# Why do airplanes start descending when we initiate a left turn?

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.

• Why do you think that gyroscopic effects make the aircraft pitch up in a left turn? Jan 14, 2021 at 20:22
• Wouldn’t it with clockwise rotating prop? Correct me if I’m wrong. Of course im not talking about jet. Jan 14, 2021 at 20:46
• 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. Jan 14, 2021 at 22:38
• 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. Jan 15, 2021 at 2:15
• 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. Jan 15, 2021 at 2:19