Aerodynamic Lifts and Airfoils

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Independent Project Instructions: Lifts and Airfoils

From your reading, it should have become clear that the main determinant for all lift production of an aircraft is the airfoil shape utilized in the construction of the wing. Therefore, the first item to determine is what airfoil is used on your selected aircraft. For this initial task, you can use any reliable source of information you may have access to; nevertheless, one good such resource is provided online by David Lednicer's (2010) Incomplete Guide to Airfoil Usage. 

Main Airfoil (for your selected aircraft): To find the main airfoil, go to the Online Tools section of the Field Exercises and Presentation page. This is found in the Course Specific Information section. It will send you to the websites you need to go to for research on your aircraft wing- Airfoil Tools and The Incomplete Guide to Airfoil Usage.

(Notice that for a variety of aircraft, the root and the tip of the wing may have different airfoil profiles, in which case it makes sense to pick the root airfoil for all our considerations here.)

Once you have determined the type of airfoil that is used on your example aircraft, the next logical step is to determine its particular lift production behavior (i.e., its lift curve). Again, there may be various sources available, but one good tool that can be utilized for this task is the Airfoil Tools database.

Start the search for your particular airfoil either by entering its name into the text search or utilize the library links in the menu on the left. Search results will display below the search window and contain a link to further details on their right. (See picture below.)

From the airfoil details view, you can gather the main properties (e.g., thickness, camber, etc.) of your selected airfoil profile. 

To retrieve the specific lift production properties (i.e., the lift curve) of your airfoil, select the second from the bottom plot from the list of available plots (see picture). This plot will be for a Reynolds number Re = 106, which is a good estimate for most of our small GA aircraft in this project (for more on selecting an appropriate Re, see the discussion below). Once the correct plot is selected, use the "Update plots" button to create your lift curve.

Besides the diagram with your airfoil's specific lift curve (labeled Cl/alpha), you can also retrieve a tabulated form of the x-foil calculations by using the "Details" link in the list of available plots. When reading your lift data, remember that the depicted relationship between AOA and CL is two-directional for most part of the curve; that is, besides looking up the CL value for a specific given AOA, you can also retrieve the required AOA to achieve a certain CL (see picture below).

Your presentation should include the following information:

A. Main airfoil for your selected aircraft

· Stall AOA and associated CLmax

· Zero-lift AOA

· Comparison of your main airfoil to a symmetric airfoil

B. Resultant attribute discussions for your selected aircraft

· Calculated stall speed

· How does lift change with airspeed if a constant AOA and altitude are held? (provide specific examples using the correct formula’s)

· How does lift change with altitude if a constant AOA and airspeed are held? (provide specific examples using the correct formula’s)

· How do the required CL and AOA for your specific aircraft (at a specific weight) change with changes in airspeed? (provide specific examples using the correct formula’s)

· What happens if the required CL is larger than CLmax of your airfoil, and what speed regime is usually associated with that condition?

The key to answering some of the questions is to provide specific calculation examples for your aircraft that showcase the points made. This means you should use actual formulas and example numbers to prove your statements. There are various forms in which you could do so, including self-generated tables and graphs or comparisons of key cases (e.g., comparing a low, medium, and high-altitude case). 

Lift Formula