Ground Effect

The ground effect is defined as the change in flight characteristics when aircraft or flying organisms approach the surface at close. This situation reduces drag while increasing lift. This force is typically felt up to a height equal to one wingspan.

Effect of Ground Effect on Flight Characteristics

1. Takeoff

During takeoff, the ground effect provides additional lift to aircraft due to air compressed beneath the wings. This reduces the required takeoff distance.

2. Turns and Maneuvers

The ground effect facilitates aircraft control during turns, especially at low altitudes. Thanks to the additional lift, airplane can complete turns with less energy expenditure. Simultaneously, by increasing the aircraft’s critical angle of attack, it prevents the aircraft from entering "stalls" during low-speed maneuvers.

3. High Altitudes

At high altitudes, the amount of air compressed beneath the wings decreases. This prevents aircraft from benefiting from the ground effect at elevated altitudes.

4. Landing

During landing, when the aircraft enters the ground effect, it encounters a phenomenon known as "float". To counteract this force and successfully touch down, pilots set the aircraft’s engines to "idle" position.

Principle of Ground Effect

Restriction of Wingtip Vortices

The ground effect fundamentally restricts the formation of wingtip vortices. Near the ground, wingtip vortices cannot grow as large as they do in free air. This reduces the strength of the downwash effect. The image below shows the size of wingtip vortices at different altitudes. As seen in the image, the wingtip vortices of the aircraft close to the ground have not fully developed complete.

Size of wingtip vortices at different altitudes - Engineer Element

Reduction of Downwash and Aerodynamic Drag

The air flowing over the wing and the wingtip vortices exit the wing at a downward angle. This causes the airflow over the wing to be directed downward reason. Since the direction of lift is always perpendicular to the direction of the airflow over the wing, the lift vector begins to tilt backward. This results in a force known as induced drag. By restricting the wingtip vortices, the effects of this phenomenon are minimized.

Illustration of downwash effects on a Cirrus SR-22 at high altitude - Boldmethod

Illustration of downwash effects on a Cirrus SR-22 at low altitude - Boldmethod

Ground Effect on Different Wing Configurations

The ground effect affects low-wing and high-wing aircraft differently. This can be better understood by examining two examples: the Cessna 172 and the Piper Warrior.

The Cessna 172 has its wings approximately 2 meters above the ground when parked, which is about 20% of its wingspan. This results in the induced drag just before landing being equivalent to approximately 60% of the induced drag at high altitude.

The Piper Warrior’s wings are approximately 1 meter above the ground when parked, which is about 10% of its wingspan. This results in the induced drag just before landing being equivalent to approximately 40% of the induced drag at high altitude. This difference allows the Piper Warrior to benefit more from the ground effect than the Cessna 172.

Comparison of ground effect between Cessna 172 and Piper Warrior aircraft - Boldmethod