When leaving ground effect, why is there a nose-up change in moment?

Question

The Airplane Flight Handbook states:

The aircraft leaving ground effect will

1. Require an increase in AOA to maintain the same CL
2. Experience an increase in induced drag and thrust required
3. Experience a decrease in stability and a nose-up change in moment
4. Experience a reduction in static source pressure and increase in indicated airspeed

I'm having a hard time understanding the number 3 here.

What's the aerodynamic reason for the nose-up change in moment after leaving the ground effect?

Is it because the downwash from the wings increases as the aircraft leaves the ground effect, thus placing a greater downward pressure on the horizontal stabilizer?

Answer 1

Koyovis' explanation is correct and deserves to be the accepted answer. However, as always, much depends on the details of the specific configuration - generalizations like those in your question deserve to be treated with some caution.

Ground effect does not always result in a pitch-down change. The Handley-Page Victor was famous for its ability to flare all by itself when entering ground effect. From Wikipedia:

One unusual flight characteristic of the early Victor was its self-landing capability; once lined up with the runway, the aircraft would naturally flare as the wing entered into ground effect while the tail continued to sink, giving a cushioned landing without any command or intervention by the pilot.

The key factors were the T-tail and the low crescent wing: Since the wing would enter ground effect much earlier than the tail surface, its lift would increase earlier. The low wing in combination with the high tail would add a pitch-up change in ground effect.

Why is the lift curve slope different in ground effect? The proximity of the ground reduces not only the downwash angle but also the induced angle ahead of the wing. A highly cambered wing with low angle of attack will experience a reduction in lift. However, with a positive angle of attack the airflow below the wing will be partially blocked by the ground, so pressure will increase below the wing and force more air to flow around the leading edge and over the wing, resulting in an increase of the lift curve slope.

With a low tail, the handbook is correct, however: Since the tail flies in the downwash of the wing, a reduced downwash results in a positive angle of attack change at the tail, increasing lift there and resulting in a pitch-down change. This comes on top of the change in the pressure distribution over the wing which shifts the center of pressure backwards, adding more nose-down pitch, as described by Koyovis. Leaving ground effect will lower the angle of attack at the tail and makes itself felt as a pitch-up change.

The F/A-18 has suffered from this effect and needed a kludge to restore its pitch control power to the out-of-ground-effect value, namely toed-in rudders. As Jan Roskam explains in his book "Roskam's Airplane War Stories" (War story 108):

When the first F-18 fighter […] was flight tested at Patuxent River, it became evident that the airplane would not rotate at the predicted speed. This made the field performance of the airplane unacceptable. The problem was traced to an error in the calculation of aerodynamic forces in ground effect. This is particularly severe in case of a low placed horizontal stabilizer. As a result there was insufficient down-load capability to effect early rotation during the takeoff ground roll.

The problem was fixed by toe-in of the rudders. A squat-switch on the main gear biasses the rudders to deflect inward while on the ground. This creates enough positive pressure over the aft fuselage to effect early rotation.

This fix, although impressive, came at a price. All flight control software had to be revalidated. Also, the squat-switches represented additional system complexity.

Answer 2

The aerodynamic centre of a wing shifts when it is in ground effect. Out of ground effect, the wing produces lift by accelerating air downwards. In ground effect the ground prevents the air from being deflected downwards, and more lift is produced due to increased pressure underneath the wing. This pressure has a different distribution than out-of-ground effect pressure, hence the change in pitching moment.

Consider a cross section of a wing: free stream lifting force centre is at the quarter chord point. If the wing was supported by just static pressure only, like a hovercraft, the force centre point would be at 50% chord.

Actually not sure now if the horizontal tail has much influence but I'll leave the original answer underneath:

Ground effect works by increasing static pressure underneath the wing and the horizontal tail. The aircraft flies out of ground effect in a nose-up attitude, and depending on tail configuration it can be possible for the main wing to be out of ground effect while the horizontal tail still experiences increased pressure underneath, therefore creating a nose-down pitching moment. Once the horizontal tail flies out of ground effect as well, the nose-down pitching moment disappears. I would not expect that T-tails have this change in pitching moment but I have never flown one.

Answer 3

As you may already know, when an aircraft is in ground effect the upwash, downwash and wing tip vortices are all reduced which means the wing behaves as though it's aspect ratio has been increased that is to say the induced drag and angle of attack are reduced for the coefficient of lift.

An aircraft can become destabilised climbing out of ground effect because if a constant angle of attack is maintained it may experience a loss of lift which in turn would further increase the nose up movement.

In conditions such as high temperature and a high all up weight it could prove impossible to climb out of ground effect.