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I was reading about the ERCO aircraft which apparently has a rudder-aileron linkage system that enables co-ordinated turns without the need for explicit rudder inputs. I've heard of Piper's having bungee cords for the same purpose.

Two questions:

  1. Does such a physical linkage lead to perfectly co-ordinated turns under all conditions? Or is it only a coarse approximation & a skilled pilot can execute a better turn?

  2. Why don't a lot more civil aircraft automate this co-ordinated turn feature? Is it because it is hard to automate or more that one wants the pilot to retain the ability to slip when necessary & execute un-coordinated turns sometimes?

Well, even if the latter were true, since 99% of all turns are co-ordinated one could just offer an override for the automated co-ordination feature, right?

A physical linkage is rather crude, but with a servo / feedback loop wouldn't it be possible to entirely eliminate the rudder pedals?

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The mechanical linkage from control column (aileron) only addresses one of the things that affect coordination, the adverse yaw. It is the most significant one, but not the only. Other reasons you may need some rudder deflection in coordinated flight include:

  • Compensating for P-factor. At high angle of attack (slow flight) the propeller blade going up sees a lower angle of attack than the propeller blade going down, creating asymmetry of thrust. The effect depends on engine power and speed, making it too complex to compensate by just mechanical linkages.
  • The rotating propeller slipstream also creates asymmetrical force on the rudder, requiring compensation depending on engine power and speed.
  • In tail-draggers, the propeller precession creates rather significant yawing moment as the tail lifts off that needs to be connected using rudder, otherwise the plane could ground-loop. Tail-draggers generally require active control by rudder, because they are directionally unstable on the ground.
  • In multi-engine aircraft, asymmetric thrust in the event of engine failure. For multi-engine aircraft this is the most significant yawing moment that the rudder must be able to compensate.
  • And last but not least, rudder is needed to damp the Dutch roll. I've never heard that mentioned as problem in utility aircraft, but it is a big problem for transport aircraft and a reason why they usually come with a yaw damper.

The yaw damper is a feedback controller that monitors the lateral acceleration and other parameters (yaw and roll rate) and applies rudder to damp the dutch roll, and also to provide coordination to turns flown by autopilot where usually the autopilot directly commands the ailerons and rudder is left up the the yaw damper. So yes, providing coordination is possible, but not via simple mechanical linkages. And of course the rudder pedals still need to be there so uncoordinated flight is possible for any of the reasons already listed in Ron Beyer's answer.

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  • $\begingroup$ For the compensation described in your bullet points #1 and #2 (P-factor / slipstream) is the compensation rudder provided by continuous assymetric input via pedals all through the flight duration (that sounds fatigue inducing) or is there some other way to provide a sustained assymetric rudder input? Something like a trim tab / trim wheel? $\endgroup$ – curious_cat Jan 3 '16 at 13:45
  • $\begingroup$ @curious_cat, well, both effects are significant at slow speed, not high. But yes, rudder (and aileron) usually have trim as well. $\endgroup$ – Jan Hudec Jan 3 '16 at 14:59
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The ERCO Ercoupe does not have rudder pedals, and even arm-less pilot Jessica Cox used one to get a pilots license.

The biggest problem that I see with designs like that is when you want to do an uncoordinated maneuver like a cross-wind landing using the wing-low technique. This requires you to turn the controls into the wind (wing low) and then use the rudder in the opposite direction to keep you aligned with the runway. Even crab-style cross-wind landings require you to kick in the rudder to straighten up before touching the pavement. The Ercoupe's gear is designed to handle landing while crabbed, but its a bit rough for the occupants.

Another operation that I like to use opposite rudder for is emergency descents. By tilting the aircraft into the dive and using the rudder to keep you going in the right direction you can achieve a pretty decent sink rate without going "into the red", especially in rough air.

Another issue I can think of is steering. Tricycle GA aircraft use rudder pedals to steer the aircraft and caster-wheel or tail-dragger aircraft require differential braking and rudder authority. The Ercoupe actually uses the control wheel for taxi, which makes most pilots laugh :).

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    $\begingroup$ This still does not explain why there is no linkage that would adjust the neutral point to provide coordination, so force on pedals would only be used to achieve uncoordinated flight. And does not address how well the mechanical linkage actually provides coordination in the first place. $\endgroup$ – Jan Hudec Jan 3 '16 at 12:58
  • $\begingroup$ Yeah, good info but the question wasn't answered. $\endgroup$ – Ryan Mortensen Jun 27 '16 at 15:58

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