Originally Posted by
Vessbot
In a simplified picture, yes. Trim sets AOA and therefore airspeed. A complicating factor in a more fleshed-out picture is that thrust also effects trim. Under-wing engines have a nose-up thrust couple, in other words increasing thrust is the equivalent to trimming for a higher AOA, a.k.a. a slower airspeed. Opposite for high-mounted engines. GA prop planes generally also have a nose-up thrust couple, from the downwash over the horizontal tail. There are also other more minor contributors to trim, like landing gear. (Drag below the CG, tends to pitch down.)
It is false that trimming sets an attitude as Intruder mentioned above. It sets AOA.
Also, it's true in a simplified sense that thrust sets climb/descend angle. But in reality, let's say you're trimmed and stabilized level at an intermediate thrust setting, and then increase thrust while touching nothing else.
1. The first thing that'll happen is that it'll pitch up or down depending on the direction of the thrust couple, and begin to tend to stabilize around the new trimmed speed (which is slower if you're in a 737 or the like).
2. Over the longer term, as a separate effect, the higher thrust will increase the speed, which will increase the lift, which will deflect the flight path upward, establishing a climb. In other words, half of a phugoid oscillation. This initial climb will probably be steeper than the new steady state (i.e., it overshot), so the rearward component of weight will decrease the speed, which will decrease the lift, which will deflect the flight path downward, completing the first phugoid cycle. This will continue for at least a few cycles of decreasing overshoots, until it stabilizes at the new steady state climb angle commensurate with the new excess thrust.
Obviously this is not a nice way to fly, so in real life the climb is established in conjunction with the practiced use of the right amount of elevator and trim.
The new steady state speed, again, will be that established by the trim change from the thrust couple. If you were in an aerodynamically "perfect" plane with no thrust couple (no downwash effect, jet engine that's not high or low but goes perfectly through the CG) then the trimmed speed wouldn't change and would be exactly what we started the whole exercise with.