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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.

I was thinking that the need for bank into the crosswind is due to the need to compensate for the sideslip force (from the yaw due to downwind rudder) which would otherwise (without the bank) turn the aircraft downwind.

But it seems to me that using asymmetric thrust to induce the same sideslip would produce the same tendency to turn if the wings were not banked into the crosswind to compensate. Why not??

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  • $\begingroup$ I added some tags. I also added a paragraph at end, then decided to roll back as I am uneasy about adding substantial content to other people's questions; if you (original asker) think it improves the question feel free to add it back. You can see past edits by clicking on the "edited 1 hour ago" (or whatever) button. $\endgroup$ – quiet flyer May 8 at 14:54
  • $\begingroup$ related: this two flights in which differential thrust was used to control the aircraft trajectory. $\endgroup$ – Manu H May 13 at 10:06
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The use of asymmetric thrust when landing a twin in a crosswind effectively accomplishes the same thing that rudder input does. It induces a yawing moment about the aircraft’s vertical axis, the same as pushing on the rudder pedals does. It helps to alleviate excess rudder pressure on the flight controls during the approach, making it a little more comfortable for the pilot to fly the approach. You are still required to apply aileron input into the wind in order to hold the runway centerline. Using asymmetric thrust during a Crosswind Landing does not alleviate aileron pressure or required bank angle to track the runway centerline.

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    $\begingroup$ That's as I thought. The only difference would be that when using asymmetric thrust, the rudder would not be as deflected. The vertical stabilizer (as well as the entire fuselage because of the sideslip angle) would still be producing lateral aerodynamic force in the downwind direction. So there would still be a need to drop the upwind wing to counteract that force to maintain a constant heading. $\endgroup$ – Charles Bretana Apr 13 '18 at 14:23
  • $\begingroup$ @CharlesBretana -- Actually there would be an even greater need-- see new answer-- $\endgroup$ – quiet flyer May 7 at 21:26
  • $\begingroup$ You may mention response time: pedals/rudder react quicker than engines. $\endgroup$ – Manu H May 13 at 10:08
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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?

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  • $\begingroup$ Disclaimer: I don't fly twin-engine aircraft. $\endgroup$ – quiet flyer May 7 at 21:44
  • $\begingroup$ In your case (wind from left), with aileron input to induce a left-yaw, the purpose in increasing in throttle on the left engine is to increase lift on the left-wing while counteracting the yaw reducing the amount of rudder input needed to maintain alignment with the center-line. More lift -> higher the force provided to counteract the crosswind with the same bank angle. (AMEL-Instrument) $\endgroup$ – David C. Rankin May 7 at 21:59
  • $\begingroup$ @DavidC.Rankin -- increasing lift on the left wing in particular would seem to increase, not decrease, the left aileron input required to hold the bank angle. Total lift cannot exceed vector sum of aircraft weight plus whatever horizontal component of lift is required to neutralize aerodynamic sideforce. If the aircraft has any dihedral, it would seem increasing lift on right wing, not left wing, would be the most effective way to increase sideforce to left without increasing bank angle. $\endgroup$ – quiet flyer May 7 at 22:07
  • $\begingroup$ @DavidC.Rankin -- after giving your comment some thought I still don't see how increasing power on the left engine would decrease the required bank angle to deal with the crosswind from the left. Plus given the whole issue of the sideforce from the rudder as outlined in my answer, it seems counterproductive-- the required bank angle would actually have to be increased, compared to what would be needed if both engines were run at the same throttle setting. $\endgroup$ – quiet flyer May 7 at 22:09
  • $\begingroup$ @DavidC.Rankin -- It seems to me that the ONLY thing that can be accomplished by increasing thrust on the upwind engine is a decrease in required rudder deflection. What am I missing? $\endgroup$ – quiet flyer May 7 at 22:11

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