Originally Posted by
Chris Scott
Perhaps we have to distinguish between the in-flight situation, when the a/c tends to rotate around its C of G, and the on-the-ground situation where its weight is mostly being supported by its main-wheels on the ground?
In the latter case (on the ground), it will rotate around the main-wheel axles as a pivot point.** Despite the chaotic airflow created by the fan-reverser deployment, the rearward thrust vector it produces is presumably directed above the pivot point. That should produce a slight pitch-up, but I cannot pretend to have noticed it on the A310 or A320.
I guess I'm saying this two years too late, but I don't think this can be the whole answer. If it one only needed consider the torque about the main gear axles, the brakes on the main gear would have zero moment arm, and therefore have no effect on pitch.
If the gear were not on the runway, a rearward thrust vector below the CG would tend to produce a nose-down pitch. Gear rolling on the runway would do nothing to offset that; they'd just roll a little slower as the nose pitched down.
But I can't seem to sort through this entirely in my head -- obviously a downward force forward of the gear but aft of the CG would create a nose-down pitch.