Ground effect and wake vortices

Change the induced flow near the surface

A nearby ground plane changes the wing’s trailing flow and induced downwash.

Why induced drag reduces

Near the ground, the vortex system and downwash are constrained. At the same lift, induced drag is reduced compared with flight well clear of the surface. The effect depends on height relative to span.

The change is strongest close to the surface and decays with height. It does not remove parasite drag or guarantee an aircraft can climb out of ground effect at its present speed and thrust.

Lift-off and flare consequences

A pilot may lift off into a low-drag condition but lack excess thrust when induced drag increases during climb away. Acceleration and configuration must follow the approved take-off technique.

During landing, reduced induced drag can extend float if excess speed is carried into the flare. Pitching-moment and static-source changes may also occur, depending on the aircraft.

A thrust-margin calculation during climb away

Use the laboratory’s fictional out-of-ground-effect induced drag of 5 kN and parasite drag of 3 kN. At h/b = 0.05, the supplied factor is 0.64/1.64 = 0.390, so induced drag is about 1.95 kN and total drag 4.95 kN. Farther from the ground the total tends toward 8 kN.

If only 6 kN of thrust is available at the same selected speed, there is initially about 1.05 kN of pathwise surplus before any climb component is added. That apparent surplus disappears as the induced-flow reduction is lost. The numbers illustrate why a low-speed lift-off can be misleading about sustainable climb performance.

Increase height-to-span in the laboratory while holding lift and the supplied outside drag components fixed. Find where total drag reaches 6 kN. Do not convert that crossing into an aircraft height limit: the factor is approximate and the model omits acceleration, engine variation, configuration, runway technique and the aircraft’s certified take-off data.

Follow the vortex system

Wake strength relates to the lift the generating aircraft needs, not simply to how loud its engines are.

Generation and motion

A finite lifting wing generates trailing vortices. Heavy, slow, clean conditions generally require high circulation and can produce strong wake. Vortices tend to descend initially and are transported by the ambient wind.

Near the ground, lateral motion, wind and terrain change where the wake remains. A light crosswind can leave a vortex close to a runway corridor. Visual appearance is not a reliable measure of remaining strength.

Roll control has limits

A following aircraft entering a vortex can encounter a rolling moment that exceeds available roll-control authority. Size, span, encounter geometry and separation all affect the consequence.

Use published wake-separation requirements and avoidance procedures. The diagram is causal, not a device for computing reduced separation or predicting vortex dissipation time.

Follow a wake in the air mass

Consider a heavy aircraft passing above a runway while a light crosswind transports its wake sideways. The follower’s ground track does not reveal the vortex location by itself: the wake has descended and drifted since generation. Near the ground the two vortices can move differently relative to the runway corridor.

A slower generating aircraft at the same weight needs greater CL and circulation in the simple lifting picture. A follower with a smaller span may encounter a larger gradient in the swirling flow across its own wing, creating a strong rolling moment. Full aileron is not a reliable guarantee that this moment can be opposed.

Use the ground-effect curve only for its stated fixed-lift induced-flow comparison. It does not calculate a wake’s position or decay. For the operational wake problem, name the missing facts: generating aircraft category and path, elapsed time, wind, approach or departure geometry, and the applicable published separation and avoidance procedures. A useful explanation separates that hazard assessment from the near-ground drag benefit.

Can you explain and apply the mechanism?

Four original questions. Work from the stated facts, then compare your reasoning with the explanation.