This experiment is done on a wind tunnel. For general information of this windtunnel and the data that can be obtained from it, go through the Wind Tunnel Fundamentals tab.

Lab Objectives:

• Measure the Lift and Drag on an Unsymmetric airfoil mounted on a load cell balance, in the form of Force coefficients, at varying angles of attack and constant freestream velocity (far-field airspeed).

• Plot the force coefficients versus the angle of airfoil inclination (angle of attack) as in Fig below.

 

• Verify that a linear trend is obtained for Lift coefficient versus angle of attack, as in Fig. above.

LIFT AND DRAG OF AN AIRFOIL

            In this lab the characteristics of airfoil lift and drag coefficients as a function of Angle of Attack will be investigated. The airfoils are not only useful for aircraft. The investigation described herein applies to many fluid dynamic scenarios like wind turbine blades, wings on F1 cars, helicopter blades, propeller blades and hydrofoils. An Airfoil is a body designed to produce lift from the movement of the fluid around it. Specifically lift is a result of circulation in the flow produced by the airfoil.

Aerofoil characteristics on different angle of attack

The variation of lift coefficient with angle of attack for an airfoil is shown in figure we can see that lift coefficient CL varies linearly with angle of attack α. The slope of this straight line is called lift slope. In this region the flow moves smoothly over the airfoil and flow is attached over most of the surface. As the α becomes large, the flow tends to separate from the top surface of the airfoil creating a large wake of dead air behind the airfoil. Inside this separated region, flow is recalculating and part of the flow is actually moving in the opposite direction so-called reversed flow.

This separated flow is due to the viscous effects and the consequence of this separated flow at high α is decrease in lift and a large increase in drag. Under such condition, the airfoil is said to be Stalled. The maximum value of CL which occurs prior to the stall is an important aspect of airfoil performance. Experimental data for drag coefficient CD is also shown in the figure. The physical source of this drag is both skin friction drag and pressure drag due to flow separation (form drag).  The sum of these two effects yields the profile drag coefficient CD for an airfoil.