Drag in Aviation – Parasite Drag
In flight every part of an aircraft is exposed to airflow will produce an aerodynamic force.One that will aid flight (a.k.a lift) and one that will oppose flight (a.k.a drag) . The term drag refers to air resistance and in this case, to an airplane as it travels through the air. Drag acts in the opposite direction of motion or flight path.Drag acts to slow the aircraft hence affects fuel consumption. For this reason, drag reduction is a high priority for designers to reduce drag, that’s why features like streamlining , flush riveting , polishing of surfaces and many other features are attempts to reduce drag.
Drag or Total drag is what we have seen in forces acting on an airplane.The total drag are the various drag forces acting on an airplane.Total drag comprises of parasitic drag and induced drag, for a pilot this is all that is required. Parasite drag is that drag force that is not directly associated with the development of lift and those drag forces associated with the production of lift is called induced drag.
Parasite drag comprises form drag , skin friction and interference drag, once again for a pilot this is all that is required. Sometimes you might see profile drag that basically combines form and skin friction as one group. We are going to take a closer look at each one of these elements to understand parasite drag.
Skin friction as its name suggests, its the friction forces between the surface of the aircraft and the air flowing around it, remember in the boundary layer lesson we spoke about the air slowing down on the surface of the airfoil,its important to understand that the airfoil is not the only place where boundary layer is found, its all over the aircraft. The magnitude of skin friction depends on certain factors :
a. surface area of the aircraft – the whole surface of the aircraft experiences skin friction as it moves through the air
b. nature of the boundary layer – a turbulent boundary layer is a lot thicker and thus causes more drag.
c. nature of the surface of the aircraft – noticed how fighter aircraft’s like the F-22 are so smooth compared to a Cessna 152, the raptor is well polished no rivets can be seen. That all refers to skin friction flush riveting and polishing helps smooth the surface and thus reducing skin friction. Its also important that a smooth surface delays the transition point in the boundary layer which in turn reduces skin friction.So, a rough surface will bring the transition point forward causing the air to become turbulent earlier.
d. airspeed – increasing the airspeed, increases the skin friction.
Form drag is next on the list of elements that causes drag or total drag. The best way to understand form drag is when you stick your hand out of a moving car, when your palm is facing the road you will face skin friction and when your palm faces the direction of motion you will face form drag. Form drag is the turbulent wake caused due to the form of an object in airflow. Form drag is depends on the frontal area presented to the airflow and the shape of the object.Vortices’s are produced behind an object , which could be from the airfoil or the aircraft itself. The size of the turbulent wake is the indication of the magnitude of form drag. Marked airflow separation occurs when an airfoil is at a very high angles of attack. This creates a pressure gradient on the upper surface of the airfoil too severe to allow the boundary layer to adhere to the surface and the separation point may move well forward. The separated air from an airfoil is very turbulent and increases the form drag.To reduce this drag, any delay of the separation point reduces form drag. Streamlining is one way that helps reducing form drag. Adding smooth contours and curvatures (fairings) will reduce form drag.
Interference drag is the drag that is found at any joint of an aircraft. Every aircraft of this generation has wings attached to the fuselage, the point where they is a joint and because of that joint it causes drag which is called interference drag. As it is not directly associated with the production of lift its is apart of parasite drag. To reduce this drag filleting , fairings that blend the two surfaces together and streamlining causes interference drag to reduce.
Designers have a hard time reducing skin friction, as no surface can be perfectly smooth. Its the most significant out of all the various types of parasitic drag. Do remember there are more types of parasitic drag but this is all that is required in general or commercial aviation. Do remember the key points,remember the vector we call drag, is the sum of parasite and induced drag.We have only discussed about parasite drag, in the next lesson we will take a look at induced drag.
