Turning moment

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In aviation, turning moment is a term for a torque around the vertical axis (z-axis) of an aircraft .

In horizontal flight, a turning moment is only produced by operating the rudder .

Negative turning moment

The negative turning moment caused by actuating the ailerons is mostly undesirable . It was discovered by the Wright brothers during test flights with their double-decker glider. With an aileron deflection, the aircraft "rolls" as desired, but also executes a movement along the vertical axis that is contrary to the intuitive expectation : the lowered aileron on the raised wing generates increased lift and increased aerodynamic drag , which in turn causes a significant braking effect. As a result, the aircraft yaws "negative" in the direction of the braked wing and thus in the direction of roll.

Example: An aileron deflection to the right causes the aircraft to roll to the right, because the lift on the right wing is reduced and the lift on the left increases. However, since the increase in lift on the left side is accompanied by an increase in aerodynamic drag, the aircraft initially yaws briefly to the left until it turns into a right-hand bend while sliding.

Coordinated turn

Flight of a coordinated right turn with wing twisting and rudder to compensate for the negative turning moment (Doppeldecker-Glider, Wilbur Wright , October 24, 1902)

The negative turning moment can be counteracted by synchronous actuation of the aileron and rudder: For example, with the actuation of the control stick to the left and the resulting lowering of the aileron on the right, a left-hand deflection of the rudder should always take place simultaneously (synchronously) to avoid yawing Avoid aircraft longitudinal axis to the right. Therefore, the two rudder deflections are usually synchronized. This also applies to a turn to the right.

Constructive countermeasures

In larger aircraft, the negative turning moment is automatically counteracted by springs or spoilers . The negative turning moment is counteracted constructively by differential aileron deflections. With a normal aileron, the amount by which the aileron on one side is raised (inside curve) is identical to the amount by which the other aileron is lowered (outside of the curve). With a differential aileron, the deflection of the aileron as it rises is greater than the deflection of the aileron as it goes down. The rising aileron is on the side on which the wing is supposed to lower (decrease in lift). To increase the positive and reduce the negative turning torque, the air resistance should also be increased on the lowered side - hence the greater deflection of the aileron. On the side where the wing should lift (i.e. the outside of the curve), lowering the aileron should increase the lift of the wing, but if possible not its air resistance. The increase in resistance cannot be completely avoided from a technical point of view, but it is compensated for by the significantly smaller downward deflection.

The negative turning moment should not be confused with the rudder reversal effect .