GURNEY FLAP.docx

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GURNEY FLAP

seminar report submitted to Mahatma Gandhi University, Kottayam

in partial fulfillment of the requirements for the award of the degree of

BACHELOR OF TECHNOLOGY

IN

MECHANICAL ENGINEERING

Submitted by

SREEJITH KC

(Reg. No. !!"!#"$"%

SNM INSTITUTE OF MANAGEMENT AND TECHNOLOGY

M&'&NK&R&

) *, M**+&KUNN&M, N. )&R&-**R, RN&KU'&M/0123!0

MAHATMA GANDHI UNIVERSITY, KOTTAYAM

NOVEMBER 2014


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DEPARTMENT OF MECHANICAL ENGINEERING

SNM INSTITUTE OF MANAGEMENT AND TECHNOLOGY

M&'&NK&R& ) *, M**+&KUNN&M, N. )&R&-**R,

RN&KU'&M/0123!0

CERTIFICATE

+his is to 4ertify that the seminar report entitled GURNEY FLAP is a

bonafide re4ord of seminar done by SREEJITH.K.C (Register

No.!!"!#"$") in partial fulfillment of the requirements for the award of

degree of 5a4helor of +e4hnology in Me4hani4al ngineering under Mahatma

Gandhi University, Kottayam.

Ass.P!"#. N$!$%$&$& '$$)!) P!"# .G*"!+* T"$s

nternal Guide ead of the 6epartment

7ternal 7aminer nterna l7aminer


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ACKNO-LEDGEMENT

8ith great pleasure, e7press my deep sense of gratitude to

)R*9G*RG +*M&: ;ead of Me4hani4al engineering department&N&N 5&++&+R ;Me4hani4al

engineering department< as my seminar 4oordinator for his guidan4e and

en4ouragement during the 4ourse of this seminar.

e7press my heartfelt gratitude to &sst.)rof. &M&'.-.K

;Me4hani4al engineering department< for his valuable assistan4e and advi4es

for presenting this seminar.

also e7press my than=s to all other fa4ulty members of me4hani4al

engineering department for giving their valuable 4o/operation.

e7press my gratitude to all my friends for their help, 4o/operation

and en4ouragement.

!


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ABSTRACT

& Gurney 9lap is the simplest trailing edge devi4e whi4h 4an be used as a

high lift devi4e for low speed appli4ations li=e mi4ro air vehi4les, gliders,

wind turbines et4. +he Gurney 9lap is named after &meri4an aerodynamist

6an Gurney who introdu4ed it in the form of a verti4al tab atta4hed to the

trailing edge of an ordinary aerofoil. +his modifi4ation ma=es the flap

4apable of produ4ing higher lift for4e at lower velo4ities.

+his paper is based on the e7perimental analysis of Gurney 9lap and from

the results of the e7periment an empiri4al relation for the optimum flap height

has been proposed. +he paper 4ontains a vivid des4ription of the hysterisis

effe4ts of the flow on the flap. +he paper also mentions the advantages,

disadvantages and appli4ations of the flap.

$


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CONTENTS

TITLES

! .&?KN*8'6GMN+@@@@@@@@@@@@@@@@@@@@...i

$. &5:+R&?+@@@@@@@@@@@@@@@@@@@@@@@@@@ii

CHAPTERS

!. ntrodu4tion@@@@@@@@@@@@@@@@@@@@@@@@.!

$ . Need for high lift devi4es@@@@@@@@@@@@@@@@@@...$

2 :tru4ture and 8or=ing of Gurney flap@@@@@@@@@@@@@@2

A . )ressure distributions over the Gurney flap@@@@@@@@@@@@0

3 . 7perimental setup@@@@@@@@@@@@@@@@@@@@.....#

0 . 7perimental pro4edure and results@@@@@@@@@@@@@@...1

0.! -ariation of 4oeffi4ient of lift@@@@@@@@@@@@@..@1

0.$ ffe4t on drag 4oeffi4ient@@@@@@@@@@@@@@@....B

0.2 *verall performan4e@@@@@@@@@@@@@@@@@..!"

0.A 9lap height optimiCation@@@@@@@@@@@@@@@....!!

0.3ysteresis effe4t@@@@@@@@@..@.@@@@@@@...@!$

#. &dvantages of Gurney flaps@@@@@@@@@@@@@@@@@.!1

1. 6isadvantages of Gurney flaps@@@@@@@@@@@@@@@@.!B

B. &ppli4ations of Gurney flaps@@@@@@@@@@@@@@@@@$"

!" .?urrent resear4h areas in Gurney flaps@@@@@@@@@@@@@.$!

!!. ?on4lusion@@@@@@@@@@@@@@@@@@@@@@@....$$

Referen4es @@@@@@@@@@@@@@@@@@@@@@@@.$2

2


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LIST OF FIGURES

9ig 2. ! 4onstru4tion of Gurney flap

9ig 2.$ 4harting the flow

9ig 2.2-orti4es behind the flap

9ig A.! )ressure distribution of Gurney flap and plain aerofoil

9ig 0.! 'ift 4urve for plain aerofoil and Gurney flap

9ig 0.$ 6rag 4urves for plain aerofoil and Gurney flap(ReD03"""%

9ig 0.2 *verall performan4e 4urves

9ig 0.A 'aminar separation bubble

9ig 0.3 :hort bubble hysteresis

9ig 0.0 6istin4tion between long bubble and short bubble hysteresis

9ig. 0.# Glide ratio for 4lean aerofoil (ReD03"""%

9ig. 0.1 Glide ratio for 3.3mm Gurney flap (ReD03"""%

9ig. 0.B Glide ratio for 4lean aerofoil (ReD!!""""%

9ig.0.!" Glide ratio for 3.3mm Gurney flap (ReD!!","""%

A


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CHAPTER 1

INTRODUCTION

igh lift devi4es are one of the most important aerodynami4 devi4es atta4hed

to air4rafts and other flying ma4hines. &s the name indi4ates these are

intended to produ4e higher lift for4e than 4onventional wings or

aerofoils.Generally two types of high lift devi4es are used in pra4ti4e. +he

first type wor=s on the prin4iple of in4reasing the aerofoil geometry, whi4h is

the 4amber of aerofoil. +he se4ond =ind of devi4es wor= on the prin4iple of

energiCing the boundary layer. & gurney flap is a typi4al and simple high lift

devi4e whi4h wor=s on the prin4iple of 4hanging the effe4tive 4amber of the

aerofoil.

Gurney flaps are generally intended to perform at lower speeds. t was

proposed by &meri4an aero dynamist 6an Gurney. &4tually the invention was

in late !B#"s but only now this flap has been subEe4ted to detailed

e7perimental analysis.

~ 1 ~

6ept. of Ma4hani4al engineering :NMM+ Malian=ara


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CHAPTER 2

NEED FOR HIGH LIFT DEVICES

9rom the basi4 prin4iples of aerodynami4s, the lift for4e produ4ed by an

aerofoil is dire4tly proportional to the velo4ity of flow. 9or an air4raft when

landing or ta=e off, the velo4ity is desirable to be lower to redu4e the length

of runway required .5ut for this some additional high lift devi4es has to be

in4orporated in the wings to generate the ne4essary lift for4e ,at lower

velo4ities.

More over the appli4ation of high lift devi4es redu4es the stalling speed of

the air4raft. :talling speed of the air4raft is the minimum speed required to

produ4e the ne4essary lift, so that the air4raft is in equilibrium. & redu4ed

stalling speed ma=es the air4raft to land, ta=e off or even fly at lower speeds.

~ 2 ~



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CHAPTER

STRUCTURE AND -ORKING OF GURNEY FLAP

& gurney flap is in the form of a simple verti4al tab atta4hed to the pressure

side of the trailing edge of the wing as shown below.

9ig 2. ! 4onstru4tion of Gurney flap

n the pi4ture it has been shown that a metal plate bolted to the aerofoil

serves as the verti4al flap. &part from the 4onventional flaps the gurney flap

is not movable whi4h simplifies its appli4a . +he wor=ing prin4iple of the

gurney flap is based on the formation of two 4ounter rotating vorti4es as

shown in fig .$ 9or the proper e7planation of the wor=ing of the flap let us

4onsider the Kutta/Fou=ovs=i equation for the lift for4e developed by an

aerofoil. +hat is the lift for4e developed 'DH-HI

8here Ddensity of fluid

-Dvelo4ity of fluid

ID4ir4ulation around the flap

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9ig 2.$ 4harting the flow

+hus it is 4lear from the equation that as 4ir4ulation or number of vorti4es

in4reases, the lift for4e also in4reases. 5ut in this 4ase the vorti4es are 4ounter

rotating. +hus one vorte7 will produ4e 4ir4ulation su4h that the lift produ4ed

by it gets added to the total lift. +he other vorte7 generates lift in the opposite

dire4tion. 5ut the fa4t is that the useful lift produ4ing vorte7 overshadows the

other one. +hus produ4ing a net higher lift for4e. +he following figure reveals

this.

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9ig 2.2-orti4es behind the flap

n the pi4ture, the useful lift produ4ing vorte7 is the bigger one at the bottom.

+he siCe of the opposite lift produ4ing vorte7 is small. +hus the useful lift

produ4ing vorte7 predominates. &4tually the reason behind its dominan4y is

that the air flowing past the bottom side of the flap suffers more downward

defle4tion than the air flowing through the upper surfa4e due to the presen4e

of flap.

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CHAPTER 4

PRESSURE DISTRIBUTION OVER THE GURNEY FLAP

9ig A.! )ressure distribution of Gurney flap and plain aerofoil

n the figure white dotted 4urve indi4ates the pressure distribution over an

ordinary aerofoil while bla4= dotted 4urve shows pressure distribution over a

gurney flap of height !J of the 4hord length. +he a7is of the 4urve

represents distan4e along the flap as a fra4tion of the 4hord length and > a7is

shows the pressure 4oeffi4ient. f we are ta=ing the pressure differen4e

between upper and lower surfa4es for a gurney flap, it is found to be greater

than the ordinary aerofoil. :in4e pressure differen4e between upper and lower

surfa4es is responsible for the lift, the above 4urves prove that a gurney flap

produ4es higher lift than ordinary aerofoil.

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CHAPTER /

EPERIMENTAL SETUP

+he e7periment on the flap was 4ondu4ted in a low speed open Eet wind

tunnel. *pen Eet wind tunnel was preferred be4ause of the ease of ta=ing

measurements from it. +he test se4tion was a ".A3#m square se4tion and was

!.$m long.+he velo4ity range for the air in tunnel was ranging from AmLs to

!3mLs.+he aerofoil was made of balsa wood and its surfa4e was polished and

4oated with water proof paint +he aerofoil was re4tangular in plan. 9ollowing

were the important dimensions of the aerofoil.

:panD".A3#m

?hordD".!3Am

Ma7imum thi4=ness D!"mm at !3J 4hord

Ma7imum thi4=ness to 4hord ratioD"."03

+he e7perimental setup was in4orporated with a pyramidal balan4e with

digital read out to measure the for4es a4ting on the flap a44urately.

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CHAPTER

EPERIMENTAL PROCEDURE AND RESULTS

&s far as an aerofoil is 4onsidered, the fo4us will be on the lift 4oeffi4ient,

drag 4oeffi4ient and the glide ratio or the lift to drag ratio. +he above

parameters are defined below.

?oeffi4ient of lift ?'D'L(!L$H-$H:%

?oeffi4ient of drag ?6D'L(!L$H-$H:%

Glide ratioD'L6D ?'L ?6

8here, 'Dlift for4e

6Ddrag for4e

Ddensity of fluid

-Dvelo4ity of flow

:Dsurfa4e area of the flap

8e are 4on4erned with the variation of the lift 4oeffi4ient, drag 4oeffi4ient

and glide ratio with the angle of atta4=. 9or that the angle of atta4= was

4hanged in step and for ea4h value of angle of atta4=, the required parameters

were 4al4ulated and the 4urves showing their variation with respe4t to the

angle of atta4= was plotted.

.1 V$!)$)"& "# "*##))*& "# 3)#

+he 4oeffi4ient of lift shows an in4reasing trend with respe4t to the angle of

atta4= till a parti4ular angle of atta4=. &fter that the 4oeffi4ient of lift drops as

the angle of atta4= in4reases whi4h is due to the separation of the flow and

~ 8 ~



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formation of eddies. +he figure below shows the variation of the lift

4oeffi4ient for Gurney flaps with different heights.

9ig 0.! 'ift 4urve for plain aerofoil and Gurney flap.

n the figure the lift 4urves are plotted for plain aerofoil, Amm gurney flap

and Bmm gurney flap. 9rom the figure it 4an be understood that the lift

4oeffi4ient in4reases as the flap height in4reases. Ma7imum lift 4oeffi4ient is

obtained as $.! for the Bmm flap at an angle of atta4= of about !"deg. &fter

this point stalling o44urs and 4oeffi4ient lift de4reases. +he 4urve was plotted

for a Reynolds number of !!",""".

*ne important fa4t that 4an be understood this 4urve is that even after stalling

4ondition a gurney flap produ4es higher lift 4oeffi4ient than a plain aerofoil.

.2 E##* "& !$+ "*##))*&

&s the angle of atta4= in4reases the drag 4oeffi4ient also in4reases. +he rate

of in4rease is low till the stalling angle of atta4= and after that the 4oeffi4ient

of drag drasti4ally in4reases. +his is due to the separation effe4ts and

formation of eddies. 9igure below shows the variation of drag 4oeffi4ient.

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9ig 0.$ 6rag 4urves for plain aerofoil and Gurney flap(ReD03"""%

&s the flap height in4reases, the 4oeffi4ient of drag also in4reases. +hus the

Bmm flap has the largest drag 4oeffi4ient.

. O5*!$33 6*!#"!$&*

f any =ind of aerofoil has to be applied onto flying obEe4ts, the value

of 4oeffi4ients of drag and lift must be 4onsidered simultaneously be4ause .:o

to have an idea about the overall performan4e of the flap ,we go for the ratio

between lift and drag 4oeffi4ients whi4h is 4alled glide ratio.

&lso the aim of evaluating the overall performan4e of the flap is to

have an idea about the optimum height of the flap. +hus for plotting the

overall performan4e 4urve the a7is variable is flap height and the

dependent variables are lift 4oeffi4ient, drag 4oeffi4ient and the glide ratio.

9igure below shows the performan4e 4urves.

~ 10 ~



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9ig 0.2 *verall performan4e 4urves

9rom the 4urves it is 4lear that 4oeffi4ient of lift in4reases more faster than

4oeffi4ient of drag .en4e till a parti4ular flap height we get glide ratio

in4reasing with flap height. &fter that optimum flap height the glide ratio

de4reases and hen4e performan4e degrades.

.4 F3$6 *)+ "6))7$)"&

+his step provides an empiri4al formula for determining the flap height

a44ording to the velo4ity of flow, the 4hord length of flap et4.9rom the

analysis of the performan4e 4urves and the boundary layer height at the

trailing edge of the flap,it was found that the optimum flap height is about the

B"J of the boundary layer thi4=ness at the trailing edge.

+he boundary layer thi4=ness 4an be evaluated from the following relation.

tD".2127 L (Re".$%

8here 7Ddistan4e along 4hord.

~ 11 ~



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&fter substituting for Reynolds number in terms of the appro7imate value of

the vis4osity and density of air and the 4hord length we get the following

relation for optimum flap height.

opt D 2#.!33H4".1 U/".$H!""" mm

8here 4 D 4hord length

UDfree stream velo4ity

Now it is possible to design the gurney flap for any given 4hord length and

flow velo4ity.

./ H%s*!*s)s *##*

ysteresis effe4t 4ause the slopes of 4oeffi4ient 4urves to be different for

in4reasing and de4reasing angle of atta4=s. &4tually the e7periment is

4ondu4ted in two stages. n the first stage angle of atta4= is in4reased in a

stepwise manner and the lift 4urve is plotted. n the se4ond stage the angle of

atta4= is de4reased in a stepwise manner and another lift 4urve is plotted in

the same graph. +he two 4urves will not 4oin4ide due to hysteresis effe4t. +he

hysteresis effe4t is due to a separation phenomenon of the flow 4alled laminar

separation bubble.

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./.1 L$)&$! s*6$!$)"& '8''3*

& laminar separation bubble may be defined as a region of lo4ally separated

flow.

8hen fluid flows past an aerofoil, after some distan4e from the leading edge,

due to the building of adverse pressure gradient , the flow separates. +he

separation o44urs in the transition region of the boundary layer near to the

end of laminar region.

&fter moving some distan4e the flow reatta4hes in the turbulent region of the

boundary layer. +hus in effe4t there e7ists a lo4ally separated region with a

mass of fluid rotating within it 4alled laminar separation bubble. 9igure

below shows a laminar separation bubble.

9ig 0.A 'aminar separation bubble

&t the stalling angle of atta4=, the flow fails to reatta4h and 4omplete

separation of flow o44urs.6epending on the siCe of bubble, it may be 4alled

as a long bubble or short bubble. 9or a long bubble, the separated area will be

larger and it 4auses higher drag on the flap.

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./.2 S"! '8''3* %s*!*s)s

t generally o44urs at the stalling angle of atta4=. 8e already have the glide

ratio 4urve in whi4h the angle of atta4= was in4reasing and the flow separates

4ompletely at stalling angle of atta4=. Now if we de4rease the angle of atta4=,

the reatta4hment of flow ta=es pla4e at a lesser angle of atta4=. +hus a

hysteresis loop is formed at the region of stalling angle of atta4= as shown in

the figure below.

9ig 0.3 :hort bubble hysteresis

n the figure the glide ratio 4urve has been sele4ted for e7planation. 5ut the

lift 4urve and drag 4urve also follow similar trends. n the fig. upstro=e means

in4reasing angle of atta4= and down stro=e means the de4reasing angle of

atta4=.

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./. L"&+ '8''3* %s*!*s)s

+he long bubble hysteresis 4an o44ur at any angle below the stalling angle of

atta4=. n the upstro=e as the angle of atta4= is in4reased, the long bubble

grows larger in siCe due to very adverse pressure gradient. 5ut when the angle

of atta4= rea4hes near the stalling value the long bubble bursts to a 4luster of

short bubbles. :in4e the siCe of bubble be4omes smaller, a slight

improvement in the lift 4oeffi4ient and glide ratio has been observed.

6uring the down stro=e the reunion of short bubbles to long bubble o44urs

only at lesser angle of atta4=s whi4h produ4es a long bubble hysteresis loop

below the stalling angle of atta4=. +he figure below 4learly shows the

different regions of operation of separation bubbles.

9ig 0.0 6istin4tion between long bubble and short bubble hysteresis

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./.4 F*$8!*s "# %s*!*s)s *##*s

&lthough the predi4tion of hysteresis effe4ts is 4umbersomeO it has been

observed that the hysteresis effe4t strongly depends on the geometry of

aerofoil and the Reynolds number of flow, the figures !! to !A reveal that.

9ig. 0.# Glide ratio for 4lean aerofoil (ReD03"""%

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9ig. 0.1 Glide ratio for 3.3mm Gurney flap (ReD03"""%

9ig. 0.B Glide ratio for 4lean aerofoil (ReD!!""""%

9ig. 0.!" Glide ratio for 3.3mm Gurney flap (ReD!!","""%

t is 4lear from the above figures that as Reynolds number in4reases

hysteresis effe4ts be4ome severe and the siCe of hysteresis loops in4reases.

&lso the siCes of hysteresis loops are larger for a plain aerofoil than a gurney

flap. +hus the gurney flap is effe4tive in redu4ing the harmful hysteresis

effe4ts. f hysteresis effe4ts are there in the wings of an aero plane it may be

harmful to its stability.

~ 17 ~



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CHAPTER 9

ADVANTAGES

Up to A"J in4rease in lift

mproved glide ratio

No moving parts

?an easily be fitted

?heap

ysteresis effe4ts are redu4ed to some e7tent

~ 18 ~



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CHAPTER :

DISADVANTAGES

& gurney flap 4annot be applied for very high speed and supersoni4

appli4ations.

Gurney flap 4auses in4reased vibration

&4tually the above disadvantage is due to a phenomenon 4alled vorte7

shedding. t has already been stated that two 4ounter rotating vorti4es

are produ4ed due to the flap. n fa4t, those vorti4es have some

os4illatory 4hara4teristi4s also. :o alternatively lift for4es are produ4ed

in either dire4tion whi4h 4auses vibration. 5ut this 4an be eliminated

by having some additional modifi4ations in the trailing edge.

~ 19 ~



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CHAPTER 10

CURRENT RESEARCH AREAS IN GURNEY FLAPS

&ppli4ation to heli4opter rotors

f a Gurney flap 4an be in4orporated on a heli4opter rotor su44essfully,

the speed of the rotor 4an be redu4ed to produ4e the same lift.

&ppli4ation to delta wing air4rafts

&4tive Gurney flaps for ra4e 4ars

9or ra4e 4ars the speed will be varying throughout the tra4=. :o

the optimum height of the flap =eeps on 4hanging. 5y the spe4ial

material of Gurney flap it 4an be rendered as an a4tive one that is a

flap whi4h is 4apable of 4hanging the height a44ording to the real time

velo4ity of 4ar and thus =eeping the optimum height for all the time.

~ 21 ~



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CHAPTER 11

CONCLUSION

9rom the e7periment 4ondu4ted it has been proved that a Gurney flap

su44essfully wor=s as a high lift devi4e at lower velo4ities. :in4e it is very

easy to be fitted and does not 4ontain any moving parts, it promises a bright

future for mi4ro air vehi4les, wind turbines, gliders et4.

~ 22 ~



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REFERENCES

L.B!"

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