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For the CRJ700 for example, reducing thrust causes a pitch up. Why is that?

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    $\begingroup$ It may be because the engines create a bit of nose down pitch at higher power based on the thrust vector being slightly higher than the CG. $\endgroup$ Commented Nov 25, 2023 at 17:53

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The engines are mounted above the vertical center of gravity position. That means there is a vertical moment arm from the thrust which causes a pitch down when thrust is increased and a pitch up when thrust is decreased. From a stabilized trimmed condition, e.g. during straight and level flight your pitch trim counteracts the nose down effect. When you then suddenly reduce thrust that trimmed condition is no longer in balance and the stabilizer pitches the plane up due to the reduced nose down moment.

Most airliners with engines mounted under the wing have the exact opposite effect. There the engines are below the center of gravity and any increase in thrust causes a pitch up and a decrease causes a pitch down.

There are other effects at play as well such as the jet wash and airspeed trim stability but the vertical moment arm is the largest factor during a sudden change in thrust.

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At least in the CRJ 700, I'd suggest that it's not primarily from the engines being mounted above the CG. They are probably a little above the CG, but only minutely.

But when we look at a side view, we quickly see that the engine's orientation almost certainly have more effect:

enter image description here

The engines are well behind the CG, and the front of each engine is tilted up by what looks like around 10 degrees. That being the case, I'd expect increased thrust to give a nose-down pitch, regardless of the height at which they were mounted.

Photo Credit

https://www.delta.com/us/en/aircraft/bombardier/crj-700

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    $\begingroup$ The engine orientation is there to align them with the local flow and should be around 4 degrees. Both vertical position and angle add up to create the pitch effect. $\endgroup$ Commented Nov 26, 2023 at 8:18

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