The principle of ground effect explains why airplanes float during landing. It is a concept that pilots need to be familiar with because ground effect affects how aircraft act on takeoff and landing. If you’re preparing for a checkride, you should know how to explain ground effect. You should also know that ground effect can cause your airplane to float and float and float down the runway when you’re trying to have a nice landing.
In some takeoff situations, ground effect can be used to an advantage to improve acceleration if the runway is long enough. In soft-field situations, where you want to take the weight off the wheels during the ground run, ground effect can be used to get off the ground before true flying speed is reached.
Not Just a Cushion Of Air
The internet is full of information on ground effect, with some explanations better than others. When I think back on my private pilot flying lessons, I vaguely remember my CFI talking about the “cushion of air” near the ground on landing, and when learning soft-field takeoffs. I don’t feel like I got an explanation of why ground effect occurs, and how the surface of the ground restricts the airflow around the wing.
The Pilot’s Handbook of Aeronautical Knowledge (PHAK) Chapter 5 delves into the aerodynamic forces that act on the aircraft, and has several pages of explanations and illustrations about ground effect. You can also find explanations of ground effect on Skybrary, AOPA, and Bold Method.
A common struggle with explaining aviation concepts is not being able to understand the relationships between things. Any explanation of ground effect requires an understanding of wingtip vortices, induced drag, and downwash. Before we get into ground effect, let’s understand these concepts:
➡️ Wingtip vortices: Circular patterns, “mini invisible tornadoes” of rotating air left behind a wing as lift is generated. Normally, wingtip vortices create drag. Vortices are strongest at higher angles of attack when the difference in pressure above the wing (low pressure) and below the wing (high pressure) is greatest.
➡️ Induced drag: If you ever heard a flight instructor say the phrase, “you don’t get anything for free,” they are likely referring to induced drag, the inevitable consequence of lift. Induced drag is sometimes called vortex drag or lift-induced drag.
➡️ Downwash: As wingtip vortices roll off the trailing edge of the wing, they angle down. This downward movement of air is called downwash. Another way of thinking about downwash is the change in the direction of air that is created by a propeller, wing, or helicopter blade. When you have more downwash, you have more induced drag.
Now that we have a basic understanding of these concepts we can understand what happens when an aircraft is close to the ground (about half the length of the aircraft’s wingspan above the ground). When an aircraft is close to the ground, induce drag is lessened for two reasons (1) wingtip vortices cannot fully develop because they are interrupted by the ground or water surface and (2) downwash is reduced.
When we explain ground effect it should not talking be about floating on an imaginary cushion of air down a runway. Any explanation of ground effect should mention reduced induced drag. Close to the ground, vortex formation is stopped before it’s fully developed, and the downwash is reduced.