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Why Airplanes Fly
Aerodynamics
Institute of Computational Fluid Dynamics
Aerodynamics?
• Aero – means Air
• Dynamics – means Motion
• Aerodynamics – the study of air in motion
• Important Point: Air is “Viscous”
Uh, that means it’s Sticky
Forces and Vectors
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A force may be thought of as a push or pull in a specific direction.
A force is a vector quantity so a force has both a magnitude and a direction.
When describing forces, we have to specify both the magnitude and the
direction.
Airplane Parts
Vertical Stabilizer
Empennage
(Tail)
Wing
Horizontal Stabilizer
Engine
Fuselage (Body)
4 Forces of Flight
Weight
Center of Gravity (CG)
Drag
Newton’s 1st Law Applies
Thrust
Newton’s 3rd Law Applies
Lift
Lift
Thrust
Drag
Weight
Newton’s 3rd Law Applies
Think About It
It’s “Tug-a-War”
The motion of the airplane through the air
depends on the relative strength and direction of
the forces we’ve discussed.
If the 4 forces are balanced, the aircraft cruises
at constant velocity and altitude.
If the forces are unbalanced, the aircraft
accelerates in the direction of the largest force.
More on Lift … Lift in Depth
Streak Lines 10° AOA
AOA: Angle Of Attack - the angle that the wing meets the oncoming air.
Note: Downwash
Air Accelerated
Down
Newton’s 2nd and 3rd Laws Apply
Airflow Around a Wing
AOA
0°
3°
Notice that:
• Air on top arrives well before
air below. “accelerated”
(stretched in the diagram)
• Air below decelerated (arrives
after “free stream”)
(compressed in the diagram)
8°
Circulation Pattern
“Bound Vortex”
Pressure Field
Result of the accelerated flow on top
and decelerated flow on bottom.
Forces on the Airfoil
Forces act along the entire surface.
Net Force
Which way does the lifting
force actually work?
Combining all the forces.
Lift Relates to AOA
Zero Lift at Zero AOA
2D Airflow Over an Airfoil
Institute of Computational Fluid Dynamics
Airflow at Selected AOA
Institute of Computational Fluid Dynamics
AOA: 0°
8°
20°
Wrapping Up Forces & Lift
• Lift = Pressure * Wing Area
• Pressure is dependant on:
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Airfoil Shape
Wing Velocity through the air
Angle that wing meets air (AOA)
Air density (more dense = more pressure)
• Newton’s
3rd Law?
“For
every Action
there is and Equal and Opposite Reaction”
The more air moved down by the wing (Action)
the more lift is generated (Reaction)
“Equal and Opposite”
Flight Without Thrust?
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Lift is dependant on Velocity
Velocity is generated by Thrust
Lift is dependant on Thrust
No Thrust = No Lift
If Thrust is so critical to generate Velocity
Then … How do GLIDERS FLY
Gliders
Glider Climbing
400’ per minute
Glider Descending
100’ per minute
Due to Gravity
Air Rising: 500’ per minute
Glider Descending: 100’ per minute
-------------------------------------------------Glider Climbing: 400’ per minute
Rising Air
500’ per minute
Stability and Control
• Add cool graphics
Stability and Control
Aircraft Motion
Aircraft move in 3D (dimensions)
(Cars move in just 1D)
• Pitch (up and down)
• Yaw (left and right)
• Roll (well uh, roll)
• ALL motion occurs around the balance point,
called the Center of Gravity (CG)
Glider Demonstration
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Fly gliders
Observe difference in flight
Find physical differences in planes
Explore why they work
The Gliders
What are the differences?
Dihedral
Balanced Forces
Unbalanced Forces
Plane slips towards low wing,
increasing lift on that wing.
Lift
Lift
CG
CG
CG Location
Down
Stable
Lift
CG
Unstable
Down
Stable vs. Unstable
• Stability is not always desirable.
• Why?
• Where would you want:
– Stable airplanes?
– Unstable airplanes?
Control
• A simple matter of affecting the forces
that we have just discussed!
• Control results from manipulating various
control surfaces on the plane to change
the forces.
Control Surfaces
Rudder
Spoilers
(Decrease Lift
Increase and Drag)
Elevators
(Yaw)
(Pitch)
Ailerons
(Roll)
Slats
(Increase Lift)
Flaps
(Increase Lift
and Drag)
Aerodynamic Myths
1. Biplanes have 2 wings to get twice the lift.
2. “Air Pockets” cause airplanes to drop
suddenly while flying.
3. Lift is generated by the wing’s top, curved,
surface which causes low pressure.
4. Bumblebees can’t fly.
Russian Helicopter
Questions?
References
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“The Beginner’s Guide to Aeronautics”, NASA
http://www.grc.nasa.gov/WWW/K-12/airplane/
“IRROTATIONAL PLANE FLOWS OF AN INVISCID FLUID”;
Colombini, Marco; UNIVERSITY OF GENOA
http://www.diam.unige.it/~irro/
Institute of Computational Fluid Dynamics
http://www.icfd.co.jp/index-e.htm
See How It Flies; Denker, John S.
http://www.av8n.com/how/#mytoc
“Bumblebees finally cleared for takeoff”; Wang, Z. Jane; Cornell
http://www.news.cornell.edu/releases/March00/APS_Wang.hrs.html
DNS of Separated Flow around NACA0012 Airfoil; Shih, et al. (1992,
1995); University of Texas
http://www.uta.edu/faculty/hshan/research/gallery.shtml
Bumblebees Do Fly
BACKUP MATERIAL
FoilSim
Items to Bring
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Gliders
Glue
Demo plane
Propellers
Vortex Generation
DNS of Separated Flow around NACA0012 Airfoil
Stream Lines
Streak Lines -12° AOA
Streak Lines 0° AOA
Velocity Vectors
2D Airflow Over an Airfoil
CG Location
Lift
CG
Stable
Down
Lift
CG
Unstable
Down