When we are talking about longitudinal stability, does that mean stability about the longitudinal axis? If it does, then don't the ailerons sort that out? Pitch is concerned with the lateral axis.
Or am I barking up the wrong tree?
The latter, I'm afraid.
Longitudinal stability, and longitudinal motion, is considered to be motion in the fore-aft/up-down/pitch combinations. So that the phugoid and short period pitch oscillation modes are considered 'longitudinal'.
Lateral stability is to do with the roll axis primarily, although the couple nature of roll and yaw for most conventional designs means that we generally speak of Lateral-Directional motion, which includes the roll/yaw/sideways motions.
I believe Milt was referring to an aircraft\'s STATIC stability - its tendancy to re-establish its trim speed when disturbed.
Tail download is not a prerequisite for speed stability either. This can be seen by considering that I can build a computer model which accurately predicts aircraft speed stability behaviour and response (or indeed an analytical model too) by reference solely to aircraft-level stability derivatives. Therefore, the contribution made by all the components of the aircraft to the derivatives are INDIVIDUALLY irrelevant. What matters is the total aircraft-level effect.
For speed stability you\'ll find things like the derivatives of CL, CD and Cm with respect to both angle of attack and speed are what matter - and what matters from the tailplane in contributing to these derivatives are the tailplane lift-curve slope, the tail volume (coefficient) and any special interference effects. The actual amount of lift on the tail does not affect the stability.
Consider the following thought-experiment (which I\'m sure I\'ve mentioned before, but it\'s the simplest way I can visualise it for people):
You have an aircraft in-trim, at a given speed etc, with a download of 1t on the tail. You disturb it in speed and see what happens.
Now take the same aircraft, but now using a reaction-force-system - a compressed air vent or something, doesn\'t matter really - you relieve the load on the tail such that there\'s now 1/2t of trim lift, and 1/2t of \'puffer\' force, which together trim the aircraft. Introduce the same disturbance as before. You\'ll find (and if you think about the disturbance as a point by point event, much as a simulation would calculate it) that the forces generated by the disturbance are identical in both cases. Therefore the motion will bve the same, and the speed stability did not depend on absolute tail lift.
What matters is the DELTA forces and moments arising from the disturbance. And those don\'t depend on the absolute values, but on other design characteristics, such as those mentioned above.