Slotted Wings, Flaps, and High Lift Devices
selective deployment of spoilers and wing flaps on
this Boeing 747 reduced the strength of the trailing vortices.
The wings on most modern-day airplanes
are equipped with control devices not only to steer the airplane, but also
to improve its flying capabilities at specific times, particularly during
landing and takeoff. Devices such as slotted wings and flaps increase lift
when an aircraft requires it the most. They are also excellent examples of
how different people working in isolation from each other can arrive at
similar solutions for a problem simultaneously.
Just before a
wing stalls, the airflow "burbles," or becomes turbulent
over the upper surface of the wing. It reduces the efficiency of the
The design feature generally known as
the slotted wing is a long slot that runs lengthwise along the
wing, either at the leading edge of the wing or at its trailing edge. It
creates greater lift, but also increases drag. It was invented nearly
simultaneously in three different placesóby two individuals working
independently in Germany and also by a research team in the United
In 1917, German pilot G. V. Lachmann
crashed his airplane into the ground after stalling it. A stall happens
when a wing no longer generates sufficient lift to keep the airplane in
the air. This can happen because the plane is travelling too slowly and/or
because the angle of the wing to the airflow is too sharp (such as what
happens in a climb). This latter situation is commonly called too high an
angle of attack. Lachmann was lying in his hospital bed recovering from
his injuries when he started thinking about airplane wings. He surmised
that if a wing was made up of several smaller wings, separated by open
spaces or "slots" that ran straight outward from the fuselage parallel to
each other, then air would flow between the slots at high angles of attack
at low speeds. The wing would act like a group of separate wings, each
operating at a normal angle of attack. In normal, level flight, air would
pass over the slots, not through them, and this slotted wing would act
like a normal wing. As a result, a plane equipped with a slotted wing
would not stall as easily as one without it. Lachmann made some model
tests and applied for a patent for his slotted wing design in February
1918, but his patent was rejected because the patent authorities argued
that the slots would destroy the wing lift. Lachmann had to conduct
further tests to prove his doubters wrong.
Around the same time, the British firm
Handley Page was trying to solve a problem similar to stalling; just
before a wing stalls, the airflow "burbles," or becomes turbulent over
the upper surface of the wing, increasing drag and decreasing lift.
Handley Page engineers tried slots that ran chord-wise, or front to back,
to reduce this burbling. But they soon found that a slot near the leading
edge of the wing and running parallel to the span increased lift
dramatically, by an astounding 60 percent.
Handley Page engineers made a number of
different tests, including a retractable slat (a piece of metal
that ran along the length of the wing from the fuselage) in front of the
wing that could be extended (pushed forward from its position at the front
of the wing) so that a slot would open up between the wing and the slat.
Another design involved a multi-slotted wing that increased lift by 300
percent; it looked like a venetian blind, but it also increased drag and
had other drawbacks. In the meantime, the German Lachmann was ultimately
able to gain a patent for his design and soon joined forces with Handley
At the same time as Lachmann's
theorizing and the Handley Page company's experiments, O. Mader of the
German airplane manufacturer Junkers was also testing a wing design to
reduce burbling and increase lift, but in a slightly different way.
Mader's approach involved mounting an auxiliary airfoil behind the
main wing. It had a larger slot between it and the main wing, running
parallel to the main wing and auxiliary airfoil, but worked in a manner
similar to the Lachmann and Handley Page designs. Junkers incorporated
slotted wings in some of its aircraft. Meanwhile, leading-edge slots were
incorporated into military airplanes in the United States and Britain.
The full impact of the slotted wing
design was not realized until it was incorporated with another lift
device, the flap. Flaps are extensions on the trailing edge of a
wing that the pilot extends during landing and takeoff to increase lift.
When they are extended, they move downward, increasing the camber (making
the wing shape curvier) and forcing the airflow down, providing lift.
Ailerons, developed in 1908 and essentially the first flaps, were large
surfaces on the rear of a wing that provided lateral (sideways) control of
an airplane: lowering an aileron on one wing increased the lift on that
wing and raising the aileron on the other wing decreased it on that wing.
These ailerons were essentially evolved versions of the wing warping
control system that the Wright brothers developed.
flap increases the camber of the wing for the portion of the wing to which
it is attached.
This increases the lift. It also increases the drag.
By 1914, flaps were introduced on the
British S.E.-4 biplane, but pilots rarely used them because they did not
increase performance all that much. By the 1920s, flaps were combined with
slots to provide lift and to eliminate the drag caused by the slots in
normal flight. They were used in some commercial aircraft but remained
rare for several years despite their ability to improve performance. In
1920, Orville Wright and J. M. H. Jacobs invented the split flap,
which consisted of a hinged section on the trailing edge of the underside
of the wing. The split flap was simple and also had the benefit of
increasing drag, which helped a pilot descend toward the runway at a
steeper rate than current wings would allow and thus made landing
are four types of flaps: plain, split, Fowler, and slotted.
In the mid-1920s, Harland D. Fowler, a
U.S. engineer who worked for the Army Air Corps as well as numerous
aircraft manufacturers throughout the decade, used his own time and money
to develop a new kind of flap. Fowler's flap did not simply hinge down
from the wing, but actually slid back from the wing and then rotated down,
while creating a slot between it and the wing. The Fowler flap had
the benefit of actually increasing the wing area in addition to increasing
the wing's lift. Fowler built a wing and tested it on several airplanes in
1927-1929. He had a hard time convincing others that his design was useful
and during the Great Depression, he had to work as a salesman to support
his research. He persisted in pushing his design and ultimately got it
adopted on an unsuccessful Martin bomber and then the Lockheed 14
twin-engine airliner in 1937.
By the 1930s, flaps were incorporated
into many passenger planes and became more sophisticated. The Italian
Piaggio company introduced the double-slotted flap in 1937. As its
name implies, it had two slots, improving performance even more. It
dramatically improved lift, like the Fowler flap, but also increased drag,
making it more useful during landing than takeoff. Its main advantage was
that it was simpler and lighter than the Fowler flap. Another innovation
was the leading-edge flap that extended forward of the wing. Boeing
also introduced the triple-slotted flap with its 727 airliner,
which is still in wide service today, particularly for express package
Today, large commercial passenger jets
have many complicated control systems for improving controllability during
the takeoff and landing phases of flight. A passenger looking out a window
over the wing just before landing will often notice the forward edge of
the wing extending outward and down and the trailing edges extending
backward and down, opening up gaps (the slots) in the wing. This
complicated system of controls represents the highly evolved descendent of
the ideas of Lachmann, Mader, and the engineers of Handley Page during
that remarkably productive period from 1917-1921.