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Calculating the Air Resistance Force in an Aerodynamic System

Key Takeaways

  • Air resistance force, also known as drag force, is simply the resistance that an object experiences due to the air when in motion. 

  • Using the drag equation, the air resistance force can be calculated, which is beneficial in understanding lift and power requirements.

  • CFD can help maintain accuracy in air resistance force calculations, which is a crucial measurement for improving aerodynamic performance. 

Drag force graphic

When an object comes in contact with the moving air, it encounters some kind of resistance. This can be experienced during a simple stroll along a walkway on a windy day, as the wind resists the motion of the object in contact and tries to push back. This resistance plays an important factor in the design of aerodynamic devices like airplanes.

Also known as drag, the air resistance force is the direct result of the collision of air particles with the surface of a flying object. This pushes the object in the opposite direction of the motion, slowing it down. Understanding how this force acts and finding a way to minimize its effects is crucial for efficient aerodynamic system design. 

The Effect of Air Resistance Force on an Aircraft

Air resistance force, or drag, is the force that acts opposite to the motion of the object, causing it to slow down. In aircraft, the air resistance force is responsible for interfering with lift and thrust to slow a vehicle down. This resistant force can be generated in a moving object because of multiple reasons:

  1. Pressure: Due to the motion, the air particles become compressed and push back the approaching front surface of an object. This induces a backward force called pressure drag. The intensity of the drag is directly proportional to the shape and size of the object as well as its speed.  

  2. Skin friction: When the air particles and the surface of the object collide in motion, there is friction. The particle near the surface slows down and attaches to the surface to create a boundary layer. As the boundary layers become turbulent, there is increased shear stress on the surface, which increases the skin-friction drag and reduces the speed.

  3. Lift: Lift is generated when there is a pressure difference between the top and bottom surface of the airfoil. However, when the angle of attack of the airfoil is higher and the speed of the flight relatively slower, the air deflects downwards, inducing a drag force. 

  4. Wave: Wave drag is experienced by the aircraft moving at supersonic and transonic speeds due to the formation of shock waves along the leading and trailing edges of an airfoil. 

As detailed, the effect of the air resistance force is mostly negative on the aerodynamic performance of the aircraft, and mitigating this is one of the primary concerns for engineers. 

Calculating the Air Resistance Force

By determining the air resistance force in an aircraft, it is possible to calculate the desired lift and power requirements. However, this analysis requires the accurate knowledge of air flow behavior, velocity, density, and their interaction with the airfoil. During the design and testing phase, test scale models of the aircraft can be tested in wind tunnels, which can replicate real air flow conditions. The air resistance force, moment, and pressure can be directly calculated, allowing engineers to make necessary optimizations to the airfoil.

The calculation of the air resistance force can then be done by simply using the drag equation:

Air resistance force

Note that:

Fd is the air resistance force

Cd is the drag coefficient

A is the cross-section area

ρ is the density of fluid/ air

V is the flow velocity

The power requirement (Pd) to overcome this resistance force can then be calculated using the following expression:

Power requirement to overcome the air resistance force

Simplifying Aerodynamic Force Calculations for Design Efficiency

Aerodynamic design is much more than a simple numerical calculation of lift or resistant forces. It requires a full-scale knowledge of flow conditions and behaviors, airfoil design, boundary conditions, and many other factors that influence aerodynamic performance. CFD meshing and simulation is an ideal technique to analyze the effectiveness of your design under different flow conditions. With the CFD tools from Cadence, a high-performance and cost-effective aerodynamic system can be designed.

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