Does an airliner use elevator in a coordinated turn?

[Leo Liu]

So I understand that for an airliner to make a turn, the plane has to use ailerons to roll to one side to shift the lift to create a centripetal acceleration, and because of the drag difference of the two wings, the rudder might also turn to that direction to correct for the adverse yaw.

I'd like to know if it is necessary to use elevator in this process. My friend, a Cessna pilot, told me elevator was needed for a coordinated turn. However, I cannot see how this facilitates turning and how the plane flight path can stay level when its elevator is deflected. Could someone help me understand it better?

 

[anorlunda]

When banking to make a turn, there is less lift from the wings. That would make the airplane lose altitude unless countered with elevator action.

"Coordinated turn" usually (but not necessarily) implies constant altitude.

 

[jrmichler]

I, a Cessna, Piper, Aeronca, Beechcraft, and Taylorcraft pilot, can verify what your Cessna pilot friend told you. It helps if you visualize a really steep turn, such as 60 degrees bank. You have to pull hard on the elevator because the airplane is experiencing 2 G's acceleration, but the trim is set for 1 G. The effect is still there at lesser bank angles, just not as obvious. If the airplane enters a turn without elevator action, the lift of the wings stays constant, but the lift force component perpendicular to the Earth decreases, causing the airplane to lose altitude. If you want a demonstration, ask your friend to take you for a ride, demonstrate a 45 degree banked turn, then let go of the controls in the middle of the turn.

"Coordinated turn" normally means turning without skidding, which requires rudder action to counteract adverse yaw. An airplane in a traffic pattern will turn from downwind to base to final using coordinated turns while losing altitude. An airplane in level flight makes coordinated turns without changing altitude.

 

[Baluncore]

In short, to turn without loss of altitude requires the vertical component of lift remain constant.
As the plane is banked, the vertical component of lift is reduced, so the elevators must then be employed to restore the vertical lift component. If airspeed is to be maintained during the turn, additional engine power will be needed to counteract the increased lift and drag.

 

[anorlunda]

Another thing to remember is that given an angle of bank 0<A<90 degrees, the rudder motion produces a component of force that lifts the tail in addition to turning the plane. In the extreme case of 90 degree bank, the elevators give "rudder" action and the rudder gives "elevator" action.

 

[Baluncore]

During the turn, the "plane" of the aircraft is flying at a fixed altitude along the concave inside-surface, of an inverted cone, (apex down).

To follow that surface, the nose of the aeroplane must be rising, as the tail is dropping, all while the aircraft is turning to maintain height.

 

[Lnewqban]

So I understand that for an airliner to make a turn, the plane has to use ailerons to roll to one side to shift the lift to create a centripetal acceleration, and because of the drag difference of the two wings, the rudder might also turn to that direction to correct for the adverse yaw.
…

The elevator commands the angle of attack of the wings.
If the aircraft is trimmed to keep level flight with leveled wings, there is no reason for remaining on level flight by simply rolling over with no modification of the elevator.
If a simple roll happens alone, the airplane will initiate a descending trajectory.

 

[Lnewqban]

So I understand that for an airliner to make a turn, the plane has to use ailerons to roll to one side to shift the lift to create a centripetal acceleration...

Please, excuse me for insisting on this point.
Centripetal acceleration implies that the vector that is perpendicular to the trajectory is continuously rotating about a point.

That is not the case for simple banking of the wings by aileron control (roll) input only.
By doing only that, up to let's say a roll angle of 20°, the pilot is inducing the airplane to perform sideways like a pendulum.

The lateral component of the originally vertical lift vector can't induce a horizontally circular trajectory, but a transversal one (in combination with the dihedral of the wings, which would tend to level the wings again at the bottom of the pendulum oscillation).

In the meanwhile, the nose of the airplane continuously drops, because (for pitch control reasons) the CG of the aircraft is located ahead of the center of lift of the wings, and now the balancing down-lift vector of the stabilizer or tail is also tilted 20° and its vertical component is weaker.

The up-elevator input induces a loop contained in the plane perpendicular to the wings, by increasing the lift as much as needed to compensate for all the above and to keep a leveled circular trajectory.
By doing so, the pilot is inducing the airplane to perform like a pendulum which mass describes a circle rather than swinging back and forth.

The lift vector then points to the imaginary spatial pivot point of that rotating or conical pendulum, while the new centripetal acceleration points to the center of the horizontal circular trajectory.

Please, see:
https://en.wikipedia.org/wiki/Conical_pendulum

Following pictures are copied from
https://www.icao.int/safety/loci/AUPRTATablet/index.html#_64

 

[anorlunda]

Your analysis never mentions the rudder. A coordinated turn is not possible without rudder action.