Dynamic Lift And Magnus Effect: Applications

Dynamic Lift

Dynamic lift is the force that acts on a body by virtue of its motion through a fluid. For example, during the game of cricket, tennis, baseball or golf, we have noticed that a spinning ball deviates from its parabolic trajectory during its motion in the air.

Explanation

The explanation for the deviation for a spinning and a non-spinning ball is given on the basis of the Bernoulli’s principle. For a non-spinning ball, the streamlines around the ball move relative to the fluid (air). From the symmetry of streamlines, it can be incurred that the pressure difference at points above and below the ball at corresponding points is zero as the velocity of the fluid above and below the ball at the corresponding points is the same. Thus, there is no upward or downward force acting on the ball due to air. For a spinning ball, air is dragged along a spinning ball as it moves. If the ball moves forward, air relatively moves in the backward direction, therefore, the relative velocity of the air above the ball is larger than that below the ball. The streamlines thus get rarified below and crowded above the ball. The difference in the velocity between the corresponding points above and below the ball result in a pressure difference between the two faces and a net upward force acts on the ball. This results in a dynamic lift that the ball experiences which is termed as the Magnus effect.

Dynamic Lift

Magnus effect

The Magnus effect is an effect in which a spinning ball or a cylinder curves away from its principle path of flight as can be seen in the image above.

Applications

Dynamic lift holds great importance in many fields such as aerodynamics and many ball sports. The dynamic lift is given consideration while designing the rotor ships and airplanes.

Magnus Effect

The figure above shows an aerofoil, which is a solid piece shaped to provide an upward dynamic lift when it moves horizontally through the air. We can recall that the cross section of the wings of an airplane looks like the aerofoil with streamlines around it. When the aerofoil moves against the wind, the orientation of the wing relative to flow direction causes the streamlines to crowd together above the wing more than those below it. The flow speed on top is higher than that below it. Thus an upward force results in the dynamic lift of the wings.

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Practise This Question

Air is streaming past a horizontal air plane wing such that its speed is 120m/s over the upper surface and 90 m/s at the lower surface. If the density of air is 1.3 kg per metre3 and the wing is 10 m long and has an average width of 2 m, then the difference of the pressure on the two sides of the wing is