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Is the rudder moving by FBW in direction of roll or in the opposite direction?

I understand that the rudder is used due to the ARI system (ailerons - rudder interconnect), but don't have any idea how much or in what direction it is used throughout the process ?

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  • $\begingroup$ I struggle to understand what you're asking... Are you asking if you need to use the rudder (but on an FBW aircraft is a tricky question...) during an aileron roll to compensate for adverse yaw on the F-16? $\endgroup$
    – Marco Sanfilippo
    Sep 16, 2019 at 13:58
  • $\begingroup$ Exactly. I understand that the rudder is used due ARI system (ailerons - rudder interconnect), but don't have any idea how much or in what direction is used, or only if barell rolled the rolls and pulling some G's. Some pilots tell me that no rudder another's tell me the rudder is used in slow flight only. $\endgroup$
    – George Geo
    Sep 16, 2019 at 15:05
  • $\begingroup$ I still do not understand if you are asking about whether the pilot needs to manually input rudder commands because the FCS does not always provide correct rudder inputs, or if the rudder is needed at all, through pilot or computer inputs. I have heard and read that an F-16 pilot may not need to touch the rudder for 90% of a mission, but that is because the FCS does it for them, not because the F-16 does not usually need rudder deflection. $\endgroup$
    – AEhere supports Monica
    Sep 17, 2019 at 9:05
  • $\begingroup$ Only " if the rudder is needed at all, through computer inputs" and direction in which is deflected. $\endgroup$
    – George Geo
    Sep 17, 2019 at 12:29
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    $\begingroup$ @GeorgeGeo The control laws will generate the rudder input. Generally it's to generate a stability-axis roll and minimize adverse sideslip via feedback and feedforwards. The actions may be counter-intuitive some times to achieve the desired results. $\endgroup$
    – JZYL
    Sep 17, 2019 at 14:54

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Adverse yaw from aileron deflection, at roll initiation, is a function of wing span: a longer wing span creates more adverse yaw. And a long wing span is not what an F-16 possesses.

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Its main problem at lower airspeed is creating a roll rate at all with such short wings. Deflecting the rudder may help increasing the roll rate - for a left roll, it would be deflected Trailing Edge Right, creating adverse yaw. Which might or might be compensated by asymmetric aileron deflection, depending on the priorities posed upon the manoeuvres of a fighter aircraft at low speed. Of course, it makes much more sense to deflect the horizontal tail such that it helps in the roll.

At high roll rates, adverse yaw is created by the difference in local angle of attack at left and right aileron. Plus an aeroplane with low wing span will start to nutate: it wants to spin about its rotation axis with the highest moment of inertia, so tends to want to convert roll into pitch from inertial coupling. So high roll rates are a different kettle of fish in aeroplanes with short wing span, with lots of work to do for the fly-by-wire computers.

In the F-16, is rudder used to arrest the adverse yaw effects...

Most probably not. Adverse yaw from aileron deflection is not a large issue on fighter aircraft.

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In normal positively-loaded flight in the F-16, we can be sure that when the pilot moves (presses) the control stick to the left to give a left roll input, the computer does not move the rudder to the right.

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  • $\begingroup$ Not true. IF IT FLY SLOW that is what computer do $\endgroup$
    – George Geo
    Sep 29, 2019 at 16:20
  • $\begingroup$ You mean, the opposite of what I do in a sailplane? Are you sure about that? Why? $\endgroup$
    – quiet flyer
    Sep 29, 2019 at 16:23
  • $\begingroup$ To keep the nose above the horizon line. But only at the slower speed. But it is done by FLCS not pilot itself. And if is put the negative G (unload) doses the same way at any point in time and at good maneuver speed. $\endgroup$
    – George Geo
    Sep 29, 2019 at 16:30
  • $\begingroup$ I am skeptical about this. You are saying that the computer commands a sideslip to keep the nose up? Do you have a source? $\endgroup$
    – quiet flyer
    Sep 29, 2019 at 16:45
  • $\begingroup$ As an aside, note that a high rate of roll around an aircraft's longitudinal axis tends to automatically convert angle-of-attack into sideslip, even in the absence of any aerodynamic adverse yaw torque. $\endgroup$
    – quiet flyer
    Sep 29, 2019 at 17:36

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