Subsonic Aerodynamics | Ground Effect | PPL(A) Principles of Flight

All content is copyrighted. Personal or group use only. Copying or printing is strictly prohibited.

What is Ground Effect?

Ground effect is the change in aerodynamic behaviour that occurs when an aircraft flies close to the surface. The ground physically interrupts the normal formation of wingtip vortices, altering the airflow around the wing and producing measurable changes in lift, drag, and pitch behaviour.

Ground effect becomes significant at heights below one wingspan above the surface, and is most pronounced below half the wingspan height.

Ground Effect Threshold — Exam Point

Ground effect is significant below one wingspan height, and most pronounced below half the wingspan height. These two figures are both examined — know both.

---

Why Ground Effect Occurs — The Mechanism

On a wing in free air, the pressure difference between the lower and upper surfaces causes air to spill around the wingtips, rolling up into wingtip vortices. These vortices induce a downward component of airflow (downwash) across the wing, which:

• Tilts the local relative airflow downward
• Increases the induced angle of attack required to generate lift
• Tilts the lift vector rearward, producing induced drag

When the aircraft descends close to the ground, the surface physically restricts the downward flow from the wingtip vortices. The vortices cannot develop fully — they are compressed and weakened. This produces a cascade of aerodynamic changes.

---

Aerodynamic Effects of Entering Ground Effect

As the aircraft descends into ground effect:

• Wingtip vortices are suppressed — the ground blocks the downward airflow that feeds vortex formation
• Downwash decreases — with weaker vortices, the downward component of airflow across the wing is reduced
• Induced angle of attack decreases — less downwash means a smaller angle of attack is needed to generate the same lift
• The lift vector tilts forward — as the induced angle of attack reduces, the lift vector rotates forward, reducing its rearward (drag) component
• Induced drag decreases — the forward tilt of the lift vector directly reduces induced drag
• Lift increases — at the same pitch attitude and airspeed, the wing generates more lift in ground effect than in free air due to the reduced induced angle of attack and forward-tilted lift vector

Efficiency Gain in Ground Effect

In ground effect, the wing operates more efficiently — the same lift is generated with less induced drag. This is why aircraft can become airborne at a lower speed than required to sustain flight out of ground effect, and why they may float during landing.

---

Effect on the Horizontal Stabiliser

Ground effect does not only affect the wing — it also changes the airflow reaching the horizontal stabiliser (tailplane):

• In normal flight, the wing’s downwash tilts the airflow downward before it reaches the tailplane, giving the tailplane a reduced effective angle of attack
• The tailplane uses this reduced AoA to generate a tail-down aerodynamic load — this balances the nose-down pitching moment of the wing and keeps the aircraft in trim
• In ground effect, downwash from the wing decreases — the airflow arriving at the tailplane is less deflected downward
• The tailplane’s effective angle of attack increases, but this means it generates less tail-down load than in free air — reducing the nose-up balancing force
• The result is a nose-down pitch tendency as the tail-down balancing force weakens

This pitch-down tendency in ground effect directly influences flare technique — the pilot must apply back pressure to arrest the descent and hold the nose up for touchdown.

Pitch Change in Ground Effect

As the aircraft enters ground effect on approach, expect a nose-down pitch tendency caused by increased stabiliser effectiveness. Failure to apply sufficient back pressure during the flare will result in a firm or early touchdown.

---

Takeoff Implications

Ground effect has important consequences during takeoff:

• Induced drag is reduced while the aircraft is still close to the ground
• The aircraft may become airborne before reaching the speed required to sustain flight in free air
• As the aircraft climbs out of ground effect, induced drag increases sharply
• If the aircraft is underpowered, overweight, or the pilot climbs too steeply, the sudden increase in induced drag can prevent further climb — or cause the aircraft to settle back onto the runway

Takeoff Out of Ground Effect

An aircraft that lifts off within ground effect may be flying at a speed below the free-air climb speed. Climbing too steeply or too early can take the aircraft out of ground effect before it has sufficient airspeed — induced drag increases suddenly and the aircraft may be unable to climb, or may sink back to the surface.

---

Landing Implications

Ground effect has equally important consequences during landing:

• As the aircraft descends through half-wingspan height on final approach, induced drag decreases and lift increases
• The aircraft floats — it continues to fly at a height and speed where, in free air, it would be descending more steeply
• This float extends the landing roll and can cause the aircraft to touch down further along the runway than planned
• In a crosswind, the float phase increases exposure to drift before touchdown

Floating in Ground Effect

Excess speed on approach significantly worsens the float in ground effect. A fast approach combined with ground effect cushioning can use a large portion of the available runway before touchdown occurs. Always fly a stabilised approach at the correct speed.

---

Leaving Ground Effect

When the aircraft climbs out of ground effect — either during takeoff or a go-around — the aerodynamic environment changes in the opposite direction:

• Wingtip vortices reform freely — without the ground restriction, vortices develop fully
• Downwash increases — stronger vortices induce more downward airflow across the wing
• Induced angle of attack increases — more angle of attack is required to maintain the same lift
• Induced drag increases — the lift vector tilts rearward again as the induced angle of attack grows
• More thrust is required to maintain the same climb rate and airspeed

This transition is most critical during takeoff when the aircraft is slow, heavy, and close to its performance limits.

---

Summary — Ground Effect at a Glance

Parameter	In Ground Effect	Out of Ground Effect
Wingtip vortices	Suppressed	Fully developed
Downwash	Reduced	Normal
Induced angle of attack	Decreased	Normal
Induced drag	Reduced	Normal (higher)
Lift (same speed/attitude)	Increased	Normal
Stabiliser effectiveness	Increased	Normal
Pitch tendency	Nose-down	Neutral
Takeoff risk	May lift off below climb speed	Drag increases on climb-out
Landing risk	Aircraft floats, longer ground roll	Normal approach behaviour