Why is it that T-tails want to climb like on a PA-44? It seems like it always wants to climb [closed]

Because of the location of the horizontal stabilizer, it isn't as affected by the power setting. Thus, it's likely that you are trimming to compensate for power reductions when you don't need to. I would make sure you trim well, and ensure you are trimming based on what the airplane needs rather than what you would put in on a Cessna.

It's important to remember that airplanes trim to a speed. If the trim control and tab is at position X, the airplane will seek a speed of Y, pitching up or down as required until it's at its trim speed. The trim control is effectively a speed setting dial, in terms of what the airplane wants to do if left on its own. If the airplane wants to climb, it's because it has surplus thrust at its trim speed, T tail or no. If you are trimmed at 120 kt in the Seminole and it wants to climb, it means you have too much power for level flight at 120 kt. If you don't want it to climb, either reduce power or trim to an airspeed that gives level flight at that power setting.

Normally located horizontal tails in tractor airplanes are subjected to trim speed changes when thrust is changed, because the thrust changes modify tail down force, the tail being in the propellers wash. Adding power increases the downforce and reduces the airplane's trim speed. If you're trimmed at 90 kt in a 172 at a certain power setting and add more power, the airplane will pitch up as it seeks to maintain its 90 kt trim speed. But the stronger slipstream from the prop "fools" the tail into thinking its still too fast even when at 90kt, and will seek a slightly slower speed, say 87 kt. Opposite happens when reducing power.

A T tail, being up mostly out of the propeller stream, is much less affected by power changes, only to the extent that there are pitching moments due to the prop's thrust line; in this they they are more like a jet with tail mounted engines (add thrust without touching the controls and it pitches down slightly due to the thrust increase, accelerates, than starts to pitch up and go about seeking its trim speed while climbing).

So when you add power to climb with a T tail, there isn't the immediate up pitching tendency from the propeller wash "blowing harder" on the tail; there is only the pitching tendency from the increased airspeed itself as the airplane initially accelerates from the added thrust, and the increased free stream airspeed "blows harder" on the tail. So overall there is a much more mild response to power changes and the airplane has a stronger tendency to accelerate a bit before pitching up.

In a normal tail you add throttle and it pitches up almost right away, but once you've stabilized in the climb, you may find your trimmed speed is a bit lower than before and you have to add a bit of nose down trim to keep the same trim speed as when you started.

With a T tail you add throttle and initially there is little pitch change until the airplane accelerates and then starts to pitch up. You can help it along with a bit of up elevator minimize the initial speed overshoot and hunt that results. However, when it's all settled down in the climb and everything's back in equilibrium, you should still be pretty much as the same trim speed as you started. If the trim speed changed it's only to the extent that the engine's thrust line increased or decreased the thrust line's pitching tendency, if there is one.

The other effects of T tails are related to its location that takes it in and out of the stronger downwash flow coming from the wings. When the airplane is pitched up on takeoff for example, the tail starts off high enough to be minimally affected by the wing's downwash, but then as the tail comes down at rotation, it descends into the downwash and suddenly sees more negative angle-of-attack and therefore more downforce than before. This results in the phenomenon common on T tails where you have to pull fairly hard to lift the nose but then relax some of the pull as soon as the airplane lifts off to keep from overpitching.