Transcript Aerodynamics - SlideFinder

Basic Aerodynamics
Misconceptions and Theory
Dr. Paul Kutler, CFI
Tuesday, April 24, 2012
Advanced Flyers
Palo Alto Airport
Airbus 380
An aerodynamics challenge
FA-18 Condensation Pattern
Aerodynamics involves multiple flow regimes
Legacy Aircraft
Aerodynamics is a maturing science
Outline
Terms and Definitions
Forces Acting on Airplane
Lift
Drag
Concluding remarks
Terms and Nomenclature
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Airfoil
Angle of attack
Angle of incidence
Aspect Ratio
Boundary Layer
Camber
Chord
Mean camber line
Pressure coefficient
Leading edge
Relative wind
Reynolds Number
Thickness
Trailing edge
Wing planform
Wingspan
Force Diagram
Airfoil Definitions
Definition of Lift, Drag & Moment
L = 1/2  V2 CL S
D = 1/2  V2 CD S
M = 1/2  V2 CM S c
A Misconception
 A fluid element that splits at the leading edge and
travels over and under the airfoil will meet at the
trailing edge.
 The distance traveled over the top is greater than over the
bottom.
 It must therefore travel faster over the top to meet at the
trailing edge.
 According to Bernoulli’s equation, the pressure is lower on
the top than on the bottom.
 Hence, lift is produced.
How Lift is Produced
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Continuity equation
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Bernoulli’s equation
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Pressure differential
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Lift is produced
The Truth
 A fluid element moving over the top surface leaves
the trailing edge long before the fluid element
moving over the bottom surface reaches the
trailing edge.
 The two elements do not meet at the trailing edge.
 This result has been validated both experimentally
and computationally.
Airfoil Lift Curve (cl vs. )
Lift Curve - Cambered &
Symmetric Airfoils
Slow Flight and Steep Turns
L = 1/2  V2 CL S
Outcome versus Action
Slow Flight
Lift equals weight
Velocity is decreased
CL must increase
 must be increased on the lift curve
Velocity can be reduced until CL max is
reached
Beyond that, a stall results
Slow Flight and Steep Turns
L = 1/2  V2 CL S
Outcome versus Action
(Concluded)
Steep Turns (“Bank, yank and crank”)
Lift vector is rotated inward (“bank”) by the bank
angle reducing the vertical component of lift
Lift equals weight divided by cosine 
Either V (“crank”), CL (“yank”) or both must be
increased to replenish lift
To increase CL, increase  (“yank”) on the lift curve
To increase V, give it some gas
More effective since lift is proportional to the velocity
squared
Stalling Airfoil
Effect of Bank Angle on Stall
Speed
L = 1/2  V2 CL S
 equals the bank angle
At stall CL equals CLmax
L = W / cos 
Thus
Vstall = [2 W / ( CL max S cos )]
1/2
Airplane thus stalls at a higher speed
Load factor increases in a bank
Thus as load factor increases, Vstall increases
This is what’s taught in the “Pilot’s Handbook”
Surface Oil Flow - Grumman Yankee
 = 40, 110 , & 240
Airfoil Pressure Distribution
NACA 0012, M
∞
= 0.345,  = 3.930
Supercritical Airfoil &
Pressure Distribution
Drag of an Airfoil
D = Df + Dp + Dw
D = total drag on airfoil
Df = skin friction drag
Dp = pressure drag due to
flow separation
Dw = wave drag (for transonic
and supersonic flows)
Skin Friction Drag
The flow at the surface of the airfoil adheres to
the surface (“no-slip condition”)
A “boundary layer” is created-a thin viscous
region near the airfoil surface
Friction of the air at the surface creates a
shear stress
The velocity profile in the boundary layer goes
from zero at the wall to 99% of the freestream value
 =  (dV/dy)wall
 is the dynamic viscosity of air [3.73 (10) -7
sl/f/s]
The Boundary Layer
Two types of viscous flows
Laminar
Streamlines are smooth and regular
Fluid element moves smoothly along streamline
Produces less drag
Turbulent
Streamlines break up
Fluid element moves in a random, irregular and
tortuous fashion
Produces more drag
w laminar < w turbulent
Reynolds Number
Rex =  V∞ x / 
Ratio of inertia to viscous forces
Infinite vs. Finite Wings
AR = b2 / S
Finite Wings
The Origin of Downwash
The Origin of Induced Drag
Di = L sin i
Change in Lift Curve Slope
for Finite Wings
Ground Effect
Occurs during landing and takeoff
Gives a feeling of “floating” or “riding on a
cushion of air” between wing and ground
In fact, there is no cushion of air
Its effect is to increase the lift of the wing and
reduce the induced drag
The ground diminishes the strength of the wing
tip vortices and reduces the amount of
downwash
The effective angle of attack is increased and
lift increases
Wing Dihedral ()
Wings are bent upward
through an angle , called
the dihedral angle
Dihedral provides lateral
stability, i.e., an airplane in
a bank will return to its
equilibrium position
This is a result of the lift on
the higher wing being less
than the lift on the lower
wing providing a restoring
rolling moment
Leading Edge Cuff
“Wing Droop”
 A fixed aerodynamic
device
 Droop causes air flow
to attach better to
upper surface
 Stall speed is lowered
 Produces a gentler
stall onset
 Cruise speed is
slightly lower
 Used on Cirrus and
Columbia aircraft
Vortex Generators
A fixed aerodynamic surface consisting of a
small vane or wing that creates a vortex to
delay flow separation and wing stall
The created tip vortex draws energetic air from
outside the boundary layer into the boundary
layer to reduce flow separation
Drag is increased and cruise speed is reduced
Airflow over the control
surfaces is maintained
Stall Strip
A fixed aerodynamic device employed on fixedwing aircraft to modify wing’s characteristics
Ensures wing root stalls before wing tip
Minimizes spinning and gives more aileron
control
Can be used as an alternative to washout
Used to trip the boundary layer at high angles
of attack causing
flow separation
Winglets
Reduced induced drag
Equivalent to extending
wingspan 1/2 of winglet
height
Less wing bending moment
and less wing weight than
extending wing
Hinders spanwise flow and
pressure drop at the wing
tip
Looks modern/esthetically
pleasing
Boeing 737 Winglet
Mach Tuck
Winglets
(Concluded)
Aviation Partners Spiroid Winglet
Reduces fuel by 10%
Drag of a Finite Wing
D = Df + Dp + Dw + Di
D = total drag on wing
Df = skin friction drag
Dp = pressure drag due to
flow separation
Dw = wave drag (for transonic
and supersonic flows)
Di = Induced drag (drag due to
lift)
Drag of a Wing
(Continued)
Induced drag - drag due to
lift
Parasite drag - drag due to
non-lifting surfaces
Profile drag
Skin friction
Pressure drag (“Form drag”)
Interference drag (e.g., wingfuselage, wing-pylon)
Flaps
A Mechanism for High Lift
Effect of Flaps on Lift Curve
High Lift Devices
1.
2.
3.
4.
5.
6.
7.
8.
9.
No flap
Plain flap
Split flap
L. E. slat
Single slotted flap
Double-slotted flap
Double-slotted flap
with slat
Double-slotted flap
with slat and
boundary layer
suction
Not shown - Fowler
flap
What’s Next on the Agenda
Boeing 787 Dreamliner
Boeing 787
What’s Next on the Agenda
Boeing Blended Wing-Body Configuration
Boeing 797
Concluding Remarks
What was not discussed
Transonic flow
Drag-divergence Mach number
Supersonic flow
Wave drag
Swept wings
Compressibility effects
The history of aerodynamics
Airbus 380 Interior
Good aerodynamics results in improved creature comforts
Questions and Answers
Backup Slides
HondaJet
HondaJet
Engine Position
The “Sweet Spot”
Location where the engine coexists with the wing
and enjoys favorable interference effects
The reason - “Transonic Area Rule”
Richard Whitcomb - NASA Scientist
The total cross-sectional area must vary smoothly
from the nose to tail to minimize the wave drag
Wave drag is created by shock waves that appear
over the aircraft as a result of local regions of
embedded supersonic flow
HondaJet
Aerodynamics
Engine inlet is positioned at 75% chord
As the cross-sectional area decreases at the trailing
edge of the wing, the engine adds area thus
yielding a smooth area variation
This engine position also slows the flow and
decreases the wing-shock strength
The critical Mach number is thus increased from
.70 to .73
The pylon is positioned near the outer portion of
the nacelle and cambered inward to follow the flow
direction
During stall, separation starts outboard of the
pylon; separation does not occur between the
pylon and fuselage
HondaJet
Aerodynamics
(Continued)
Natural laminar flow fuselage nose
Following the area rule, the nose expands
from its tip and then contracts as the
windshield emerges.
As the wing is approached, the fuselage
cross-sectional area increases smoothly;
this helps maintain the laminar flow
HondaJet
Aerodynamics
(Concluded)
Natural laminar flow wing
Utilizes integral, machined panels that
minimizes the number of parts for smoother
flow when mated together
Employs winglets to reduce induced drag
30% more efficient than other business jets
Eagle in Flight
cl = 2 L/
 V2 S
Turbulator
Variable
Twist
Adaptive
Dihedral
Winglets
Smart Structures
b/2
c
Tail ?
cd,i = cl2 /
π AR
STOL/VTOL
Capabilities
Variable
Camber
Smooth
Fairings
Elastic Flaps
Tilting
Minimized Noise
Control
& Detectability
Center
Retractable Landing Gear