X-Plane 11.36 adds a new parameter to the steering/castor setup for nose and tailwheels, that makes it possible to add a castoring range onto a steered wheel.

Up and including to 11.35, a wheel could be one of three things:

  • Fixed: a main gear wheel is usually fixed and does not steer.
  • Steered: a nose or tail wheel that is actuated with a tiller or rudder linkage is steered up to a certain angle, where the maximum angle can be dependent on the speed.
  • Free Castoring: on many taildraggers and also some modern tricycle aircraft like the Cirrus SR22 or Columbia 400 there is no wheel steering at all, they instead rely on rudder and differential braking to steer on the ground, with the wheel just trailing.

11.36 adds another option, that is commonly found on aircraft like the C152, C172, etc.:

  • Non-rigid steering with castoring range: The wheel can be steered through rudder linkage up to a (small) deflection angle. By using differential brakes, the plane can turn tighter, in which case the wheel deflects further than it would from rudder linkage alone.

The new parameter is found in Plane Maker as: “castor limit (deg) This is the maximum amount that the nosewheel or tailwheel of the aircraft can steer from full rudder deflection and pulling the plane into a turn with differential brake. Enter zero if the nose wheel does not steer more than the rudder-linked values or the tail wheel is not limited in castoring at all.”

That means, the nose wheel steering limit (fast and slow) for the 172 is 10 degrees, as can be found in the TCDS of the aircraft (or the POH).
However, the new castor limit is 45 degrees.
Now what happens is:

  • With the rudder pedal, the nosewheel is deflected with the rudder up to 10 degrees left or right.
  • At 10 degrees deflection, differential braking can be used. If the differential braking causes a tighter turn, the wheel will turn in, following the steering impulse from the differential braking, and allow a tighter turn without slipping the nose wheel, to a maximum of the “castor limit” of 45 degrees.
  • While in this situation, relaxing the differential braking will not immediately cause the wheel to spring back to 10 degrees. It will come out of this deflection depending on the forces that turn the plane. So you can get the plane out of the turn by relaxing the rudder pedal deflection – once the rudder is not hard over one way anymore, the springs will pull on the wheel once again, or you can increase thrust, apply brakes or even diff-brake in the other direction.

Note that all of that also works on a tailwheel as well.

For users without brake pedal axes, the “auto-toe brakes” follows the existing “left and right brake power to help with steering (if no rudder pedals present)” setting. This way, the toe brakes can be activated on full rudder deflection, leading to the tighter turn naturally.
If that parameter is 0, no auto-toe braking happens. The parameter is ignored if the user has hardware pedals with independent toe-brake axes.

Do not use the “tailwheel spring force” parameter on anything but a tailwheel any more. In the past, you could abuse it to get auto toe-braking, but you can get this explicitly with the toe brake parameter. The tailwheel spring force has unintended side effects if used on planes that aren’t tail-wheel configuration, so it must be 0 unless you actually have a taildragger with a spring steering.

Use the deflection time parameter to model the delay introduced into the nosewheel steering by a non-rigid linkage (bungee cords in a Cessna). Wheel deflection follows the rudder (or tiller) deflection with the speed constant “nosewheel steering full deflection time (sec)”. This can be used for a hydraulic actuator that takes time, or the bungees pulling the wheel around rather than a fixed linkage. That time would be 0 in a Piper with fixed linkage.

If the castor limit is 0 (which it is for existing planes not modified in Plane Maker 11.36) the nosewheel steering will work as it did before, so it is suitable for Airliners or Pipers.
If the castor limit is non-zero and applied to a castoring wheel (which has no steering angle) it will limit how far the wheel can be deflected by castoring. While not particularly useful for tail-wheels, this is useful for castoring nose-wheels, like on a Grumman or Cirrus.