Engineering Expert Witness Blog

     Last week we talked about Daniel Bernoulli and his famous Bernoulli Principle, which is the cornerstone of fluid dynamics.  As we’ll see in this week’s installment, the Bernoulli Principle doesn’t just apply to water flowing inside pipes.  Let’s consider another instance in which it is instrumental, that of an airplane wing. 

     Figure 1 shows the side view of a wing with arrows indicating direction of air flow as the plane moves through the air.

Figure 1 – A Side View of an Airplane Wing

     Even though he lived more than 100 years before the first airplane, Bernoulli’s Principle can be used to explain why such a contraption can fly. You see, when comparing air flowing above and beneath a wing, its very shape makes the air flow want to travel faster along the top than it does on the bottom.

     Bernoulli’s principle comes into play with the airplane wing just as it did in last week’s water pipe flow example.  That is, the total energy of flow is the same at all points as the air flows above and below the wing.  Now, if air flow speeds up on top of the wing, then the flow’s kinetic energy increases along with it.  And remember last week’s analogy of change for $100?  Well, something has to give, so in this example the increase in kinetic energy is accomplished at the expense of pressure energy, but the total energy remains the same.  This decrease in pressure energy then translates into a drop in pressure on top of the wing.  The higher pressure beneath the wing overcomes the lower pressure above the wing.  This imbalance is what provides the plane’s lift, enabling it to get off the ground once it achieves a high enough speed on its race down the runway.

     The Wright brothers, men ahead of their time, were actually among the first aeronautical engineers.  They possessed remarkably advanced knowledge of mathematics and mechanical engineering principles.  They also understood what Bernoulli taught, and they used his Principle to design and test the shapes of wings on their gliders and planes.  They met with success when they determined that the wing’s shape was crucial to supplying lift.  In fact, they determined that, depending on the wing’s shape, it would provide the plane with the most lift for the least amount of air speed, allowing them to use a lighter engine to drive the propellers.  Weight is always a factor when flying, and the ability to use a lighter engine went a long way towards getting their first plane off the ground.

     That’s it for Fluid Mechanics.  Next week we’ll continue with a discussion of heat transfer, which is the study of how heat moves through vacuums, gases, liquids, and solid objects.

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