Opposite roll with rudder

[uhntissbaby111]

Noticed a strange issue today while just flying around NTTR. I was messing around and flying straight and level and kicked in a bit of left rudder to watch what the yaw string did. As expected the nose yawed left and rolled slightly left. So I put in some right stick to keep it level. I went throttle idle and after a few seconds of the speed decreasing, the aircraft started suddenly rolling right. I managed to consistently reproduce, and the same issue happens with rudder. Here are the flight conditions and steps below. Not sure if this is an error or if this is correct behavior. Although to me, it seems like an error.

 

Conditions: NTTR, 0530 local time, standard temp and pressure, F-14A, Roll SAS off, no weapons only 2 drop tanks, about 12,000lbs of fuel, altitude ranging from 6000-9000 ft MSL

 

Here is how I managed to reproduce:

1) Fly straight and level to 375-400kts IAS

2) Kick in left or right rudder to get full ball deflection, and reduce throttle to idle

3) Add appropriate lateral stick to keep wings level

4) Once the speed decreases to around 350, the aircraft will begin to roll in the opposite direction abruptly

 

If unable to reproduce, let me know and I'll record my screen and upload a video

[TLTeo]

What was your AoA? Above 15 units or so, aileron input results in roll reversal

[uhntissbaby111]

28 minutes ago, TLTeo said:

What was your AoA? Above 15 units or so, aileron input results in roll reversal

That's not it. I barely had any AOA on the aircraft. And when I neutralize the stick it still keeps rolling. I went ahead and made a quick video. I'm not able to reproduce it 100%, but I managed to do it on the first try. I tried a few more times after and couldn't get it. So you only need to watch the first 15-20 seconds or so.

 

 

Edited March 21, 2021 by uhntissbaby111

[Quid]

I remember testing this close to two years ago; if memory serves, as the plane starts dropping into negative AoA, the rudder roll direction reverses.  Try this: maintain positive AoA and start rudder rolling, then, start pushing the stick forward and see as the plane goes to negative if it reverses and starts rolling in the other direction; I thought that was the cause.

[uhntissbaby111]

16 hours ago, Quid said:

I remember testing this close to two years ago; if memory serves, as the plane starts dropping into negative AoA, the rudder roll direction reverses.  Try this: maintain positive AoA and start rudder rolling, then, start pushing the stick forward and see as the plane goes to negative if it reverses and starts rolling in the other direction; I thought that was the cause.

Ok yup! That's it. As soon as some negative AOA gets on the jet the roll reverses. It feels a bit sudden, like a switch is flipped at a certain value and it just rolls the opposite way. I'm assuming this is correct F-14 behavior then?

[Quid]

1 hour ago, uhntissbaby111 said:

Ok yup! That's it. As soon as some negative AOA gets on the jet the roll reverses. It feels a bit sudden, like a switch is flipped at a certain value and it just rolls the opposite way. I'm assuming this is correct F-14 behavior then?

No idea if it's correct or not; I'm only aware of lateral (stick) control reversal being highlighted in the NATOPS, nothing with rudder reversal, but that doesn't mean it didn't happen - HB may have learned about this behavior from their SMEs or from some other study; not sure, they'd have to chime in.

[Skysurfer]

That is indeed very odd.

[Golo]

Logically its a form of a side slip roll just reversed. If you are in a slip the opposite wing of the rudder will be high lift wing and if you pull (increase AOA, therefore increase lift even more) it will try to roll you in the direction of the rudder deflection. So it makes sense that that if you do pushover (decrease AOA, maybe even go -AOA) it would reverse the roll, because it would become higher lifting wing, just in negative lift.

I would not say its odd. It happens in F/A-18 and F-5 as well its just more subtle and smooth. F-14 snap rolls into it, if that is correct I dont know, but from the way F-14 is build with its variable wing sweep I would say it make sense. 

[Xtorris]

On 3/21/2021 at 12:22 AM, Quid said:

I remember testing this close to two years ago; if memory serves, as the plane starts dropping into negative AoA, the rudder roll direction reverses.  Try this: maintain positive AoA and start rudder rolling, then, start pushing the stick forward and see as the plane goes to negative if it reverses and starts rolling in the other direction; I thought that was the cause.

This right here. The negative AOA 

 

"The F-14 exhibits positive dihedral effect throughout the positive AOA envelope (tending to roll away from sideslip), but negative dihedral effect at negative AOA." ~F-14B NATOPS 11.5.2 Dihedral Effect

 

"At negative AOA, dihedral effect is negative such that a right rudder input will produce a left roll." ~F-14B NATOPS 11.5.2 Dihedral Effect

[Skysurfer]

Just now, Pacientis said:

This right here. The negative AOA 

 

"The F-14 exhibits positive dihedral effect throughout the positive AOA envelope (tending to roll away from sideslip), but negative dihedral effect at negative AOA." ~F-14B NATOPS 11.5.2 Dihedral Effect

 

"At negative AOA, dihedral effect is negative such that a right rudder input will produce a left roll." ~F-14B NATOPS 11.5.2 Dihedral Effect

 

Nice, great find!

[Quid]

1 minute ago, Pacientis said:

This right here. The negative AOA 

 

"The F-14 exhibits positive dihedral effect throughout the positive AOA envelope (tending to roll away from sideslip), but negative dihedral effect at negative AOA." ~F-14B NATOPS 11.5.2 Dihedral Effect

 

"At negative AOA, dihedral effect is negative such that a right rudder input will produce a left roll." ~F-14B NATOPS 11.5.2 Dihedral Effect

Completely missed that.  Thanks!

[Xtorris]

Yeah, I actually sat down and read the entire F-14B NATOPS manual (excluding some stuff that HB didn't model into the game). Learned quite a bit, and even discovered a few quirks the plane has.

 

Sadly enough, I enjoyed reading it.