The altitude at which an airplane flies will affect its flight characteristics. At high altitudes, they typically experience better fuel efficiency. The air is less dense at high altitudes, so airplanes generate less drag. At low altitudes, conversely, the dense air results in more drag and poorer fuel efficiency. Ground effect, however, can reduce the amount of drag an airplane generates at very low altitudes by 25% to 50%.
An Introduction to Ground Effect
Ground effect is a phenomenon that involves reduced drag and increased lift when an airplane flies near the surface. Because it involves flying at very low altitudes, ground effect is usually felt during landings and takeoffs. It typically occurs at very low altitudes — usually about one wingspan or less. If an airplane has a wingspan of 150 feet, for instance, it will experience ground effect when flying at 150 feet or lower.
How Lift Is Generated
To better understand ground effect, you must familiarize yourself with the process of generating lift. Airplanes generate lift from air moving over and under their wings. When an airplane moves forward, air will flow over and under its wings. Not only does this generate lift, though; it also produces vortices.
Vortices are pockets of swirling air. They occur most frequently at the wingtips. Vortices induce drag, which can slow down airplanes and make them less fuel efficient.
What Happens During Ground Effect
Airplanes behave differently close to the surface. When airplanes get very close to the ground, less air will flow under their wings. The end result is weaker vortices at the wingtips and weaker downwash — a phenomenon referred to as ground effect.
With weaker vortices and downwash, ground effect increases lift and decreases drag. Pilots can typically feel this one-two combination while flying. During takeoffs, the airplane may lift off the runway sooner than expected. During landings, the opposite happens: the airplane may seem to float longer than expected as the pilot approaches the runway.
Factors That Affect Ground Effect
Several factors can affect ground effect. As previously mentioned, wingspans play a role in this phenomenon. Airplanes with long wingspans have a greater ground effect range than their counterparts with short wingspans. They will essentially experience ground effect at higher altitudes.
The type of surface over which an airplane is flying will affect ground effect. Ground effect is typically more pronounced when flying over hard and smooth surfaces like runways. It’s weaker, conversely, when flying over rough terrain like dirt.
The weight of an airplane will influence its ground effect. Heavy airplanes require more lift to stay in the air, which in turn affects how noticeable ground effect is to them.
Finally, the use of wing flaps and landing gear play a role in ground effect. Devices such as these change the way in which air flows over airplanes.