AD of Wing &Highlift

7/28/2019 AD of Wing &Highlift

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SECTION 1. AERODYNAMICS OF LIFTING SURFACES

THEME 7. AERODYNAMICS OF THE WING HIGH-LIFT DEVICES

Swept wings of rather small area with an airfoil of rather small camber and

relative thickness are applied in modern aircraft with the purpose of flight speed

increasing. Such wings can not provide large lift on landing modes because of early

flow stall. The problem of increasing lifting properties for modern wings at high angles

of attack for shortening of take-off and landing distance is very actual now. For this

purpose wings are equipped with special design elements which allow to increase the

value ofC in the area of critical angles of attackya max st . These elements working onmodes of takeoff, landing and maneuver are calledwing high-lift devices.

The set of effective high-lift devices applied in aircraft is wide enough (table 7.1).

There distinguish rigid, jet, combination high-lift devices and high-lift devices based on

the boundary layer control (BLC).

The high-lift devices are installed on the leading and trailing wing edges. The

high-lift devices of the wing trailing edge are realized by flaps of various types (Fig.

7.1): simple flap, one-slotted flap, Fowler extension flap, double-slotted flap, plane flap

etc.

Flaps are applied to increase the lift of an airplane at keeping of its position

(keeping the angle of attack). They are extended while taking off and landing. The lift

grows due to increase of wing camber.

Extension flaps consisting of several sections are used on modern airplanes.

Multi-section configuration allows bending the wing smoothly, and air jets streaming on

the upper surfaces of sections through slots, providing smooth continuous flow at high

angles of sections deflection. The theoretical substantiation of multi-slotted flaps was

given by

S. A. Chaplygin. Such flaps additionally increase lift due to the growth of wing area.

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Fig. 7.1. High-lift devices of the wing trailing edge:

a) - flap Cy h l dev =. . .0 7 ; b) - one-slotted flap;c) - one-slotted extended flap

flap = 30 oCy h l dev =. . .1 1 ;

d) - double-slotted flap Cy h l dev =. . .1 4 ; e) - Fowler flap;f) - plane flap Cy h l dev = . . . .0 8 0 9 .flap = 60 o

An angle between chords of main flap section in deflected and non-deflected

positions is called flap setting flap . It is measured in a plane, perpendicular to axis ofrotation; flap > 0 if flap is deflected downwards.

The flap are used not only for improvement of take-off and landing

characteristics, but also for direct control of lift, rational redistribution of loading which

effects a wing, and also for drag reduction.

The high-lift devices of the wing leading edge are usually made as the deflected

slats (Fig. 7.2): movable slat, Krueger slat, deflecting nose etc.

The slats are intended for prevention of premature flow stalling from wing. It is

reached due to wing camber at the leading edge and jet blowing onto the upper wing

surface through a slot.

An angle characterizing turn of coordinate system related with the slat at its

deflection is called slat setting slat.The slat is the wing-shaped and locates along the wing leading edge. At

increasing of angle under the influence of sucking force the slat is put forward intooperative location.

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Fig. 7.2. High-lift devices of the wing leading edge:

a) - sliding slat; b) - extended slat Cy h l dev = . . . .0 6 0 9 ;c) - deflected nose Cy h l dev = . . . .0 55 0 75 . = 60 o

Choice of high-lift devices in each particular case is determined by such criteria,

as increment of the lift coefficient Cy h l dev. . provided with it (Fig. 7.3, 7.4) andinevitable drag increment. The high-lift devices type allowing to receive the required

take-off and landing characteristics of the airplane should be got out right at the

beginning of the designing process.

Fig. 7.3. Influence of deflection of split flap,

flap and slotted wing onto C fya = ( ) Fig. 7.4. Influence of slat deflection

onto C fya = ( )

The major factor causing an increasing of a wing C factor at deflection of high-

lift devices is the growing of its cross-sections concavity. The growth of C is also

promoted by increase of the wing area at using movable flaps.

ya

ya

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84

C

Let's consider the influence of high-lift

devices deflection of the trailing edge onto

structure of flow about the wing. Comparison

of pressure factor distributions chordwise

at non-deflected and extended flaps (fig. 7.5)

shows, that the flap deflection causes an

essential growth of rarefaction along total

upper wing surface, and not just on its

deflected part. The appreciable increase of

overpressure is observed along the total lower

surface. As a result the lift coefficient

increases.

p

For effective realization of factor C

increasing it is necessary to provide attached

flow about wing with the extended high-lift devices. As it's known, this is promoted by

boundary layer control (BLC) by increasing of kinetic energy of decelerated air layer

(blown off) or its removal from the flow (suction) (Fig. 7.6). The change of dependence

of lift coefficient is similar to slat application (Fig. 7.4). The control system of

circulation

ya

Cy h l dev = . . .0 6 0 8

Fig. 7.5. Pressure factor distribution

along airfoil outline with flap and

without it

. at C = 0 3. , systems with flow blowing-off fromslot on a wing tail part (Fig. 7.7) and system of blower of wing surface by jets from the

engine (Fig. 7.8) are also examples of jet high-lift devices. The intensity of blower

(blowing-off) is characterized by a factor of momentum:

Cm V

q S

kgs

ms

Nm

m

s j

j =

22

, (7.1)

where is the air consumption per second, V is the jet speed, is the wing area

maintained by high-lift devices, q is the dynamic pressure.

ms j Sj


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Fig. 7.6. Systems for boundary layer control Cy h l dev = . . . .0 6 0 8 :a) - suction through a slot, b) - distributed suction through the porous or

punched surface, c) - blow-off from a slot.

Fig. 7.7. Systems with flow blow-off from a slot on wing tail part:

a) - flap with blowing of the upper surface Cy h l dev = . . 7 8 , C 2 ;b) - jet flap Cy h l dev = . . 4 5 ; c) - ejector flap Cy h l dev = . . 6 7, C 2 .

Fig. 7.8. A system of wing surface blowing by engine jets:

) - blowing of the flap upper surface flap = 3 , C 2 , Cy h l dev . . 8 ;b) flap lower surface flap = 40 60o o , Cy h l dev =. . ...6 7.

The spoilers are panels installed on the wing which can be deflected outside to

spoil the flow over the wing. They are made as rotary or extended (fig. 7.9) and

installed both on the upper and on the lower wing surfaces. Spoiler either turbulizes or

stalls the flow depending on altitude of its moving out. The pressure redistributes both

on the upper and on the lower surfaces.

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Fig. 7.9. Spoilers: a) - rotary; b) - extended.

Spoilers are used for roll control (instead of ailerons).

Spoilers are also applied for shortening of run at landing and aborted takeoff. In

such case they are mounted on the wing upper surface directly ahead of flaps and

deflected simultaneously on both wings. It causes flow stalling from the wing upper

surface and high-lift devices. As a result, the lift coefficient C abruptly decreases and

the drag coefficient C grows, loading onto wheels also grows, that allows to increase

braking force considerably. Such spoilers are called ground spoilers. For landing angles

of attack .

y

x

Cy h l dev = . . . . . . .0 7 0 75Generally, a type and span of high-lift devices, wing plan form, panel flap chord

, flap chord , type of wing airfoil and its relative thicknessbflap bflap , etc. influence

. value.Cy h l dev.

For swept wings the effectiveness of high-lift devices is abruptly reduced at

angles close to st . Similar effect is caused by aspect ratio decreasing.

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The table 7.1. High-lift devices.

High-lift devicesIncrease of

maximum lift

Angle of

basic airfoil at

max. liltRemarks

Basic airfoil

- 15

Effects of all high-lift devices

depend on shape of basic airfoil.

Plain or camber

flap

50 % 12

Increase camber. Much drag when

fully lowered. Nose-down pitching

moment.

Split flap

60 % 14

Increase camber. Even more dragthan plain flap. Nose-down pitching

moment.

Zap flap

90 % 13

Increase camber and wing area.

Much drag. Nose-down pitching

moment.

Slotted flap

65 % 16

Control of boundary layer. Increase

camber. Stalling delayed. Not so

much drag.

Double-slotted flap

70 % 18

Same as single-slotted flap only

more so. Treble slots sometimes

used.

Fowler flap

90 % 15

Increase camber and wing area. Best

flaps for lift. Complicatedmechanism. Nose-down pitching

moment.

Double-slotted

Fowler flap

100 % 20

Same as Fowler flap only more so.

Treble slots sometimes used.

Krueger slat50 % 25

Nose-flap hinging about leading

edge. Reduces lift at small

deflections. Nose-up pitching

moment.

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Table 7.1. High-lift devices.

High-lift devicesIncrease of

maximum lift

Angle of

basic airfoil at

max. liltRemarks

Slotted wing

40 % 20

Controls boundary layer. Slight

extra drag at high speeds.

Fixed slat

50 % 20

Controls boundary layer. Extra drag

at high speeds. Nose-up pitching

moment.

Movable slat

60 % 22

Controls boundary layer. Increasescamber and area. Greater angles of

attack. Nose-up pitching moment.

Slat and slotted

75 % 25

More control of boundary layer.

Increased camber and area. Pitching

moment can be neutralized.

Slat and double-

slotted Fowler flap

120 % 28

Complicated mechanisms. The best

combination for lift; treble slots may

be used. Pitching moment can be

neutralized.

Blown flap

80 % 16

Effect depends very much on details

of arrangement.

Jet flap

60 % ?

Depends even more on angle and

velocity of jet.

Note. Since the effects of these devices depend upon the shape of the basic

airfoil, and the exact design of the devices themselves, the values given can only be

considered as approximations. To simplify the diagram the airfoils and the flaps have

been set at small angles, and not at the angles giving maximum lift.

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THEME 8. WING PROFILE DRAG

The profile drag is the sum of surface- friction drag and drag of pressure caused

by pressure redistribution along the streamlined surface due to viscosity influence

(sometimes latter item is called form drag).It is necessary to mean that surface-friction drag is the main part of profile drag of

streamlined bodies (therefore it is often considered that C Cxp x fr). This circumstance

is taken into account in approximate methods ofC calculation. It is possible to adopt,

that does not depend on angles of attack in modes of attached flow and then

calculation of C is performed at

xp

Cxp

xp = 0 (small change of C on angles of attack istaken into account at definition of induced drag, having put an effective aspect ratio

xp

eff , or separate items at polar calculating). In range of Mach numbers less than 4 5... all drag components (wave, induced, profile) can be determined separately from each

other. At that the wave and induced drag are well calculated without the account of

viscosity. However at (zone of hypersonic speeds) there are effects of

viscous interaction, which cause the necessity of the account of viscosity and pressure

mutual influence, that makes wave and profile drag inter-related.

M 4 5.. .

Below we shall consider the method of calculation for streamlined bodies at

(without the account of viscous interaction).M 4 5.. .The most widespread engineering method of C calculation is method CAGI.

According to this method the profile drag is determined as surface-friction drag of a flatplate with introduction of correction multipliers which are taking into account an

additional part of drag from pressure forces. According to CAGI method the wing

profile drag is determined by the formula

xp

C xp f c 2 (8.1)

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where is the drag coefficient of friction of one side of a flat plate in a flow of

incompressible fluid at identical to wing: Reynolds number

f

Re and position of a point

of laminar boundary layer transition into turbulent ; the factor double value takes intoxt


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account flow about the upper and lower surfaces; is the multiplier which is takinginto account a compressibility (Mach number );M c is the factor taking intoaccount contribution of pressure forces into profile drag.

Generally ,f c and are also the function of xt , Re , , M i.e.

( ) f xf tRe, ; ( )c tf c x, ; ( ) tf M x, . At that Re = V l

, where length

of a mean aerodynamic chord is used as characteristic lengthbA l. It is convenient to

write Reynolds number as a function dependent on Mach number and flight altitude

( )Re = =Vb M b f H A A , (8.2)where ( )f H a , is the speed of a sound anda is the kinematic factor ofviscosity are determined under the tables of standard atmosphere depending on flight

altitude. Or

f H H H( ) . + 2 33 1 12 535 102 7 , [ ]m1 (8.3)The most complex and insufficiently investigated is the definition of position of

transition point xT . From the standpoint of drag decreasing it is desirable to have the

body (wing) streamlined completely by laminar flow (i.e. xt = 1). Only profile C andinduced drags exist in subsonic flow. Polar formula is written as

, where C

xp

Cxi

C C AC xa x ya+0 2 Cx0 xp . The parameter is determined asKmaxK

ACxmax = 1

2 0and at this mode C C Cxa x xp=2 20 , i.e. the profile drag is a half of

full drag). However it practically can not be achieved. Any irregularities, rivets, welded

seams etc. are a source of turbulence. As a rule, at a preliminary designing stage the

precise value of xt is not known. Usually one assumes that the body (wing) is

streamlined completely by turbulent flow (xt = 0 ), that overestimates full drag andrequired thrust of the power plant. At actual value ( xt > 0 ) the excess of a thrust(power) is received which can go onto increasing of maneuverable properties of the

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airplane. Nevertheless, it is necessary to note deep researches, which are being

performed on decreasing of C . In case ofxp x = 0 it is possible to assume thefollowing computational formulae forC definition:xp

( )Cf = 0 087

1 62

,

lg Re ,; c c c+ +1 2 9 2 ; c M

M= ++

1 5

1 0 2

2

2.. (8.4)

If the value xt 0 is known, then it is necessary to address to the diagrams. It isalso possible to use approximate formulae (at xt 0 5. ):

( )( )C xf t t

+0 087

1 6

11 33

2

,

lg Re ,

,

Rex ;

c x xce c et t= + + 1 2 92 4 2 4, ; (8.5)

( ) tM

x M c M+ +

+

1

1 0 20 055 1 5

2

2 2

,, .

If there are various sources of turbulence on a streamlined surface (design

superstructures, joints of skin sheets, riveted and welded seams, slot of high-lift devices

of the wing leading edge etc.), then it is necessary to locate the point of transition in a

place of source presence.

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