The Theory of Lift

By Craig Skinner

In 1738, a Swiss mathematician by the name of Daniel Bernoulli discovered the principle that bares his name and is used to explain why planes fly. Bernoulli's principle states that the energy of a mass of air is constant and is equal to it's velocity plus it's pressure.

Energy of a mass or air (constant) = Velocity + Pressure

For energy to remain the same, any change in either the velocity or pressure must be offset by an equal change in the other. Therefor, (and this is the important bit) if velocity increases then pressure decreases.

Now lets apply Bernoulli's Principle to an airplane wing. It was discovered that when a flat plate was inclined against an airflow, the velocity over the top increased and the pressure decreased (remember that we have to keep the total energy of the airflow constant and any increase in velocity must coincide with a decrease in pressure).

Diagram 1
It was also soon discovered that a curved surface provided much greater lift and had the added advantage of providing a certain amount of thickness that was necessary for structural strength. Now lets look at the pressure distribution of a typical airfoil. There is higher pressure on the underside of the wing and lower pressure on the top of the wing and this results in the wing being pushed upwards. The lowest pressure on the top surface is located around the 30 to 35% mark of the cord (the cord is a line drawn from the leading edge to the trailing edge) and this corresponds to location of the Center of Pressure. The Center of Pressure is the theoretical point in which all of the lift is located and should be positioned close to the Center of Gravity (the theoretical point through which all of the planes weight is located) in order to have a properly balanced airplane.

Diagram2
Well, that's lift in a nutshell. Air goes over top of wing, air speeds up, pressure drops, lower pressure on top and higher pressure on bottom, wing goes up. There are just two more things that I am going to mention. First the lift, and drag, increase as the Angle of Attack (angle between the cord and the relative air flow) increases up to the angle at which a stall occurs but that is another paper in it-self. Lastly, and not to disappoint the techno wizards, here is the formula for lift.

Lift = CL * P * V2 * S

  • CL = Shape of Airfoil and the Angle of Atta
  • S = Surface area of the wing
  • P = Air Density
  • V = Airspeed