Airfoil LiftAn Introduction

Chris Cameron

3/17/10

Table of Contents:

1. Audience description Pg. 3

2. Introduction Pg. 3

3. Generating Lift Pg. 4

3.1 Force Due to Pressure Pg. 4

3.2 Creating the Pressure Difference Pg. 4

4. Drag Pg. 5

5. Variations to Improve Lift to Drag Ratio Pg. 5

5.1 Angle of Attack Pg. 5

5.2 Laminar vs. Turbulent Flow Pg. 6

6. Conclusion Pg. 6

7. Works Cited Pg. 7

1. Audience description:

The audience for this process description is first semester students who are considering aerospace engineering as a major. This audience has a basic understanding of how lift is produced and also of the math and physics behind it. The process described here is a more detailed explanation designed to help them further understand the concept and better decide if they want to pursue aerospace engineering as a major. As such, this process description should be very detailed, but use simple enough terms that someone with a high school level education can understand the material.

2. Introduction:

An airfoil is a specifically shaped body created to produce a force perpendicular to the flow over the body. This force is known as “lift” and is created by a pressure difference between the top and bottom of the airfoil (Figure-1). Airfoils are used in many different roles, from propeller motors to windmills. However we are going to discuss them as used on airplanes to keep them airborne. Airfoils on airplanes are more commonly known as the “wings.” Thrust is produced by various means to propel the airplane forward which creates the flow over the airfoil. Once the velocity of the plane becomes large enough, the lift being produced by the airfoil overcomes the force of gravity and flight is achieved.

Figure-1: Airfoil Producing Lift

3. Generating lift:

The most important concept of an airfoil is how the lift is produced. This can be explained by how pressure differences are created and the force they produce.

3.1 Force Due to Pressure:

As stated above, the lift produced by an airfoil is due to the pressure difference between the top and bottom of the airfoil. The most basic example of pressure creating a force is that in a balloon. You fill the inside of balloon with air which raises the pressure and causes the air to try to escape to the lower pressure outside of the balloon. The skin of balloon restricts it from the air from escaping but is able to deform, stretching and allowing the balloon to become larger. A similar effect is created on an airfoil. The bottom of the airfoil is analogous to the inside of the balloon and the top to the outside. Again the pressure difference is created but because the airfoil more is rigid and cannot be bent easily, as a result, the pressure pushes up on the airfoil instead.

To be more specific, pressure is the force per area acting on a surface. Normally objects are in pressure equilibrium, with equal amounts of pressure pushing in all directions. This causes the effects to cancel out and not be felt. However when a pressure differential is created there are now two forces of different strength acting on opposite sides of the object. The difference between these pressures, multiplied by the area over which they are acting, yields the force due to the pressure difference.

3.2 Creating the Pressure Difference:

Pressure can be broken up into different components, namely the static and dynamic pressures. In the balloon example above, adding the air into the balloon increased its static pressure. Airfoils however make use of dynamic pressure to create their pressure difference. The relationship of static and dynamic pressures is governed by Bernoulli’s equation (shown below).

In this equation the 12ρ v2 is the dynamic pressure while the other two terms relate to the

static pressure. The “v” in the dynamic pressure equation is velocity, and is the key to creating the pressure difference. Airfoils are designed to increase the speed of the air passing over the top surface, while slowing the air on the bottom surface. This difference in velocities causes a much higher dynamic pressure on the top surface. Because the sum of dynamic and static pressure must remain constant, the higher dynamic pressure lowers the static pressure. This static pressure is the one which is felt and a change in which is responsible for the creation of lift.

4. Drag:

One side effect of lift is drag. This is the force that works against the motion of the airfoil trying to “drag” it back with the flow. While drag is associated with friction, it is also produced by the same method of creating the pressure difference on the airfoil. While higher lift is often good, it is usually associated with increased drag. It is important when using airfoils to optimize your lift while keeping drag at a minimum.

5. Variations to Improve Lift to Drag Ratio:

The ultimate goal when using airfoils is to maximize the lift to drag ratio. The two most common methods are varying the angle of attack, and changing between laminar and turbulent flow.

5.1 Angle of Attack:

An important concept for airfoils is the angle of attack. Increasing the angle of attack will at first increase the lift. This is because the airfoil will allow the air to flow less smoothly over its surface creating a higher difference in velocities and therefore pressures as well. However changing the velocity also introduces a loss to the flow. A separation can begin to occur off the back of the airfoil where the flow stagnates (Figure-2). In addition the force generated by an airfoil is always perpendicular to its cord line. As the angle of attack increases the component of the force that is acting as drag also increases. Most importantly, separated flow reduces the efficiency of the airfoil and once the critical angle of attack is reached any further increase in angle of attack will cause lift to be lost. This is known as a “stall.”

Figure-2: Flow Separation

5.2 Laminar vs. Turbulent Flow:

One other important concept concerning airfoils is laminar vs. turbulent flow (Figure-3). Depending on the design of the airfoil, either one of these flows can occur. Laminar flow travels in a straight line with little to no mixing, while turbulent flow has a lot of variation, bouncing around and mixing. A good example of this is a calm stream flowing along for laminar flow, and raging rapids to demonstrate turbulent flow. In a typical situation (slower than the speed of sound) laminar flow is usually the best option for lift. This has very little deviation from general direction of flow, therefore creating the least amount of drag. However there are times when turbulent flow is more useful for creating lift. Turbulent flow becomes useful either at very high speeds or at high angles of attack. This is because turbulent flow has more inertia due to its bouncing nature, which allows the flow to resist separation from the airfoil.

Figure-3: Laminar vs. Turbulent Flow

6. Conclusion:

Airfoils are used to generate lift by creating a pressure difference. This is done by producing different velocities on the top and bottom of the airfoil. The one downside to creating lift is that drag is also produced. By varying the angle of attack the lift to drag ratio can be optimized. Also, turbulent flow can be used when excessive separation would occur in laminar flow.

7. Works Cited:

Figure-1: Courtesy of www.aviation-history.com

Figure-2: Courtesy of wings.avkids.com

Figure-3: Courtesy of www.cvphysiology.com

"Airfoil ." Wikipedia, the Free Encyclopedia. Web. 17 Mar. 2010. <http://en.wikipedia.org/wiki/Airfoil>.

McCormick, Barnes Warnock. Aerodynamics, Aeronautics, and Flight Mechanics. New York, N.Y.: J. Wiley and Sons, 1995. Web.

Wilcox, David C. Basic Fluid Mechanics. La Canada, Calif.: DCW Industries, 2007. Print.