In addition to primary flight controls, most
airplanes have another group called secondary
controls. These include trim devices of various
types and wing flaps. The trim devices are
adjusted so that the aircraft remains balanced in
moveable surfaces on the trailing edge of the wing
similar in shape to the ailerons. they are usually
larger in surface area. They are located on inboard
end if the wing next to the fuselage. Both sides are
activated together so they do not produce a rolling
action like the ailerons.
usually deployed in "degree" increments. In small
aircraft deployment is usually in 10 degree
increments from zero degrees (non-deployed) to 40
degrees maximum. Larger or more sophisticated
aircraft may have a different range of settings.
Normally, the flaps operate electrically through a 4
or 5 position switch located on the instrument
panel. In earlier aircraft the flaps were operated
using a manual flap handle.
flaps increases both the lift and drag of the wing.
Flap activation increases the angle of attack across
the wing / flap section. At 10 degrees, more lift
than drag is produced. As the flap angle is
increased more drag and less lift is produced for
each increment of deployment.
use of flaps is in landing. They permit a steeper
decent without increase in airspeed. Flaps may be
used in certain take-off situations (usually 10°) on
short or soft fields.
Flaps are now fitted to most aircraft because:hey
permit a slower landing speed, which decreases
the required landing distance.
They permit a comparatively steep angle of
descent without an increase in speed. This makes
it possible to safely clear obstacles when
making a landing approach to a small field.
They may also be used to shorten the takeoff
distance and provide a steeper climb path.
This term describes the maximum velocity at
which flaps can be deployed. The VFE is shown on
the air speed indicator as the top end of the
Flaps are high lift devices which, in effect, increase the
camber of the wing and, in some cases, as with the Fowler Flap, also increase
the effective wing area. Their use gives better take-off performance and permits
steeper approach angles and lower approach and landing speeds.
When deflected, flaps increase the upper camber of the wing,
increasing the negative pressure on the top of the wing. At the same time, they
allow a build up of pressure below the wing. During take-off, flap settings of
10 degrees to 20 degrees are used to give better take-off performance and a
better angle of climb, especially valuable when climbing out over
However, not all airplane manufacturers recommend the use of
flaps during take-off. They can be used only on those airplanes, which have
sufficient take-off power to overcome the extra drag that extended flaps
produce. The recommendations of the manufacturer should, therefore, always be
Flaps do indeed increase drag. The greater the flap deflection.
the greater the drag. At a point of about half of their full travel, the
increased drag surpasses the increased lift and the flaps become air brakes.
Most flaps can be extended to 40 degrees from the chord of the wing. At settings
between 20 degrees and 40 degrees, the essential function of the flaps is to
improve the landing capabilities, by steepening the glide without increasing the
glide speed. In an approach over obstacles, the use of flaps permits the pilot
to touch down much nearer the threshold of the runway. Flaps also permit a
slower landing speed and act as air brakes when the airplane is rolling to a
stop after landing, thus reducing the need for excessive braking action. As a
result, there is less wear on the undercarriage, wheels and tires. Lower landing
speeds also reduce the possibility of ground looping during the landing
Plain and split flaps increase the lift of a wing, but at the
same time, they greatly increase the drag. For all practical purposes, they are
of value only in approach and landing. They should not normally be employed for
take-off because the extra drag reduces acceleration.
Slotted flaps, on the other hand, including such types as Fowler
and Zap, produce lift in excess of drag and their partial use is therefore
recommended for take-off.
From the standpoint of aerodynamic efficiency, the Fowler Flap
is generally considered to offer the most advantages and the fewest
disadvantages, especially on larger airplanes, while double slotted flaps have
won wide approval for smaller types.
On STOL airplanes, a combination of double slotted flaps and
leading edge slats are common.
Changes in flap setting affect the trim of an airplane. As flaps
are lowered, the centre of pressure moves rearward creating a nose down,
pitching moment. However, in some airplanes, the change in airflow over the
tailplane as flaps are lowered, is such that the total moment created is nose up
and it becomes necessary to trim the airplane "nose down".
The airplane is apt to lose considerable height when the flaps
are raised. At low altitudes, therefore, the flaps should be raised
Most airplanes are placarded to show a maximum speed above
which the flaps must not be lowered. The flaps are not designed to withstand
the loads imposed by high speeds. Structural failure may result from severe
strain if the flaps are selected "down" at higher than the specified
When the flaps have been lowered for a landing, they should not
ordinarily be raised until the airplane is on the ground. If a landing has been
missed, the flaps should not be raised until the power has been applied and the
airplane has regained normal climbing speed. It is then advisable to raise the
flaps in stages.
How much flap should be used in landing? Generally speaking, an
airplane should be landed as slowly as is consistent with safety. This usually
calls for the use of full flaps. The use of flaps affects the wing airfoil in
two ways. Both lift and drag are increased. The Increased lift results in a
lower stalling speed and permits a lower touchdown speed. The increased drag
permits a steeper approach angle without increasing airspeed. The extra drag of
full flaps results in a shorter landing roll.
An airplane that lands at 50 knots with full flaps selected may
have a landing speed as fast as 70 knots with flaps up. If a swerve occurs
during the landing roll, the centrifugal force unleashed at 70 knots is twice
what it would be at 50 knots, since centrifugal force increases as the square of
the speed. It follows then, that a slower landing speed reduces the potential
for loss of control during the landing roll. It also means less strain on the
tires, brakes and landing gear and reduces fatigue on the airframe
There are, of course, factors, which at times call for variance
from the procedure of using full flaps on landing. These factors would include
the airplane's all-up-weight, the position of the C.G., the approach path to
landing, the desired rate of descent and any unfavourable wind conditions, such
as a strong cross wind component, gusty winds and extreme turbulence. With
pilot learns to assess these various factors as a guide to flap
In some airplanes, in a crosswind condition, the use of full
flap may be inadvisable. Flaps present a greater surface for the wind to act
upon when the airplane is rolling on the ground. The wing on the side from which
the wind is blowing will tend to rise. In addition, cross wind acting on full
flaps increases the weather vaning tendencies, although in an airplane with very
effective rudder control even at slow speeds, the problem is not so severe.
However, in many airplanes, the selection of full flaps deflects the airflow
from passing over the empennage, making the elevator and rudder surfaces
ineffective. Positive control of the airplane on the ground is greatly hampered.
Since maintaining control of the airplane throughout the landing roll is of
utmost importance, it may be advisable to use less flaps in cross wind
conditions. In any case, it is very important to maintain the crosswind
correction throughout the landing roll.
tab is a small, adjustable
hinged surface on the
trailing edge of the
aileron, rudder, or
elevator control surfaces.
Trim tabs are labour
saving devices that enable
the pilot to release
manual pressure on the
Some airplanes have trim
tabs on all three control
surfaces that are
adjustable from the
cockpit; others have them
only on the elevator and
rudder; and some have them
only on the elevator. Some
trim tabs are the
The tab is moved in the
direction opposite that of
the primary control
surface, to relieve
pressure on the control
wheel or rudder control.
For example, consider the
situation in which we wish
to adjust the elevator
trim for level flight.
("Level flight" is the
attitude of the airplane
that will maintain a
constant altitude.) Assume
that back pressure is
required on the control
wheel to maintain level
flight and that we wish to
adjust the elevator trim
tab to relieve this
pressure. Since we are
holding back pressure, the
elevator will be in the
"up" position. The trim
tab must then be adjusted
downward so that the
airflow striking the tab
will hold the elevators in
the desired position.
Conversely, if forward
pressure is being held,
the elevators will be in
the down position, so the
tab must be moved upward
to relieve this pressure.
In this example, we are
talking about the tab
itself and not the cockpit
Rudder and aileron trim
tabs operate on the same
principle as the elevator
trim tab to relieve
pressure on the rudder
pedals and sideward
pressure on the control
The tabs are usually
controlled by a wheel
which is often situated on
the floor between the two
front seats. Some aircraft
have the trim controlled
by a small rocker switch
on the control column. The
aircraft should be trimmed
after every change in
attitude or power setting.
It takes a little practice
to trim an aircraft,
but in the end it is done
other wing additions
The type of operation for which an airplane is intended has a
very important bearing on the selection of the shape and design of the wing for
that airplane. Wing fences, slots, slats, spoilers, speed brakes and
flaps are additions to the wing that perform a variety of functions
related to control of the boundary layer, increase of the planform area (thus
affecting lift and drag) and reduction of aircraft velocity during landing and
Wing fences are fin-like vertical surfaces attached to the upper
surface of the wing, that are used to control the airflow. On swept wing
airplanes, they are located about two-thirds of the way out towards the wing tip
and prevent the drifting of air toward the tip of the wing at high angles of
attack. On straight wing airplanes, they control the airflow in the flap area.
In both cases, they give better slow speed handling and stall
Slats are auxiliary airfoils fitted to the leading edge
of the wing. At high angles of attack, they automatically move out ahead of the
wing. The angle of attack of the slat being less than that of the mainplane,
there is a smooth airflow over the slat which tends to smooth out the eddies
forming over the wing. Slats are usually fitted to the leading edge near the
wing tips to improve lateral control. The Socata Rallye is an example of a light
aircraft that utilizes leading edge slats.
Slots are passageways built into the wing a short
distance from the leading edge in such a way that, at high angles of attack, the
air flows through the slot and over the wing, tending to smooth out the
turbulence due to eddies.
spoiler on an F4 Phantom wing
Spoilers are devices fitted to the wing which increase drag and
decrease lift. They usually consist of a long narrow strip of metal arranged
spanwise along the top surface of the airfoil. In some airplanes, they are
linked to the ailerons and work in unison with the ailerons for lateral control.
As such, they open on the side of the upgoing aileron, spoil the lift on that
wing and help drive the wing down and help the airplane to roll into a
In some airplanes, spoilers have replaced ailerons as a means of
roll control. The spoiler moves only upward in contrast to the aileron that
moves upward to decrease lift and downward to increase lift. The spoiler moves
only up, spoiling the wing lift. By using spoilers for roll control, full span
flaps can be used to increase low speed lift.
Spoilers can also be connected to the brake controls and. when
so fitted, work symmetrically across the airplane for producing drag and
destroying lift after landing, thereby transferring all the weight of the
airplane to the wheels and making braking action more effective.
RF-84K Thunderflash speed brake
Speed brakes are a feature on some high performance airplanes.
They are a device designed to facilitate optimum descent without decreasing
power enough to shock cool the engine and are especially advantageous in
airplanes with high service ceilings. They are also of use in setting up the
right approach speed and descent pattern in the landing configuration. The
brakes, when extended, create drag without altering the curvature of the wing
and are usually fitted far enough back along the chord so as not to disrupt too
much lift and in a position laterally where they will not disturb the airflow
over the tailplane. They are usually small metal blades housed in a fitting
concealed in the wing that, when activated from the cockpit, pivot up to form a
plate. On some types of aircraft, speed brakes are incorporated into the rear
fuselage and consist of two hinged doors that open into the