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.
This is because when you roll left (or right), the lift changes from pushing up to pushing up + left. This is what makes you turn. Since some of that lift is being used to the left, not as much is pushing up, causing the plane to lose a bit of altitude. On FlyByWire aircraft, since it is computer controlled, the computer automatically compensates for this by adjusting elevators to pitch up slightly.