Airfoil

Understanding the Airfoil

Mr. Presti

Northport Middle School Technology

What is aerodynamics?

The word comes from two Greek words: aerios = concerning the air. dynamis = meaning powerful.

Aerodynamics is the study of forces and the resulting motion of objects through the air.

4 Forces of Flight

1. Gravity (Weight): Pulls the plane down. Gravity pulls everything near or on Earth towards the center of Earth.

2. Drag: Holds the plane back. Drag is the resisting force called friction. Friction will continue to push against objects until it stops. Water and air cause friction.

4 Forces of Flight (Continued)

4. Thrust: Pushes the plane forward. Forward driving force of a propeller attached to an engine, or the forward force produced by gases escaping rearward from a jet or rocket engine.

3. Lift: Pushes the plane up. Slower air on the underside of the wing creates more air pressure than the upper curved part of the wing (faster moving air)…creating lift.

Forces Acting on a Plane

Tests the Effects

To test the effects of pressure and airspeed, take a regular piece of notebook paper and hold it up like the girl at the left.

Blow over it and watch what happens.

When you blow across an airfoil you are increasing the speed at which the air moves across the top of the airfoil. This decreases the pressure and causes lift.

Straight and Level Flight

In order for an airplane to fly straight and level, the following relationships must be true:

Thrust = Drag Lift = Weight Q1 - What would happen if Drag is greater than Thrust?

Q2 - What would happen if Lift is greater than Weight?

A Few Words About Fluid• A principal concept in aerodynamics is the idea

that air is a fluid (like all gases, air flows and behaves in a similar manner to water and other liquids).

• Basic aerodynamic tests are sometimes performed underwater.

• Lift and Drag can exist only in the presence of a moving fluid (object is stationary and the fluid is moving OR fluid is still and the object is moving through it).

A Few Words About Fluid

Consequently, neither lift nor drag can be created in space (where there is no fluid). This explains why spacecraft don't have wings unless the spaceship spends at least some of its time in air.

Who’s Theory on Lift Is Correct?

1. Bernoulli – Focused his studies on the curvature of the wing, and the differing air pressure over the top and bottom of the wing.

2. Newton – Focused his studies on the deflection of air or fluid on an object and its reaction. (Newton’s 3rd Law)

How Lift is Created (Bernoulli)

• The Longer Path Explanation: holds that the top surface of a wing is more curved than the bottom surface. Air particles that approach the leading edge of the wing must travel either over or under the wing.


• Let's assume that two nearby particles split up at the leading edge, and then come back together at the trailing edge of the wing.

• Since the particle traveling over the top goes a longer distance in the same amount of time, it must be traveling faster.


• Bernoulli's equation, a fundamental of fluid dynamics, states that as the speed of a fluid flow increases, its pressure decreases.

• The Longer Path explanation deduces that this faster moving air develops a lower pressure on the top surface, while the slower moving air maintains a higher pressure on the bottom surface.

Why is it not entirely correct?(HAND-OUT) (Bernoulli)

There are several flaws in this theory, although this is a very common explanation found in high school textbooks and even encyclopedias:

The assumption that the two air particles described above rejoin each other at the trailing edge of the wing is groundless. In fact, these two air particles have no "knowledge" of each other's presence at all, and there is no logical reason why these particles should end up at the rear of the wing at the same moment in time.

For many types of wings, the top surface is longer than the bottom. However, many wings are symmetric (shaped identically on the top and bottom surfaces). This explanation also predicts that planes should not be able to fly upside down, although we know that many planes have this ability.

How Lift is Created (Newtonian)

• Newton’s 3rd Law: For every action there is an equal, and opposite, reaction.

• You can see a good example of this by watching two skaters at an ice rink. If one pushes on the other, both move -- one due to the action force and the other due to the reaction force.

Wing

Air Particles

How Lift is Created (Newtonian)

• Each individual air particle bounces off the bottom surface of the wing and is deflected downward.

• As the air particles strike the bottom surface of the wing, they impart some of their momentum to the wing, thus incrementally nudging the wing upward with every molecular impact.

Wing

Air Particles

Why is it not entirely correct?(HAND-OUT) (Newtonian)

The Newtonian explanation provides a pretty intuitive picture of how the wing turns the air flowing past it, with a couple of exceptions:

The top surface of the wing is left completely out of the picture. The top surface of a wing contributes greatly to turning the fluid flow. When only the bottom surface of the wing is considered, the resulting lift calculations are very inaccurate.

Almost a hundred years after Newton's theory of ship hulls, a man named Leonhard Euler noticed that fluid moving toward an object will actually deflect before it even hits the surface, so it doesn't get a chance to bounce off the surface at all. It seemed that air did not behave like individual shotgun pellets after all. Instead, air molecules interact and influence each other in a way that is difficult to predict using simplified methods. This influence also extends far beyond the air immediately surrounding the wing.

Angle of Attack

The angle of attack is the angle that the wing presents to oncoming air.

This angle controls the amount of lift that the wing generates (although it is not the only factor).

What other factors control lift?

Air Plane Parts

Problem Statement

To design a model airplane that can fly as high as possible and indefinitely with a constant power source.

Building Your Plane

1. Research (Generate Ideas)

2. Fuselage Construction

3. Vertical/Horizontal Stabilizers

4. Motor Mount

5. Airfoil Construction (Wing Design)

6. Control Surfaces

7. Test

Resources (Links) Airplane Parts with worksheet (quiz).

http://www.grc.nasa.gov/WWW/K-12/BGA/Dan/airplane_parts_act.htm

Wing Geometry and Definitionshttp://www.grc.nasa.gov/WWW/K-12/airplane/geom.html

Wing Area Problemshttp://www.grc.nasa.gov/WWW/K-12/BGA/Melissa/wing_area_act.htm

Inventing Flight for Schoolshttp://www.inventingflightschools.org/index.html

Wright Brothers (Simulator, Movie Clips, Story Board)http://dsc.discovery.com/convergence/wright/wright.html

History of Flight (Activity Sheet)

1. When did the Wright brothers achieve their “historic” flight? (be specific)

2. Where did the historic flight take place?

3. Why did they bring their “flying machine” to this particular site? (3 reasons)

4. Describe their “flying machine”? (size, weight, engine, ect..)

5. How many times were they able to achieve flight that day?

6. What was the distance of their first attempt, and how long was it up in the air for?

7. What was the furthest distance they were able to travel that day?

http://sln.fi.edu/flights/first/during.html

Airfoil

Technology

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