35 years before I became an aircraft mechanic, I asked myself this same question. My friends and I finally decided that some serious magic is involved. Today, watching a 747 rise off the runway is still a magical sight. Physics provides a more accurate explanation though.
Air traveling over a wing is affected by Bernoulli’s principle just as it is by passing through a constriction in a tube. The constriction forces the air to speed up, causing a drop in pressure.
Air traveling above and below a wing must reach the trailing edge in precisely the same amount of time. The curved shape of the upper wing creates a longer route for the air to travel. It’s forced to accelerate in order to catch up with the bottom, creating the low pressure area. What we call “lift” is the resulting difference in pressure.
This interpretation of Bernoulli’s principle has been taught to students and pilots since world war two. Unfortunately, the explanation is not completely accurate. As aircraft speed increases, the pressure differential does increase. Wind tunnel testing shows that airflow does speed up as it passes over the wing, but it actually reaches the trailing edge sooner than the lower airflow does. The “equal transit time” theory is quite incorrect for most situations.
When combined with “circulation theory”, lift is described more accurately. Aeronautical engineers calculate lift through the interactions of airflow and vortexes. The mathematics are so horrific that it is largely ignored by most others.
According to my FAA Airframe Handbook, 75% of lift occurs over the top of the wing. The remaining 25% actually pushes up from beneath. A “high lift” wing has a larger upper curve (positive camber) than an ordinary wing. Engineers must strike a balance between lift, drag, and efficiency, depending on the requirements of the aircraft.
None of this explains why an aircraft can fly inverted, which is commonly done in many types. If the Bernoulli principle were fully accurate, inverted flight would be suicidal. The “angle of attack” of the wing allows this to be possible. In very simple terms, this refers to the nose-up attitude of the aircraft. A pilot can actually use the fuselage instead of the wing to create lift by flying sideways, with an appropriate angle of attack. Even a sheet of plywood can generate lift.
This illustrates a common misconception about the required shape of a wing. Because speed and angle of attack are the major controlling factors in lift, it’s not essential for a wing to have a curved upper surface. A perfectly symmetrical wing has identical lift compared to a cambered one, when flown at a typical angle of attack. Only at steeper angles does a cambered wing produce more lift. The curved surface and the wing thickness merely give airflow a better chance to adhere to the wing (maintaining laminar flow), avoiding a stall.
The upper camber of most wings is only 1 or 2 degrees a barely perceptible amount. Some wings are completely symmetrical, especially those designed for aerobatics. Many modern wings have no camber at all on the upper surface, including the Boeing 777. This wing actually has a cambered LOWER surface, enabling more fuel storage. Under Bernoulli’s assumptions this would make no sense at all.
The amount of lift increases rapidly as the angle of attack increases. If a pilot increases the angle of attack too much, however, eddy’s will form in place of the low pressure area. Lift is replaced with drag, causing the wing to stall. Wisely designed wings are shaped so a stall happens progressively instead of suddenly and completely.
Because lift is also a reaction force, Newton’s Laws are very helpful in it’s description. The opposite force of air accelerating downward is the upward force of lift. If there is no down-wash of air leaving the trailing edge of the wing, there can be no lift.
A variety of auxiliary devices exist which can increase lift during the slower speeds of takeoff and landing. Leading edge flaps extend from the wing, which in effect, increase the positive camber. Trailing edge flaps can be lowered (not raised) to direct more air downwards. Some aircraft can lower both ailerons simultaneously to increase lift even further. Such devices allow the maximum angle of attack to be exceeded without stalling. Spoilers can be deployed from the upper wing to “spoil” the low pressure zone, destroying lift, which enable shorter landing runs.
The blades of a propeller and the rotors of a helicopter create lift in exactly the same manner, as they are miniature wings in themselves. The reason a helicopter can sit on the ground at full rotor rpm and not take off is because the angle of attack of the rotors are at a neutral position until the pilot wants to take off. Rotor rpm remains constant during flight, meaning lift is controlled by varying the angle of attack of the blades.
The amount of controversy that rages on this subject is surprising. Over simplification and inaccuracies are perpetuated in countless textbooks, magazines, schools and websites. These views are so entrenched, agreement will not happen overnight. The disagreement is polarized into two camps Bernoulli’s and Newton’s.
No one can win this argument for the simple reason that neither principle can describe lift on it’s own. Downward acceleration of air masses (Newton), forces of pressure differences (Bernoulli), circulation theory, speed, and angle of attack must all be considered together. Once this is accepted and understood, the explanation of lift is much easier to understand.
Convincing a six year old no magic is involved, that’s a whole different challenge.
