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Today I watched a video about the Fly-by-Wire (FBW) system (see below) and at 3:20 there is a left turn initiated in a Cessna 152. Then it is shown that with a left turn (only by ailerons), in aircraft with a mechanical flight control system, it will start a descend (and a slip).

I understand why the aircraft starts flying a slipping turn, but I don't understand why it start descending. Because gyroscopic effect would make the aircraft to pitch up in a left turn.

It is also said that each aircraft with mechanical flight control system, either little Cessna or Boeing 737, will have that tendency. Can somebody explain that descending tendency?

My guess is that the turn is initiated without power increase, so to stay with the same indicated airspeed the aircraft must be pitched down, but I'm not sure of that.

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  • $\begingroup$ Why do you think that gyroscopic effects make the aircraft pitch up in a left turn? $\endgroup$
    – Liam Baron
    Jan 14 at 20:22
  • $\begingroup$ Wouldn’t it with clockwise rotating prop? Correct me if I’m wrong. Of course im not talking about jet. $\endgroup$
    – Konrad
    Jan 14 at 20:46
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    $\begingroup$ The rate of rotation is too slow to notice that sort of thing in that circumstance. The only time you notice precession from a propeller is things like taking off a taildragger and raising the tail off the ground abruptly early in the roll. You will get a swing to the left from that. A noticeable pitching action from precession would have to come from a hard rudder yawing input, not from rolling into a turn where the rotation about the yaw axis is very gentle. $\endgroup$
    – John K
    Jan 14 at 22:38
  • $\begingroup$ There is a problem with this question. That video is not from any real plane. It is from a computer simulation. I guarantee you that if you do the same maneuver (without using the rudder pedals) in a real Cessna 152, you will find that the displacement of the slip-skid ball is much more related to roll rate (and likewise to aileron displacement), and much less related to bank angle, than is shown in this video. $\endgroup$ Jan 15 at 2:15
  • $\begingroup$ In particular in the interval from 3:42 to 3:46 the aircraft is actually rolling at a substantial roll rate toward the high wingtip, yet the ball remains displaced toward the low wingtip. This would not happen in a real Cessna 152. $\endgroup$ Jan 15 at 2:19
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This is because when you roll left (or right), the lift changes from pushing up to pushing up + left (or + right). This is what makes you turn. Since some of that lift is being used to the left (or right), not as much is pushing up, causing the plane to lose a bit of altitude. On FlyByWire aircraft, since they are computer-controlled, the computer automatically compensates for this by adjusting elevators to pitch up slightly.

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Imagine a line extending up from the center of the wing perpendicular to the chord line of the wing. This is the lift vector. It points more-or-less straight up in level flight. In this scenario, 100% of the aircraft's lift is pointing up. If we roll into a turn, now some of that lift is pointing sideways- that pulls the aircraft sideways, which pulls it through the turn. That horizontal component of lift is subtracted from the vertical component, and that means that weight now exceeds lift. This causes a descent. For the same reason, when you roll out of a turn without elevator input, you'll balloon upward. To fly a level turn, we need to increase lift. To do this, we can (assuming the wing is not near the critical AOA) increase the angle of attack by putting back pressure on the elevator. By doing this, we increase lift and stop the descent. https://www.boldmethod.com/learn-to-fly/aerodynamics/the-aerodynamics-of-a-turn-in-an-airplane/

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  • $\begingroup$ Re "If we roll into a turn, now some of that lift is pointing sideways- that pulls the aircraft sideways, which causes the slip." -- a steady sideways force simply represents the centripetal force that makes the aircraft follow a circular path. There's no reason for that to automatically make the aircraft slip sideways through the air (except for the effect of yaw rotational inertia causing some slight delay in getting the required yaw rotation rate established as the turn is first starting.) $\endgroup$ Jun 1 at 12:53
  • $\begingroup$ So you have to look to other aerodynamic effects to explain any sideslipping tendency during a sustained turn. (I.e., once the bank angle has been established and maintained for more than just a few seconds, yaw rotational inertial shouldn't be a factor any more, so the explanation that the sideways "push" from the banked wing should automatically tend to cause a sideslip doesn't really hold up.) $\endgroup$ Jun 1 at 12:55
  • $\begingroup$ Revised it to be a bit more accurate. $\endgroup$
    – MD88Fan
    Jun 1 at 13:43
  • $\begingroup$ Much better explanation than mine :) $\endgroup$
    – tizmataz77
    Jun 1 at 15:19

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