June 20, 2010
June 20, 2010
June 23, 2010
15.594.1 - 15.594.10
Force Balance Design for Educational Wind Tunnels
A typical educational wind tunnel need only measure the lift and drag forces and be able to control the angle of attack of the mounted model. This paper presents a simple design for the balance apparatus to measure lift and drag. Two load cells are used in the apparatus, one to measure the vertical force (lift) and one to measure the horizontal force (drag). This can be used for aerodynamics studies of airplane and airfoil models, as well as ground vehicles such as trucks and cars. If desired, a third load cell can be added to enable measurements of the pitching moment. A linear actuator is used to change the angle of attack of plane and airfoil models. The actuator is mounted inside the force balance apparatus, using a parallel four-bar linkage so that the angle of attack is linearly related to the actuator position. The linear actuator can be controlled by a computer, and the lift, drag, and angle of attack data recorded on the same computer. This design is simple and easy to fabricate, can be added to an old wind tunnel in need of an upgrade or replacement or included as part of a new wind tunnel built from scratch. Complete details of the design including engineering drawings are included in the paper. The design is also low cost compared to commercial wind tunnel balances and gives sufficient accuracy for educational and some research purposes. A counterweight is used to mechanically zero the force readout. The new load balance has been tested and found accurate. It has sufficient precision to perform a sting drag measurement as well. A quick-connect mechanism is used for mounting and removing models.
Wind tunnels are a useful tool not only for aerospace engineers, but also for mechanical and civil engineers. From an educational point of view, in addition to measuring the aerodynamic forces on airplanes, wings, cars, trucks, bridges, and buildings, they can also be used to measure the aerodynamic forces on sports balls, partially open valves, and anything else that can be mounted on the mounting sting. There are many excellent references in the literature on wind tunnel testing and basic aerodynamics1-10 appropriate for undergraduate students. Blevins’ handbook7 is particularly recommended as a source of drag coefficient data, and the NACA report8 a useful source of lift and drag data for airfoils. Most NACA reports can be found online on the NASA technical reports server (ntrs.nasa.gov).
Wind tunnel testing is not cheap, however. In addition to the cost of the wind tunnel itself, there is the requirement for the infrastructure to house the wind tunnel and the cost of electricity to run the tests. While a crude wind tunnel can be constructed relatively cheaply from a large fan and sheet metal, a wind tunnel is only useful for flow visualization without a force balance.
A full 6-component force and moment balance for a wind tunnel can easily cost in the tens of thousands of dollars, perhaps as much as the rest of the wind tunnel and equipment costs. This paper presents a design for a simple 2-component wind tunnel balance that can be easily fabricated at a basic machine shop, has accuracy sufficient for educational and even some research purposes, and costs about a thousand dollars.
Morris, M., & Post, S. (2010, June), Force Balance Design For Educational Wind Tunnels Paper presented at 2010 Annual Conference & Exposition, Louisville, Kentucky. https://peer.asee.org/15891
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