why airplanes fly

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