If so instructed by your teacher, print out a worksheet
page for these problems.
Actual Airflow vs. Ideal Airflow
Complete the following steps in the order
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.
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
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
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.
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.
Test different materials to use for the tufts and the ways to
attach them to the airfoil section.
- Which material is the most sensitive to airflow?
- What is the most efficient way to bond the tuft to the
- Where is the best place to put the tufts?
- 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?
- Would you be able to design and implement such a device for
the test airfoil?
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
LINKS TO RELATED AREAS
Wind Tunnel Experiments
home-built tunnel (NASA BAALS)
Beginner's Guide to Wind Tunnels