The pitch trim system in conventional planes is still a “negative feedback control system” for airspeed. In terms of control theory, the setpoint (i.e. the trim wheel position) is the airspeed.
For a specific loading configuration, you could mark the trim wheel in knots 
What confuses people is the phugoid behavior i.e. it can take a while for the speed to stabilise following a big power change.
Are you sure that theory takes the effects of changing propeller slipstream with power changes into account? You could have a plane where flying 100 kts full power would exert a high downward force on the elevator, but 100 kts power off in a descent would not give that force. I imagine the trim setting wouldn’t be the same in both cases of flying 100 kts. Or am I missing something?
Are you sure that theory takes the effects of changing propeller slipstream with power changes into account?
It is first-order only, but on say my TB20 the IAS is the same within 1-2kt, from cruise power to no power.
Patrick wrote:
I guess my question is: Does the instructor have a point or is he stuck in his “PPL training scheme” where they need to instill the very concept of trimming into their students, who mostly fly C152 anyway?
IMO the instructor has a point. Trimming is essential for accurate flying, but it also free up brain cells to be used for other stuff than “fighting” the stick to maintain alt and speed. Trimming simply makes it much easier to fly.
A C152, VMC, in pattern, might lead you to the wrong conclusion that trimming isn’t necessary, because you’re constantly changing the pitch. Even a slightly heavier plane (take a 172, a PA28, even an Aquila or Katana), will very quickly make you reconsider. My FI at the time, when I started to develop the lazy no trim idea, said: not trimming the plane for any new flight stage is bad airmanship and will eventually bite you. It’s just bad skills and potentially dangerous. I assume you are a student without any cruise or cross country experience. Pitch, power, and then trim away the forces on the controls. You need to be able to leave the yoke alone, and the aircraft needs to remain stable. In climb, cruise, and approach. Always. Trim. The. Airplane.
what_next wrote:
That’s the one little point where I envy the fly-by-wire colleagues: All they have to do is press down on the “coolie hat” trimmer button on their sidestick and the aircraft is instantly trimmed in pitch and roll.
I remember the Libelle glider, a 1970’s design, that used to have a pushbutton pitch trimmer.
