A common misconception about wings
While lift generated by an airfoil is often attributed to Bernoulli's Principle, it cannot be used to explain this. The popular and faulty explanation is generally written as this:
A wing is flat on the underside and curved on the top (is cambered). Thus air that moves over the top of the wing must travel a longer path to get to the end of the wing and therefore gets a higher velocity. According to Bernoulli's Principle, the faster moving air has lower pressure and the difference therefore results in lift.
This explanation is flawed in a critical aspect: it assumes that two parcels of air separated at the leading edge of the wing, with one traveling over and one traveling below, must meet again at the trailing edge of the wing. This does not happen. In fact the Kutta-Joukowski Theorem shows that if this was to happen, the wing would generate no lift.
The actual mechanism generating lift on an airfoil is Newton's Third Law of Motion. An airfoil is always flown at an angle of attack against the air flow. As the wing deflects air downwards the opposing reaction force on the wing pushes it upwards. Note that nearly all of the lift arises from airflow over the top of the wing being deflected downwards, due to the Coanda Effect; the deflection due to the underside of the wing makes only a small contribution. (This is one reason why wing-mounted jet engines are suspended below the wing, rather than being placed on top of it - the disruptioin to flow over the bottom surface of the wing has much smaller effect of lift than mounting the engines above the wings.) Contrary to Bernoulli's Principle this explains why an aircraft with a thrust-to-weight ratio less than 1.0 can fly on a level flight path while being upside down (instead of being pulled dramatically towards the ground); why slim wings, such as those of the F-104 Starfighter or those of a paper plane, generate lift despite the camber being nonexistent; and why some lifting body aircraft can fly despite being very bulbous on the underside.
The airflow can be observed by solving the Navier-Stokes equations for the appropriate flow regime (turbulent or laminar) and the pressure along the wings edges directly calculated. As pressure is simply a measurement of force per unit area, integration of the pressure along the wings surface (both top and bottom) provides an overall force, which is the amount of lift provided by the wing.
Though Bernoulli's Principle cannot be used to explain the lifting mechanism, it can still be used to accurately analyze the airflow around an airfoil. If you know either the air pressure or the air velocity over a wing you can use Bernoulli's equation to calculate the value of the other property. The equation is very often used this way. This high frequency of use has been cited as the reason the misconception has arisen.
