Flaps are devices used to improve the lift
characteristics of a wing and are mounted
on the trailing edges of the wings of a fixed-wing
aircraft to reduce the speed at which the
aircraft can be safely flown and to increase
the angle of descent for landing. They shorten
takeoff and landing distances. Flaps do this
by lowering the stall speed and increasing
the drag.
Extending flaps increases the camber or curvature
of the wing, raising the maximum lift coefficient
— the lift a wing can generate. This allows
the aircraft to generate as much lift, but
at a lower speed, reducing the stalling speed
of the aircraft, or the minimum speed at which
the aircraft will maintain flight. Extending
flaps increases drag, which can be beneficial
during approach and landing, because it slows
the aircraft. On some aircraft, a useful side
effect of flap deployment is a decrease in
aircraft pitch angle which lowers the nose
thereby improving the pilot's view of the
runway over the nose of the aircraft during
landing. However the flaps may also cause
pitch-up depending on the type of flap and
the location of the wing.
There are many different types of flaps used,
with the specific choice depending on the
size, speed and complexity of the aircraft
on which they are to be used, as well as the
era in which the aircraft was designed. Plain
flaps, slotted flaps, and Fowler flaps are
the most common. Krueger flaps are positioned
on the leading edge of the wings and are used
on many jet airliners.
The Fowler, Fairey-Youngman and Gouge types
of flap increase the planform area of the
wing in addition to changing the camber. The
larger lifting surface reduces wing loading
and allows the aircraft to generate the required
lift at a lower speed and reduces stalling
speed. Although the effect is similar to increasing
the lift coefficient, increasing the planform
area of the wing does not change the lift
coefficient, which is dependent on the camber,
not the chord.
Physics explanation
The general airplane lift equation demonstrates
these relationships:
where:
L is the amount of Lift produced,
is the air density,
V is the true airspeed of the airplane or
the Velocity of the airplane, relative to
the air
S is the planform area or Surface area of
the wing and
is the lift coefficient, which is determined
by the camber of the airfoil used, the chord
of the wing and the angle at which the wing
meets the air.
Here, it can be seen that increasing the area
and lift coefficient allow a similar amount
of lift to be generated at a lower airspeed.
Extending the flaps also increases the drag
coefficient of the aircraft. Therefore, for
any given weight and airspeed, flaps increase
the drag force. Flaps increase the drag coefficient
of an aircraft due of higher induced drag
caused by the distorted spanwise lift distribution
on the wing with flaps extended. Some flaps
increase the planform area of the wing and,
for any given speed, this also increases the
parasitic drag component of total drag.
Flaps during takeoff
Depending on the aircraft type, flaps may
be partially extended for takeoff. When used
during takeoff, flaps trade runway distance
for climb rate—using flaps reduces ground
roll and the climb rate. The amount of flap
used on takeoff is specific to each type of
aircraft, and the manufacturer will suggest
limits and may indicate the reduction in climb
rate to be expected. The Cessna 172S Pilot
Operating Handbook generally recommends 10°
of flaps on takeoff, especially when the ground
is rough or soft.
Flaps during landing
Flaps may be fully extended for landing to
give the aircraft a lower stall speed so the
approach to landing can be flown more slowly,
which also allows the aircraft to land in
a shorter distance. The higher lift and drag
associated with fully extended flaps allows
a steeper and slower approach to the landing
site, but imposes handling difficulties in
aircraft with very low wing loading. Winds
across the line of flight, known as crosswinds,
cause the windward side of the aircraft to
generate more lift and drag, causing the aircraft
to roll, yaw and pitch off its intended flight
path, and as a result many light aircraft
have limits on how strong the crosswind can
be while using flaps. Furthermore, once the
aircraft is on the ground, the flaps may decrease
the effectiveness of the brakes since the
wing is still generating lift and preventing
the entire weight of the aircraft from resting
on the tires, thus increasing stopping distance,
particularly in wet or icy conditions. Usually,
the pilot will raise the flaps as soon as
possible to prevent this from occurring.
Maneuvering flaps
Some gliders not only use flaps when landing,
but also in flight to optimize the camber
of the wing for the chosen speed. When thermalling,
flaps may be partially extended to reduce
the stalling speed so that the glider can
be flown more slowly and thereby reduce the
rate of sink, which lets the glider use the
rising air of the thermal more efficiently),
and to turn in a smaller circle to make best
use of the core of the thermal. At higher
speeds a negative flap setting is used to
reduce the nose-down pitching moment. This
reduces the balancing load required on the
horizontal stabilizer, which in turn reduces
the trim drag associated with keeping the
glider in longitudinal trim. Negative flap
may also be used during the initial stage
of an aerotow launch and at the end of the
landing run in order to maintain better control
by the ailerons.
Like gliders, some fighters such as the Nakajima
Ki-43 also use special flaps to improve maneuverability
during air combat, allowing the fighter to
create more lift at a given speed, allowing
for much tighter turns. The flaps used for
this must be designed specifically to handle
the greater stresses as most flaps have a
maximum speed at which they can be deployed.
Control line model aircraft built for precision
aerobatics competition usually have a type
of maneuvering flap system that moves them
in an opposing direction to the elevators,
to assist in tightening the radius of a maneuver.
Types
Plain flap: the rear portion of airfoil rotates
downwards on a simple hinge mounted at the
front of the flap. The Royal Aircraft Factory
and National Physical Laboratory in the United
Kingdom tested flaps in 1913 and 1914, but
these were never installed in an actual aircraft.
In 1916, the Fairey Aviation Company made
a number of improvements to a Sopwith Baby
they were rebuilding, including their Patent
Camber Changing Gear, making the Fairey Hamble
Baby as they renamed it, the first aircraft
to fly with flaps. These were full span plain
flaps which incorporated ailerons, making
it also the first instance of flaperons. Fairey
were not alone however, as Breguet soon incorporated
automatic flaps into the lower wing of their
Breguet 14 reconnaissance/bomber in 1917.
Due to the greater efficiency of other flap
types, the plain flap is normally only used
where simplicity is required.
Split flap: the rear portion of the lower
surface of the airfoil hinges downwards from
the leading edge of the flap, while the upper
surface stays immobile. Like the plain flap,
this can cause large changes in longitudinal
trim, pitching the nose either down or up,
and tends to produce more drag than lift.
At full deflection, a split flaps acts much
like a spoiler, producing lots of drag and
little or no lift. It was invented by Orville
Wright and James M. H. Jacobs in 1920, but
only became common in the 1930s and was then
quickly superseded. The Douglas DC-3 & C-47
used a split flap.
Slotted flap: a gap between the flap and the
wing forces high pressure air from below the
wing over the flap helping the airflow remain
attached to the flap, increasing lift compared
to a split flap. Additionally, lift across
the entire chord of the primary airfoil is
greatly increased as the velocity of air leaving
its trailing edge is raised, from the typical
non-flap 80% of freestream, to that of the
higher-speed, lower-pressure air flowing around
the leading edge of the slotted flap. Any
flap that allows air to pass between the wing
and the flap is considered a slotted flap.
The slotted flap was a result of research
at Handley-Page, a variant of the slot that
dates from the 1920s, but wasn't widely used
until much later. Some flaps use multiple
slots to further boost the effect.
Fowler flap: split flap that slides backward
flat, before hinging downward, thereby increasing
first chord, then camber. The flap may form
part of the uppersurface of the wing, like
a plain flap, or it may not, like a split
flap, but it must slide rearward before lowering.
It may provide some slot effect, but this
is not a defining feature of the type. Invented
by Harlan D. Fowler in 1924, and tested by
Fred Weick at NACA in 1932. They were first
used on the Martin 146 prototype in 1935,
and in production on the 1937 Lockheed Electra,
and is still in widespread use on modern aircraft,
often with multiple slots.
Junkers Flap: a slotted plain flap where the
flap is fixed below the trailing edge of the
wing, rotating about its forward edge, and
usually part of the Junkers Doppelflügel,
or "double-wing" style of wing trailing edge
control surfaces, which hung just below and
behind the wing's fixed trailing edge. When
not in use, it has more drag than other types,
but is more effective at creating additional
lift than a plain or split flap, while retaining
their mechanical simplicity. Invented by Otto
Mader at Junkers in the late 1920s, they were
historically most often seen on both the Ju
52/3m airliner/cargo plane, and the Ju 87
Stuka dive bomber, though the same wing control
surface can be also be found on many modern
ultralights.
Gouge flap: a type of split flap that slides
backward along curved tracks that force the
trailing edge downward, increasing chord and
camber without affecting trim or requiring
any additional mechanisms. It was invented
by Arthur Gouge for Short Brothers in 1936
and used on the Short Empire and Sunderland
flying boats, which used the very thick Shorts
A.D.5 airfoil. Short Brothers may have been
the only company to use this type.
Fairey-Youngman flap: drops down before sliding
aft and then rotating up or down. Fairey was
one of the few exponents of this design, which
was used on the Fairey Firefly and Fairey
Barracuda. When in the extended position,
it could be angled up so that the aircraft
could be dived vertically without needing
excessive trim changes.
Zap Flap or commonly, but incorrectly, Zapp
Flap: Invented by Edward F. Zaparka while
he was with Berliner/Joyce and tested on a
General Aircraft Corporation Aristocrat in
1932 and on other types periodically thereafter,
but it saw little use on production aircraft
other than on the Northrop P-61 Black Widow.
The leading edge of the flap is mounted on
a track, while a point at mid chord on the
flap is connected via an arm to a pivot just
above the track. When the flap's leading edge
moves aft along the track, the triangle formed
by the track, the shaft and the surface of
the flap gets narrower and deeper, forcing
the flap down.
Krueger flap: hinged flap, which folds out
from under the wing's leading edge while not
forming a part of the leading edge of the
wing when retracted. This increases the camber
and thickness of the wing, which in turn increases
lift and drag. This is not the same as a leading
edge droop flap, as that is formed from the
entire leading edge. Invented by Werner Krüger
in 1943 and evaluated in Goettingen, Krueger
flaps are found on many modern swept wing
airliners.
Gurney flap: A small fixed perpendicular tab
of between 1 and 2% of the wing chord, mounted
on the high pressure side of the trailing
edge of an airfoil. It was named for racing
car driver Dan Gurney who rediscovered it
in 1971, and has since used on some helicopters
such as the Sikorsky S-76B to correct control
problems without having to resort to a major
redesign. It boosts the efficiency of even
basic theoretical airfoils to the equivalent
of a conventional airfoil. The principle was
discovered in the 1930s, but was rarely used
and was then forgotten. Late marks of the
Supermarine Spitfire used a bead on the trailing
edge of the elevators, which functioned in
a similar manner.
Leading edge droop: entire leading edge of
the wing rotating downward, effectively increasing
camber, but slightly reducing chord. Most
commonly found on fighters with very thin
wings unsuited to other leading edge high
lift devices.
Blown flaps: also known as Boundary Layer
Control Systems, are systems that blow engine
air over the upper surface of any of the previously
mentioned types of flap to improve lift characteristics.
Two types exist - the original type blew air
out of channels or holes in the surface of
the flap, while newer systems simply blow
engine exhaust over the top of the flap. These
require ample reserves of power and are maintenance
intensive thus limiting their use, but they
provide lots of lift at low airspeeds. Although
invented by the British, the first production
aircraft with blown flaps was the Lockheed
F-104 Starfighter. The later type was trialled
on the Boeing YC-14 in 1976.
Controls that look like flaps, but are not:
Handley Page leading edge slats/slots may
be confused for flaps, but are mounted on
the top of the wings' leading edge and while
they may be either fixed or retractable, when
deployed they provide a slot or gap under
the slat to force air against the top of the
wing, which is absent on a Krueger flap. They
offer excellent lift and enhance controllability
at low speeds. Other types of flaps may be
equipped with one or more slots to increase
their effectiveness, a typical setup on many
modern airliners. These are known as slotted
flaps as described above. Frederick Handley
Page experimented with fore and aft slot designs
in the 20s and 30s.
Spoilers may also be confused for flaps, but
are intended to create drag and reduce lift
by "spoiling" the airflow over the wing. A
spoiler is much larger than a Gurney flap,
and can be retracted. Spoilers are usually
installed mid chord on the upper surface of
the wing, but may also be installed on the
lower surface of the wing as well.
Air Brakes are used on high performance combat
aircraft to increase drag, allowing the aircraft
to decelerate rapidly. They may be installed
either on the wings or fuselage and differ
from flaps and spoilers in that they are not
intended to reduce lift and are built strongly
enough to be deployed at much higher speeds.
Ailerons are similar to flaps, but are intended
to provide lateral control, rather than to
change the lifting characteristics of both
wings together, and so operate differentially
- when an aileron on one wing increases the
lift, the opposite aileron does not, and will
often work to decrease lift. Some aircraft
use flaperons, which combine both the functionality
of flaps and ailerons in a single control,
working together to increase lift, but to
slightly different degrees so the aircraft
will roll toward the side generating the least
lift. Flaperons were used by the Fairey Aviation
Company as early as 1916, but didn't become
common until after World War II.
See also
Air brake
Aircraft flight control system
Aileron
Circulation control wing
High-lift device
Leading-edge slats
References
Further reading
Clancy, L.J.. "6". Aerodynamics. London: Pitman
Publishing Limited. ISBN 0-273-01120-0. 
Taylor, H.A.. Fairey Aircraft since 1915.
London: Putnam. ISBN 0-370-00065-X. 
Windrow, Martin C. and René J. Francillon.
The Nakajima Ki-43 Hayabusa. Leatherhead,
Surrey, UK: Profile Publications, 1965.
