In General Aviation light twins, when landing in heavy crosswinds,
what are some considerations to using asymmetric (or Split) throttles
to induce a sideslip, to align the fuselage with the ground track and
the runway centerline instead of rudder?
I was told that this technique allows the aircraft to track down the
runway centerlines with fuselage aligned with the runway without as
much bank angle into the crosswind as would be required with symmetric
thrust using cross-controlled rudder and opposite aileron alone.
The opposite is actually true-- using differential thrust instead of rudder to hold heading in a wing-down final approach and landing actually creates a slight increase in the bank angle required to hold the aircraft on the desired track.
Consider final approach at a fixed airspeed, with a crosswind from the left. Consider that we are using the wing-down (slipping) method, aiming to hold the fuselage completely parallel to the runway centerline throughout the last part of the final approach. The angle between the relative wind and the fuselage is constrained to a fixed value. The component of aerodynamic sideforce toward the right that is created by the airflow impacting the left side of the fuselage is also constrained to a fixed value. However, if both throttles are set the same, then to hold the heading constant, we need to deflect the rudder to the right to counteract the left "weathervane" yaw torque from the sideways airflow against the vertical fin. The deflected rudder contributes an aerodynamic sideforce toward the left, which reduces the overall bank angle required. (To prevent the aircraft from turning, the bank angle must be sufficient to create a horizontal component of lift to the left that is sufficent to overcome the horizontal component of the net aerodynamic sideforce toward the right.) If we hold heading by increasing the power on the left engine, so that we can relax the right rudder input, the leftwards aerodynamic sideforce from the deflected rudder vanishes, so we'll need to use more left bank, not less.
In theory, we could reduce the required bank angle by applying differential thrust in the other direction, so that more right rudder is required, but this is undoubtedly never done in actual practice.
Clearly, the real purpose of the differential thrust is simply to relieve the pilot of the need to hold downwind rudder, i.e. to deflect the rudder opposite to the bank angle. How significant is the resulting increase in the required bank angle? Or in other words, how significant is the reduction in the required bank angle if the pilot uses the rudder rather than differential thrust in the crosswind landing?
Consider the situation of a failed engine. In an engine-out situation, the pilot is typically advised to bank about 5 degrees--rarely much more than that-- into the good engine to cancel the sideforce from the rudder and allow linear flight with the fuselage streamlined to the airflow.
If the wing-down sideslip for the crosswind landing requires substantially less rudder deflection than would be needed to compensate for a failed engine at that same airspeed-- as is undoubtedly the case-- then it follows that the reduction in bank angle achieved by maintaining the sideslip with the rudder rather than with differential throttle will be substantially less than 5 degrees. In words, not very significant. It is clear why a pilot might choose to accept this small increase in bank angle, to relieve the need to hold downwind rudder.
The sideforce from the deflected rudder itself is clearly very small compared to the sideforce from the airflow striking the side of the fuselage as the aircraft maintains the slip angle required for the wing-down crosswind landing.
Note also that "loading up" the lowered wing while "unloading" the raised wing (via the differential power setting and resulting difference in propwash velocity) will slightly increase the aileron input needed to equalize the lift vector from each wing and bring the net roll torque to zero and hold the desired bank angle.
The bottom line is that a pilot using differential thrust to reduce or eliminate the required rudder deflection during a wing-down approach to a crosswind landing is accepting a slight increase in the required bank angle, for the sake of going easy on the muscles in his downwind leg.
Related -- Why does a pilot bank up to 5 degrees into the operating engine following failure of the other engine?
