Ground effect and wingtip vortices

When a wing comes within several feet of the ground or water, the surface restricts the vertical component of airflow around the wing, altering the wing's upwash, downwash, and wingtip vortices. The PHAK's summary is direct: as the wing encounters ground effect at a constant AOA, there is a reduction in upwash, downwash, and wingtip vortices -- exactly the ingredients of induced drag.

Since induced drag dominates at low speed, and ground effect specifically reduces induced drag, the wing needs a LOWER AOA to produce the same CL close to the ground. Held at a CONSTANT AOA instead, the wing produces MORE CL near the ground than it would at altitude -- which is the aerodynamic reason an airplane can feel like it wants to fly a little early, or float a little long.

Ground effect is a strong function of height above the surface, measured relative to wingspan, not in absolute feet. The PHAK gives the specific relationship: at a wing height equal to its own span, the reduction in induced drag is only 1.4 percent -- barely noticeable. At a height equal to one-quarter of the span, the reduction is 23.5 percent. At a height equal to one-tenth of the span, it is 47.6 percent.

This is why ground effect is "most usually recognized" only right at liftoff and just before touchdown: a large reduction in induced drag only happens when the wing is genuinely close to the ground, which for most training airplanes means within roughly a wingspan of the runway.

An airplane climbing out of ground effect experiences the reverse of an airplane entering it during landing. The PHAK lists the changes precisely: an increase in AOA is required to maintain the same CL, induced drag and thrust required both increase, stability decreases with a nose-up change in moment, and static source pressure drops, which increases the INDICATED airspeed shown in the cockpit.

That last point has a real trap in it: because ground effect increases local pressure at the static source in most cases, the indicated airspeed reads LOWER while still in ground effect than the true performance actually supports. A pilot who forces the airplane off the ground below the manufacturer's recommended speed can become airborne while still in ground effect, then find the airplane unable to sustain flight -- or unable to climb -- once it rises out of it, especially at high density altitude, high gross weight, and high temperature.

On approach, ground effect is most noticeable at altitudes below the wingspan. If the airplane arrives there at a constant AOA, it experiences an increase in CL and a reduction in thrust required -- the classic "floating" feeling during the flare. Because drag is reduced and the airplane no longer decelerates the way it would out of ground effect with power off, any excess airspeed carried into the flare can translate into a surprisingly long float down the runway.

As the airplane nears touchdown, a power reduction is normally needed specifically to offset the extra lift ground effect is producing -- without it, the airplane tends to climb back away from the desired glidepath instead of settling onto the runway.

As covered in the four-forces lesson, wingtip vortices are strongest when the generating airplane is heavy, slow, and clean -- exactly the condition on departure and final approach. Near the ground, those vortices behave in specific, predictable ways that let you avoid them by geometry rather than luck.

Wind carries vortices with it: a 10-knot wind drifts them at roughly 1,000 feet per minute in the wind direction, which is why wind speed and direction belong in your mental picture of where an intended takeoff or landing point actually sits relative to another airplane's wake.