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In the usual aircraft configuration, the center of lift is located behind the CG producing negative pitch moment (the horizontal stabilizer counteracts it by producing downforce - positive pitching moment). If the aircraft rolls to one side the vertical component of lift on the wings would decrease, so by that logic the aircraft should pitch up.

Why does it pitch down? What am I missing?

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    $\begingroup$ Notice at higher bank angles rudder into turn pitches nose down more and more as well. Very important to stay well above higher stall speeds in steep turns, also helps to relax rudder a bit and let adverse yaw hold nose up. At very high angles, opposite rudder may be needed. But this is advanced and should not diminish in any way the importance of coordinated turns for safety, especially near stall. $\endgroup$ – Robert DiGiovanni Jan 31 at 20:15
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The answer is that the aircraft doesn't actually pitch down. When banking (or rolling) into a turn, the angle of attack (or the angle between the center of the wing and the flight path of the aircraft) stays the same. Assuming no correction to maintain altitude is made, the aircraft will continue on the same angle of attack, just banked over by some amount. Due to the bank, the lift produced by the wing is no longer vertical, instead, it is directly perpendicular (at least for our purposes) to the wings. Therefore, if the airplane is in a 30 degree bank, then the lift vector will also be 30 degrees from vertical. Doing some vector analysis, then you can determine that if the angle of attack hasn't changed the new vertical component of lift will be about 86% of what it was in level flight. Therefore, the plane would begin losing altitude. This altitude loss is what you see as the airplane pitching down.

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    $\begingroup$ Of course the aircraft does actually pitch down. What you described is the first part, but then the longitudinal stability kicks in, the angle of attack will increase as the flight path angle turns down, but pitch does not, it will cause reduction of the down-lift on the elevator (and increase of lift on the main wing) and thus pitch-down moment. $\endgroup$ – Jan Hudec Jan 31 at 20:13
  • $\begingroup$ @JanHudec Exactly what I was looking for, thanks. In a turn AOA gets little higher -> increases lift on the wings and decreases downforce produced by the tailplane -> negative pitch moment and nose drops. I would mark yours as an accepted answer but it is a comment.. $\endgroup$ – Darjan Feb 1 at 21:31
  • $\begingroup$ @Darjan this doesn't happen. The downforce produced by the tailplane is never reduced! It is the lack of lift from the main wing (because of the non-vertical lift vector) that leads to the nose drooping. The tail in fact then must create MORE downforce when the pilot pulls back on the stick to maintain altitude. $\endgroup$ – PilotDan Feb 1 at 23:17
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    $\begingroup$ @PilotDan, if the pilot changes the control input, what the aircraft does obviously depends on that change. If the pilot pulls back on the stick enough to maintain altitude, the aircraft raises the nose, because it needs higher angle of attack now. However, usual understanding of “tends to pitch” is “in absence of control input”—and in that case the sequence is bank decreases vertical lift, aircraft accelerates downward (without pitching yet), this increases angle of attack and that finally pitches the aircraft down due to decrease of tail downforce along with increase of wing lift. $\endgroup$ – Jan Hudec Feb 2 at 21:48
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Darjan, first the basics. An aircraft in straight and level flight has the center of all lifts balanced with the center of gravity. A plane with the center of gravity set forward of the WING center of lift requires down force on the tail to balance it.

Now roll 45 degrees. Gravity vector is no longer aligned with elevator trim. If you roll 90 degrees the rudder is now you "elevator" relative to CG. Unless you apply opposite rudder, a forward CG set nose will drop. This, among other reasons, is why aerobatic planes like their CG further back.

Agree with @PilotDan that bank will also cause reduced vertical lift, but a forward set CG will also drop the nose. Many pilots will try to pull harder on the yoke to maintain altitude, adding power seemed to work better for steep turns, as well as reducing bank angle.

Bottom line, forward set CG is not a panacea of safety in all cases. Published forward CG limit is best adhered too.

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But tail cannot produce pitch down moment as well. Now tail has to push agains centrifugal force. But gravity is still pulling nose if the flight is not coordinated so it will go down.

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