User:J Doug McLean/sandbox

Overview
A fluid flowing around the surface of a solid object applies a force on it. It doesn't matter whether the object is moving through a stationary fluid (e.g. an aircraft flying through the air) or whether the object is stationary and the fluid is moving (e.g. a wing in a wind tunnel) or whether both are moving (e.g. a sailboat using the wind to move forward). Lift is the component of this force that is perpendicular to the oncoming flow direction. Lift is always accompanied by a drag force, which is the component of the surface force parallel to the flow direction.

Lift is mostly associated with the wings of fixed-wing aircraft, although it is more widely generated by many other streamlined bodies such as propellers, kites, helicopter rotors, racing car wings, maritime sails, wind turbines, and by sailboat keels, ship's rudders, and hydrofoils in water. Lift is also used by flying and gliding animals, especially by birds, bats, and insects, and even in the plant world by the seeds of certain trees. While the common meaning of the word "lift" assumes that lift opposes weight, lift can be in any direction with respect to gravity, since it is defined with respect to the direction of flow rather than to the direction of gravity. When an aircraft is cruising in straight and level flight, most of the lift opposes gravity. However, when an aircraft is climbing, descending, or banking in a turn the lift is tilted with respect to the vertical. Lift may also act as downforce in some aerobatic manoeuvres, or on the wing on a racing car. Lift may also be largely horizontal, for instance on a sailing ship.

The lift discussed in this article is mainly in relation to airfoils, although marine hydrofoils and propellers share the same physical principles and work in the same way, despite differences between air and water such as density, compressibility, and viscosity.

The flow around a lifting airfoil is a fluid mechanics phenomenon that can be understood on essentially two levels:

1) The level of the rigorous science represented by the mathematical theories, which are based on established laws of physics and represent the flow accurately, but which require solving partial differential equations, and

2) The level of qualitative physical explanations, which attempt to interpret the physics without solving equations. Correctly explaining lift in these qualitative terms is difficult because the cause-and-effect relationships involved are subtle .  A comprehensive explanation that captures all of the essential aspects is necessarily complex.  There are also many simplified explanations.  But simplifying the explanation of lift is inherently problematic, and all of the known simplified explanations leave significant parts of the phenomenon unexplained and have other significant flaws .   These issues are discussed further in connection with each of the simplified explanations presented below.

Simplified physical explanations of lift on an airfoil


An airfoil is a streamlined shape that is capable of generating significantly more lift than drag. A flat plate can generate lift, but not as much as a streamlined airfoil, and with somewhat higher drag.

Many different simplified explanations have been proposed. Most follow one of two basic approaches, based either on Newton's laws of motion or on Bernoulli's principle.

Explanation based on flow deflection and Newton's laws


An airfoil generates lift by exerting a downward force on the air as it flows past. According to Newton's third law, the air must exert an equal and opposite (upward) force on the airfoil, which is lift.

The airflow changes direction as it passes the airfoil and follows a path that is curved downward. According to Newton's second law, this change in flow direction requires a downward force applied to the air by the airfoil. Then Newton's third law requires the air to exert an upward force on the airfoil; thus a reaction force, lift, is generated opposite to the directional change. In the case of an airplane wing, the wing exerts a downward force on the air and the air exerts an upward force on the wing.

The downward turning of the flow is not produced solely by the lower surface of the airfoil, and the air flow above the airfoil accounts for much of the downward-turning action (reference?).

This explanation is correct as far as it goes but is incomplete. It doesn't explain how the airfoil can impart downward turning to a much deeper swath of the flow than it actually touches. Furthermore, it doesn't mention that the lift force is exerted by pressure differences, and doesn't explain how those pressure differences are sustained.

Controversy regarding the Coandă effect
Some versions of the flow-deflection explanation of lift cite the Coandă effect as the reason the flow is able to follow the convex upper surface of the airfoil. The conventional definition in the aerodynamics field is that the Coandă effect refers to the tendency of a fluid jet to stay attached to an adjacent surface that curves away from the flow, and the resultant entrainment of ambient air into the flow.

More broadly, some consider the effect to include the tendency of any fluid boundary layer to adhere to a curved surface, not just the boundary layer accompanying a fluid jet. It is in this broader sense that the Coandă effect is used by some popular references to explain why airflow remains attached to the top side of an airfoil. This is a controversial use of the term "Coandă effect"; the flow following the upper surface simply reflects an absence of boundary-layer separation, thus it is not an example of the Coandă effect.

Explanations based on an increase in flow speed and Bernoulli's principle
There are two common versions of this explanation, one based on "equal transit time", and one based on "streamtube pinching".

Equal transit time


The "equal transit time" explanation starts by arguing that the flow over the upper surface is faster than the flow over the lower surface because the path length over the upper surface is longer and must be traversed in equal transit time. Bernoulli's principle states that under certain conditions increased flow speed is associated with reduced pressure. It is concluded that the reduced pressure over the upper surface results in upward lift.

A serious flaw in the equal transit time explanation is that it doesn't correctly explain what causes the flow to speed up. The longer-path-length explanation is simply wrong. No difference in path length is needed, and even when there is a difference, it's typically much too small to explain the observed speed difference. This is because the assumption of equal transit time is wrong. There is no physical principle that requires equal transit time and experimental results show that this assumption is false. In fact, the air moving over the top of an airfoil generating lift moves much faster than the equal transit theory predicts. . The much higher flow speed over the upper surface can be clearly seen in the animated flow visualization adjacent to The wider flow around the airfoil.

Obstruction of the flow


Like the equal transit time explanation, the "streamtube pinching" explanation argues that the flow over the upper surface is faster than the flow over the lower surface, but gives a different reason for the difference in speed. It argues that the curved upper surface acts as more of an obstacle to the flow, forcing the streamlines to pinch closer together, making the streamtubes narrower. When streamtubes become narrower, conservation of mass requires that flow speed must increase. Reduced upper-surface pressure and upward lift follow from the higher speed by Bernoulli's principle, just as in the equal transit time explanation.

One serious flaw in the streamtube pinching explanation is that it doesn't really explain how streamtube pinching comes about at all, let alone why it's greater over the upper surface than the lower surface. Another flaw is that conservation of mass isn't a satisfying physical reason why the flow would speed up. Really explaining why something speeds up requires identifying the force that makes it accelerate.

Issues common to both versions of the Bernoulli-based explanation
A serious flaw common to all the Bernoulli-based explanations is that they imply that a speed difference can arise from causes other than a pressure difference, and that the speed difference then leads to a pressure difference, by Bernoulli’s principle. This implied one-way causation is a misconception. The real relationship between pressure and flow speed is a mutual interaction.

In addition, as explained below under a more comprehensive physical explanation, producing a lift force requires maintaining pressure differences in both the vertical and horizontal directions. The Bernoulli-only explanations don't explain how the pressure differences in the vertical direction are sustained. That is, they leave out the flow-deflection part of the interaction.

Although the two simple explanations above are incorrect, there is nothing incorrect about Bernoulli's principle or the fact that the air goes faster on the top of the wing. In airfoil flows, Bernoulli's principle is applicable to the flow outside the boundary layer and can be used correctly as part of a more complicated explanation of lift.