Welcome to the CPL Aerodynamics question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.
When entering ground effect why would the aircraft become more stable? I'm guessing that the horizontal stabiliser has less downwash from the wings so the nose pitches down about it's COG. But how does that increase longitudinal stability? I thought the aircraft was more stable when the stabiliser was producing more of a moment.
And also why does the nose pitch up and the aircraft becomes less stable when leaving ground effect.
This story is a bit long-winded but, hopefully, it helps. If I only have confused the issue for you, ask some more questions and we can progressively get to a position which might be more enlightening than confusing. Some parts of the story may seem a bit heretical but that is due to the basic pilot training stories being simplified to the point where they can get to be a bit over-simplified.
Some points to consider with ground effect transitions (ie on takeoff, climbing IGE [in ground effect] to OGE [out of ground effect], on landing, descending OGE to IGE). The discussion certainly is not all encompassing and is intended just to give you a skim-the-surface idea of the complex relationships involved and their implications for pilot handling problems -
(a) for modest heights, the lift curve tends to move up and left a little, resulting in a lower alpha required to obtain the same CL. We tend to see this as a modest lift increase. CLmax, though, tends to be reduced in ground effect so that can have an effect on stalling. As the height above the ground of the lifting surfaces reduce, compared to the spans, the effects change somewhat but that is more the concern with WIG (wing in ground effect aircraft) rather than what we routinely are flying.
(b) the tailplane sees a varying upwash/downwash component in the airflow and the effect is very dependent on the surface geometries, especially the distance between wing and tail. Tailplane effects can be difficult to quantify as separate components but can have considerable influence on a particular aircraft's transition characteristics.
(c) for the wing, pressure distributions alter significantly as the aircraft moves into ground effect. The presence of TE flaps can have a very significant effect. Spanwise load distribution and CLmax values change and these can have undesirable handling consequences depending on the specifics of the aircraft design. There is also a variation in the centre of pressure. These things lead to changes in pitching moment which have to be trimmed out by the pilot's use of the elevator in a fairly dynamic environment. Is it any surprise that student pilots can have lots of difficulty getting on top of, particularly, the final landing phase, flare and touchdown ?
(d) as the wing moves further into ground effect (typically lower than 40% height/span, we can start to see some strange effects on lift. These effects are exploited in the design of ground effect aircraft which are the real low flyers but, due to geometry, are not a real concern to ourselves.
(e) drag-wise, the lift dependent (induced) drag will reduce due to the change in orientation of the lift vector and the profile drag due to changes in flow velocities. Overall, we see a reduction in aircraft drag.
(f) pitching moment changes are very design specific and have to do with OGE lift being due mainly to upwash/downwash flows and IGE lift more due to flows under the wing and tail. This, then, can lead to strange observations on takeoff and landing due to the wing and tailplane leaving/entering ground effect at different times during the takeoff and landing. This gives rise to different contributions of wing/tail lift and overall aircraft pitching moment. Very much dependent on the specific aircraft design.
What this all means is that there is no one-size-fits-all story - things depend heavily on the configuration and design of the specific aircraft we might happen to be endeavouring to make do our bidding at the time.
The usual observations (but this is not set in concrete for all aircraft) are that -
(a) leaving ground effect, there is a nose up change in pitching moment and nose down on entering. John Farley (the Harrier Chief TP) observed during a talk to the RAF Historical Society that the Harrier, on encountering ground effect, displayed a quite strong nose down change in the pitching moment when conducting a conventional landing. Apparently, a conventional landing in the Harrier was not for the faint-hearted but that is another story. Sadly, John, a thoroughly charming fellow, left our company several years ago in his mid-80s. You could do well for your flying theory education by reading some of his posts on PPRuNe (posting as John Farley). As an aside, he and I got on very well even though, on one occasion, he saw fit to give me a friendly clip over the ear when I dared step a little out of line in a discussion on G-LOC.
Some of the T-tails see this reversed. Apparently, the HP Victor was somewhat renowned for flaring and landing itself without any specific pilot input due to the T-tail and the specifics of the wing planform. Conversely, some low tail aircraft had initial flight test problems transitioning ground effect which required investigation and redesign. The F/A-18 was in this category as initial tests presented some problems on rotation.
(b) entering GE, the drag reduces. Leaving GE, it increases.
(c) entering GE, the lift increases, Leaving GE, it decreases.
(d) IGE we need to adjust the angle of attack to control lift and we tend to see differences in stall angles.
Overall, we usually need to adjust elevator inputs to control pitching moment changes and we need to be very aware of the lift and drag changes (speed control and flare height are pretty important on landing)
Another problem is that we can find ourselves able to fly IGE and accelerate but, should we move OGE, we might find the whole thing stagnating and a risk of stall and LOC.
When entering ground effect why would the aircraft become more stable
Longitudinal static stability is a measure of pilot perception of stick push/pull loads (that's the basis of flight test assessment of static stability - the usual graphics of cups and balls only goes so far ...). So, if the aircraft is trimmed and then (for whatever reason) gets slower, the pilot has to hold a pull force to maintain that lower speed for the aircraft to be considered stable. If the pull force is too low, or a push force is required, the aircraft is considered to be unstable. The reverse applies if the aircraft (for whatever reason) gets faster. Stable requires that the pilot hold a push force to keep the higher speed. The higher the stick loads required to hold the out of trim condition, the more stable is the aircraft in that configuration.
If you know what you are doing, you can fly a statically unstable aircraft for a short while but it is very tiring and mentally demanding. Most pilots would not be up to the task and would lose the aircraft due to a departure from controlled flight. Not so with a dynamically unstable aircraft. That's going to be crash, burn, die every time unless you have a very fast computer doing the sums.
Many years ago I did a flight test short course and the TP instructor related a tale of how his FTE (whom he subsequently married) misjigged the loading and they took off in an aircraft with the CG much too far aft and sufficient to make it statically unstable. Recognising the problem and knowing what to do, he managed to stagger around the circuit and throw the aircraft back on the ground. His comments to his FTE were not disclosed to us but one can imagine ....
As described above, transitioning OGE to IGE usually sees a nose down pitching moment change. Probably the easiest way to relate this to the static stability concept is to compare it to what happens with CG movement. As CG moves aft, static stability reduces. CG moving aft is going to give you an increasing tendency to reduce nose down pitching moments or increase nose up pitching moments. This is closely related to stick loads and, in particular, stick force gradients (ie how many lbs/kt off trim speed I require to maintain various off trim speeds). So, as pilots, we would tend to relate an increasing nose down pitching, or decreasing nose up pitching, with increasing static stability. It is important to keep in mind that static stability relates closely to what the pilot perceives.
This is what allows various flight test fixes to improve static (and dynamic) stability if the flight test program shows the aircraft to have unsatisfactory stability characteristics. You might like to look up elevator downsprings and bobweights on the net - the fix changes the stick loads and "tricks" the pilot into seeing the result as an increase in stability.
Indeed, most light aircraft are intentionally designed with tailplanes which are on the small side (less weight, less drag). The resulting unsatisfactory longitudinal static stability characteristics measured at flight test are then "fixed" by introducing an elevator down spring to give the pilot the feel of an acceptably stable aircraft. Have a look during your next preflight. If the elevator is full down against the stops, it has a downspring (which causes the tailplane to sit down). Perhaps you may have wondered about this observation when you would (should ?) know that elevators are very carefully balanced to give a slight nose down characteristic. The only light aircraft I can bring to mind which was designed without the intention of a downspring fix is the Cessna 303. This is secondhand information as I have never flown one and, as far as I can recall, never even seen one in the flesh.
Because of the geometry variation effects on the aircraft's interaction with GE transitions, in dynamic situations, eg strong crosswind takeoffs and landings, the effects on longitudinal and also lateral directional stability can create hazardous conditions. In some circumstances, the lateral directional modes can be excited (along with the pilot's eyebrow raising in response) - we have seen a number of uncommanded ground contacts and runway excursions during takeoff and landing as a consequence.
why does the nose pitch up and the aircraft becomes less stable when leaving ground effect
As described above, transitioning IGE to OGE, usually sees a nose up pitching moment change. A similar argument for stability perceptions can be made as for the landing case.