0

Aerodynamics of Wingtip Devices

Simon Li

Glenforest Secondary School

Physics Extended Essay

Total Word Count: 3943

1

Table of Contents

Abstract…………………………………………………………………………….…….2

Introduction…………………...…………………………………………………….……3

Background and Research………………………………………………………………...3

Research Questions…….…………..……………………………………………………..7

Hypothesis………………………………………………………………………….……..7

Experiment Overview……………....……………………………………………………..7

Variables…………………….…………………………………………………………….7

Materials……….………………………………………………………………………….8

Procedure…………………...……………………………..………………………………8

Results……………………………………………………………………………………11

Conclusion and Evaluation………………………………………………………………15

Bibliography…………...……………………………………………………………..….16

2

Abstract

This experiment will investigate into the effectiveness of wingtip devices, also known as

winglets, which help reduce drag by diminishing wingtip vortices and improve fuel

efficiency. The experiment will test three variants of winglets: blended winglets, wingtip

fencings, and raked wingtips. A control wingtip without winglets will also be tested. The

results will be compared against each other, and to real world statistics and applications

to answer the question, To what extent do different typed of winglets affect the strength

of wingtip vortices? The raked wingtip is predicted to be the most effective because by

acting as an extension of the wings, it may have less negative disturbances to the

aerodynamic properties than the other winglets.

This experiment is aimed at proving that wingtip devices do work as intended. It will also

prove the relative effectiveness of the winglets as observed in real world operation. The

experiment will take place in a 1:500 scale. The results will not be extrapolated to match

those of full scale tests. This is due to the limited equipment and budget available for this

experiment. To successfully mimic full scale operation of the winglets in 1:500 scale,

reliable sources such as airframe manufacturer websites and patents will be among the

resources in the research process. These sources will provide detailed dimensions and

allow accurate construction of the mini winglets.

At the end of the investigation, the visualizations of the airflow patterns will provide

undoubted prove that the wingtip devices improve the efficiency of the wings. The raked

wingtip will have the best results in diminishing wingtip vortices. The blended wingtips

and wingtip fencings will show similar, but lower improvements to airflow than the raked

wingtip.

3

Introduction

The aerospace industry is expanding at a faster rate than ever before. The driving

force behind this rapidly developing industry is the constant technological advancements

that make flying a more efficient way of travel. This report will investigate one of these

technologies, wingtip devices. Wingtip devices, also called winglets, were first developed

in the 1970s [Understanding Winglet Technologies, Page 1]. They are designed to reduce

drag formed at the wingtips of aircrafts, thereby increasing fuel efficiency. They way

these wingtip devices manipulate air flow over the wingtips is incredibly intriguing.

Modern aircrafts are equipped with a variety of winglets, each with different structural

features. In this report, these different types of winglets will be compared to determine

how effective they are relative to each other. The experimental data will be compared

with real world applications of these winglets to determine the validity of the results. This

process will answer the question: To what extent do different typed of winglets affect

the strength of wingtip vortices? The raked wingtip is predicted to be the most effective

due to its simplicity in design with less turbulent disturbances to the airflow over the

wingtips.

Background and Research

For many decades after the first powered flight, wingtip designs remained the

same. Wingtips were simply a flat ending of the wing with no added features. However,

as larger and faster airplanes emerge, so do their wake turbulence. Wake turbulence

occurs when an aircraft disturbs the air that it passes through, and leaves a trail of

unstable air behind it. It is discovered that the majority of the wake turbulence generated

by an aircraft does not originate from the inner portions of the wings, but rather the

wingtips [Pendelton]. Heavy aircrafts with large wings can generate wake turbulence

strong enough to flip lighter aircrafts many miles behind it. This hazard has caused many

accidents and raised concerns about manufacturing and designing larger aircrafts to

support the booming demand for air travel.

4

Wake turbulence that result from wingtips of aircrafts is often in the form of

wingtip vortices. The wingtip produces a horizontal rotating column of air that has

enough energy to last for minutes [Pendelton] . The cause of the phenomenon originates

from the fundamental design of aircraft wings. The wings of airplanes are shaped to

allow air above the wing to flow faster than the air beneath it. The curvature of the wings

increases the distance travelled by the air per period of time above the wings. This is an

application of Bernoulli’s law to create an area of low pressure on top of the wing and

high pressure beneath it[Understanding Winglet Technologies, Page 2]. Since high

pressure fluids have a high tendency of flowing towards area of lower pressure, the air

beneath the wing pushes upwards to create lift. However, at the wingtips, higher pressure

air beneath the wing is able to roll around the wingtips to the top of the wing. Relative to

the aircraft, the airstream that initially only had a vector opposite to the aircraft’s motion,

now has an additional rotational vector [Understanding Winglet Technologies, Page 2].

Through vector addition, a rotating column of air forms behind the wingtip, such that the

direction of rotation is towards the fuselage of the aircraft. This phenomenon is shown by

the diagram below.

The escaped air from beneath the wing is no longer generating lift. The reduction of lift

will cause the aircraft to increase its angle of attack, abbreviated AOA. AOA is the angle

of the wings in relation to the oncoming air [Induced Drag]. An increase in AOA means

the aircraft is closer to the stalling angle, where the wings no longer generate lift. Also,

increasing AOA exposed more area of the aircraft to the airstream, resulting in more drag

due to air resistance [Induced Drag]. This drag is known as parasitic drag. Furthermore,

since the lift vector is perpendicular to the wing, the force generated by the wings at a

positive AOA will have two vector components. The first component is lift, vertical and

5

directing upwards. The second component is drag, horizontal and directing towards the

back of the aircraft. The additional drag caused by the increased AOA is known as

induced drag [Induced Drag]. Together, parasitic drag and induced drag make up the total

drag vector on an aircraft during flight, with 60% of total drag being parasitic, and 40%

being induced[Understanding Winglet Technologies, Page 3]. Induced drag is the focus

of this investigation because its main source is the lost lift near the wingtips.

In practice, there are two ways of mitigating induced drag. One way is to increase

the wing span, allowing more lift to be generated by the wings. This increases the lift to

drag ratio of the aircraft. The increased AOA due to wingtip vortices would be reduced,

resulting in less induced drag [Understanding Winglet Technologies, Page 2]. However,

adding wingspan will also add parasitic drag, as more aircraft surface is exposed to the

free stream air. Therefore, an optimal wingspan must be achieved. The second resolution

to the wingtip vortices issue is to create a wingtip device with aerodynamic properties

that limits the formation of wingtip vortices. These devices are known as winglets. In the

1970s, the first winglets were developed by the Whitcomb Company. The early winglets

from Whitcomb revealed over 20% reduction in induced drag[Understanding Winglet

Technologies, Page 1]. Today, there are many variations of winglets being used on

aircrafts around the world. The commercial aircraft sector of the European Aeronautic

Defense and Aerospace Company, also known as Airbus, incorporates wingtip fencing on

many of their production models. These types of winglets are seen on the A320 and the

A380 [balint01]. Wingtip fencing is a plate of aluminum cut

into an arrow like design, shown to the right. It is placed

perpendicular to the wing at the wingtip such that the fencing

extends above and below the wing. This design reduces

airflow rolling around the wingtips and effectively reduces wingtip vortices.

Another variant of winglets was designed contemporary to Airbus’s wingtip

fencings. Boeing developed the blended wingtips, which are currently installed on the

New Generation 737s and the Boeing 767-300 [balint01]. Unlike wingtip fencings,

blended winglets do not extend beneath wing level. The wing of the aircraft goes through

6

a transitional phase at the wingtip, gradually increasing its

angle from horizontal to vertical, as shown to the right. The

vertical section of the winglet extends well above wing level

to increase the effectiveness of the winglet. On the Boeing

767, the blended winglets are 3.4m tall [767 family-

Boeing Company]. The winglets create a barrier between the low pressured air above the

wings, and the. atmospheric pressured air beyond the wingspan. This design also prevents

high pressured air from rolling around the wingtip. Although blended winglets do not

extend beneath the wings like wingtip fencings, Boeing’s experiments determine blended

wingtips are more effective [Understanding Winglet Technology, p.4]. Also, omitting the

lower winglets from beneath the wings save weight, causing reduced stress on the

wingtip. The transitional phase of the winglet minimized the disruption in airflow over

the wings. Furthermore, it is observed that vortices do form at the end of winglets, just

like on conventional wingtips. This phenomenon occurs due to the pressure difference on

either sides of the winglet. However, these vortices are much smaller, and are well above

the airflow over the wings, leaving the flow undisturbed [Airline World]. The experiment

in this report will further investigate into this claim.

In the past decade, Boeing designed another type of winglets, the raked wingtips. They

have been installed on most of Boeing’s aircrafts developed in the 21 st century as a

replacement for blended winglets. Some of the aircrafts operating with raked winglets are

the -400 series of the Boeing 767, Boeing 777, and the newest Boeing aircraft, the 787

[Airline World]. Raked wingtips are similar to an extension of wingspan. Unlike a

normal extension, the sweep angle of the wingtip is greater than that of the wing. The

higher sweep angle along with a sharper tip allows greater lift while minimizing

increased parasitic drag. Furthermore, since more lift is generated further aft of the wing

due to the wingtips, the aircraft will fly with decreased angle of attack. A decrease in

angle of attack will reduce the strength of wingtip vortices, which means less induced

drag. Raked wingtips are becoming more common in modern aircraft design due to their

effectiveness in diminishing induced drag, and their rather simple design [Larry L

Harrick, p.11]. When designing the B767-400, Boeing extended the wingspan by 4.3m

7

from the -300 series to 51.9m, according the specifications of the different variants of

Boeing 767s on the Boeing website [767 family – Boeing Company]. The 4.3m is due to

the addition of the raked winglets. The fuselage diameter is 4.7m. Therefore, the

wingspan without the fuselage, which is not part of the wing, is 51.9−4.7=47.2 m. 4.3m

of the 47m, or approximately 11% of the wings is comprised of raked winglets. This

figure is useful in the experiment when constructing the raked wingtips.

Research question

To what extent do different typed of winglets affect the strength of wingtip vortices?

Hypothesis

It is expected that the raked wingtip will be the most effective out of all of the wingtips

because it is the most advanced type of wingtip in the world.

Experiment design overview

Since the experiment will be conducted with small scale models, it would be inaccurate

to extrapolate the results to attempt to match full scale results. Therefore, the data will

only be compared to each other to determine relative effectiveness. The experiment will

take place inside a small wind tunnel. Model aircrafts with different winglets attached

will be secured in the free stream. Smoke passing through the wind tunnel will be

adjusted to flow over the winglets to produce a visual image of the airflow over the

wingtips. Videos of different experiments will be analyzed carefully to produce the final

results.

Variables

Dependent – Types of winglets

Independent – size of wingtip vortices

Control – Constant wind speed, same aircraft model, constant wind tunnel structure, same

angle of attack

8

Materials

1 extra large piece of 1 inch thick Styrofoam

1 large piece of acrylic glass

1 bag of incense sticks

2 model Boeing 767 (1 with blended winglets, 1 with no winglets)

Table Saw

Duct tape

Liquid Glue

Large piece of cardboard

2 cores of toilet paper rolls

0.5m of string

1 box fan

4 boxes of drinking straws

Black construction paper

Black paint

2mm thin hard plastic

Scissors

Precision knife

Digital Camera

Tripod

Procedure

Building the wind tunnel

1. Cut an area of Styrofoam with length of 1.5m and width greater than that of the

box fan.

2. Secure the Styrofoam board on a table

9

3. Cut two pieces of acrylic glass using the table saw, each with length of 1.5m and a

width 3cm greater than the height of the box fan. These are the walls of the wind

tunnel.

4. Cut another piece of acrylic glass with length of 1.5m and a width greater than

that of the box fan. This is the ceiling of the wind tunnel.

5. Place the box fan at one end of the length of Styrofoam.

6. Mark the edges of the fan on the Styrofoam, and cut a 2cm deep slit along the

length of the board, ensure both slits maintain the same separation along the entire

board. Ensure the slits are not deep enough to cut clean through the Styrofoam

board.

7. Squeeze the two wind tunnel walls into the slits. The walls should be touching the

sides of the fan.

8. Use duct tape along each edge of the fan to secure it to the walls and Styrofoam

base, and to ensure an air tight fit.

9. Tape pieces of Styrofoam on top of the fan if the walls extend beyond the height

of the fan.

10. Cut straws into 5cm lengths

11. Place cut straws parallel to each other and make bundles by taping them together.

12. Place bundles of straws at the opposite end of the wind tunnel from the fan to

create a honeycomb structures. This structure will ensure linear air flow in the

wind tunnel and eliminate any turbulent air entering the tunnel.

13. Fill up the opposite end of the tunnel with straws until the entire end of the tunnel

is blocked by the honeycomb structure.

14. Place the ceiling piece of the wind tunnel over the entire structure.

15. Cut a piece of construction paper 1.5m in length and width the same as the height

of the wall. Paste it onto one of the walls of the tunnel. The black paper will allow

better visualization of the white smoke.

16. Cut another piece of construction paper 1.5m in length and ¼ of the height of the

wall in width. Use white crayon to mark every 2cm along the length and paste it

onto the other wall of the tunnel. This wall is the viewing wall.

10

17. Place aluminum foil over the Styrofoam foundation of the tunnel to prevent a fire

in case incense sticks touch the Styrofoam.

18. Optimize wind speed in the tunnel. If winds are fast, the smoke becomes less

visible. If winds are slow, the smoke is more susceptible to small areas of

turbulence. If the fan blades have only one speed, adjust the size of the outlet by

limiting the size of the intake with the divergence of parts of the fan’s intake to

the air outside the wind tunnel.

Conducting the experiment

1. Take real world dimension of wingtip fencings and raked wingtips and divide by

500 to adjust for the 1:500 aircraft model.

2. Download computer image of the winglets and shrink them to appear with the

same calculated dimensions on the monitor.

3. Trace the outlines of the winglets onto the transparent plastic.

4. Cut the plastic outline.

5. Paste the winglets on the model without winglets.

6. Cut a piece of cardboard with length of the fuselage of the model, and width twice

the diameter of the fuselage. Paste this on top of a toilet paper core

7. Pitch the aircraft model to an angle where the tail touches the ground, then paste it

onto the cardboard.

8. Paint the side of the aircraft facing the observer black, to allow better

visualization of the white smoke patterns.

9. Repeat steps 6-8 with the other aircraft model.

10. Set up camera tripod and zoom for the best view. Ensure this camera position

remains the same for the entire experiment.

11. Light up 4 incense sticks and stick them through the honeycomb structure into the

tunnel. Ensure the smoke will pass through the general area of the wingtip.

12. Place the aircraft with no winglets into the tunnel and turn on the fan.

13. Record a video of at least 15seconds of the smoke patterns.

14. Repeat steps 8 and 9 with the three other winglets installed.

11

15. Analyze the video by comparing the smoke patterns to determine the

effectiveness of each wingtip structure. Using the white reference lines, measure

and compare the period of rotation observed in the smoke pattern. As the smoke

twists, the side view will provide the top and bottom point of rotation. The

distance between these two points is half a period

Results

Upon reviewing the videos of the experiments, a smoke pattern consistently appear with

each test. This pattern resembles a sine wave from a side view. This is the visualization of

wingtip vortices. The half of one rotational period can be measured by the lateral

distance from the minimum to the maximum of the sine wave. A snapshot of the raked

wingtip experiment is shown below. Since a picture does not show the pattern as clearly

as the video, the smoke pattern is outlined in red to provide a better analysis.

Furthermore, the maximum and minimum of the wave is marked to provide more

accurate measurements.

The superimposed red line image is shown below for each winglet test.

With no winglets:

12

Blended Winglets:

Wingtip Fencing:

Raked Winglets:

The maximum and minimum of the rotation is better visualized, so raw data can be collected as

shown below:

Winglet Half Period (cm) ±0.5cm Full period (cm) ±1.0cm

None (Control) 2.0 4.0

Blended Winglet 2.5 5.0

Wingtip Fencing 2.7 5.4

Raked Winglets 4.2 8.4

A graph can be produced from this set of data to see the extent of the impact of different

types of winglets.

13

The results above show a significant change in the energy of the wingtip vortices with

different winglets. The vortices generated by the raked wingtip take twice the distance to

make one full revolution. Since the wind speed in the tunnel is constant, the rotation

speed of the vortices in different test may be compared to each other. The equation d = vt

will be applied to both the rotational vector, and the vector of the wind flow in the tunnel.

The wind speed for different tests is constant, thus v is constant.

Let t be the time it takes the control vortices to make one full rotation.

Blended wingtip period: 5.04.0

t

Wingtip Fencing period: 5.44.0

t

Raked wingtip period: 8.44.0

t

The difference in the diameter of the vortices is very minuscule. Therefore, the vortices

make one full rotation along a circumference of the same size. With distance constant,

relative rotational velocity is calculated.

Let v be the rotational velocity of the control wingtip.

14

Blended wingtip velocity: 4.05.0

v=80 % v

Wingtip Fencing velocity: 4.05.4

v=74 %v

Raked Wingtip velocity: 4.08.4

v=48 % v

Using the energy equation E=12

m v2, Energy in Joules is proportional to the square of

velocity in meters per second. With a 20% reduction in speed, or 80% of the original

speed, rotational energy would experience a 36% reduction. This result means blended

wingtip diminishes the energy lost to wingtip vortices by 36%. Using the same method,

wingtip fencings reduce the energy of wingtip vortices by 45% while blended wingtips

reduce vortices energy by 77%. A loss in the rotational speed in the vortices also means

less volume of air is rolling around the wingtips. More lift is generated and less angle of

attack is required, thus reducing induced drag. These figures can be represented on a

graph, as shown below.

15

It is clear from the graphs that raked wingtip provides the greatest reduction in wingtip

vortices and thus, it is the most effective winglet for diminishing induced drag. The

experimental results reveal a slightly greater effectiveness in wingtip fencing than

blended wingtips. However, due to the uncertainties presented in the data, the advantage

cannot be confirmed. Furthermore, all wingtips showed improvement for reducing

wingtip vortices, which is the result expected before conducting this experiment. This

find further confirms the validity of the method used for this investigation.

To further validate these results, they will be compared to real world application of

different variants of winglets. The similar effectiveness of the blended winglets and the

wingtip fencings replicates the fierce competition between the Boeing 737 with blended

wingtips, and Airbus A320 with wingtip fencings. Both these aircrafts have similar

characteristics and advantages in the aerospace industry, and earned similar numbers of

airline orders. The huge advantage of the raked wingtip revealed in the experiment

explains why the raked wingtip is used on most of Boeing’s new airplanes, driving

blended winglets to obsolescence. Boeing shows raked wingtips generate 5.5% better fuel

economy for the Boeing 777, while blended winglets generates only 3.5% for the Boeing

737 [Airline World]. With close similarity to real world applications, the results in this

experiment can be considered valid.

Conclusion and Evaluation

This experiment effectively answers the research questions: To what extent do different

typed of winglets affect the strength of wingtip vortices? The results determine that raked

wingtips are the most effective at limiting wingtip vortices. This result was exactly what

the thesis predicted. Furthermore, blended wingtips and wingtip fencings also

significantly weakens the vortices, to a lesser degree. The weaker the vortices, the less

drag are induced on the aircraft. To further validate these results, they will be compared

to real world application of different variants of winglets. The similar effectiveness of the

blended winglets and the wingtip fencings replicates the fierce competition between the

Boeing 737 with blended wingtips, and Airbus A320 with wingtip fencings. Both these

aircrafts have similar characteristics and advantages in the aerospace industry, and earned

similar numbers of airline orders. The huge advantage of the raked wingtip revealed in

16

the experiment explains why the raked wingtip is used on most of Boeing’s new

airplanes, driving blended winglets to obsolescence. Boeing shows raked wingtips

generate 5.5% better fuel economy for the Boeing 777, while blended winglets generates

only 3.5% for the Boeing 737 [Airline World]. With close similarity to real world

applications, the results in this experiment can be considered valid.

There are, however, a few aspects of the experiment that could be improved for more

sophisticated testing. There are a variety of equipment improvements if enough funding

is provided. Some improvements include larger models, greater fan speed, longer wind

tunnel, and a smoke wand. These will provide better visualizations of the vortices and

more accurate measurements. For better comparison of the vortices, more camera angles

should be used. Also, if a sophisticated filming program is available, combine the videos

together to better visualize the differences.

This experiment shows how a simple wingtip structure could improve efficiency so

significantly. With 40% of total drag on an aircraft being induced drag, engineers will

continue to research and develop better winglets. One major breakthrough in winglet

design is capable of achieving record breaking fuel efficiency and providing a great

advantage over competing airframes. It will also significantly reduce emission caused by

air travel, and make this mode of transportation the most preferable and economical way

to travel. With the implementation of these winglets in real world, fuel efficiency and

environmental control can be greatly improved.

17

Sources

"767 Family." The Boeing Company. 9 Aug. 2011.

<http://boeing.net/commercial/767family /400/400_3.html>

"Aero 17 - Blended Winglets."The Boeing Company. 8 Aug. 2011.

<http://www.boeing.com

/commercial/aeromagazine/aero_17/winglet_story.html>

balint01. "Aircraft Winglets."Airline world. 8 Aug. 2011.

<http://airlineworld.wordpress.com /2008/10/01/aircraft-winglets/>

"Induced Drag." Professional Pilot. N.p., n.d. Web. 5 Aug. 2011.

<http://selair.selkirk.ca /Training/Aerodynamics/induced.htm>

Larry L Herrick et al. ”Blunt-Leading-Edge Raked Wingtips”. US Patent

6089502. July 18, 2000.

Pendelton, Linda. "Wake Turbulence- An Invisible Enemy." AVweb » The

World's Premier Independent Aviation News Resource. Web. 3 Aug. 2011.

<http://www.avweb.com /news/airman/183095-1.html>

"Understanding Winglet Technology." Smart Cockpit.. 2 Aug. 2011.

<www.smartcockpit.com

/data/pdfs/flightops/aerodynamics/Understanding_Winglets_Technology.p

df>