PURPOSE: To illustrate the pressure difference across an airplane wing.
DESCRIPTION: A blower creates air flow past the airfoil, which can be rotated. The difference in pressure between the top and the bottom surfaces of the wing is indicated by the manometer.
Note on the myth of airfoil lift: The nature of the actual lifting force on a real airplane wing is complex. See the Demonstration Reference File for several papers which discuss this problem. One can argue that the Bernoulli effect creates a pressure difference between the top and the bottom of the airplane wing. However, this pressure difference, in the absence of air deflection downward cannot explain the lift required to keep the airplane up in the air. According to Newton's third law, there must be deflection of the air downward due to either or both: deflection of the air due to the angle of attack of the wing, and shedding of vortices at the trailing edge of the wing.
A better explanation of airplane wing lift (as well as a number of other demonstrations generally ascribed to the Bernoulli effect) involves the Coanda effect and downward deflection of the air passing over the wing. According to the Coanda effect, the air flow follows the contour of the wing, ultimately moving at an angle downward from the rear of the wing. The reaction force acting on the wing provides the wing lift.
Many good web references discuss this issue; here are a few:
- Prof. Dr. Klaus Weltner: Online Publications, excellent summaries and explanations.
- The Science Hobbyist: Airfoil Lifting Force Misconceptions in K-6 Textbooks
- A Physical Description of Flight, by David Anderson and Scott Eberhardt
- Airplane Flight Analogy, by Bill Beaty (1997)
- Coanda Effect: Understanding Why Wings Work, by Jef Raskin
Other relevant web sites are given in the list of references obtained by clicking on the link below.
REFERENCES: Available. (PIRA 2C20.50)
EQUIPMENT: Airplane wing section mounted on manometer base, blower on stand.
SETUP TIME: 10 minutes.