Transcript The lift of a wing is proportional to the amount of air diverted down

at: www.answers.com/topic/lift
Physical description of lift on an airfoil
Lift is generated in accordance with the fundamental principles of physics such as Newton's laws of motion, Bernoulli's principle, conservation of mass and the
balance of momentum (where the latter is the fluid dynamics version of Newton's second law).[3] Each of these principles can be used to explain lift on an
airfoil.[4] As a result, there are numerous different explanations with different levels of rigour and complexity. For example, there is an explanation based on
Newton’s laws of motion; and an explanation based on Bernoulli’s principle. Neither of these explanations is incorrect, but each appeals to a different audience.
[5]
To attempt a physical explanation of lift as it applies to an airplane, consider the flow around a 2-D, symmetric airfoil at positive angle of attack in a uniform free
stream. Instead of considering the case where an airfoil moves through a fluid as seen by a stationary observer, it is equivalent and simpler to consider the
picture when the observer follows the airfoil and the fluid moves past it.
Lift in an established flow
Streamlines around a NACA 0012 airfoil at moderate angle of attack.
If one assumes that the flow naturally follows the shape of an airfoil, as is the usual observation, then the explanation of lift is rather simple and can be
explained primarily in terms of pressures using Bernoulli's principle (which can be derived from Newton's second law) and conservation of mass, following the
development by John D. Anderson in Introduction to Flight. [3]
The image to the right shows the streamlines over a NACA 0012 airfoil computed using potential flow theory, a simplified model of the real flow. The flow
approaching an airfoil can be divided into two streamtubes, which are defined based on the area between streamlines. By definition, fluid never crosses a
streamline; hence mass is conserved within each streamtube. One streamtube travels over the upper surface, while the other travels over the lower surface;
dividing these two tubes is a dividing line that intersects the airfoil on the lower surface, typically near to the leading edge.
The upper stream tube constricts as it flows up and around the airfoil, the so-called upwash. From the conservation of mass, the flow speed must increase as
the area of the stream tube decreases. Relatively speaking, the bottom of the airfoil presents less of an obstruction to the free stream, and often expands as
the flow travels around the airfoil, slowing the flow below the airfoil. (Contrary to the equal transit-time explanation of lift, there is no requirement that particles
that split as they travel over the airfoil meet at the trailing edge. It is typically the case that the particle traveling over the upper surface will reach the trailing
edge long before the one traveling over the bottom.)
From Bernoulli's principle, the pressure on the upper surface where the flow is moving faster is lower than the pressure on the lower surface. The pressure
difference thus creates a net aerodynamic force, pointing upward and downstream to the flow direction. The component of the force normal to the free stream
is considered to be lift; the component parallel to the free stream is drag. In conjunction with this force by the air on the airfoil, by Newton's third law, the airfoil
imparts an equal-and-opposite force on the surrounding air that creates the downwash. Measuring the momentum transferred to the downwash is another way
to determine the amount of lift on the airfoil.
Stages of lift production
In attempting to explain why the flow follows the upper surface of the airfoil, the situation gets considerably more complex. To offer a more complete physical
picture of lift, consider the case of an airfoil accelerating from rest in a viscous flow. Lift depends entirely on the nature of viscous flow past certain bodies[6]: in
inviscid flow (i.e. assuming that viscous forces are negligible in comparison to inertial forces), there is no lift without imposing a net circulation.
When there is no flow, there is no lift and the forces acting on the airfoil are zero. At the instant when the flow is “turned on”, the flow is undeflected
downstream of the airfoil and there are two stagnation points on the airfoil (where the flow velocity is zero): one near the leading edge on the bottom surface,
and another on the upper surface near the trailing edge. The dividing line between the upper and lower streamtubes mentioned above intersects the body at
the stagnation points. Since the flow speed is zero at these points, by Bernoulli's principle the static pressure at these points is at a maximum. As long as the
second stagnation point is at its initial location on the upper surface of the wing, the circulation around the airfoil is zero and, in accordance with the Kutta–
Joukowski theorem, there is no lift. The net pressure difference between the upper and lower surfaces is zero.
The effects of viscosity are contained within a thin layer of fluid called the boundary layer, close to the body. As flow over the airfoil commences, the flow along
the lower surface turns at the sharp trailing edge and flows along the upper surface towards the upper stagnation point. The flow in the vicinity of the sharp
trailing edge is very fast and the resulting viscous forces cause the boundary layer to accumulate into a vortex on the upper side of the airfoil between the
trailing edge and the upper stagnation point.[7] This is called the starting vortex. The starting vortex and the bound vortex around the surface of the wing are
two halves of a closed loop. As the starting vortex increases in strength the bound vortex also strengthens, causing the flow over the upper surface of the airfoil
to accelerate and drive the upper stagnation point towards the sharp trailing edge. As this happens, the starting vortex is shed into the wake, [8] and is a
necessary condition to produce lift on an airfoil. If the flow were stopped, there would be a corresponding "stopping vortex".[9] Despite being an idealization of
the real world, the “vortex system” set up around a wing is both real and observable; the trailing vortex sheet most noticeably rolls up into wing-tip vortices.
The upper stagnation point continues moving downstream until it is coincident with the sharp trailing edge (a feature of the flow known as the Kutta condition).
The flow downstream of the airfoil is deflected downward from the free-stream direction and, from the reasoning above in the basic explanation, there is now a
net pressure difference between the upper and lower surfaces and an aerodynamic force is generated.
As Newton’s laws suggest, the wing must change something of the air
to get lift. Changes in the air’s momentum will result in forces on the
wing. To generate lift a wing must divert air down, lots of air. The lift of a
wing is equal to the change in momentum of the air it diverts down.
Momentum is the product of mass and velocity. The lift of a wing is
proportional to the amount of air diverted down times the downward
velocity of that air. Its that simple. (Here we have used an alternate form
of Newton’s second law that relates the acceleration of an object to its
mass and to the force on it, F=ma) For more lift the wing can either
divert more air (mass) or increase its downward velocity. This downward
velocity behind the wing is called "downwash". Figure 5 shows how the
downwash appears to the pilot (or in a wind tunnel). The figure also
shows how the downwash appears to an observer on the ground
watching the wing go by. To the pilot the air is coming off the wing at
roughly the angle of attack. To the observer on the ground, if he or she
could see the air, it would be coming off the wing almost vertically. The
greater the angle of attack, the greater the vertical velocity. Likewise, for
the same angle of attack, the greater the speed of the wing the greater
the vertical velocity. Both the increase in the speed and the increase of
the angle of attack increase the length of the vertical arrow. It is this
vertical velocity that gives the wing lift.