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Wing Design: Major Decisions

Wing Area / Wing Loading

Span / Aspect Ratio

Planform Shape Airfoils

Flaps and Other High Lift Devices Twist

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Wing Design Parameters

First Level

Span

Area Thickness

Detail Design

Planform shape Taper

Sweep

Tips

Airfoils

Twist

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Factors affecting wing size

Cruise Drag

Stall Speed

Take off and landing distance Maneuver

Instantaneous

Sustained

Fuel Volume

Hangar size

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Span Considerations Climb

Induced drag important at climb airspeeds Greater span good for rate of climb

Cruise

High altitude: induced drag significant, greater spanpreferred

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

Ground Handling

Taxiways and runway lights

Hangar size

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Wing Area

Cruise Drag Low altitude cruise favors high wing loading and low wettedarea.

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

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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Cruise Optimum Wing

For a given altitude and airspeed:

D D D C C

b o d y m isc d w

LS qe A

S q= + + +. 0

2

Differentiating wrt. wing area and setting derivative to zero gives:

d w

L

C

CeA0

2

=

This is the condition for best wing L/D

Accordingly: for optimum cruise: size wing

to fly at best wing L/D

Lopt d wC C eA= 0 S W q eAd wC= /( )0

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Cruise Optimized Wing

Wing Size Effect on L/D

6

7

8

9

10

11

12

13

14

15

16

70 80 90 100 110 120 130 140 150 160 170 180 190 200

Vkeas

L/D

Airplane with wing optimized for given equivalent

airspeed does not fly at best airplane L/D

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Optimum Wing Loading: AR=7, Cd0=.008

0

10

20

30

40

50

60

70

80

100 125 150 175 200 225 250

True Airspeed (Knots )

WingLoading(Po

undsperSquareFo

ot)

Sea Level

7500 Ft.

10,000 Ft.

20,000 Ft.

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Stall Speed Sized Wing

25

30

35

40

45

50

55

60

65

70

75

80

1.3

stallspe

ed(knots)

W/S=30

Clmax=1.4

Clmax=1.6

Clmax=1.8

Clmax= 2.0

Clmax=2.2

W/S=25

W/S=20

W/S=15

W/S=10

W/S=5

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Stall Speed Drives Takeoff Distance

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CRASH SURVIVABILITY

0

10

20

30

40

50

60

70

80

90

40 45 50 55 60 65 70 75 80

SPEED (KNOTS)

IMPAC

TANGLE

SURVIVABLE

IMPACT

NON-SURVIVABLE

IMPACT

W/S=10

STALL

W/S=15W/S=20 W/S=25 W/S=30

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CRASH SURVIVABILITY

0

10

20

30

40

50

60

70

80

90

40 45 50 55 60 65 70 75 80

SPEED (KNOTS)

IMPA

CTANGLE

SURVIVABLE

IMPACT

NON-SURVIVABLEIMPACT

W/S=10

APPROACH W/S=15 W/S=20

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Wing Thickness

Wing weight is strongly affected by thickness,

particularly for cantilever wings.

Thicker is lighter

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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Airfoils

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Airfoil Airflow

Airfoil Generates Lift By

Deflecting Streamlines Downward

Momentum Change in DeflectedAir Causes Pressure Changes

That Act on Wing

Air Flowing Over Upper Surface

at Leading Edge Does Not Meet

Neighbor Air at Trailing Edge

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Flow Very Thick Airfoil

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Flow Over Very Thin Airfoils

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Effect of Airfoil Thickness on Drag

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Typical Thickness Distributions

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Camber Effects

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Effects of Camber on Lift

Angle of Attack

Cl

Forward Camber

Aft Camber

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Effect of Camber on Drag

Effect of Camber on Airfo il Drag Polar

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0.005 0.006 0.007 0.008 0.009

Cd

Cl

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Aerodynamic Center

A point about which pitching moment doesnot vary with angle of attack.

Typically near 25% chord for airfoils inincompressible flow

Moves aft at transonic Mach Numbers

AC is at 50% chord for airfoils at supersonicMach Numbers

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Span Loading

Span loading is comprised of 2 parts: Basic,and Additional:

Basic Span Loading: Span loading when total wing lift=0

Primarily a function of twist and camber

Zero everywhere for untwisted case.

Additional Span Loading Lift due to angle of attack

Linear function of AOA in attached,incompressible flow

Primarily a function of planform (chord distributionand sweep)

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Taper effects

Positive Effects: Thicker Root

Centroid of load moved inboard => reduced

bending moment Lighter Structure

More Volume

Higher Span Efficiency

Not so Positive Effects: Structural Complexity

High local Cl (additional) outboard

Reduced Reynolds number outboard

Poor Stall Characteristics Possible

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Simple Planforms

Constant Chord Straight Taper

C d Pl f

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Compound Planforms

Constant Chord CenterSection (Semi-Tapered)

Compound Taper

S Eff t

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Sweep Effects:

Delayed Drag Rise

Aerodynamic Center Moved Aft

Heavier Structure Increased Additional Loading (both CLC and

Cl) outboard (Decreased for forward Sweep)

Pitch up at stall

Aeroelastic concerns

Induced Drag

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Induced Drag

(Drag Due to Lift)

Induced drag is determined by weight, spanloading, span efficiency.

1/2 of the total drag at best L/D 3/4 of the total drag at max. endurance (min.

power)

Most important in climb and high-altitudecruise.

Induced Drag

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Induced Drag

Wing Deflects A StreamTube of Air To Generate Lift

Stream Tube Diameter is

Approximately the WingSpan

Stream Tube Size is Not

Affected by Wing Chord

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Induced Drag is a Function of Span

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Loading, Not Aspect Ratio

Cdi = CL2/eAR

AR = b2/S => Cdi = CL2 S/e b2

Di = Cdi Sq = CL2

S2

q/

e b2

Multiplying by q/q gives: Di = CL2 S2q2/e qb2

CL2 S2q2=L2 Therefore :

Di

= L2/e qb2

Induced Drag Example: Redhawk

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Induced Drag Example: Redhawk

Redhawk AdvancedTechnology Light Airplane

Built From Cessna Cardinal

New Wing With Advanced

FeaturesHigher Aspect Ratio

Smaller Wing Area

High Lift Devices

Spoilers for Roll Control

New Wing Had Less Span

Than Original

Airplane Was Slower Than

Stock at Cruise Altitude

Very Poor Rate of Climb

R d i I d d D

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Reducing Induced Drag

Reduce Weight

Increase Span

Increase Span Efficiency (e)

Wing Tips Some Improvement possible (~ 5%)

Winglets and End Plates

Induced Drag Decreased

Parasite Drag Increased

Span Extension Usually Superior

Improve Wing Root Junction Flow

Poor Junction causes large loss of span efficiency

Wing Tips

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Wing Tips

Advancing tip:Vortex sheds outboard

Retreating tip:

Vortex sheds inboard

Wing Root Junctions

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Wing Root Junctions

Air can get very confused

Local separation / vortex shedding common

A bad wing root junction increases bothparasite drag and induced drag

Fillets and fairings

Body shaping

Flow Separation at a Wing Root

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p g

Improved Junction Fairing EliminatesSeparation

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Separation

Fully Attached Flow

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y

Stalls and Spins

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Stalls and Spins

Stalls: What is a stall?

Effect of angle of attack Effect of load factor

Airfoil Effects

3D Effects

Airplane Stall Characteristics

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Stalls and Spins

Stall/spin accidents are still a major problem.

Airplane should have good stall warning Gentle stall characteristics are important for

the average pilots safety.

Spin resistant configurations are desirable

Good low-speed flying qualities are important

to stall/spin avoidance

Pilot Induced Fatal Accidents (1994 1998)

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Pilot Induced Fatal Accidents (1994-1998)

stall spin

31%

loss of control

13%

low alt. maneuver

13%

cfit

10%

weather

7%

aerobatics/loc

5%

overstress

4%

midair

3%

pilot incap.

3%

fuel ex.

2%

other

7%takeoff abort

2%

Stall

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Stall is a loss of lift caused by flow separation on theupper (low-pressure) surface of the airfoil.

Stall is affected by:

Airfoil Geometry Planform

Wing Twist

Reynolds Number

Mach Number

For any given geometry, stall is determined by angle

of attack.

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At low angles of attack, the airflow over an airfoil is attached

The flow follows the contours of the skin

Lift varies linearly with angle of attack

At high angle of attack, flow over the uppersurface separates.

Flow no longer follows the upper-surface skin

This causes a loss of upper-surface lift

When the separated region gets large enoughthe airfoil is stalled.

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Wing Stall Progression

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g g

Wing will stall first where local lift coefficientfirst exceeds local Clmax

Stall should start inboard of about 1/3 of the

exposed semi span for acceptable roll

damping at stall.

Stall should develop progressively from rootto tip

Wing Twist

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Washout if tip is at lower AOA than root

Wash-in if tip is at higher AOA than root

Washout is used to control the spanwise

development of the stall. Amount of washout needed depends on planform

Highly tapered wings need more twist

Insufficient washout can cause dangerousroll-off at the stall

Attempting to reduce drag by eliminating washout is

likely to be dangerous

WING WASHOUT OMITTED

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Why do they do it? Perception that wing twist increases drag and

eliminating washout will make the airplane faster.

But its a misconception: Washout actually has very little effect on parasite

drag.

Removing twist may actually hurt span efficiency

and increase drag.

Danger: Airplane is likely to have dangerous stall/spin

behavior.

Roll Damping

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Stable: Lift change opposes roll Unstable: Lift change drives roll

Stall Progression on Wing:

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Good: Root=>Tip

Ailerons effective

Stable Roll Damping

Poor: Full-SpanSudden Stall

Ailerons Weak

Roll-off Likely

Very Poor: Tip => Root

Ailerons Ineffective

Unstable Roll DampingSpin likely

Spin Resistance

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Angle of Attack

Limiting

Discontinuous Leading Edge

Cuffs