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 may continuously motion, move and Deformation (physics), deform (''flow'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are M ...

at a suitable

angle of attack In fluid dynamics, angle of attack (AOA, α, or \alpha) is the angle between a Airfoil#Airfoil terminology, reference line on a body (often the chord (aircraft), chord line of an airfoil) and the vector (geometry), vector representing the relat ...

the lift (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, the foil is called an

airfoil An airfoil (American English) or aerofoil (British English) is a streamlined body that is capable of generating significantly more Lift (force), lift than Drag (physics), drag. Wings, sails and propeller blades are examples of airfoils. Foil (fl ...

or aerofoil, and if the fluid is

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

the foil is called a

hydrofoil A hydrofoil is a lifting surface, or foil, that operates in water. They are similar in appearance and purpose to aerofoils used by aeroplanes. Boats that use hydrofoil technology are also simply termed hydrofoils. As a hydrofoil craft gains sp ...

Physics of foils

A foil generates lift primarily because of its shape and

. 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: lift 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 Bernoulli's principle is a key concept in fluid dynamics that relates pressure, speed and height. For example, for a fluid flowing horizontally Bernoulli's principle states that an increase in the speed occurs simultaneously with a decrease i ...

, so for foils generating lift the resulting flowfield about the foil has a higher average velocity on one surface than on the other."...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. And since the effects of the compressibility of air at low speeds is negligible, these simplified equations can be used for airfoils as long as the airflow is substantially less than the speed of sound (up to about

Mach The Mach number (M or Ma), often only Mach, (; ) is a dimensionless quantity in fluid dynamics representing the ratio of flow velocity past a Boundary (thermodynamic), boundary to the local speed of sound. It is named after the Austrian physi ...

0.3). For hydrofoils at high speeds, of the order of according to Faltinsen,

cavitation Cavitation in fluid mechanics and engineering normally is the phenomenon in which the static pressure of a liquid reduces to below the liquid's vapor pressure, leading to the formation of small vapor-filled cavities in the liquid. When sub ...

and ''ventilation'' – with air penetrating along the strut from the water surface to the foil – may occur. Both effects may have a substantial influence on the foil's lift.

Basic design considerations

The simplest type of foil is a flat plate. When set at an angle (the

) to the flow the plate will deflect the fluid passing over and under it, and this 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, airship, or other vehicle that moves through a fluid medium (usually air or water). On an airplane, the rudder is used primarily to counter adverse yaw ...

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, ''

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 fo ...

'', and stall (see

Bird flight Bird flight is the primary mode of animal locomotion, locomotion used by most bird species in which birds take off and flight, fly. Flight assists birds with feeding, Sexual reproduction, breeding, avoiding predation, predators, and Bird migrati ...

, Fin,

Airfoil An airfoil (American English) or aerofoil (British English) is a streamlined body that is capable of generating significantly more Lift (force), lift than Drag (physics), drag. Wings, sails and propeller blades are examples of airfoils. Foil (fl ...

, Placoid scale, Tubercle, Vortex generator, Canard (close-coupled), Blown flap,

Leading edge slot A leading-edge slot is a fixed aerodynamics, aerodynamic feature of the wing of some aircraft to reduce the Stall (flight), stall speed and promote good low-speed handling qualities. A leading-edge slot is a spanwise gap in each wing, allowing ai ...

, Leading edge slats), as well as

Wingtip vortices Wingtip vortices are circular patterns of rotating air left behind a wing as it generates Lift (force), lift. The name is a misnomer because the cores of the vortex, vortices are slightly inboard of the wing tips. Wingtip vortices are sometimes ...

(see Winglet).

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, philli, pines, 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