Keith,
Excellent explanation.
Assuming the aircraft he talked about was in level flight, anyone want
to say how heavy the aircraft was?
When the centre of gravity is beyond its forward limit the tailplane is more effective due to a longer moment arm
My book tells me that: "with the COG outside the forward limit, the elevators may have to be permanently displaced upwards in order to provide sufficient balancing download from the tailplane for straight and level flight."
The first quote relates to stability:
An aircraft in level flight, and trimmed, encounters a disturbance that raises the nose slightly. The tailplane is attached to the aircraft so its Angle of Attack increases slightly. With a long momemt arm this change
in AoA is sufficient to pitch the nose back down to where it originally was.
The second quote relates to elevator effectiveness:
If the CofG is forward then the nose down pitching moment due to this
needs to overcome by changing the AoA of the Tailplane. Assuming
you do not want the aircraft's pitch to change then the only way to do this
is with elevator. If the Cof G is so far forward that full elevator deflection is
required then this is not an efficient way to fly (extra drag) and, most
certainly, is not safe.
Why are there CofG limits? The effectiveness of a tailplane (as with any
other lift surface) depends on the speed of the airflow over it and on its
Angle of Attack. The CofG limits are set so that the elevator can always
create an effective tailplane AoA at any speed likely to be encountered in flight.
So, flying slowly:
Rear CofG is determined so that there is always sufficient elevator
authority to recover from a stall (and therefore spin).
Forward CofG is determined so that there is always sufficient elevator
authority to Round Out/Flare.
PS: Sorry no pictures