I dragged out my 50 year old course notes and also some current course notes and was surprised that they all talk about +ve tailplane lift. Talk about having to give technically the wrong answer in order to pass the exam, but thats nothing new, equal transit time for the production of lift is another.
What was missing in all the course notes was any discussion about the CofG and the neutral point*. Remember that the CofG lies ahead of the wings centre of pressure, or aerodynamic centre, requiring a tailplane download to balance the moments. For an aircraft to have +ve longitudinal stability the CofG must be ahead of the neutral point. As the CofG moves aft (closer to the neutral point) it means the tailplane must produce less downwards lift. If the CofG were to lie on the neutral point the aircraft has no +ve stability and will retain whatever attitude to which it is disturbed. With the CofG aft of the neutral point the aircraft is unstable. This feature has been made use of in the modern fighter to increase manoeuvrability, the F-16 being the first. Of course this means you need full time computers to control the thing and loss of those means an ejection, a feature that civil certification authorities are somewhat adverse to.
The degree of stability is referred to as the static margin. Some figures I found on the net.
Cessna 172 0.19
Learjet 35 0.13
Boeing 747 0.27
P-51 Mustang 0.05
F-106 0.07
F-16A -0.02 (subsonic) .23 (supersonic)
F-16C 0.01 (subsonic) .26 (supersonic) increased stability due to larger tailplane
X-29 -0.33
* The neutral point is that point on the aircraft at which all the lifting forces are considered to act ie lift produced by the wings, tailplane, fuselage, nacelles etc. In other words its the aerodynamic centre of the aircraft as a whole.