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Deflection of rudders can balance moment about the centre of gravity. I have thought of non-symmetric deflection of rudders so that net sideward force is create to balance the moment about the center of gravity of the aircraft.

Rodrigo de Azevedo
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Tarun Sadasivuni
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  • Any reason you are combining the one-engine-out and the crosswind? Could it perhaps be better to ask separately? Just a thought. If you want to combine, shouldn't we know if crosswind is on same side or opposite as failed engine? Are we using the wing-down method of correction or kick-out-the-crab? Which does a jet fighter typically use anyway, or does it just land crabbed? Or is the landing speed so high that for all practical purposes there is never a crosswind component? Kind of hard to give a good answer to your question unless we know some of these things. – quiet flyer Apr 16 '20 at 17:18
  • I see grounds for some new ASE questions there, if they aren't duplicates-- you might consider asking them-- – quiet flyer Apr 16 '20 at 17:25
  • @quietflyer i will edit the question.actually i combined two different questions – Tarun Sadasivuni Apr 16 '20 at 17:28
  • Anyway welcome to ASE-- oh now I see you were asking about each thing separately not saying both were happening at once. – quiet flyer Apr 16 '20 at 17:28
  • @quietflyer thank you, and please clarify my doubt in details. – Tarun Sadasivuni Apr 16 '20 at 17:31
  • Are you essentially asking the same question as this-- which never has been answered -- https://aviation.stackexchange.com/questions/70579/fighter-jet-with-two-rudders-the-deflection-angle-it-is-not-identical-one-have – quiet flyer Apr 16 '20 at 17:33
  • @quietflyer my question is somewhat differen,im askiung how f18 tackles cross winds using gust, the reason why i mentioned unsymmetrical deflection of rudders is to know if this is possible then it can be answer to my main question – Tarun Sadasivuni Apr 16 '20 at 18:01
  • @quietflyer leave the second part and can you please answer the first one – Tarun Sadasivuni Apr 16 '20 at 18:01
  • Are you asking from a pilot standpoint, or a software programming standpoint? Because I'm pretty sure the pilot cannot deflect them differently, but I could confirm with friends if you have good reason to believe that it might be possible. – Michael Hall Apr 16 '20 at 18:33
  • Considering that deflecting the rudder creates drag as well as sideforce, to maximize yaw torque per unit of extra drag it seems that there might be an argument for deflecting only the one that moves outward and not the one that moves inward. I.e. treat the rudders like wingtip drag devices and only move the one that is on side of intended direction of yaw. Is this essentially what you are asking? If so feel free to "borrow" any part of this comment into your question. Of course I can also see an argument for deflecting two surfaces a small amount rather than one a larger amount. – quiet flyer Apr 16 '20 at 18:45
  • As being angled (V-shape), there is a chance rudder react to pitch input such as in this video. combination of both pitch input and rudder input can create asymmetric deflections. – Manu H Apr 17 '20 at 06:36
  • @ManuH -- right, the rudders are both deflected inward for takeoff. The V-shaped tilt of the tails would also make this plane a bad candidate for deflecting rudders differentially for the reason I suggested above, in any situation where you DONT want to induce a nose-up pitch torque by the rudder deflection. And I'm still not sure exactly what the original question is intended to be. – quiet flyer Apr 17 '20 at 14:50
  • "The V-shaped tilt of the tails would also make this plane a bad candidate for deflecting rudders differentially for the reason I suggested above, in any situation where you DONT want to induce a nose-up pitch torque by the rudder deflection."-- unless of course you mix an an elevator (stabilator) deflection as well... – quiet flyer Apr 17 '20 at 20:46

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The Hornet has a fly-by-wire flight control system that is run by 2 flight control computers (also known as Roll Pitch Yaw computers) and these each have 2 channels. This system gets pilot input from the controls but the computers will decide the most efficient way to impart the command signal using all of the available control surfaces. Pilot has no say in which surface moves. If they did, this aircraft would be a nightmare to control because, like most modern fighter jets, it has been designed to be inherently unstable. This allows it to be highly manoeuvrable during a dogfight.

Some of the surfaces are controlled by 2 channels (rudder, aileron, leading edge flaps). Some are quad redundant, utilising all 4 channels (Trailing Edge Flaps, Horizontal Stabs).

All of the surfaces, with the exception of the leading edge flaps from memory (because they are connected mechanically with a rod???), can deflect independently of each other, if the software/flight envelope permits. For example, if you want to roll a Hornet, some combination of all or some of the following surfaces may deflect: L and R Aileron, L and R Trailing Edge Flaps, L and R Horizontal Stabilisers. There are various sensors such as accelerometers which provide the negative feedback loop required to ensure the aircraft is getting to the required attitude at the expected rate. If it isn't, surfaces may deflect more or others may be commanded to help out. The circuitry inside the computer is basically a summing amp with the following inputs: command signal from the pilot, the position feedback from the surface and the rate feedback from the accelerometers. I think the important thing to understand is that this signal is nulled (0 volts) when the aircraft is pointing in the commanded attitude. That means no more movement of the surface.

With regards to rudder on takeoff, these accelerometers would sense an uncommanded change in the yaw axis and I suspect the flight control computers would damp this affect using the feedback method described above without the pilot really noticing much, certainly not having to stomp on a rudder pedal to compensate. There would be some sort of signal processing delay and mechanical reactive delay to correct the movement but this is all done without pilot input.

The rudders don't have to deflect symmetrically as they are controlled by different flight control computer channel pairs, one from each computer for redundancy. In fact, when the rudder fails, it is commanded to streamline and the other surfaces will compensate. The software programming inside the flight control computers that determines which surface/surfaces move and by how much.

Craig
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  • “because, like all fighter jets, it has been designed to be inherently unstable”—F-18 was probably the second one that was, after F-16. Before F-16 fighter jets used mechanical controls and were designed to be stable in pitch, otherwise they wouldn't be controllable. – Jan Hudec May 20 '20 at 09:08
  • … which means the F-15 accident actually demonstrates nothing, because F-15 did not use fly-by-wire controls. It did use differential elevators for roll control, which was important in that accident, but it was a mechanical link. – Jan Hudec May 20 '20 at 09:12
  • I'll edit the f-15 part. I was taught this in avionics school but I was taught wrong because I think my instructor must have assumed it was the E variant. – Craig May 20 '20 at 13:08
  • The Negev collision happened three years before the E variant first flew, so it indeed couldn't have been that. – Jan Hudec May 20 '20 at 19:40