Total Drag Acting on a Finite Wing in Subsonic Flow

This Topic Basically means the summation on all drag forces which are possibly occurring on a finite wing when directed motion of a fluid medium in which the speed or velocity is less than that of sound in the medium throughout the entire region under consideration which acts on an aerodynamic wing with tips that result in trailing vortices or in other words on a finite wing Any physical body being propelled through the air has drag associated with it. In aerodynamics, drag is defined as the force that opposes forward motion via the atmosphere and is parallel to the direction of the free-stream velocity of the airflow. Drag must get rid by thrust in order to achieve forward motion. Drag is generated by literally nine conditions associated with the motion of air particles over the aircraft. There are several types of drag such as form, pressure, skin friction, parasite, induced, and wave.

The drag on an airfoil is primarily due to viscous effects at low speed and regarding compressibility at high speed. In addition, at high angles of attack, the flow can separate from the upper surface and cause additional drag too. Hence as indicated in our dimensional analysis, the drag coefficient depends on basic three quantities, Reynolds number, Mach number, and the angle of attack. Typically the Reynolds number is important at low speeds, the Mach number at high speeds and the angle of attack at all speeds. Total drag is comprised of the summation of various drags that arise from several different design characteristics. Major contributions of drag come from skin friction drag, induced drag, and profile drag. Skin friction drag is the literal friction between the airs flowing over the wing's surface. Induced drag comes directly from lift, as lift increases so does the drag. Profile drag comes directly from the geometry of the wing and you would expect an airfoil to produce less drag than a cube.

Profile drag develops from the frictional resistance of the blades passing through the air. It does not change with the airfoil’s angle of attack, but increases moderately when airspeed increases. Profile drag is composed of form drag and skin friction. Form drag results from the turbulent wake caused by the separation of airflow from the surface of a structure. The amount of drag is related to both the size and shape of the structure that protrudes into the relative wind. Skin friction is caused by surface roughness as well. Even though the surface appears smooth, it may be rough when viewed under a microscope. A thin layer of air clings to the rough surface and creates small eddies that contribute to drag. Skin friction drag is caused by the actual contact of the air particles against the surface of the aircraft. This is the same as the friction between any two objects or substances etc. Because skin friction drag is an interaction between the airplane surface and the air and the magnitude of skin friction drag depends on the properties of both the solid and the gas.

Moving apart from the above mentioned topic, even helicopter blades could be taken to explain this effect. Induced drag is generated by the airflow circulation around the rotor blade as it creates lift and high pressure area beneath the blade joins the low pressure area above the blade at the trailing edge and at the rotor tips. This causes spiral, or vortex, which trails behind each blade whenever lift is being produced. These vortices deflect the airstream downwards in the vicinity of the blade and creating an increase in downwash. Therefore, the blade operates in an average relative wind conditions that is inclined downward and rearward near the blade and because the lift produced by the blade is perpendicular to the relative wind, the lift is inclined aft by the same amount. The component of lift that is acting on a rearward direction is induced drag. As the air pressure differential increases with an increase in angle of attack, stronger vortices form, and induced drag increases well enough. Since the blade’s angle of attack is usually lower at higher airspeeds, and higher at low speeds, induced drag decreases as airspeed increases and increases as airspeed decreases gradually. Induced drag is the major cause of drag at lower airspeeds here.

Parasite Drag is the type of drag which increases with the airspeed. Any loss of momentum by the airstream, due to such things as openings for engine cooling, creates additional parasite drag as explained. Because of its rapid increase with the increasing airspeed, parasite drag is the major cause of drag at higher airspeeds. Parasite drag varies with the square of the velocity; therefore, doubling the airspeed increases the parasite drag four times in average. As airspeed increases, parasite drag increases, while induced drag decreases. Profile drag remains relatively constant throughout the speed range with some increase at higher air speeds. Combining all drag forces results in a total drag curve situation. The low point on the total drag curve shows the airspeed at which the drag is minimized.

Form drag and pressure drag are virtually the same type of drag. Form or pressure drag is caused by the air that is flowing over the aircraft or the airfoil. The separation of air creates turbulence and results in pockets of low and high pressure that leave a wake behind the airplane or the airfoil. This opposes forward motion and is a component of the total drag condition. Since this drag is due to the shape, or form of the aircraft, it is also called form drag as mentioned earlier. Relevant surfaces at angles to each other as in create turbulence in the region of the joint. This occurs most frequently at the intersection of the fuselage and the wing.

Therefore by considering above mentioned facts, Total Drag on a finite wing depends on the shape and size, velocity and inclination to the flow, mass, viscosity and compressibility of air basically. Since almost all aircrafts are reluctant to have a higher drag, by changing the aspect ratio, changing tip plates and tip tanks, adjusting the thickness of the taper and altering the shape and size of the wing can reduce the total drag on a finite wing under a subsonic flow. Finally I hope there will be many new discoveries and inventions regarding this field in order to make things much effective and efficient in the years to come.