Aeromodelling

 

 

Drag is the aerodynamic force that opposes an aircraft’s motion through the air. Drag is generated by every part of the airplane and it induced depends on the shape, size, inclination, and flow conditions of the air passing the object.

 

Drags commonly caused by the airplane exposed parts in the air stream which are not lift producers called as Parasite Drag. The two major contributors to parasite drag are Form drag and Skin-friction drag.

 

 

 

Form drag refer to the resistance which is the air flow past an object, the air stream no longer get a smooth streamline flow. The amount of induced air turbulent and vortices depend upon the shape of the object.

 

Skin friction drag refer to the skin smoothness of aerodynamically structures determines the resistance of the skin to airflow. If such resistance exists, the stream line of a thin layer is disturbed and affecting the adjacent layers. This form of drag is known as Skin-friction Drag.

Induced drag is the unavoidable by-product of lift as increases the angle of attack. The greater the angle of attack, up to a critical angle, the greater the amount of lift developed and the greater the induced drag.

There are two different types of induced drag

a) Dynamic induced drag

b) Pressure induced drag

Dynamic induced drag is depending on the angle of attack of the aerofoil, when the aerofoil move across the air stream there will be a force backward (induced drag) and a force upward (lift). The amount of force in each direction will depend on the angle of attack, shown in below diagram.

However, at very high angles of attack, approaching the stall point, lift will decrease and the drag will overcome lift and thrust with an accompanying loss of speed and attitude. If the angles of attack of aerofoil almost vertical to the relative wind, the only force would be backward; that is, all dynamic drag and no lift.

Pressure-induced drag refer to left generated across the wing, the pressure below an airplane wing is higher than the pressure above it. As a result, the air at the high pressure side (bottom surface) tends to flow outwards to the wind tip. Therefore, there is a constant tendency of air to flow from bottom to top. The air flow over the top surface of a wing tends to move in towards the fuselage and off the trailing edge.

Since the airplane is constantly moving the air is forced up at the wing tips. This causes a spiral or vortex which trails behind each wingtip whenever lift is being produced. These vortices increase drag, because of the turbulence produced, and constitute induced drag.