# An Introduction to the Ground-Level Aerodynamics of Aircraft

### Key Takeaways

• Lifting characteristics are influenced by the distortion of airflow below the horizontal surfaces of the aircraft wing, which account for ground-level aerodynamics.

• It is through the reduction in the amount of induced drag that ground-level aerodynamics enhance the lift-to-drag ratio.

• Ground-level aerodynamics are maximized under calm wind conditions and over smooth, leveled, hard surface.

Ground-level aerodynamics aid pilots in making graceful landings

I have a fear of air travel, especially during flight landings. Even though the aircraft glides down from the sky by gradually lowering in altitude, I feel uncomfortable. To my slight relief, pilots never drop the plane out of the sky rapidly, rather they gracefully descend. Ground-level aerodynamics aid pilots in these graceful landings.

Ground-level aerodynamics are all about the positive influence on the lifting characteristics of aircraft wings when operating in close proximity to ground. The presence of ground beneath the horizontal surfaces of an aircraft introduce changes in the 3D flow field, which impacts the overall performance.

Let’s take a closer look at ground-level aerodynamics and how they impact flight.

## Ground-Level Aerodynamics of an Aircraft

When aircraft fly closer to the ground, the ground surface creates a positive influence on the lifting characteristics provided by the aircraft wings. The impact the ground surface produces on the aircraft wings is a result of ground-level aerodynamics. The lifting characteristics are influenced by the distortion of the airflow below the horizontal surfaces of the aircraft wing, which account for ground-level aerodynamics.

Ground-level aerodynamics are not restricted to ground or land. It can be generalized that ground-level aerodynamics come into action when an aircraft flies closer to the earth’s surface. The earth’s surface can be either ground (land) or water. Ground-level aerodynamics are more pronounced when aircraft operate in the vicinity of the ground, which is in the range of one wingspan distance from the boundary.

When underground-level aerodynamics take effect, there is an enhanced force performance observed in the lifting surface of the aircraft. The ground effect on the aircraft changes the three-dimensional flow field and associated performance variations can be seen. Ground-level aerodynamics are prominent in increasing the lift-to-drag ratio.

Let’s take a look at how ground-level aerodynamics influence the lift and drag forces acting on an aircraft.

## How Ground-Level Aerodynamics Improve Lift-to-Drag Ratio

Increased lift-to-drag ratio can be achieved either by:

• - Producing an increase in lift
• - Decreasing the induced drag

The ground-level aerodynamics of aircraft produces a reduction in the induced drag generated. It is through this reduction in the amount of induced drag that the ground-level aerodynamics enhance the lift-to-drag ratio.

### Wingtip Vortices

The effects of ground-level aerodynamics on an aircraft can be explained in terms of wingtip vortices. Wingtip vortices are the reason for induced drag and wake of turbulence in aviation. However, when the vortices are disturbed by the ground, ground-level aerodynamics effects are established.

When an aircraft is flying high, wingtip vortices are large, as there are no obstacles to prevent their expansion. When an aircraft starts flying closer to the ground at the same airspeed and angle of attack, the wingtip vortices become smaller, as the ground puts a limitation on their expansion. The flow field around the wing is altered by the ground plane. Smaller wingtip vortices lead to less downwash in the air flowing off the back of the wing.

The lesser the downwash, the less the tilting of lift vector in aircraft. The drop in downwash causes the lift vector to point up against the weight of the aircraft, which is always exerted downwards. The lift vector pointing back is less, which leads to reduced drag generation. In this way, less downwash improves the lift-to-drag ratio.

To summarize, we can say less downwash in aircraft causes:

1. An increase in the vertical lift, which opposes the weight acting downwards
2. A decrease in rear ward lift, which results in reduced drag
3. An improved lift-to-drag ratio

The ground-level aerodynamics of aircraft is the basis for all the merits brought to aviation through less downwash.

## Changes in Induced Drag

The extra lift attained by the effects of ground-level aerodynamics is originally contributed by reduced drag. The reduction in induced drag is dependent on the height of the aircraft above ground. Fluctuations in the height change the value of the lift coefficient.

As the distance of the aircraft from the ground ascends, the induced drag increases non-linearly. In a fixed-wing aircraft, the nonlinear increase in induced drag reaches the free air value at a height equal to that of the wingspan. In a helicopter, the free air value is reached at a distance equal to the rotor diameter.

Ground-level aerodynamics are maximized under calm wind conditions and over smooth, leveled, hard surfaces. Ground-level aerodynamics bring several economic benefits to the aviation industry. Cadence can help you find the ground-level aerodynamic effects of aircraft at different distances from the ground. With Cadence’s suite of CFD software, you can simulate the 3D fluid flow under ground-level aerodynamic effects.