ACD506_Day 3 Aircraft Wing Design

M. S. Ramaiah University of Applied Sciences

1Faculty of Engineering & Technology

Session delivered by:

Dr. H. K. Narahari

Aircraft Wing Design

Session 3



At the end of this session the students will be able to :

Choose Wing loading (W/S) based on different performance requirements Choose the lowest point in the feasible solution space to get lowest Thrust requirement

Design Planform considering its dependence on various design elements

Select a Wing cross-section (airfoil)

Choose appropriate High lift devices In order to reach required (L/D, CD0 etc)

Session Objectives



Overview

The configuration of the wing is fundamental to the design of the aircraft.

Interaction of the many parameters involved in wing design can be described under: Aerofoil section, including the use of high lift devices,

Planform shape and geometry : determined by the operating Mach number of the aircraft and aerofoil shape..

Overall size, that is the wing area : decided by the planform and aerofoil section.



Overview

Wing design starts with Wing loading parameter W/S

Based on historical data (to be refined later)

Constraint Diagram

Selection of Planform

Aspect ratio, root and tip chord

Sweep (LE, quarter chord and TE)

Taper ratio and twist

Selection of airfoil based on dominant requirement

eg. For a passenger a/c choose such that the cruise CL falls in the drag bucket region of airfoil

Select high lift devices accordingly



Airfoil selection : Drivers

C L max at low and higher Mach numbers.

The stalling characteristics where a gentle loss of lift is preferable, especially for light aircraft.

Drag especially in aircraft climb and cruise conditions, when the lift to drag ratio should be as high as possible, and at higher Mach numbers.

The aerofoil pitching moment characteristics which may be particularly important at higher speeds. If it is large there may be a significant trim drag penalty.



Airfoil Selection : Drivers

The depth and shape of the aerofoil it effects the structural design

Affects potential volume for fuel.

The slope of the lift curve as a function of incidence in that it effects overall aircraft attitude, especially at high values of lift coefficient, such as are required

at landing.



Shape parameters

Aerofoil characteristics are determined by several shape parameters of which the most significant are:

maximum thickness to chord ratio (t/c) and its chordwise location. Civil subsonic 8-10 % , supersonic 3-5%

Leading edge or Nose radius impacts C L max Should be larger for subsonic aircraft

Sharp for trans and supersonic aircraft

the degree and distribution of camber, if any is used.

Control surfaces are usually uncambered.



Shape Parameters

Some degree of camber is normal for a wing section as it gives better lift characteristics.

Normal flight is the criteria used for camber choice inverted flight would be possible of course

trailing edge angle, which is often best made as small as is feasible.



Aerofoil Nomenclature



Span Influences the following

Climb Induced drag important at climb airspeeds

Greater span good for rate of climb

Cruise High altitude: induced drag significant, greater span

preferred

Low Altitude: parasite drag dominates, span less important

Weight Increasing span and aspect ratio makes the wing heavier.

Optimum is a compromise between wing weight and induced drag



Wing Area Influences

Cruise Drag Low altitude cruise favors high wing loading and low wetted

area.

Higher altitude cruise favors lower wing loading and greater span.

Takeoff and Landing Increasing wing loading increases takeoff and landing roll

Roll is proportional to the square of the takeoff or landing speed



Wing Area Influences

Maneuvering Favors low wing loading, particularly for instantaneous turn rate.

Stall Speed Most light airplanes wings are sized by stall speed requirements

FAR part 23, Part 103

Survivability



Wing Thickness influences

Wing weight is strongly affected by thickness Thicker is lighter because deeper beams possible

Supersonic wave drag is a strong function of t/c

Variation of parasite drag with wing t/c is small at subsonic, subcritical speeds. Drag is primarily skin friction

Large drag increase if wing gets so thick that flow separates

Thickness taper Wing weight most strongly affected by root depth

Tapering t/c from root to tip can provide lighter wing for given parasite drag.



Wing Sweep

Delayed Drag Rise : postpone transonic drag shoot-up

Aerodynamic Center Moved Aft

Heavier Structure : torsion along with bending

Increased Additional Loading outboard (Decreased for forward Sweep)

Pitch up at stall

Aero-elastic concerns



CL Max estimation

Most 2 D airfoils have a CL max rage between 1.6 to 1.7 (in a few cases could be higher)

Aero foil at the root is usually thicker than tip, so take average between quoted CL max for root that

C L max swept wing = cos(sweep)*[ Cl max (aero foil+ (LE and TE) edge devices

Typical values for LED = 0.65

TE devices vary based on span , chord and type of device



Parameter estimation CL max for landing = 0.6 *( 1.5+ LED + TED) *

cos(sweep)

CL max for take off = 0.8 *( 1.5+ LED + TED )* cos(sweep)

Trailing edge devices are common and simpler Chord length vary from 20 to max 40 %

Twist is approximately equal to = 0.2 * AR ^0.25 * cos (sweep ) ^2



High Lift Devices

High lift devices, as opposed to drag producing devices such as spoilers, function differently : Deflection of the trailing edge and, possibly, leading edge of the

aerofoil to increase the chordwise curvature or camber.

Greater lift results at the expense of more drag and pitching moment.

Extension of the trailing edge and, possibly, leading edge to increase the chord. This effectively increases the wing area and gives higher lift with

relatively small drag penalty.



High Lift Devices

Introduction of slots between the lower and upper aerofoil surfaces. This enhances upper surface flow, delays flow separation,

and again results in more lift potential, but with a drag penalty.

Increase of camber shifts the (C L - ) curve to the left i.e the angle at which lift is zero, o, is more negative.

the AOA at which the aerofoil stalls is slightly reduced,

the maximum lift coefficient is increased.



High Lift Devices

Increase of chord results in more lift at a given angle of attack due to the effectively increased wing area.

Thus relative to the clean wing reference area there is an increase of the slope of the (C L - ) curve.

Slots, especially those in the leading edge region, delay the onset of stall.

There is an upward extension of the (C L - ) curve along its initial slope.



Effect of High Lift devices



Trailing Edge Devices

The simplest systems, such as plain and split flaps, change only the camber of the aerofoil.

More complex concepts, such as multi-slotted or Fowler flaps, not only change camber but also extend the chord.

Trailing edge devices are between 20% 40% of chord.

The maximum angle through which a flap is deflected is between 35 to 45 deg.

Some High lift devices are seen in the next slide



TE Devices



TE Devices 2



Leading Edge devices

How do we decide on LE devices? Howe suggests the following criteria for transport and combat a/c Evaluate F L.E = {(W/S) takeoff / cos ( )}

For Transport a/c if F L.E > 5500 N/m2

For Combat a/c if F L.E > 4000 N/m2

Max increase in C L LED ~= 0.65

Max increase in C L TED higher values are possible



Leading Edge Devices

Plain Hinged Nose section

Kruger Flap : Section moves forward

and outward


26Faculty of Engineering & TechnologyM. S. Ramaiah University of Applied Sciences

26

Wing : 3D effects

A 3D finite wing produces vortex flow as a resultof tip effects (shown in next slide)

The high pressure from the lower surface rolls upat the free end of the finite wing, creating the tipvortex.

This vortex flow generates a downwash,

which is distributed spanwise at varying strengths.

Lift is a reaction force to this downwash

Energy lost in the downwash appears as liftdependent induced drag , D i and its minimization is a goal of aircraft designers.



27

Wing : 3D effects

Downwash decreases for large span wings : aspect ratio

For large AR, flow can be approximated by 2D flow.



28

Effect of 3D effects



29

Typical values for max

The wing tip effect delays the stall by a fewdegrees because the outer-wing flowdistortion reduces the local angle of attack

it is shown as max. is the shift of CL max;

this value of max is determined experimentally.

Typical empirical relationship

max = 2 deg, for AR > 5 to 12,

max = 1 deg, for AR > 12 to 20,

max = 0 deg, for AR > 20.



30

Wing Planform



31

Wing Definition

aspect ratio, AR = (b b)/(b c) = (b2)/(SW)

Sweep Angle



Effect of 3D

Two-dimensional lift values are not obtained on a practical wing of finite span especially when it is swept.

The combination of finite aspect ratio, sweep and taper of the planform causes spanwise flow interactions which increase the effective angle of attack of local chordwise

sections.

this gives rise to a tendency to higher lift coefficients outboard resulting in the possibility of tip stall

Nose-up pitch when sweep is present.



Effect of 3D

Reduction of the local angles of attack outboard relative to the root can overcome this problem.

This may be done by a leading edge device, such as a droop nose, or by built-in geometric properties.

Wash out" is typically equivalent to about 2 o nose down twist at the tip.



Effect of 3D : Taper Ratio

Rectangular wings are easy to fabricate but are aerodynamically in-efficient due to wash out . This can be addressed by different means, one of which is

Taper ratio, defined as ratio between tip & root chord

However taper has some other effects as well It will change the wing lift distribution. such that the

spanwise lift distribution be elliptical.

it taper will increase the cost of the wing manufacture

it will reduce the wing weight, since the center of gravity of each wing section (left and right) will move toward fuselage center line.



Effect of 3D : Taper Ratio

wing mass moment of inertia about x-axis (longitudinal axis) will be decreased. Consequently, this will improve the aircraft lateral control.

taper will influence the aircraft static lateral stability, since the taper usually generates a sweep angle



Effect of 3D : Elliptical Loading

Improves lift efficiency and has other beneficial effects on structural design



Effect of 3D : Part-span

Leading and trailing edge high lift devices cannot occupy all of the actual wing span. further reductions of lift relative to the 2D case.

Leading edge devices are not full span because of: Presence of fuselage.

Shape of wing tip required for good cruise performance which restricts the outboard extremity of the slat.

Possible limitations in the region of engine pylons.

Trailing edge devices are limited by Presence of Fuselage

Need for ailerons



Wing Thickness effects Wing weight is strongly affected by thickness,

particularly for cantilever wings. Thicker is lighter because of deeper beams

Supersonic wave drag is a strong function of t/c,

At subsonic speeds parasitic drag not affected by t/c Low M , drag is primarily skin friction

Large t/c flow separation large drag increase

Thickness taper Wing weight most strongly affected by root depth

Tapering t/c from root to tip can provide lighter wing for given parasite drag.



Effect of t/c on drag polar

C L

C D



Laminar flow Airfoils



Effect of Camber on Airfoils



Effect of Camber on Airfoils

For the same drag penalty, we can sustain higher C L

C L

C D



Sweep angle Improves the wing aerodynamic features (lift, drag, pitching

moment) at transonic, supersonic and hypersonic speeds by delaying the compressibility effects.

Adjusting the aircraft C.G

Improves static lateral stability, but destroys elliptic loading

Impacting longitudinal and directional stability.

Increasing pilot view (especially for fighter pilots).



Typical Sweep angles



Effect Sweep and (t/c ) max

High speed drag rise is related to (t/c) max Sweep reduces effective Mach number on the

wing and postpones drag rise. Typical combinations of sweep and (t/c) max are shown

below



46

Typical Wing t/c Ratios



Subsonic Sweep (t/c) combination

Note : Transport planes fly at M = 0.85 and normally use 10-11% t/c Sweep inthe range 30-35 deg



48

Typical Aspect Ratio and Sweep



Transonic Sweep (t/c) combination



Effect of Wing Taper Ratio

Pros : Thicker Root

Centroid of load moved inboard => reduced bending moment

Lighter Structure, More Volume

Higher Span Efficiency

Cons : Structural Complexity

High local Cl (additional) outboard

Reduced Reynolds number outboard

Poor Stall Characteristics Possible



51

FINITE WING LIFT CURVE SLOPE ( 2p)

Lift curve for a finite wing has a smaller slope than corresponding curve for an infinite wing with same airfoil cross-section

Figure (a) shows infinite wing, ai = 0, so plot is CL vs. ageom or aeff and slope is a0

Figure (b) shows finite wing, ai 0

Plot CL vs. what we see, ageom, (or what would be easy to measure in a wind tunnel), not what wing sees, aeff

1. Effect of finite wing is to reduce lift curve slope

Finite wing lift slope = a = dCL/da 2p

2. At CL = 0, ai = 0, so aL=0 same for infinite or finite wings

ieff aaa



52

CALCULATING CHANGE IN LIFT SLOPE

If we know a0 (infinite wing lift slope, say from data) how can we

find finite wing lift slope, a, for wing with given AR?

eAR

a

aa

d

dC

eAR

a

aC

eAR

CaC

aC

ad

dC

L

L

LL

iL

i

L

pa

p

a

pa

aa

aa

0

0

0

0

0

0

0

1

1

const

const

const

Lift slope definition for infinite wing

Integrate

Substitute definition of ai

Solve for CL

Differentiate CL with respect to a to find lift

slope for finite wing

Note: Equation is in radians



Lift Curve slope High AR & straight wing



Lift Curve slope Low AR & straight wing



Lift curve slope- Swept wings



Typical Wing Geometry (Howe)



Typical Wing Loadings (Howe)



Typical Wing Loadings



59

Typical T/W and W/S Ranges



Subsonic Profile Drag

Drag Coefficient =



Where R is given by the figure shown below



Subsonic Body Profile Drag

S B is max cross section area of body, S S is wetted area and l B /d is body fineness ratio

Where



CL CD various A/c



Drag rise with Mach no



65

Drag rise with Mach no



Guidelines for Choice of CL

For Aspect Ratio > 5 (Transport, Bombers) CL max = const * {CL max airfoil + LE Devices + TE devices } * cos ( 0.25c)

CL max airfoil =1.5 to 1.6, LE Devices =0.6 to 0.65

For Takeoff : Const = 0.8

For Landing : Const=0.6

L/D ~= AR +10 for M



CL required for max R&E



L/D variation : Wing Parameters



Wing parameters : Summary



Session Summary

In this session the following topics were dealt with :

Wing loading (W/S) and its choice based on different performance requirements

Planform design and its dependence on various design elements

Wing cross-section (airfoil) selection

High lift devices and their use.



Thank you !



73

WING LOADING (W/S), SPAN LOADING (W/b) AND ASPECT RATIO (b2/S)

AR

SW

CeqD

D

AR

SW

Sb

SW

Sb

W

SCqb

W

eqD

D

b

W

eqD

SCqD

AR

b

S

W

b

W

D

i

D

i

i

D

2

0,

2

0

2

2

2

2

2

0,

2

0

2

0,0

1

11

1

p

p

p

Span loading (W/b), wing loading (W/S)

and AR (b2/S) are related

Zero-lift drag, D0 is proportional to wing area

Induced drag, Di, is proportional to square

of span loading

Take ratio of these drags, Di/D0

Re-write W2/(b2S) in terms of AR and substitute into drag

ratio Di/D0

1: For specified W/S (set by take-off or landing

requirements) and CD,0 (airfoil choice), increasing AR will

decrease drag due to lift relative to zero-lift drag

2: AR predominately controls ratio of induced drag to zero

lift drag, whereas span loading controls actual value of

induced drag

Documents

ACD506_Day 3 Aircraft Wing Design