aerostude
Sorry, I don’t agree - may we discuss what you posted?
<font face="Verdana, Arial, Helvetica" size="2">The centre of gravity HAS to lie BEHIND this point or you are in trouble.</font>
Why? On most aircraft the C of G will
not lie behind the C of P, although on some types, frequently for instance on seaplanes, it could. You may have assumed that the tailplane only provides lift, but it can, and does, often provide a downforce. There are also other forces/couples acting on the aircraft to consider, principally the Thrust/Drag couple. It is only when these four main forces cannot be balanced satisfactorily by themselves, that the tailplane will be called upon to provide a balancing force, either upward or downward.
There is in fact an advantage in having the C of G behind the C of P; in the event of thrust loss, there is a nose down (stabilising) pitch.
<font face="Verdana, Arial, Helvetica" size="2">If you than consider the input from the tail lift</font>
Why must it be lift? Ideally, at normal cruise speed, the tailplane will carry no load, but at high speed, it will normally be a download.
<font face="Verdana, Arial, Helvetica" size="2">if the c of g moves forward, the elevator effectiveness is increased</font>
As the distance between the C of G and the C of P increases, more elevator deflection is required, not less, thus the elevator is, by definition, less effective. Thus the forward C of G limit is determined by the minimum acceptable level of manoeuvrability.
<font face="Verdana, Arial, Helvetica" size="2"> if the c of g moves back….the aircraft is difficult to control and ultimately uncontrollable-effectively the aircraft becomes too statically stable.</font>
As the C of G moves back, stability is reduced, and the aircraft does become more difficult to control, but this is because it is becoming unstable, not too statically stable! Thus the aft C of G limit is determined by the minimum acceptable level of stability.
Regards
Bellerophon