Prerequisite Knowledge:
Drag is one of the undesirable forces acting on an aircraft; designs with more drag require more thrust to stay in the air, which requires more powerful propulsion systems. This article aims to explain the complexities of drag.
Types of Drag:
There are several types of drag that act on an aircraft, and they are as follows:
Parasite Drag:
Parasitic drag can be broken down into several categories, as stated above:
Skin Friction
Friction is a force that resists the motion of two objects sliding against each other. In the context of aerodynamic drag, skin friction is the friction between the air and the aircraft. In order to minimize skin friction, the surface of an aircraft should be kept as smooth as possible. Even something as simple as bug splatter on the leading edges of an aircraft can generate significant amounts of skin friction at high speeds
Form Drag
Form drag is a product of the shape of an object. Aerodynamic shapes should aim to slice through the air, displacing it progressively (like a boat moving through water). Designs with a blunt shape that impact the air rather than displacing it have the highest amount of form drag.
Drag is one of the undesirable forces acting on an aircraft; designs with more drag require more thrust to stay in the air, which requires more powerful propulsion systems. This article aims to explain the complexities of drag.
Types of Drag:
There are several types of drag that act on an aircraft, and they are as follows:
- Parasite drag: Drag produced by non-lift producing surfaces (e.g. the landing gear of an aircraft). This type of drag can be broken down into form drag, interference drag, and skin friction.
- Induced drag: Drag produced by lift generating surfaces (e.g. the wings).
- Wave drag: Drag produced by shockwaves along a sonic/supersonic body. This type of drag will be discussed more in the transonic /supersonic aerodynamics article.
Parasite Drag:
Parasitic drag can be broken down into several categories, as stated above:
Skin Friction
Friction is a force that resists the motion of two objects sliding against each other. In the context of aerodynamic drag, skin friction is the friction between the air and the aircraft. In order to minimize skin friction, the surface of an aircraft should be kept as smooth as possible. Even something as simple as bug splatter on the leading edges of an aircraft can generate significant amounts of skin friction at high speeds
Form Drag
Form drag is a product of the shape of an object. Aerodynamic shapes should aim to slice through the air, displacing it progressively (like a boat moving through water). Designs with a blunt shape that impact the air rather than displacing it have the highest amount of form drag.
Shapes with smaller drag coefficients are more aerodynamic.
Interference Drag
Like a boat moving through water, aircraft displace the air it moves through. This can create areas of turbulent air, which increase drag significantly. Interference drag is why modern airliners have their engines placed ahead of the wing, as they create significant amounts of interference drag that can disrupt the flow of air over the wing.
Like a boat moving through water, aircraft displace the air it moves through. This can create areas of turbulent air, which increase drag significantly. Interference drag is why modern airliners have their engines placed ahead of the wing, as they create significant amounts of interference drag that can disrupt the flow of air over the wing.
Induced Drag:
Induced drag is drag that is produced by lift generating surfaces on the aircraft (primarily the wings).
In the lift generation article, you learned that wings generate lift by creating a pressure difference between the upper and lower surfaces. Since fluids naturally want to balance out pressures, the higher pressure below the wing will generally attempt to curl over the wingtip, in order to reach the low pressure above the wing and balance the pressures. This creates spiralling vortices at an aircraft's wingtips, known as wake vortices.
Induced drag is drag that is produced by lift generating surfaces on the aircraft (primarily the wings).
In the lift generation article, you learned that wings generate lift by creating a pressure difference between the upper and lower surfaces. Since fluids naturally want to balance out pressures, the higher pressure below the wing will generally attempt to curl over the wingtip, in order to reach the low pressure above the wing and balance the pressures. This creates spiralling vortices at an aircraft's wingtips, known as wake vortices.
These vortices produce significant amounts of drag, and can extend behind the aircraft for miles, posing a threat to other aircraft. Wake vortices will be covered in more detail in a separate aerodynamics article.
The Drag Equation
The drag equation is actually the exact same as the lift equation, seen in the previous article:
The Drag Equation
The drag equation is actually the exact same as the lift equation, seen in the previous article:
The variables are the exact same as well. The reason for this is that the drag equation works the same as the lift equation, it uses the principle of fluid continuity to calculate the force exerted by drag. One should know the basics of this formula, and note a few things. One of the biggest things is that drag is proportional to fluid density, the coefficient of drag, and the area of the surface, but drag is also proportional to the fluid velocity squared. This means that the velocity of the object relative to the fluid has the biggest effect on the drag force.
That's it for drag! The next article will cover an aircraft's primary flight controls.
That's it for drag! The next article will cover an aircraft's primary flight controls.