ACD506_Day 3 Aircraft Wing Design
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Transcript of ACD506_Day 3 Aircraft Wing Design
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M. S. Ramaiah University of Applied Sciences
1Faculty of Engineering & Technology
Session delivered by:
Dr. H. K. Narahari
Aircraft Wing Design
Session 3
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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
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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.
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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
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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.
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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.
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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.
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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.
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Aerofoil Nomenclature
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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
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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
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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
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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.
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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
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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
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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
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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.
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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.
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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.
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Effect of High Lift devices
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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
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TE Devices
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TE Devices 2
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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
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Leading Edge Devices
Plain Hinged Nose section
Kruger Flap : Section moves forward
and outward
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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.
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Wing : 3D effects
Downwash decreases for large span wings : aspect ratio
For large AR, flow can be approximated by 2D flow.
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Effect of 3D effects
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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.
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Wing Planform
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Wing Definition
aspect ratio, AR = (b b)/(b c) = (b2)/(SW)
Sweep Angle
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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.
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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.
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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.
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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
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Effect of 3D : Elliptical Loading
Improves lift efficiency and has other beneficial effects on structural design
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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
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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.
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Effect of t/c on drag polar
C L
C D
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Laminar flow Airfoils
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Effect of Camber on Airfoils
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Effect of Camber on Airfoils
For the same drag penalty, we can sustain higher C L
C L
C D
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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).
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Typical Sweep angles
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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
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Typical Wing t/c Ratios
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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
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Typical Aspect Ratio and Sweep
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Transonic Sweep (t/c) combination
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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
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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
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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
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Lift Curve slope High AR & straight wing
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Lift Curve slope Low AR & straight wing
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Lift curve slope- Swept wings
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Typical Wing Geometry (Howe)
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Typical Wing Loadings (Howe)
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Typical Wing Loadings
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Typical T/W and W/S Ranges
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Subsonic Profile Drag
Drag Coefficient =
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Where R is given by the figure shown below
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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
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CL CD various A/c
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Drag rise with Mach no
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Drag rise with Mach no
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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
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CL required for max R&E
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CL required for max R&E
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L/D variation : Wing Parameters
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Wing parameters : Summary
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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.
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Thank you !
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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