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How Wings Really Generate Lift

By Edited Mar 18, 2016 0 0

It is a popular misconception that the lift from a wing can be explained by two particles passing at different speeds around a wing.  This story is fluid mechanically and physically incorrect.  It is the aim of this article to introduce the correct explanation for the generation of lift, without delving deep into the mathematics or physical principles behind it.

To address first the issue regarding the popular equal transit time theory which is taught to many of us at high school.  This theory is based upon Bernoulli's equation for an inviscid, incompressible flow - an idealised flow.  Basically, due to the upper surface of the wing being longer than the lower surface, because of it's curvature, the particle on the upper surface must travel faster to meet up with a particle on the trailing edge.  Bernoulli's equation is used to relate pressure and velocity, whereby, faster speed means lower pressure.  The theory concludes that the pressure difference between the upper and lower surfaces causes lift.

This is fairly straightforward to teach, and simple to understand.  However, it is fundamentally flawed in that real flows are not incompressible, nor inviscid.

In an inviscid flow (a flow without any viscosity/friction), imagine air particles passing around a body.  An easy concept is to visuable this type of flow around a cylinder.  If there is no viscosity, the fluid will pass around the cylinder, and be in contact with the surface at all points.  There will be no separation of the fluid and no wake - like you see at the rear of a boat.

At all points on the surface of the cylinder, the fluid pressure acts perpendicular to the surface.  It is possible to obtain the lift and drag by integrating (adding up) all the surface pressures in the lift and drag directions.  For an inviscid flow, the lack of a wake means that the resulting forces are zero.

This is fundamentally important to know.  For an inviscid flow, there is no resulting force, regardless of the angle of attack.

When we introduce viscosity, the air flow over the cylinder changes.  The first force that appears is drag.  When the cylinder rotates, it acts on the air flow, and through the action of viscosity, it causes lift.

In aerodynamics, this introduces a complicated concept known as circulation.  To save confused, circulation basically impliess that if there is rotation in the flow, there is lift.

Whilst this theory is a little more complex than the common misconception, take this away with you as your understanding and explanation.

Lift is generated on a wing through the action of friction causing the air to be turned around the wing.  The turning is called circulation, which results in lift.

There is a special equation in simplified aerodynamics that relates lift to the circulation, called the Kutta-Joukowski theorem.



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