What is Drag? All Explained Here!- Part 2
Hope you have gone through the previous article on Lift Induced drag and Parasite Drag. Today will discuss remaining drag types including Skin friction, Foam, Pressure, Interference and Total drag.
Skin Friction Drag
Skin friction drag is the drag that caused by the adjacent air layer to the moving body. Normally air is at rest before it gets interfered by the aircraft, when the aircraft moves through the air it induced a movement to the air particles adjacent to the aircraft structure and tries to carry the air particles with the aircraft. Energy is transferred to the air particles from the aircraft by causing a loss and this loss is called Skin friction drag. When the aircraft surface is rough, more skin friction is created and by making the exterior surface smooth we can reduce the skin friction drag.
When the object moves forward, displaced air changes it’s direction and velocity. Energy to do such changes are extracted from the object and by changing the shape of the object we can reduce the change of direction and velocity of air particles in order to reduce the losses. By making the aircraft parts more streamliner it’s able to reduce the foam drag.
Let’s take a moving cube as an example and we know it creates a huge foam drag due to the poor frontal shape. Due to the opposition created by the front face of a cube, air particles change their speeds to a very low value while their static pressure increases. Frontal face encounters a pressure greater than atmospheric pressure and aft face of the cube will encounter a pressure lower than atmospheric pressure. This pressure difference creates a sucking effect in the opposite direction to the movement. This sucking effect is known as the pressure drag.
If we consider the drag created by the whole aircraft, it is greater than the sum of drag created by individual components. Reason for this is the formed junctions and generating of eddies in the junctions when the air flow passes through them such as wing/fuselage, wing/horizontal stabilizer, wing/nacelle. In order to reduce the interference drag filleting, fairing and streamlining are good solutions.
Total drag is the sum of all drags. As shown in the diagram induced drag predominates at slow speeds and decreases with the increment of airspeed and become insignificant at high speeds. Form drag is insignificant at low speeds and increases to be the most significant at high speeds. There is a point where the parasite and induced drag become equal and this point is called the minimum drag speed(max L/D ratio). As high thrust is required in both low and high speeds moderate thrust is adequate in medium airspeeds.
The graph shows Coefficient of drag vs AOA for few aircraft and note that coefficient of drag is high at both the extremes. Drag is high at low angles of attack and high angles of attack. For most of the aerofoils, small positive angles will be the best angle of attack with the minimum drag.