Lift (force)

• 1. Introduction

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  Lift is the force that holds an airplane up, and almost everything you have been told about how it works is incomplete. It is the component of a fluid's force on an object that points perpendicular to the oncoming flow. Its quieter partner, drag, points parallel to that flow. Lift conventionally pushes upward to counter gravity, yet it can point in any direction perpendicular to the flow. On a sailing ship it can be largely horizontal. At the top of an aerobatic loop it becomes downforce.

  The puzzle is that lift is everywhere and yet stubbornly hard to explain. Birds, bats, and insects use it. So do the seeds of certain trees, helicopter rotors, racing car wings, wind turbines, and the keels of sailboats. Two famous explanations compete for the public's attention, one built on Newton's laws and one built on Bernoulli's principle. Each captures something real. Each, on its own, is wrong. Why are the simple stories misleading, what does a cambered wing actually do to the air, and how can a plane fly upside down? Those questions sit at the heart of what follows.

• 2. Lift That Needs No Motion

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  Buoyancy lets a balloon hang in the sky without moving at all. This is aerostatic lift, where an internal fluid is lighter than the surrounding fluid. Blimps, dirigibles, boats, and submarines all rely on it, and none of them need forward speed to stay afloat. It stands apart from dynamic lift, the kind a moving wing generates.

  Planing lift works a third way, with only the lower portion of a body immersed in a liquid flow. Motorboats, surfboards, windsurfers, sailboats, and water-skis all skim across the surface on this principle. The name of the force changes with the medium it acts in. In air it is an aerodynamic force. In water or any other liquid it is a hydrodynamic force, though marine hydrofoils and propellers obey the same physics as an airfoil despite differences in density, compressibility, and viscosity.

• 3. The Two Famous Stories

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  An airfoil generates lift by pushing the air downward as it flows past. Newton's third law then demands that the air push back up on the airfoil with an equal and opposite force, and that reaction is lift. As the airflow approaches the wing it curves upward, then changes direction and follows a path curved downward. Much of that downward turning happens in the air above the airfoil, not just along the lower surface. The explanation is correct, but it leaves out how a wing can turn a far deeper swath of air than it ever touches, and it never mentions the pressure differences that actually carry the force.

  Bernoulli's principle anchors the rival story, which comes in two versions. The "equal transit time" version claims air over the longer upper surface must speed up to rejoin its partner at the trailing edge, lowering the pressure there. This is simply false. No physical principle requires equal transit times, experiments show the times are not equal, and the air over the top actually moves much faster than that idea predicts. The "obstruction" version says the curved upper surface pinches the streamtubes narrower, forcing higher speed by conservation of mass. But it cannot explain how flat plates, symmetric airfoils, or wings flying upside down generate lift at all.

  Both Bernoulli stories share a deeper flaw. They imply one-way causation, where a speed difference arises first and then creates a pressure difference. The truth is a mutual interaction between pressure and speed. The simple versions leave out the flow-deflection half of that exchange, which is why each explains only a fragment of the whole.

• 4. The Coanda Controversy

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  Some versions of the flow-deflection story invoke the Coanda effect to explain why air clings to the convex top of a wing. In aerodynamics, the Coanda effect properly refers to a fluid jet staying attached to a surface that curves away from it, drawing surrounding air into the flow. A broader, looser usage stretches the term to cover any boundary layer adhering to a curved surface, and it is this broad sense that popular references reach for.

  The broad usage is contested. Flow following the upper surface simply reflects an absence of boundary-layer separation, so it is not really the Coanda effect at all. Naming the phenomenon does not explain it. A fluid's ability to follow a curved path does not depend on shear forces, viscosity, or even the presence of a boundary layer. Air can adhere to both surfaces of an airfoil and generate lift in inviscid flow, where viscosity is absent entirely.

• 5. Pressure And Curved Air

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  Pressure is the normal force per unit area that air exerts on itself and on every surface it touches, and it is always positive, always pushing inward, never pulling. The net upward force on a wing shows up as a pressure difference, with average pressure on the upper surface lower than the average pressure below. The pressure beneath pushes up harder than the reduced pressure above pushes down, and the difference is lift.

  Leonhard Euler captured the link between curved flow and pressure in 1754, in what is sometimes called the streamline curvature theorem. When a fluid follows a curved path, a pressure gradient runs perpendicular to the flow, with higher pressure on the outside of the curve and lower pressure on the inside. The relation involves the density, the velocity, and the radius of curvature. Tighter curves and higher speeds create larger pressure differences, and for perfectly straight flow the pressure difference vanishes to zero.

• 6. Angle, Camber, And Stall

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  A symmetrical airfoil at zero angle of attack generates no lift at all. The angle of attack is the angle between the airfoil's chord line and the oncoming flow. As it increases, the air deflects through a larger angle and lift grows, roughly in proportion to the angle for small values. Camber changes the picture: a cambered airfoil, with its upper surface more convex than the lower, generates lift even at zero angle of attack, because the trailing edge points downward and turns the air down.

  Camber also reveals how a plane can fly upside down. Invert a cambered airfoil and adjust the angle of attack, and the lift force points upward again. Pushing the angle higher cannot continue forever. Lift climbs to a maximum at a critical angle of attack, after which the upper-surface flow separates from the wing, deflection drops, and lift falls away. The wing is then said to be stalled.

  Stall is set by boundary-layer separation. When the boundary layer can no longer cling to the upper surface, it peels off and leaves a region of recirculating flow above the wing. Lift drops sharply at angles past the stall, though it never falls to zero. For single-element airfoils the maximum lift coefficient is generally less than 1.5. With high-lift slotted flaps and leading-edge devices deployed, it can exceed 3.0.

• 7. The Skin Of The Wing

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  No surface is truly smooth on the scale of air molecules. Molecules striking the wing bounce off in random directions, and viewed as a continuous material, the air cannot slide along the surface. Its velocity drops to nearly zero right at the skin, a rule known as the no-slip condition. Between that motionless film and the moving air beyond sits a thin boundary layer in shearing motion. Viscosity resists the shear and produces skin friction drag, and over most airfoils this boundary layer is naturally turbulent, which raises that drag further.

  Under normal flight the boundary layer stays attached to both surfaces all the way to the trailing edge. Compared with inviscid theory, it trims the lift by a modest amount and adds a viscosity-related pressure drag on top of skin friction. Together these make up the profile drag. A different story unfolds around bluff bodies that lack a streamlined shape. A circular cylinder sheds vortices alternately from its sides in a Karman vortex street, producing a fluctuating lift force even when the mean force is negligible. The frequency is set by the dimensionless Strouhal number, which depends on the flow's Reynolds number, and the resulting vortex-induced vibrations can threaten collapse in tall structures like industrial chimneys.

• 8. What The Wing Does To The Sky

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  A wing reshapes the air across a region far wider than itself. Ahead of the airfoil the flow deflects upward, above and below it deflects downward, and far behind it returns to the same state as the oncoming stream. The flow over the top speeds up while the flow underneath slows down, and together these establish a net circulatory component around the wing. Above sits a diffuse region of low pressure, below a diffuse region of high pressure, and the pressure difference acting on the surface is only part of this larger field.

  The cause runs in both directions at once. Non-uniform pressure pushes air from high pressure toward low, accelerating it and turning it downward, so the pressure field is the cause of the downward deflection. Yet the pressure differences themselves can exist only because the air has mass to push against, resisting changes in speed and direction. Air motion responds to pressure, and pressure is sustained by the air's inertia. This is why lift depends on air density, and why the Coanda claim that viscosity drives the downward turning is false.

  Sustaining lift means maintaining this pattern in both the vertical and horizontal directions, which requires both downward turning and the speed changes that Bernoulli's principle describes. Each simple explanation fails by choosing only one half. Lift is proportional to air density and roughly to the square of the flow speed, and engineers fold these factors into a single lift coefficient. To predict lift precisely, the conservation laws become partial differential equations solved by computational fluid dynamics, with the Reynolds-averaged Navier-Stokes equations landing within a few percent of real lift, while inviscid Euler or potential-flow methods can miss it by 10 to 20 percent below stall and grossly overestimate it above. The same field reaches all the way down to the earth, where the pressure disturbance below a plane forms a patch of slightly-higher-than-ambient pressure on the ground, spread so wide that it adds up to the full weight of the aircraft, a balance Lanchester recognized early in the development of modern aerodynamics.