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Lift (Force)Lift consists of the sum of all the aerodynamic forces normal to the direction of the external airflow. It can be explained principally by two basic laws of physics: Newton's third law of motion and Bernoulli's principle. Newton's third law Lift is created as an airstream passes by an airfoil which deflects the air flow downward. The force created by this deflection of the air creates an equal and opposing force upward on an airfoil (see Newton's third law.) The deflection of air flow downward during the creation of lift is known as downwash. (Note: Confusingly, the term "downwash" has two somewhat different meanings with regard to aircraft. See downwash for a more complete explanation.) It is important to note that lift is not simply a reaction force in which the air molecules "bounce off" the bottom of the airfoil. Rather, both the top and bottom surfaces of the airfoil play important roles in deflecting the airflow downward. In fact, the acceleration of the air during the creation of lift is most accurately described as a turning of the airflow. Nearly any shape will produce lift if tilted with respect to the air flow direction (inclined) or cambered (curved). However, most shapes will be very inefficient and create a great deal of drag. One of the primary goals of airfoil design is to devise a shape that produces the most lift while producing the least drag. Invariably, the creation of lift creates drag - this is called lift-induced drag, or just induced drag. One unsolved research question in airfoil design is why the airflow "sticks" to the wing as it changes direction - this is known as the Coanda Effect. Bernoulli's principle The force on the wing can also be examined in terms of the pressure differences above and below the wing. (This method of explanation is mathematically equivalent to the Newton's 3rd law explanation as developed above.) The relationship between the velocities and pressures above and below the wing are nearly predicted by Bernoulli's equation. Simplified, the equation states that: pressure + 1/2 * density * velocity squared = constant or static pressure + dynamic pressure = constant. The differences between the Bernoulli-predicted values and the true values are small and related to viscosity, which is neglected in the Bernoulli equation. Fluid dynamics A third way of conceptualizing lift is a mathematical construction called circulation. Again, it is mathematically equivalent to the two explanations above. It is often used by practicing aerodynamicists as a convenient quantity, but is not often useful for a layperson. The circulation is the line integral of the velocity of the air, in a closed loop around the boundary of an airfoil. It can be understood as the total amount of "spinning" of air around the airfoil. When the vorticity is known, the section lift can be calculated using: -
where is the air density, is the free-stream airspeed, and is the circulation. The boundary layer is a thin region close to the airfoil, defined for convenience in order to ignore viscosity outside the boundary layer. There are two types of flow in the boundary layer. Around the leading edge the air flows smoothly and behaves like a stack of sheets (laminae) sliding over each other - laminar flow. Further along the wing there is a transition to a turbulent flow. The laminar layer produces less drag, but the turbulent layer is more stable, that is, less likely to move away from the surface. As flow speed increases, the boundary layer starts to separate at the trailing edge of the wing and a vortex begins to form, moves back and then leaves the surface. This is the starting vortex which disrupts the symmetry of the air flow, causing differences in flow pressure and speed between the upper and lower surfaces of the wing - lift. The vortex extends in a closed circuit of two real vortices trailing from near the wing tips (wing-bound vortex) and the starting vortex, forming a horseshoe shape and sometimes called the horseshoe vortex system. Aerodynamicists are one of the most frequent users of dimensionless numbers. The coefficient of lift is one such term. When the coefficient of lift is known, for instance from tables of airfoil data, lift can be calculated using the Lift Equation: -
where: - is the coefficient of lift,
- is the density of air (1.225 kg/m3 at sea level)*
- V is the freestream velocity, that is the airspeed far from the lifting surface
- A is the surface area of the lifting surface
- L is the lift force produced.
* Note that at altitudes other than sea level, the density can be found using the Barometric formula Compare with: Drag equation. External links
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