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? – Liam Baron Jan 14 at 20:22
• Wouldn’t it with clockwise rotating prop? Correct me if I’m wrong. Of course im not talking about jet. – Konrad Jan 14 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. – John K Jan 14 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. – quiet flyer Jan 15 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. – quiet flyer Jan 15 at 2:19

2 Answers

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.

Most conventional aircraft are designed so that their Center of Gravity (COG) is forward of their Center of Lift (COL). When an aircraft banks for a turn, weight and centrifugal force combine to increase the Load acting through the COG. In order for the Aircraft not to lose altitude, the vertical component of Lift must continue to be equal to weight. The horizontal component of Lift must be more than the centrifugal force in order to affect a turn. The combined load on the aircraft in a 60° bank coordinated turn can be twice the normal weight of the aircraft.

In straight, level and unaccelerated flight, the aircraft acts similar to a lever with the COL acting as a fulcrum. The moment of the COG toward the front is counterbalanced by a downward force on the tail created by the negative lift of the horizontal stabilizer. This negative lift is created by the airspeed of the relative wind against, and the angle of attack of the horizontal stabilizer.

In a normal banking turn without an increase in power, airspeed remains constant or slightly decreases with an increase in induced drag. When the force of the COG and the force of the COL increase with no corresponding increase of the force on the horizontal stabilizer, the airplane will pitch nose down around the COL.

• Centrifugal force is not a real force. As long as you choose the CG as the fulcrum for your torque calculations, there is no need to take "centrifugal force" into consideration when determining the aircraft's rotational acceleration in any given instance. – quiet flyer Jan 15 at 2:24
• @quietflyer - Centrifugal force may not be a real force. But it a convenient way to describe the inertial force the centripetal force acting towards the center of the turn is counteracting. – Dean F. Jan 15 at 2:28
• So you are suggesting that if the aircraft is designed so that the tail is producing an upforce rather than a downforce, the following events will not happen if we roll into a turn without applying any back pressure on the stick or yoke? a) vertical component of lift is temporarily decreased, b) flight path curves (accelerates) toward the earth, c) aircraft pitches down in synch with the changing direction of the flight path, d) airspeed starts to rise. ? I think an incorrect application of the idea of "centrifugal force" has led you in the wrong direction. – quiet flyer Jan 15 at 18:49
• "In a normal banking turn without an increase in power, airspeed remains constant or slightly decreases with an increase in induced drag" -- but what are we doing with the stick or yoke in the fore-and-aft sense? The point of the video is that if you don't move the stick or yoke aft as you enter the turn, and the computer doesn't do it for you, then aircraft will tend to descend. This would be associated with an increase in airspeed. You have to specify what you are doing with the stick or yoke in the fore-and-aft sense for your comment to have any meaning. Were you assuming constant alt? – quiet flyer Jan 15 at 18:53