Ground Effect - ATPL 081 — Principles of Flight

The three-dimensional story so far assumed the wing flies in open air, free to trail its vortex system as deep as it likes. Within roughly a wingspan of the runway that assumption fails: the ground is in the way. The result is ground effect: the aircraft flying measurably better in the last few metres than anywhere else, and every flight passes through it twice (once climbing out, once flaring to land). This lesson explains the mechanism and then follows it into the flare, the lift-off, and the differences a wing's height above the ground makes.

What the ground does to the vortex system

The tip vortices and the downwash they feed need room below and behind the wing to develop. Close to the ground that room is gone: the surface blocks the downward flow, the tip vortices cannot grow to full strength, and the downwash at the wing is reduced. The whole airflow pattern flattens: streamlines that would have been deflected steeply downward now leave the wing more nearly horizontally, with a region of raised pressure beneath, often called a cushion, though it is not air compressed under the wing: the mechanism is the vortex system being suppressed.

Now replay the induced-drag lesson with a weaker downwash. Less downwash means a smaller induced angle of attack, so the effective airflow is less tilted, the lift vector leans back less, and the induced drag coefficient falls: at the same geometric angle of attack the wing produces more lift and less drag. The effect switches on gradually with height: barely noticeable at one wingspan above the surface, clear below half a wingspan, and strong in the flare: at a height of about a tenth of the span, the induced drag coefficient can be roughly halved.

Effective alpha and the lift curve

The lift side of the same mechanism: with the downwash reduced, the effective angle of attack rises at any given geometric angle: the wing meets air that is less bent downward, so it works at more of the angle the pilot set. On the $C_L$ –α graph this steepens the curve: in ground effect the wing reaches any given $C_L$ at a lower geometric angle of attack than in free air. And because the section still lets go at its own effective angle, that steeper curve tops out earlier: the stalling angle of attack, $\alpha_{crit}$ , is reduced in ground effect. An aircraft held off in the flare stalls at a smaller angle of attack, which, holding off level over the runway, also means a lower nose attitude than the free-air figure suggests.

Entering and leaving ground effect

Because the effect switches on and off within a wingspan of the surface, every take-off and landing crosses its boundary, and the crossing itself is felt. Entering ground effect in the flare, the aircraft suddenly flies better: lift rises, induced drag falls, and the aircraft's surplus energy (excess speed, extra lift, missing drag, plus any residual thrust) makes it float: the induced drag that would have decayed its speed is suddenly missing, so the speed holds and the wing keeps flying down the runway. The reduced downwash also reaches the tailplane: the tail's steady down-force depends partly on the wing's downwash arriving from above, so as that downwash weakens, the down-force weakens with it, and many aircraft pitch gently nose-down onto the runway attitude in the flare.

Leaving ground effect after lift-off reverses the same mechanism, and this direction is the dangerous one. Climbing through roughly half a wingspan, the downwash re-establishes: the effective angle of attack drops, lift falls away, and the induced drag rises just as the aircraft is slow, heavy and close to the ground. And the tailplane feels the change too: the returning downwash restores the tail's down-force, giving a nose-UP pitching tendency at exactly the moment more angle of attack is the last thing the aircraft needs: a tendency to hold against, not to chase. An aircraft hauled off the runway early (possible, because in ground effect it lifts off below its free-air flying speed) can find itself out of ground effect with insufficient speed to climb, and settle back onto the runway. That failure mode is why the scheduled rotation speed guarantees flight clear of ground effect: rotate at it, and the boundary passes unnoticed.

Take-off and landing

Gather the consequences by phase:

Take-off — in ground effect the aircraft accelerates better (less induced drag) and can lift off below its free-air flying speed; the risks live at the boundary, where climbing out of the effect raises drag and lowers lift at the worst possible moment. Rotate at the computed speed and the boundary passes unnoticed; rotate early and the aircraft may be unable to keep flying once clear of ground effect.
Landing — entering ground effect the aircraft floats: lift up, drag down, speed bleeding off only slowly. Carried excess speed translates almost directly into extra landing distance, which is why stabilised approach speeds matter most exactly where the runway is short.

High wing, low wing

How strongly an aeroplane feels all of this depends on how close its wing (not its wheels) is to the ground:

Low-wing — the wing rides deep in ground effect during take-off and landing: pronounced float, a clear lift-off-early temptation, and noticeable pitch changes at the boundary.
High-wing — the wing sits a whole fuselage-height higher, so every effect is weaker: less float, a smaller lift bonus, milder changes on entering and leaving.

Same physics, different distance, which is why the difference shows up in each type's landing technique.

What the exam asks

This lesson is 'explain the mechanism, then apply it to the runway'. The overview, following the learning objectives:

Explain the influence of ground effect on wing tip vortices, downwash, airflow pattern, lift and drag

Describe its influence on the induced angle of attack and on $C_{D\,i}$ : both reduced

Describe its influence on the effective angle of attack and $C_L$ : both raised at a given geometric angle

Describe its influence on the stalling angle of attack: $\alpha_{crit}$ is reduced

Explain the effects of entering ground effect (float, gentle pitch-down) and leaving it (lift down, drag up, nose-up tendency, settling risk)

Describe the influence on take-off and landing characteristics and performance

Describe the difference between high- and low-wing aeroplanes: the wing's height above the ground decides the strength