A foil is a solid object with a shape such that when placed in a moving

fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shear ...

at a suitable angle of attack the

lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobil ...

(force generated perpendicular to the fluid flow) is substantially larger than the drag (force generated parallel to the fluid flow). If the fluid is a

gas Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...

, the foil is called an airfoil or aerofoil, and if the fluid is

water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as a ...

the foil is called a hydrofoil.

Physics of foils

A foil generates lift primarily because of its shape and angle of attack. When oriented at a suitable angle, the foil deflects the oncoming fluid, resulting in a force on the foil in the direction opposite to the deflection. This force can be resolved into two components:

and drag. This "turning" of the fluid in the vicinity of the foil creates curved streamlines which results in lower pressure on one side and higher pressure on the other. This pressure difference is accompanied by a velocity difference, via

Bernoulli's principle In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy. The principle is named after the Swiss mathematici ...

, so for foils with positive angles-of attack, and other than flat-plates, the resulting flowfield about the foil has a higher average velocity on the upper surface than on the lower surface."...the effect of the wing is to give the air stream a downward velocity component. The reaction force of the deflected air mass must then act on the wing to give it an equal and opposite upward component." In: "The cause of the aerodynamic lifting force is the downward acceleration of air by the airfoil..." "''...if a streamline is curved, there must be a pressure gradient across the streamline...''" A more detailed description of the flowfield is given by the simplified Navier–Stokes equations, applicable when the fluid is

incompressible In fluid mechanics or more generally continuum mechanics, incompressible flow ( isochoric flow) refers to a flow in which the material density is constant within a fluid parcel—an infinitesimal volume that moves with the flow velocity. An eq ...

. However, since the effects of the compressibility of air at low speeds is negligible, these simplified equations can be used for both airfoils and hydrofoils as long as the fluid flow is substantially less than the speed of sound (up to about Mach 0.3).

Basic design considerations

The simplest type of foil is a flat plate. When set at an angle (the angle of attack) to the flow the plate will deflect the fluid passing over and under it, and this

deflection Deflection or deflexion may refer to: Board games * Deflection (chess), a tactic that forces an opposing chess piece to leave a square * Khet (game), formerly ''Deflexion'', an Egyptian-themed chess-like game using lasers Mechanics * Deflection ...

will result in a lift force on the plate. However, while it does generate lift, it also generates a large amount of drag. Since even a flat plate can generate lift, a significant factor in foil design is the minimization of drag. An example of this is the

rudder A rudder is a primary control surface used to steer a ship, boat, submarine, hovercraft, aircraft, or other vehicle that moves through a fluid medium (generally aircraft, air or watercraft, water). On an aircraft the rudder is used primarily to ...

of a boat or aircraft. When designing a rudder a key design factor is the minimization of drag in its neutral position, which is balanced with the need to produce sufficient lift with which to turn the craft at a reasonable rate. Other types of foils, both natural and man-made, seen both in air and water, have features that delay or control the onset of

lift-induced drag In aerodynamics, lift-induced drag, induced drag, vortex drag, or sometimes drag due to lift, is an aerodynamic drag force that occurs whenever a moving object redirects the airflow coming at it. This drag force occurs in airplanes due to wings or ...

, ''

flow separation In fluid dynamics, flow separation or boundary layer separation is the detachment of a boundary layer from a surface into a wake. A boundary layer exists whenever there is relative movement between a fluid and a solid surface with viscous f ...

'', and stall (see

Bird flight Bird flight is the primary mode of locomotion used by most bird species in which birds take off and fly. Flight assists birds with feeding, breeding, avoiding predators, and migrating. Bird flight is one of the most complex forms of locomo ...

Fin A fin is a thin component or appendage attached to a larger body or structure. Fins typically function as foils that produce lift or thrust, or provide the ability to steer or stabilize motion while traveling in water, air, or other fluids. Fin ...

, Airfoil,

Placoid scale A fish scale is a small rigid plate that grows out of the skin of a fish. The skin of most jawed fishes is covered with these protective scales, which can also provide effective camouflage through the use of reflection and colouration, as ...

Tubercle In anatomy, a tubercle (literally 'small tuber', Latin for 'lump') is any round nodule, small eminence, or warty outgrowth found on external or internal organs of a plant or an animal. In plants A tubercle is generally a wart-like projection ...

Vortex generator A vortex generator (VG) is an aerodynamic device, consisting of a small vane usually attached to a lifting surface (or airfoil, such as an aircraft wing) or a rotor blade of a wind turbine.

, Canard (close-coupled),

Blown flap Blown flaps, or jet flaps, are powered aerodynamic high-lift devices used on the wings of certain aircraft to improve their low-speed flight characteristics. They use air blown through nozzles to shape the airflow over the rear edge of the wing, ...

Leading edge slot A leading-edge slot is a fixed aerodynamic feature of the wing of some aircraft to reduce the stall speed and promote good low-speed handling qualities. A leading-edge slot is a spanwise gap in each wing, allowing air to flow from below the wing ...

Leading edge slats Slats are aerodynamic surfaces on the leading edge of the wing of a fixed-wing aircraft which, when deployed, allow the wing to operate at a higher angle of attack. A higher coefficient of lift is produced as a result of angle of attack and speed ...

), as well as

Wingtip vortices Wingtip vortices are circular patterns of rotating air left behind a wing as it generates lift.Clancy, L.J., ''Aerodynamics'', section 5.14 One wingtip vortex trails from the tip of each wing. Wingtip vortices are sometimes named ''trailing ...

(see

Winglet Wingtip devices are intended to improve the efficiency of fixed-wing aircraft by reducing drag. Although there are several types of wing tip devices which function in different manners, their intended effect is always to reduce an aircraft' ...

Lifted ability in air and water

The weight a foil can lift is proportional to its lift coefficient, the density of the fluid, the foil area and its speed squared. The following shows the lifting ability of a flat plate with span 10 metres and area 10 square metres moving at a speed of 10 m/s at different altitudes and water depths. It uses the lift at an altitude of 11 km as a datum to show how the lift increases with decreasing altitude (increasing air density). It also shows the influence of ground effect and then the effect of increase in density going from air to water. „Lifted_Weight_as_a_Function_of_Altitude_and_Depth_by_Rolf_Steinegger“ https://doi.org/10.21256/zhaw-4058 height 11 km: lift 1.0 (datum for comparison) 5 m 3.4 in ground effect 4.1 water surface-planing 1,280 just submerged 1,420 depth 5 m 2,840 10 km 2,860

References

External links

Lift from Flow Turning

Penguin can fly

{{sic, phillipines, hide=y thresher shark swim towards scuba divers

Swimming with Wild Dolphins

Fluid dynamics Aerodynamics

Physics of foils

Basic design considerations

Lifted ability in air and water

See also

References

External links