Aerodynamics - SlideFinder
Download
Report
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
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
•
Continuity equation
•
Bernoulli’s equation
•
Pressure differential
•
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