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
• 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
Fuselage (Body)
Wing
Horizontal Stabilizer
Vertical Stabilizer Empennage(Tail)
Engine
4 Forces of Flight
Weight
Center of Gravity (CG)
Drag
Newton’s 1st Law Applies
Thrust
Newton’s 3rd Law Applies
Lift
Newton’s 3rd Law Applies
Weight
Lift
Thrust Drag
Think About ItIt’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
Newton’s 2nd and 3rd Laws Apply
Note: DownwashAir Accelerated Down
AOA: Angle Of Attack - the angle that the wing meets the oncoming air.
Airflow Around a Wing
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)
0°
AOA
3°
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.
Which way does the liftingforce actually work?
Net Force
Combining all the forces.
Lift Relates to AOA
Zero Lift at Zero AOA
2D Airflow Over an AirfoilInstitute of Computational Fluid Dynamics
AOA: 0°
Airflow at Selected AOA
AOA: 8°AOA: 20°
Institute of Computational Fluid Dynamics
Wrapping Up Forces & Lift
• Lift = Pressure * Wing Area• Pressure is dependant on:
– 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?
• 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 Descending100’ per minuteDue to Gravity
Glider Climbing400’ per minute
Rising Air500’ per minute
Air Rising: 500’ per minuteGlider Descending: 100’ per minute--------------------------------------------------
Glider Climbing: 400’ 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
• Fly gliders
• Observe difference in flight
• Find physical differences in planes
• Explore why they work
The Gliders
What are the differences?
DihedralBalanced Forces
Unbalanced Forces
Plane slips towards low wing, increasing lift on that wing.
CG Location
Stable
Lift
CG
Lift
CG
Lift
CG
Lift
CG
Down
Down
Lift
CG
Lift
Lift
CGDown
Lift
CG Down
Down
Lift
CG
Lift
Down
Lift
CG
Lift
Down
Lift
CG
Lift
Unstable
Down
Lift
CG
Unstable
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.
Slats(Increase Lift)
Control SurfacesRudder
(Yaw)Elevators(Pitch)
Ailerons(Roll)
Flaps(Increase Lift
and Drag)
Spoilers(Decrease Lift
Increase 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
• “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 Dynamicshttp://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; Cornellhttp://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
• 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 LocationLift
CG DownStable
Down
Lift
CG Unstable