flight can make it
difficult to move and hold that control surface in
the deflected position.
A control surface might also be too sensitive for
similar reasons. Several different tabs are used to
aid with these types of problems.
Trims tabs are designed to allow the pilot to be
able to take his or her hands and feet off of the
controls and have the aircraft maintain its flight
condition.
Most trim tabs are small movable surfaces
located on the trailing edge of a primary flight
control surface
 Often, it is difficult to move a primary control
surface due to its surface area and the speed of
the air rushing over it.
Deflecting a balance tab hinged at the trailing
edge of the control surface in the opposite
direction of the desired control surface movement
causes a force to position the surface in the
proper direction with reduced force to do so.
Servo Tabs
 Vortex generators
are small airfoil sections usually attached to the
upper surface of a wing.
They are designed to promote positive laminar
airflow over the wing and control surfaces.

Usually made of aluminum and installed in a

spanwise line or lines, the vortices created by
these devices swirl downward assisting
maintenance of the boundary layer of air flowing
over the wing.