DRAG AND LIFT FORCE
A fluid flowing around the surface of an object exerts a force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the force parallel to the flow direction. Lift conventionally acts in an upward direction in order to counter the force of gravity, but it can act in any direction at right angles to the flow.
If the surrounding fluid is air, the force is called an aerodynamic force. In water or any other liquid, it is called a hydrodynamic force.
Drag and Lift
Drag is defined as the resisting force acting on an immersed body in the direction of flow. Drag is responsible for boundary layer, flow separation.
Lift is defined as a lifting force acting on an immersed body perpendicular to the direction of flow. For a symmetrical body there will be no lift force.
Drag
It is the resisting force acting on a body moving through a fluid (air or water).
It is of two types: SURFACE DRAG and FORM DRAG.
SURFACE DRAG:
Depends mainly on smoothness of surface of the object moving through the fluid. Friction Drag, also known as Skin Friction Drag, is drag caused by the friction of a fluid against the surface of an object that is moving through it. It is directly proportional to the area of the surface in contact with the fluid and increases with the square of the velocity.
Example: wearing racing suits in skiing and speed-skating.
FORM DRAG:
Depends mainly on the cross-sectional area of the body presented to the fluid. Form Drag, also known as Pressure Drag or Profile Drag, is the drag caused by the departure of the border layer from a surface and the wake created by that separation. It is mainly dependent upon the shape of the object.
Example: Bicyclist in upright v. crouched position, swimmer related to buoyancy and how high the body sits in the water.
No matter how smooth the surface of an airfoil seems, any surface is rough on the measure of air molecules. Air molecules flying into the surface bounce off the rough surface in arbitrary directions relative to their original velocities. The result is that when the air is viewed as a constant material, it is seen to be unable to slide along the surface, and the air's velocity relative to the airfoil decreases to nearly zero at the surface (i.e., the air molecules "stick" to the surface instead of sliding along it), something known as the no-slip condition. Because the air at the surface has near-zero velocity but the air away from the surface is moving, there is a thin border layer in which air close to the surface is subjected to a clipping motion. The air's viscosity counterattacks the shearing, giving rise to a shear stress at the airfoil's surface called skin friction drag. Over most of the surface of most airfoils, the boundary layer is obviously turbulent, which increases skin friction drag.
Under usual flight conditions, the border layer remains attached to both the upper and lower surfaces all the way to the irregular edge, and its effect on the rest of the flow is modest. Compared to the predictions of in-viscid flow theory, in which there is no boundary layer, the attached boundary layer reduces the lift by a uncertain amount and modifies the pressure distribution somewhat, which results in a viscosity-related pressure drag over and above the skin friction drag. The total of the skin friction drags and the viscosity-related pressure drag is usually called the profile drag.
DRAG EQUATION
In fluid dynamics, the drag equation is a formula used to calculate the force of drag experienced by an object due to movement through a fully enclosing fluid.The drag equation states that drag D is equal to the drag coefficient Cd times the density r times half of the velocity V squared times the reference area A.
Cd is the drag coefficient which is a dimensionless coefficient related to objects shape, geometry and taking into account both skin friction and form drag. If the fluid is liquid, it depends on the Reynolds number, while if it is gas it depends on both the Reynolds number and the mach number.
Examples of drag include the component of the net aerodynamic or hydrodynamic force acting opposite to the direction of movement of a solid object such as cars, aircraft and boat hulls; or acting in the same geographical direction of motion as the solid, as for sails attached to a downwind sail boat.
Lift
It represents a net force that acts perpendicular to the direction of the relative motion of the fluid. It is created by different pressures on opposite sides of an object due to fluid flow past the object.
lift can be in any direction with respect to gravity, since it is defined with respect to the direction of flow rather than to the direction of gravity. When an aircraft is cruising in straight and level flight, most of the lift opposes gravity. However, when an aircraft is climbing, descending, or banking in a turn the lift is tilted with respect to the vertical. Lift may also act as downforce in some aerobatic scheme’s, or on the wing on a racing car. Lift may also be largely horizontal, for example on a sailing ship.
Lift is a result of pressure differences and depends on angle of attack, airfoil shape, air density, and airspeed.
Following factors affect the lift.
Pressure differences
Pressure is the normal force per unit area exerted by the air on itself and on surfaces that it touches. The lift force is transmitted through the pressure, which acts perpendicular to the surface of the airfoil. Thus, the net force states itself as pressure differences. The direction of the net force suggests that the average pressure on the higher surface of the airfoil is lower than the average pressure on the base.
These pressure differences arise in combination with the curved airflow. When a fluid follows a curved path, there is a pressure gradient perpendicular to the flow direction with higher pressure on the outside of the curve and lower pressure on the inside.
Example: Airplane wing (hydrofoil)
Angle of attack
Angle of attack of an airfoil
The angle of attack is the angle between the chord line of an airfoil and the approaching airflow. A symmetrical airfoil will generate zero lift at zero angle of attack. But as the angle of attack increases, the air is deflected through a larger angle and the vertical component of the wind speed velocity increases, resulting in more lift. For small angles a symmetrical airfoil will generate a lift force unevenly relational to the angle of attack.
As the angle of attack increases, the lift reaches a maximum at some angle; increasing the angle of attack past this critical angle of attack causes the upper-surface flow to separate from the wing; there is less deflection downward so the airfoil generates less lift. The airfoil is said to be delayed.
Airfoil shape
An airfoil with curvature compared to a symmetrical airfoil
The maximum lift force that can be generated by an airfoil at a given airspeed depends on the shape of the airfoil, especially the amount of camber (curvature such that the upper surface is more convex than the lower surface, as illustrated at right). Increasing the camber generally increases the supreme lift at a given airspeed.
Cambered airfoils will generate lift at zero angle of attack. When the chord line is horizontal, the rambling edge has a downward direction and since the air follows the trailing edge it is deflected downward. When a cambered airfoil is upside down, the angle of attack can be adjusted so that the lift force is upwards. This explains how a plane can fly upside down.
FLOW CONDITIONS
The ambient flow conditions which affect lift include the fluid density, viscosity and speed of flow. Density is affected by temperature, and by the medium's acoustic velocity - i.e., by compressible effects.
AIR SPEED AND DENSITY
Lift is proportional to the density of the air and roughly proportional to the square of the flow speed. Lift also depends on the size of the wing, being generally proportional to the wing's area projected in the lift direction. In calculations it is convenient to quantify lift in terms of a lift coefficient based on these factors.
LIFT EQUATION
Lift depends on the size of the wing, being nearly proportional to the wing area. It is frequently suitable to compute the lift of a given airfoil by its lift coefficient, which describes its general lift in terms of a unit of the wing.
The lift equation states that lift L is equal to the lift coefficient Cl times the density r times half of the velocity V squared times the wing area A.
If the value of coefficient of lift of a wing at a specified angle of attack is given, then the lift produced for specific flow conditions can be determined by the equation :
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