Some assorted points from this thread.
I thoroughly agree with Capt PB's comments on physical models as applied to aerodynamics or any other bits of physics. So please take what follows in the spirit of "if you want to extend the model and are prepared to accept the complexity that goes with it, this might help", rather than just nit-picking on my part.
The definition of lift and drag as used in standard aerodynamics texts is always with respect to the direction of incident airflow. Lift is the component perpendicular to the incident freestream airflow, drag is parallel to it. That's substantially the same as using the 'flight path' as Capt PB suggests.
However, the difference between flight path and
local airflow is vital in the classic explanation of induced drag, and I hope JF will forgive me for saying that his model of induced drag is oft quoted but misleading.
When a wing of finite length generates lift, it creates a vortex which leaves the wingtips in what we all know as wake vortices. Just as the winds around a Low pressure area can be felt hundreds or thousands of miles away, so the effect of the vortices is wide ranging. Not only does it affect the air flow behind the wing, but also in front of it.
If you follow through the direction of the vortices, you'll see that there's a general downward movement within the wingspan, and an upward movement beyond the tips. Thus the incident flow approaches the wing
not along the flight path, but from a direction slightly above the flight path by an angle known as the 'induced angle of attack'. It actually varies along the span -- it's greater closer to the tip and therefore the vortex, but it's convenient to picture it as an average downflow. This induced angle of attack is not the same as the true angle of attack of the aerofoil. For a start, it varies with the aspect ratio of the wing. It's typically much smaller, by a factor of about half the aspect ratio. So for an aspect ratio of 10, an AOA of 10 degrees gives an induced AOA of about 2 degrees.
Because lift is conventionally defined as being perpendicular to the bulk incident airflow, the effect of the induced AOA and the tilted airflow as it approaches the wing is like tilting the lift vector of an infinite aerofoil back by the induced AOA, as well as reducing the effective AOA. It's the picture of that tilted lift vector that often leads to the misconception that as the AOA increases so the lift vector tilts back perpendicular to the chord line. But the effect is nowhere near as great as that.
On thrust/drag couples I have to disagree slightly with Capt PB, although it may be a question of interpretation. There's a difference between the instantaneous and transient effect of a force, causing an angular acceleration about the pitch axis, and the steady state change in equilibrium position.
From the point of view of 'pitch up' or 'pitch down' tendency, I think it's usually the latter that concerns us, because the time scales for AOA pitch stability are pretty short.
If you apply an unbalanced force to a body that's in equilibrium (linear and rotational) the resultant motion does indeed depend on the position of the centre of mass (CM). The force will cause an acceleration. If you don't mind using non-inertial frames of reference and you're in to double-entry book-keeping, you can add an 'inertial force' at the CM in the opposite direction and look at the effect of that couple.
However, forces rarely stay unbalanced for long. As the airspeed increases, the drag increases about the appropriate point as determined by the aerodynamics of the body, not by the position of the CM. As soon as the acceleration stops, the drag has increased to equal the thrust, and the thrust-drag couple is balanced in the usual way by the pitch-stability mechanism of the aeroplane.
In the case of your glider and JATCO pod example, the immediate angular acceleration might be pitch down, but the 'glider' will rapidly accelerate to a point where the increased drag at the end of the pole makes a bigger impact than inertial force, and the natural stability from the decalage of the mainplane and tailplane is left to counter the now pitch-up couple. The steady state condition will be a higher AOA than before the JATCO pod was fired, and a much higher pitch attitude of course if the aircraft is now climbing. The details of the forces and geometry will determine the relative sizes of the effects and whether the pilot perceives this as a pitch down followed by a pitch up, or just a pitch up.
The example might seem trivial, but it's for these reasons that the pitch-up/down effects of flap extension get quite complicated.
Another good example is the mythical dependence of wings-level engine-out controllability of a twin on CM position. The effect is purely about aerodynamic couples and thrust balancing and has nothing to do with the position of the CM, contrary to what we're told in most standard texts.
By the way, is the UserName 'Mrs Pit Bull' taken? Bookwormess is announcing the readiness of dinner and I can't help but think that 'Mrs Pit Bull' might be more fitting.
