What Bernoulli Missed About Flight

GrokSoup writes: "How come planes can fly upside-down? The Bernoulli principle as applied to flight -- air moves faster over the top of the wing creates low pressure, sucking the plane upward -- always bugged me because it didn't explain inverted flight. Turns out I'm not the only one. The current New Scientist has an entertaining interview with Fermilab physicist David Anderson who explains why the Bernoulli explanation is only partial: lift from wing-shape is the least significant component of lift. Much more important is the wing's angle of attack."

Comments

Here is Anderson's article

[K-Man]

Excerpt from the article on lift. As I suspected, he's just popularizing the usual aerodynamics.

Almost everyone today has flown in an airplane. Many ask the simple question "what makes an airplane fly?" The answer one frequently gets is misleading and often just plain wrong. We hope that the answers provided here will clarify many misconceptions about lift and that you will adopt our explanation when explaining lift to others. We are going to show you that lift is easier to understand if one starts with Newton's laws rather than the Bernoulli principle. We will also show you that the popular explanation that most of us were taught is misleading at best and that lift is due to the wing diverting air down. Most of this diverted air is pulled down from above the wing.

Let us start by defining three descriptions of lift commonly used in textbooks and training manuals. The first we will call the Mathematical Aerodynamics Description of lift, which is used by aeronautical engineers. This description uses complex mathematics and/or computer simulations to calculate the lift of a wing. It often uses a mathematical concept called "circulation" to calculate the acceleration of the air over the wing. Circulation is a measure of the apparent rotation of the air around the wing. While useful for calculations of lift, this description does not lend themselves to an intuitive understanding of flight.

The second description we will call the Popular Description, which is based on the Bernoulli principle. The primary advantage of this description is that it is easy to understand and has been taught for many years. Because of its simplicity, it is used to describe lift in most flight training manuals. The major disadvantage is that it relies on the "principle of equal transit times", or at least on the assumption that because the air must travel farther over the top of the wing it must go faster. This description focuses on the shape of the wing and prevents one from understanding such important phenomena as inverted flight, power, ground effect, and the dependence of lift on the angle of attack of the wing.

The third description, which we are advocating here, we will call the Physical Description of lift. This description of lift is based primarily on Newton's three laws and a phenomenon called the Coanda effect. This description is uniquely useful for understanding the phenomena associated with flight. It is useful for an accurate understanding the relationships in flight, such as how power increases with load or how the stall speed increases with altitude. It is also a useful tool for making rough estimates ("back-of-the-envelope calculations") of lift. The Physical Description of lift is also of great use to a pilot who needs an intuitive understanding of how to fly the airplane.

This is an old debate/flamewar

[K-Man]

People have debated this on the net for years, and not without reason. The bernoulli effect got put into textbooks years ago, and everyone was convinced that a wing cross-section was some magical shape that produced lift without drag, or with little drag, and that simpler shapes, like a fan blade, simply didn't have the right stuff to hold up an honest-to-goodness airplane.

The truth is much simpler. Aircraft stay up by accelerating air downwards. The viscosity of air allows it to be pushed down by a flat object held at a positive angle of attack. Any such object will have a net difference in pressure between its top and bottom, referred to as lift (and drag). Making the wing teardrop-shaped makes the flow more laminar and reduces drag, but that's really just streamlining.

Bernoulli Effect

[qwertykid]

The Bernoulli effect in itself is not strong enough to provide the lift for the aircraft. It merely is responsible for holding the airflow to the upper surface of the wing with a given angle of attack. I fly indoor model airplanes (rubber band powered free-flight) as a hobby, and i can tell you that we certainly do not have flat-bottomed airfoils. Our wings are simply ribs and spars covered on the top (covering the bottom also adds too much weight and decreases the effectiveness of the angle of attack). Without the bernoulli effect, we would not be able to have such an angle of attack in our wings without causing turbulence on the top of the wing when the airflow doesnt hold to the surface. this phenomenon disrupts the airflow over the top of the wing... most of us call it a "stall".

--qwertykid

Re:Finally, an explanation that makes more sense.

[Placido]

There's a gap between the wing and the flaps because it reduces turbulence. If there wasn't any gap there would be a huge amount of wind resistance and turbulence creating severe problems for the flight of the aircraft. The gap also allows the air from the bottom of the wing to flow over the top of the flaps creating extra lift.


Types of Flaps
Flaps come in several varieties:

• Plain flaps are mounted on simple hinges. The trailing edge of the wing simply pivots downward. Plain flaps are common on small aircraft because they're simple and inexpensive.
• Split flaps hang down from the trailing edge of the wing, but the top surface of the wing doesn't move.
• Slotted flaps work much like plain flaps. But they leave a gap between the flap and the wing, allowing air to flow from the bottom of the wing over the top surface of the flap. This airflow dramatically increases lift at low airspeed.
• Fowler flaps are the most complicated and efficient arrangement. They move backward and downward as they're deployed, increasing both the wing's area and its curvature. Large jet aircraft usually have Fowler flaps.

There's also leading edge flaps which are cool.


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