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Actual Airflow vs. Ideal Airflow

Problems

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If so instructed by your teacher, print out a worksheet page for these problems.


Complete the following steps in the order given:

Step 1.
Launch the FoilSim software and create a symmetrical wing. Print out the wing and measure it. It should fit the test section of your wind tunnel. Adjust the size of the airfoil section if needed to obtain a size that will fit into the test area. Use carbon paper to trace the outline of the foil onto manila paper, and cut out the manila paper to create a template of the airfoil shape. Use woodworking tools to shape a balsa wood blank to fit the airfoil section desired. The template will be used to check for accurate reproduction of the desired shape. Take time to ensure a faithful reproduction of the design. Sand with a fine abrasive to ensure as smooth a surface finish as possible.

Step 2.
Return to the FoilSim software and test the airfoil section at the following angles of attack: 0, 5, 10, 15, and 20 degrees. Notice that the characteristic stall signature is not apparent in the simulation at any angle of attack. FoilSim was created with the assumption of ideal airflow and will not depict turbulence which results from boundary layer separation. Now enable the Stall Model in FoilSim and repeat the angles. A stall now appears, but it is only a model, not an actual computed separation. In order to observe actual airflow, we will need to place a model in a wind tunnel for testing.

Step 3.
Return to the model of the airfoil. Attach fine threads to the top surface using SMALL AMOUNTS of white glue. Place the threads along the chord line from the leading edge back to the trailing edge. Install several lines of threads aligned so that they do not interfere with each other. This process is called tufting and will allow for a visual indication of the airflow along the surface. Mount the airfoil into the test section of your wind tunnel.

Step 4.
Perform tests on the section at the following angles of attack: 0, 5, 10, 15, and 20 degrees. Look for the turbulence in the tufts which indicates a stall condition. Stall will begin at the trailing edge and, as the angle increases, the stall will advance towards the leading edge. At a 10 degree angle of attack, the airfoil should not experience stalling. By 20 degrees, the foil should be completely stalled. Now, beginning from 10 degrees, increase the angle of attack by increments of one degree to observe the stall progression. A modern airfoil design will not begin to stall prior to a 12 degree angle of attack and will not enter a fully stalled condition before 17 degrees. Attempt to verify this in your own testing.

Step 5.
Test different materials to use for the tufts and the ways to attach them to the airfoil section.

  1. Which material is the most sensitive to airflow?
  2. What is the most efficient way to bond the tuft to the surface?
  3. Where is the best place to put the tufts?
  4. Many modern aircraft wings incorporate stall warning transducers to notify the pilot that the airplane is approaching a stall condition. What would be a good threshold to activate the warning for this model wing?
  5. Would you be able to design and implement such a device for the test airfoil?

Step 6.
In a technical report, explain why actual airflow differs from ideal airflow. Look at boundary layers to help with the concept of airflow separation. This explains why the flow of air in actual world conditions does not behave as predicted in ideal airflow predictions.


 LINKS TO RELATED AREAS

Wind Tunnel Experiments http://www.grc.nasa.gov/WWW/K-12/airplane/judtalk.html

Another home-built tunnel (NASA BAALS) http://www.fi.edu/flights/first/makebigger/index.html

Beginner's Guide to Wind Tunnels http://www..grc.nasa.gov/WWW/K-12/airplane/bgt.html


Related Pages:
Standards
Worksheet
FoilSim Activity Index
Aerodynamics Index

 

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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: Thu, Jun 12 04:48:26 PM EDT 2014

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