Hey everyone,
Can somebody please shine some light on this topic; Effects of entering and leaving ground effect on longitudinal stability?

G'day,

Upwash and downwash effects on the relative airflow are reduced in ground effect, because (to put it simply) the ground simply gets in the way and physically prevents the formation of strong vortices. This reduction in up- and downwash means the relative airflow (RAF) is not deflected downwards so dramatically as the wing moves through it. Since lift acts perpendicular to the relative airflow, as the direction of the RAF changes, so too does the direction of the lift vector.

Downwash means the lift vector is oriented more towards the rear, resulting in a component of the lift actually acting opposite to the direction of motion. This is induced drag.

More downwash means a more rearward orientation of the lift vector and more induced drag.
Less downwash means less rearward orientation of the lift vector and less induced drag.

Therefore, since ground effect reduces downwash, it also reduces induced drag.

The result of this change on the aircraft's handling can be summarised as follows:
Leaving ground effect causes a reduction in longitudinal stability and a nose up pitching moment
Entering ground effect causes an increase in longitudinal stability and a nose down pitching moment.

These are the worst combinations of effects on the aircraft as you transit in and out of ground effect. In fact that's an easy way to help you remember which is which.

I don't know if you have already seen this topic, but there are some good answers in here too :
www.bobtait.com.au/forum/aerodynamics/4959-ground-effect#7734

Or a nice plain language article here as well:
code7700.com/ground_effect.html

Cheers,

Rich

Awww thanks Richard for the clarification

But why is it exactly that longitudinal stability is considered to be decreased when leaving ground effect? Is it as a result of the induced drag causing a reduction in speed therefore decreasing elevator authority and its stabilising effects? Or is a loss of longitudinal stability taken to have occurred because a more level attitude was the pilots intention/was the reference point and a pitch away from that is instability? Or maybe something else?

Cheers

Probably not a simple answer question.

However, longitudinal static stability has to do with the elevator forces which the pilot senses when maintaining an off trim speed (which relate to the desire of the aircraft to pitch up/down). Think what force you need to hold the IAS below or above trim speed. Anything which affects these forces will be sensed as a change in LSS by the pilot (even if he/she doesn't think of things that way).

Transitioning IGE/OGE or OGE/IGE will see significant and progressive changes in upwash/downwash around the wing and tail airflows. The net effect on pitching moment will determine what effect exists for LSS. Typically, during the takeoff rotation, as you transition IGE/OGE, you will be holding a degree of pull force on the stick. A nose up change in pitching moment will give you the sensation of a reduction in LSS as the pull force on the stick reduces.

Don't worry too much if the following is outside your study background but it is included for those who might be interested - the main effects transitioning to IGE are

(a) a reduction in the downwash angle at the tail

(b) an increase in the wing-body lift slope

(c) an increase in the tail lift slope
.
Stability near the ground becomes a matter of estimating/measuring these three effects.

The important things to be figured out are the elevator angle and control force necessary to maintain Clmax in level flight close to the ground. Usually, the elevator angle required to trim at CLmax IGE is larger than for OGE. This often becomes the critical design case for the forward CG limit.

While not related, specifically, to GE, the same sort of phenomenon gives rise to the need for stability augmentation systems seen typically on some of the piston to turboprop conversions. At low speed, high power, and high nose-up pitch attitude (ie missed approach, particularly), there is a vertical force on the propeller which gives a signficant nose up pitching moment and can cause the aircraft to become statically unstable without some assistance from an elevator circuit variable down spring arrangement.