Q: Why is the Hornet's rudder the only one that works the other way around? (below ~4.7 AoA)

[Tom Kazansky]

hi,

can someone please explain to me, why the Hornet's rudder is the only one that works the other way around?

 

With every other fixed-wing module I've testet, the aircraft rolls to the direction where the rudder is moved to.

Tested everything F-86, F-5E, F-14/15C&E/16, MiG-29G, L-39....

In some of them I can even do a 360° roll right after take off, only with the use of full rudder in the desired direction.

 

In the Hornet it works the other way around by a very slow roll rate. When I press down full left rudder, the Hornet rolls slowly to the right. And vice versa of course. Even with the FCS gain switch set to override. (Is there another option to turn down (not off) the fly-by-wire / FCS?)

 

(And: yes, my rudder-axis-binding is correct)

 

EDIT: thanks to @Hulkbust44, we see it is depending on speed. It changes significantly at around 260 kts from expected behaviour below and the opposite above this speed. Added this to the title.

EDIT2: thanks to @Figaro9, we see it's depending on the AoA. Adjusted the title.

Edited April 15 by Tom Kazansky

[Phantom711]

The whole point of an FBW is, that the aircraft is NOT supposed to do a movement, that you did not command. 
So if you move the left rudder pedal, the aircraft will move it‘s control surfaces in order to do a turn around its vertical axis. You did not command a roll, so it will compensate for any undesired roll movement automatically. The very slight opposite roll is probably some overcompensation by the system in order to accomodate for the increased lift on the opposite wing.

 

EDIT: I am surprised though, that you say it works in an F-16.

Edited April 13 by Phantom711

[Tom Kazansky]

1 hour ago, Phantom711 said:

EDIT: I am surprised though, that you say it works in an F-16.

yep. you are not the only one.

[Foka]

1. Rudder is not for controlling roll, but yaw.

2. Pushing right rudder should turn aircraft to the right.
 

Verify in bindings, that you don't have double bindings and your rudder does not control roll. Also verify if you press right rudder if rudder in DCS moving this same way or opposite.

[Tom Kazansky]

15 minutes ago, Foka said:

1. Rudder is not for controlling roll, but yaw.

2. Pushing right rudder should turn aircraft to the right.
 

Verify in bindings, that you don't have double bindings and your rudder does not control roll. Also verify if you press right rudder if rudder in DCS moving this same way or opposite.

As stated in OP, bindings are correct. Just try full rudder and tell me what you see please.

[Phantom711]

@Foka

Sorry, but you are missing the point here. I think the OP is aware of what roll and yaw is.

BUT: In (some) non-FBW aircraft you can make the aircraft roll by using the rudder.

[SMH]

Yeah, you're right. Seems wrong. 

[Foka]

Experience in tech support shows that even user says he done/checked/verified something, doesn't mean he did.

[Tom Kazansky]

3 minutes ago, Foka said:

Experience in tech support shows that even user says he done/checked/verified something, doesn't mean he did.

That's correct. But not the case here.

 

[SMH]

17 minutes ago, Foka said:

Experience in tech support shows that even user says he done/checked/verified something, doesn't mean he did.

Maybe you should take your own advice and actually try it.

[Hulkbust44]

Sounds correct to me. The pedals in the Hornet are a yaw command.

Push the right pedal, nose yaws to the right, to keep the track the FCS compensates by rolling the opposite direction.

Take a mustang, give it a good boot of rudder and see how you need to move the stick to keep her straight.

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[Tom Kazansky]

8 minutes ago, Hulkbust44 said:

Take a mustang, give it a good boot of rudder and see how you need to move the stick to keep her straight.

Ok, but what happens if you just use and hold rudder and no stick? Does it really roll to the opposite side?

All planes I tested rolled to the side of the rudder I pressed. All but the Hornet.

[SMH]

47 minutes ago, Hulkbust44 said:

Sounds correct to me. The pedals in the Hornet are a yaw command.

Push the right pedal, nose yaws to the right, to keep the track the FCS compensates by rolling the opposite direction.

Take a mustang, give it a good boot of rudder and see how you need to move the stick to keep her straight.

Sent from my moto g stylus 5G (2022) using Tapatalk
 

Wrong. (And the FCS isn't an autopilot. It doesn't know nor care where the plane's "track" is.)

Try it in the F-16. Also a FBW jet. 

[Hulkbust44]

Ok, but what happens if you just use and hold rudder and no stick? Does it really roll to the opposite side?
All planes I tested rolled to the side of the rudder I pressed. All but the Hornet.

Yeah, because nothing has the fly by wire to the degree that the Hornet does.

Certainly not with the yaw logic.

Aerodynamically, what does kicking the rudder do? You're yawing the nose and pushing one wing out in front of the other, and the inboard wing will have it's airflow blanked. This causes the outer wing to create more lift inducing a roll. Opposite aileron will of course counteract that, and that's what the FCS is doing as it's logic is to execute a level yaw. This makes it very good for spread out suppressive fire on gun runs.

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[Hulkbust44]

Wrong. (And the FCS isn't an autopilot. It doesn't know nor care where the plane's "track" is.)

Try it in the F-16. Also a FBW jet. 

Not a track, but a higher speed that's basically what it's doing.

F-16 is a completely different system. No where near the same rudder logic. (The thing gives up past some 22 alpha)


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[Hulkbust44]

Okay, so this behavior is what I would expect if the FCS was deflecting surfaces to compensate, watching the FCS format, it is not. Not at all. The rudders are the only surfaces moving.
260 knots and below (until you get to AoA scheduling) it rolls in the direction of the pedals. Somewhere between 260/270 KIAS it does switch, the aircraft rolls away from the pedal input.

The airspeed distinction was missing from the OP, this is peculiar.

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[Tom Kazansky]

237 knts

6 hours ago, Hulkbust44 said:

Aerodynamically, what does kicking the rudder do? You're yawing the nose and pushing one wing out in front of the other, and the inboard wing will have it's airflow blanked. This causes the outer wing to create more lift inducing a roll. Opposite aileron will of course counteract that, and that's what the FCS is doing as it's logic is to execute a level yaw.

 

5 hours ago, Hulkbust44 said:

Okay, so this behavior is what I would expect if the FCS was deflecting surfaces to compensate, watching the FCS format, it is not. Not at all. The rudders are the only surfaces moving.
260 knots and below (until you get to AoA scheduling) it rolls in the direction of the pedals. Somewhere between 260/270 KIAS it does switch, the aircraft rolls away from the pedal input.

The airspeed distinction was missing from the OP, this is peculiar.

thank you @Hulkbust44

although I was aware of the aerodynamics, it is helpful to explain it here, so we learn about the expected behaviour.

thank you also for your observation about the speed differences, and finally, but most important, you are correct:

the other control surfaces are not moving at all, so there is no way the FCS/fly-by-wire compensates anything. (certainly, the thrust of the two engines is always symmetrical and this is not changed.)

 

It is interesting what happens below 236-260 knots (maybe depending on altitude? haven't tested that yet, just jumped to the default Freeflight mission):

the DCS Hornet goes from a (e.g.) left rudder induced roll to a right roll just by increasing speed, without any movement of the other control surfaces.

so my initial question stands:

Why is the Hornet's rudder the only one that works the other way around? (above 260 kts)

(have to change the title)

Edited April 14 by Tom Kazansky

[oldcrusty]

Here's my gamer aerodynamics guru's take on this:  At slower airspeed (say below 260) the 'blanked out' part of the wing (referring to Hulkbust's description is larger then at at higher speed.  The rudder deflection and yaw is less pronounced at higher speed so the lift difference between the wings is small.  The rudder sits on top of longitudinal axis and when deflected it would cause a tiny roll in the opposite direction... I guess, lol.  

I would think that FCS should counter this roll at any airspeed but it doesn't.   BTW, the interconnect between the rudder and stick during the turn seems to work fine but that's a separate logic.  So... any real gurus, SMEs input?

EDIT:  I don't have a Viper but I'm curious about it's rudder effectiveness at higher speeds... should be nil

Edited April 14 by oldcrusty

[Tom Kazansky]

1 hour ago, oldcrusty said:

Here's my gamer aerodynamics guru's take on this:  At slower airspeed (say below 260) the 'blanked out' part of the wing (referring to Hulkbust's description is larger then at at higher speed.  The rudder deflection and yaw is less pronounced at higher speed so the lift difference between the wings is small.  The rudder sits on top of longitudinal axis and when deflected it would cause a tiny roll in the opposite direction... I guess, lol.  

I would think that FCS should counter this roll at any airspeed but it doesn't.   BTW, the interconnect between the rudder and stick during the turn seems to work fine but that's a separate logic.  So... any real gurus, SMEs input?

EDIT:  I don't have a Viper but I'm curious about it's rudder effectiveness at higher speeds... should be nil

 

if so (which I doubt), this should be equal for all planes. but the Hornet is the only plane with an opposite role (above 260 kts). so it does not make sense.

Edited April 14 by Tom Kazansky

[Figaro9]

This behavior is not speed but AOA-dependent according my today’s tests.Above ~4.7 alpha the roll is in direction of the rudder input. Bellow ~ 4.7 roll is very slightly in the opposite direction. Can’t tell why. Probably the outboard angled stabilizers play a role here…

[Tom Kazansky]

1 hour ago, Figaro9 said:

This behavior is not speed but AOA-dependent according my today’s tests.Above ~4.7 alpha the roll is in direction of the rudder input. Bellow ~ 4.7 roll is very slightly in the opposite direction. Can’t tell why. Probably the outboard angled stabilizers play a role here…

interesting, thank you. will check tomorrow, but it seems I'll have to change the title of the thread once again

[Tom Kazansky]

13 hours ago, Figaro9 said:

This behavior is not speed but AOA-dependent according my today’s tests.Above ~4.7 alpha the roll is in direction of the rudder input. Bellow ~ 4.7 roll is very slightly in the opposite direction. Can’t tell why. Probably the outboard angled stabilizers play a role here…

checked that today and I can confirm your statement.

was able to control the direction of roll (with pressed and held rudder) just by adjusting the AoA with the elevators at any (tested) speed.

thanks, again

(got to change the title... again, we are getting further here)

Edited April 15 by Tom Kazansky

[Tom Kazansky]

I have to correct myself: the FCS of the Hornet does not do "nothing" with the control surfaces, it does adjust the rudder itself while changing AoA (of course not to the opposite direction).

I did not manage to reduce the influence of the FCS to stop that, not with the Gain switch, nor the Manual Spin Recovery Mode switch (thanks @DummyCatz )

I fanally saw some unclassified aerodynamic formulas* (thanks again @DummyCatz) that explain that the Hornet is able to create such opposite rolls at certain states and all that lead me to my cautious and tentative conclusion that it's ok for me at the moment.

Thanks to all of you for your interest and helpful replies.

 

(* I'm not quite sure I'm allowed to post here, so I don't) EDIT: the link should be ok, I guess:

"AERODYNAMIC PARAMETERS OF HIGH-ANGLE-OF-ATTACK RESEARCH VEHICLE (HARV) ESTIMATED FROM FLIGHT DATA", NASA TM 202692, https://ntrs.nasa.gov/api/citations/19900019262/downloads/19900019262.pdf

Edited April 15 by Tom Kazansky

[Tom Kazansky]

17 hours ago, Figaro9 said:

This behavior is not speed but AOA-dependent according my today’s tests.Above ~4.7 alpha the roll is in direction of the rudder input. Bellow ~ 4.7 roll is very slightly in the opposite direction. Can’t tell why. Probably the outboard angled stabilizers play a role here…

Couldn't stop thinking about that, so I came to the following:

let's assume hypothetically(!) we could increase the angle of both of the vertical stabilizers even more to the outboard sides, and consider them almost like wings:

so a (e.g.) left rudder input would look exactly like a right aileron input (if those stabilizers were wings). The right control surface would point upwards and the left surface downwards. The result would be a right roll.

You get what I mean?

So this would explain why a smaller angle of the stabilizers to the outboard side could induce a smaller (but noticeable) roll to the opposite side of the rudder input.

(Does not explain why it is AoA dependent, but ok.)

Edited April 15 by Tom Kazansky

[Tom Kazansky]

... the first plane with an even bigger angle of stabilizers to the outside, that came to my mind was the YF-23 Black Widow II and I found the following interesting behaviour:

while taxiing on the ground the YF-23 moves its stabilizers clockwise in a right taxi-turn. correct me if I'm wrong, but this should mean a right rudder press.

the control surfaces ot the much more perpendicular (i.e. vertical) stablilizers of the Hornet move "counterclockwise" (or to the right, in this regard) with a right rudder press.

 

This tells me, that if the stabilizers are installed with an higher and higher angle to the outboard sides, the effect I described in my previous post becomes so pronounced, that it is the better solution to invert the stabilizers' movement to get the roll/yaw you want to get.

Here is a YT video of the right taxi turn (starting at 34min 50s), you might need to slow the replay speed down to see it better:

https://www.youtube.com/watch?v=PYLiMYGBE2Q#t=34m50s

I admit that's a far excursion, and not a proof by no means, but it helps me understand what could be going on, till proven otherwise.

Edited April 15 by Tom Kazansky