Level: Middle school to H.S.
Time involvement: 10 to 20 minutes.
The Wright Brothers, being self-taught scientists, well understood that trying to study the effects of air on various wing shapes by attaching them to a bicycle was a frustrating unpredictable experiment. Hence, they build a wind tunnel powered by a motor. It allowed them to measure important characteristics of lift and drag for numerous shapes without building a full-scale wing. NASA, major universities, and aircraft manufacturers all employ wind tunnels to test experimental designs. Ranging from tabletop models to huge models that can contain an entire aircraft and create supersonic speeds, they provide engineers with valuable data that improve the design of wings, propellers and even rocket ships.
The “Poor Man’s” Wind tunnel is simple and easily reproduced for classrooms. It matters not whether the sample wing is fixed with air moving past or the air is stationary and the wing moves. Fashioned from a small rectangular frame of PVC pipe, two test samples are supported on sets of parallel fishing lines strung vertically across the frame. The samples are a round tubular shape and simple wing airfoil.
Discussion: Before beginning the demonstration the three essential factors that determine the lift generated by a wing are explained. The factors are: Shape, Speed, and Angle of Attack. Since the goal of a wing is to create a lifting force when moved through the air the shape must “force” the air to move faster on the top compared to the bottom. (See the Bernoulli Demonstration for a more complete explanation.) Since the tubular shape is symmetrical the airflow above and below is identical, therefore no difference in airflow and no difference in pressure and no lift. The airfoil shaped one, on the other hand, is curved on top and relatively flat on the bottom, (much like a bird’s wing). This causes a faster flow rate on top with the consequent reduced pressure, hence a lifting force. Speed or movement is required. Airplanes with fixed wings remain safely on the ground while parked. Only when moving at sufficient speed on the runway will the wings generate enough lift for flight.
The last factor, angle of attack, simply means how the angle the wing meets the air. A wing meeting the air in a level manner will generate some lift. A wing meeting the air with the leading edge raised will generate more lift. If the angle of attack exceeds 20-25 degrees the airflow on the top becomes disturbed and may even become reversed near the trailing edge. The condition is referred to as a stall. For training purposes even student pilots are trained to recognize and cause deliberate stalls and recovery. Inadvertent stalls occurring near the ground do not allow sufficient altitude to recover and have caused accidents with tragic results.
How to make the wind tunnel work! Since the demonstration will probably occur in a room with relatively still air the demonstrator creates the wind. (No, you don’t use your lungs!) First, assure that you are in an area clear of people, bookcases, hanging decorations, etc. By swinging the PVC frame in a horizontal arc of about 180 degrees it will be possible to generate enough “wind” to make your wing take off. If the first attempt is mediocre increase the speed. If it is still mediocre increase the angle of attack by tipping the frame slightly backward. Point out to your students that only the airfoil shape lifts. The tubular shape is unaffected by an increase in speed or angle of attack. With a little practice the airfoil shape will slide up to the top of the frame and drop back when the swinging motion stops. It is not necessary to make continuous 360 turns. In fact, it will be difficult for the student to keep track of the behavior of the two shapes and you risk the chance of losing your balance.
Notes: Students may want to try the demonstration. Use discretion. It takes some practice to get it right. This can be a WOW! demonstration of a complex aspect of aviation if carefully constructed and demonstrated. The tubular frame does not have to be completely glued together. If the PVC elbows are glued to only the sides, the whole affair can be disassembled and stored. By pre-measuring the fishlines they can be wrapped around a piece of cardboard and the airfoil shapes stored in a small box.