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Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13”
CFD ANALYSIS OF AIRCRAFT WING FLAPS
1
Mrs.S.Vandaarkuzhali, 2R.Manikandan,
1
AssoProfessor,2PG Students
Department Of Mechanical Engineering,
Mailam Engineering College, Mailam.
E-Mail: [email protected],2 [email protected]
ABSTRACT
A Flap is the simplest trailing edge device which can be used as a high lift device for low
speed applications like micro air vehicles, gliders, wind turbines etc. The Gurney Flap is named
after American aero dynamist Dan Gurney who introduced it in the form of a vertical tab
attached to the trailing edge of an ordinary aerofoil.
This modification makes the flap capable of producing higher lift force at lower
velocities.This paper is based on the ―CFD ANALYSIS OF AIRCRAFT WING FLAPS‖ and
from the results of the experiment an empirical relation for the optimum flap height has been
proposed. And also‖Comparison Between NACA
palf tuohtiw dna htiw liofria 0015‖.The
paper contains a vivid description of the hysteresis effects of the flow on the flap. The paper also
mentions the advantages, disadvantages and applications of the flap.
aerofoil geometry, which is the camber of
1.INTRODUCTION
High lift devices are one of the most
aerofoil. The second kind of devices work
important aerodynamic devices attached to
on the principle of energizing the boundary
aircrafts and other flying machines. As the
layer. A gurney flap is a typical and simple
name indicates these are intended to produce
high lift device which works on the principle
higher lift force than conventional wings or
of changing the effective camber of the
aerofoil. Generally two types of high lift
airfoil.
devices are used in practice. The first type
The Gurney flap is a vertical tab
works on the principle of increasing the
added to the trailing edge on the pressure
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side of a wing. Car racer Dan Gurney is
The first application of the flap was
credited as the inventor in the early 1970s,
in 1971, after Gurney retired from driving
although no patent could be granted. The
and began managing his own racing team
Gurney flap is a simple device, consisting of
full-time. His driver, Bobby Unser, had been
a short strip, on the order of 1–5% of airfoil
testing a new Gurney designed car at
chord in height, fitted perpendicular to the
Phoenix International Raceway, and was
pressure surface or the chord-line along the
unhappy with the car's performance on the
trailing edge of a wing.The most common
track. Gurney needed to do something to
application of this device is in racing-car
restore his driver's confidence before the
spoilers, where it is used to increase the
race, and recalled experiments conducted in
down-force increase in lift and a slight
the 1950s by certain racing teams with
reduction in drag. Larger lift increments
"spoilers" affixed to the rear of the
were observed for greater flap heights, but
bodywork to cancel lift. (At that level of
the
development, the spoilers were not thought
drag
increased
noticeably beyond
heights of approximately 2%C.
of as potential performance enhancers—
merely devices to cancel out destabilizing
2.HISTORY OF GURNEY FLAPS
and potentially deadly aerodynamic lift.)
by
Gurney decided to try adding a "spoiler" to
automobile racing icon Dan Gurney, was a
the trailing edge of the rear wing. The
right-angle piece of sheet metal, rigidly
device was fabricated and fitted in under an
fixed to the top trailing edge of the rear wing
hour, but Unser's test laps with the modified
on his open wheel racing cars of the early
wing turned in equally poor times. When
1970s. The device was installed pointing
Unser was able to speak to Gurney in
upwards to increase down force generated
confidence, he disclosed that the lap times
by the wing, improving traction. He field
with the new wing were slowed because it
tested it and found it allowed a car to
was now producing so much down force that
negotiate turns at higher speed, while also
the car was under steering. All that was
achieving higher speed in the straight
needed was to balance this by adding
sections of the track.
additional down force in front.
The
original
application,
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Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13”
Unser realized the value of this
2.1 GURNEY FLAP
breakthrough immediately and wanted to
―Gurney‖ is a device the teams use to adjust
conceal it from the competition, including
the downforce generated by the wing.
his brother Al. Not wanting to call attention
to the devices, Gurney left them out in the
―Gurney‖, also called a ―Gurney flap‖ or
open. To conceal his true intent, Gurney
―wickerbill,‖ is a trailing edge flap.
deceived inquisitive competitors by telling
Airplanes use flaps for increased lift during
them the blunted trailing edge was intended
take off and landing.
to prevent injury and damage when pushing
the car by hand. Some copied the design,
and some of them even ―improved‖ it by
pointing the flap downwards, which actually
hurt performance.
Gurney was able to use the device in
racing for several years before its true
purpose became known. Later, he discussed
FIG- 1 Gurney flap with trailing edge of
his ideas with aerodynamicist and wing
wing.
designer
The Gurney Flap (or wickerbill) is a small
Bob Liebeck of Douglas Aircraft Company.
flat tab projecting from the trailing edge of a
Liebeck tested the device, which he later
wing. Typically it is set at a right angle to
named the ―Gurney flap,‖ and confirmed
the pressure side surface of the airfoil and
Gurney’s field test results using a 1.25%
projects 1% to 2% of the wing chord This
chord flap on a Newman symmetric airfoil.
trailing edge device can improve the
His 1976 AIAA paper (76-406) ―On the
performance of a simple airfoil to nearly the
design of subsonic airfoils for high lift‖
same level as a complex high-performance
introduced the concept to the aerodynamics
design.
community.
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3.FLEXIBLE
EXTENDED TRAILING
EDGE
A Biologically-Inspired Concept
FIG 2.-Gurney flaps airfoil.
The device operates by increasing
pressure on the pressure side, decreasing
pressure on the suction side, and helping the
FIG 4-Example for flexible extended
boundary layer flow stay attached all the
trailing edge.
way to the trailing edge on the suction side
3.1THEORY OF OPERATION
of the airfoil. Common applications occur in
auto racing, helicopter horizontal stabilizers,
The Gurney flap increases
and aircraft where high lift is essential, such
the
maximum lift coefficient (CL,max), decreases
as banner-towing airplanes.
the angle of attack for zero lift (α0), and
increases the nosedown pitching moment
(CM), which is consistent with an increase in
camber of the airfoil. It also typically
increases
the
drag
coefficient
(Cd),
especially at low angles of attack, although
for thick airfoils, a reduction in drag has
been reported. A net benefit in overall lift to
FIG 3.-Trailing edge flow field for an airfoil
drag ratio is possible if the flap is sized
with Gurney flap .
appropriately based on the boundary layer
thickness
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wings to generate the necessary lift force ,at
lower velocities.
More over the application of high
lift devices reduces the stalling speed of the
aircraft. Stalling speed of the aircraft is the
minimum speed required to produce the
necessary lift, so that the aircraft is in
FIG 5- Wing Mounted in Test Section
equilibrium. A reduced stalling speed makes
the aircraft to land, take off or even fly at
The Gurney flap increases lift by altering the
low speed.
Kutta condition at the trailing edge. The
wake behind the flap is a pair of counter-
3.3.HELICOPTER APPLICATION
rotating vortices that are alternately shed in
Gurney flaps have found wide
a von Kármán vortex street. In addition to
these spanwise vortices shed behind the flap,
application
on
helicopter
horizontal
chordwise vortices shed from in front of the
stabilizers, because they operate over a very
flap become important at high angles of
wide range of both positive and negative
attack.The increased pressure on the lower
angles of attack. At one extreme, in a high-
surface ahead of the flap means the upper
powered climb, the negative angle of attack
surface suction can be reduced while
of the horizontal stabilizer can be as high as
produced lift.
-25°; at the other extreme, in autorotation, it
may be +15°. As a result, at least half of all
3.2NEED FOR HIGH LIFT DEVICE
modern helicopters built in the West have
them in one form or another.
From the basic principles of
aerodynamics, the lift force produced by an
aerofoil is directly proportional to the
velocity of flow. For an aircraft when
landing or take off, the velocity is desirable
to be lower to reduce the length of runway
required .But for this some additional high
lift devices has to be incorporated in the
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low angles of attack. The double gurney flap
reduces the control input required to
transition from hover to forward flight.
3.4PRESSURE DISTRIBUTION OVER
THE GURNEY FLAPS
The device basically operates by
increasing pressure on the pressure side of
the wing, decreasing pressure on the suction
side, and helping the boundary layer flow
T
stay attached all the way to the trailing edge
FIG 6- Gurney flaps in Holicopter for
on the suction side of the airfoil. At the same
time, a long wake downstream of the flap
horizontal stabilizer.
containing a pair of counter-rotating vortices
can delay or eliminate the flow separation
he Gurney flap was first applied to
near the trailing edge on the upper surface
the Sikorsky S-76B variant,when flight
(aircraft wing) or lower surface (racing car
testing revealed the horizontal stabilizer
wing). Correspondingly, the total suction on
from the original S-76 did not provide
the airfoil is increased.
sufficient lift. Engineers fitted a Gurney flap
to the NACA 2412 inverted airfoil to resolve
the
problem
without
redesigning
the
stabilizer from scratch. A Gurney flap was
also fitted to the Bell JetRanger to correct an
angle of incidence problem in the design
that was too difficult to correct directly.
The Eurocopter AS355 TwinStar
FIG 6- Effect of Gurney flaps.
helicopter uses a double Gurney flap that
projects from both surfaces of the vertical
stabilizer. This is used to correct a problem
3.5.DIMENSIONS OF FLAPS
with lift reversal in thick airfoil sections at
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The experiment on the flap was
(1)
Mass is conserved ,
(2)
Newton’s second law ( force = mass
conducted in a low speed open jet wind
* acceleration ),
tunnel. Open jet wind tunnel was preferred
(3)
because of the ease of taking measurements
Energy is conserved.
These
fundamental
physical
from it. The test section was a 0.457m
principles can be expressed in terms of basic
square section and was 1.2m long.The
mathematical equations, which in their most
velocity range for the air in tunnel was
general form are either integral equations or
ranging from 4m/s to 15m/s.The aerofoil
partial differential equations. Computational
was made of balsa wood and its surface was
fluid dynamics is the art of replacing the
polished and coated with water proof paint
integrals or the partial derivatives ( as the
The aerofoil was rectangular in plan.
case may be ) in these equations with
Following were the important dimensions of
discredited algebraic forms, which in turn
the aerofoil.
are solved to obtain numbers for the flow

¢-Span=0.457m

¢-Chord=0.154m

¢ -Maximum thickness =10mm
field values at discrete points in time and/or
space. The end product of CFD is indeed a
collection of numbers, in contrast to a
closed-from analytical solution. However, in
at 15% chord

the long run, the objective of most
¢-Maximum thickness to chord
engineering
ratio=0.065
analyses,
closed
form
or
otherwise, is a quantitative description of the
problem, i.e., numbers. The instrument
The experimental setup was incorporated
which has allowed the practical growth of
with a pyramidal balance with digital read
CFD is the high-speed digital computer.
out to measure the forces acting on the flap
CFD
accurately.
solutions
generally
require
the
repetitive manipulation of many thousands,
even millions, of numbers, a task that is
4.INTRODUCTION TO CFD
humanly impossible without the aid of a
To answer this question, we note that
computer. Therefore, advances in CFD, and
the physical aspects of any fluid flow are
its application to problems of more and
governed by three fundamental principles:
more
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and
sophistication,
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Proceedings of the “National Conference on Emerging Trends In Mechanical Engineering 2k13”
intimately related to advances in computer
the flow field in a reasonable time. As a
hardware, particularly in regard to storage
result of these factors, computational fluid
and execution speed. This is why the
dynamics is established industrial design
strongest force driving the development of
tool, helping to reduce design timescales and
new supercomputers is coming from the
improve
CFD community.
engineering world. CFD provides the cost-
the
process
throughout
the
effective and accurate alternative to scale
4.1 History of CFD
model testing, with variations on the
Computers have been used to solve
simulation
fluid flow problems for many years.
being
performed
quickly,
offering obvious advantages.
Numerous programs have been written to
4.2 Application of CFD
solve either specific problems, or specific
classes of problems, or specific classes of
CFD is the analysis of systems
problem. From the mid-1970’s the complex
involving fluid flow, heat transfer and
mathematics required to generalize the
associated phenomena such as chemical
algorithms began to be understood, and
reaction by means of computer based
general
were
simulation. This technique is powerful and
developed. These began to appear in the
spans a wide range of industrial and non-
early 1980’s and required what were then
industrial application areas. Some examples
very powerful computers, as well as an in-
are:
purpose
CFD
solves
depth knowledge of fluid dynamics, and

Aerospace

Automobile and Engine

Industrial Manufacturing

Naval Architecture

Civil Engineering
models mean that the process of creating a

Environment Application
CFD model and analyzing the result is much

Health and Safety
large amounts of time to set up simulations.
Consequently CFD was a tool used almost
exclusively in research. Recent advances in
computing power, together with powerful
graphics and interactive 3-D manipulation of
4.3 Methodology of CFD
less labour-intensive, reducing the time and
there the cost. Advanced solvers contain
CFD codes are structured around the
algorithms which enables robust solution of
numerical algorithms that can tackle fluid
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flow problem. All codes contain three
up. Remember Bernoulli The slower air on
elements:
top is at a higher pressure and presses down
on the wing surface. The force a wing

Pre-processor

Solver

Post-processor
produces depends on the airfoil shape, the
area of the wing, and the square of its speed
through the air.
4.4 Pre-processor:

It consists the input of a flow problem
to a CFD program by a user. The user
activities at the pre-processing stage
involve.

Definition of the geometry of the
On the second picture is a racing car wing
region of interest: the computational
at a high angle of attack. At high angles of
domain
attack, air is unable to follow the contour of

Grid generation
the lower wing surface and can detach

Selection
of
the
physical
(stall), lowering the efficiency (downforce)
and
of
chemical phenomena that need to be
the
wing
and
adding
drag.
modelled

Definition of fluid properties

Specification of appropriate boundary
conditions.
A small lip on the trailing edge,
The first picture shows a racing car
shown in the third picture, causes a lower
wing which generates downforce or negative
pressure just behind it which sucks the lower
lift as it moves through the air. The air has
flow back up to the wing surface. The
to accelerate to go around the lower side of
Gurney flap causes some extra drag, but the
the wing and loses pressure when it speeds
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wing can be run at a higher angle of attack
There are mainly three distinct
and produces more downforce.
streams of numerical solution techniques;
finite element finite difference and spectral
Designers can only use limited
methods. In outline the numerical methods
amount of the wing on a racecar because of
that from the basis of solver perform
rules limiting the number and dimensions of
wings. Side pods and tires get in the way
and they just can't be left out
5.COMPARISION WITH AND WITHOUT
GURNEY FLAP
5.1
WITHOUT
GURNEY
FLAP:
5.2 LIFT AND DRAG FOR WITHOUT
GURNEY FLAP
a)DRAG FORCE VECTOR:
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zone name
Pressure
(N)
Upper side of 8823.7261
force Viscous
Total force (N)
Total coefficient
2827.6292
11651.355
0.58711792
2827.6292
11975.162
0.60343472
5608.9031
23626.518
1.1905526
force(N)
the airfoil
Lower side of 9193.8884
the airfoil
Net
18017.615
b)LIFT FORCE VECTOR:
zone name
Pressure
force Viscous force(N) Total force (N)
Total coefficient
(N)
Upper side of 468804.26
6.4666261
468810.73
23.623619
-0.71912061
-468656.44
-23.615844
5.7475055
154.28458
0.007774481
the airfoil
Lower side of -468655.72
the airfoil
Net
148.53707
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5.3 WITH GURNEY FLAP
5.4 LIFT AND DRAG FOR WITH
GURNEY FLAP
a) DRAG FORCE VECTOR:
zone name
Pressure
(N)
Gurney
flap 75014.62
force Viscous
Total force (N)
force(N)
Total
coefficient
33.325835
75047.946
3.7817054
3375.5778
-25474.154
-1.283656
2163.026
21308.691
1.0737561
5571.9296
70882.482
3.5718055
portion
Upper side of -28849.732
the airfoil
Lower side of 19145.665
the airfoil
Net
65310.553
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b.)LIFT FORCE VECTOR:
zone name
Pressure
force Viscous
(N)
Total coefficient
-26.104107
-15950.99
-0.80377875
56.97324
1094942.1
55.17471
-1.4946828
189102.24
9.5289615
29.37445
1268093.4
63.899893
force(N)
flap -15924.886
Gurney
Total force (N)
portion
Upper side of 1094885.2
the airfoil
Lower side of 189103.74
the airfoil
1268064
Net

5.5. AIRFOIL DATA

velocity=180m/s

airfoil =0015 ACAN

angle of attck is zero
Application to helicopter rotors .
If a Gurney flap can be incorporated
on a helicopter rotor successfully, the
dna
speed of the rotor can be reduced to
4 % of
produce the same lift.
chord
6.CONCLUSION

Application to delta wing aircrafts

Active Gurney flaps for race cars
The analysis carried out on the chord at the
For race cars the speed will be
trailing edge of the airfoil with NACA
varying throughout the track. So the
airfoil with and without flap 0015.The above
optimum height of the flap keeps on
figures are comparisons between naca
changing. By the special material of
distributed at the pressure in upper and
Gurney flap it can be rendered as an
lower surface have equal pressure shown in
active one that is a flap which is
figure. The pressure distribution in the upper
capable of changing the height
surface less one unlike shown in figure and
according to the real time velocity of
angle of attack is zero.
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car and thus keeping the optimum
height for all the time.
REFERENCE
1. L.Brown and A.Filippone (2003),Aerofoil
at low speeds with Gurney Flaps, The
Aeronautical Journal, No.2800, pages 539 to
546
2.
L J Clancy, Aerodynamics, Longman
Group,
1996
Edition
3.http://aerodyn.org/HighLift/gurney.html
4. http://www.allamericanracers.com
5.http://www.as.go.dlr.de/Transsonium.
6. http://www.cfd.tu-berlin.des
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