Aerodynamics Lab 6A This lab tends to take a lot of time. For this reason, the lab lectures (aerodynamics and propulsion) are given during a normal class period before the actual lab. Winter Quarter

Airfoil Terminology This is some of the terminology typically associated with airfoils. Point out the listed features, and explain that the camber of an airfoil is the maximum distance between the mean or “camber line” and the chord line (in this diagram that is the horizontal line immediately below the camber line.) The amount of camber an airfoil has is important, and will be described in detail later. Winter Quarter

Angle of Attack The Angle of Attack is the angle that is formed between the chord line of an airfoil and the Relative Wind velocity vector. In this lab, the Relative Wind vector can be taken as being parallel to the ground (since the wind source is a fan set in a fixed position.) Winter Quarter

Lift and Drag Lift is defined as a force normal to the relative wind Drag is a force parallel to the relative wind Winter Quarter

How is Lift Produced ? Air moving over the top of the wing has a higher velocity than the air on the bottom High Velocity = Low Pressure Low Velocity = High Pressure The resulting pressure difference causes a force that pushes up on the wing (aka lift) This is one of the most common theories about how airplanes fly – it is known as Bernoulli’s Principal. Winter Quarter

How Angle of Attack and Camber Affect Lift This is a plot of the coefficient of lift versus the angle of attack. From the graph, it can be seen that the lift increases linearly until it reaches some maximum value, after which the airfoil “stalls” and the lift decreases drastically. This behavior is useful in determining the maximum angle at which a particular wing is capable of flying. This plot also illustrates the behavior of a cambered airfoil. As you can see, this particular airfoil is capable of producing lift even at a zero degree angle of attack (and even at some negative angles of attack.) This is one of the main differences between cambered and un-cambered Airfoils (other than the obvious difference in shape.) Winter Quarter

What About a Symmetric (no camber) Airfoil? This is a plot of a symmetric airfoil (i.e. one without camber.) As you can see, the airfoil does not produce any lift at a zero degree angle of attack. In other words, this airfoil must be set at some positive angle of attack to produce any lift. Winter Quarter

Bottom Line: Cambered Vs Symmetric Cambered airfoils produced lift at zero angle of attack. Symmetric (no camber) airfoils do not produce lift at zero angle of attack This just reiterates the two main points. Winter Quarter

What Happens to an Airfoil when it Stalls ? This is an illustration of a wing section that is operating at an angle of attack that is too high. You can see that the airflow on the top surface separates almost immediately, and begins to swirl – creating a region of relatively slow moving air. From the Bernoulli Principal, we know that this region of slow moving air corresponds to high pressure. However, for a wing to produce lift, a low pressure region must exist on the top surface. As a result, once the airflow separates from the wing (or the wing becomes “stalled”), it will no produce any usable lift. Flow over the top surface separates from the airfoil, resulting in a high pressure wake region Winter Quarter

Sources of Additional Airfoil Information How Airplanes work: http://www.howstuffworks.com/airplane4.htm NASA’s FoilSim II airfoil simulation program: http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html Airfoil Database (Hint: look at low Reynolds number airfoils for the upcoming lab) : http://www.aae.uiuc.edu/m-selig/ads/coord_database.html These are several websites that provide additional information on airfoils, and airplanes in general. The third reference is especially useful for this lab, as it has hundreds of different airfoils for a variety of different applications. With some thought, the students should be able to use their AutoCAD knowledge to produce airfoil patterns to use in constructing their wing sections based on the coordinates provided at this site. Reynolds number is a non-dimensional number frequently used in aeronautical engineering, it is simply the ratio of inertial to viscous forces. In most aeronautical applications, the Reynolds number is relatively high (since the viscosity of air is low, and the flight velocity is quite high). However, low Reynolds numbers are sometimes found when dealing with small wings, at low speeds – just like what will be encountered in this lab. Winter Quarter