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

[Figaro9]

1 hour ago, Tom Kazansky said:

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.)

 

Yes, that's how I thought about it too. 4.7 alpha is close to the best L/D ratio. It is possible that drag and lift difference becomes large enough above that to outweigh the lifting effect of the canted stab. Pure speculation though... But it might also explain why the roll rate increases with higher AOA.

Edited April 15 by Figaro9

[jaylw314]

8 hours ago, Figaro9 said:

Yes, that's how I thought about it too. 4.7 alpha is close to the best L/D ratio. It is possible that drag and lift difference becomes large enough above that to outweigh the lifting effect of the canted stab. Pure speculation though... But it might also explain why the roll rate increases with higher AOA.

 

I can't say this behavior accurate, but it's interesting that the NATOPS suggests below 5deg AOA and above Mach 0.6 is an area where the Hornet is more vulnerable to out-of-control flight.  I read that as excessive stick at low AOA could result in significant adverse (opposite) yaw, but I don't know if adverse roll with excessive rudder would follow   Or it could just be a coincidence

Edited April 15 by jaylw314

[Akiazusa]

Hi,

Just tested, if you try activating the MSRM at 40-50 AOA, you'll also get an opposite roll by the rudder input, while bypassing the flight computer.

[Chaffee]

On 4/15/2024 at 12:34 PM, jaylw314 said:

I can't say this behavior accurate, but it's interesting that the NATOPS suggests below 5deg AOA and above Mach 0.6 is an area where the Hornet is more vulnerable to out-of-control flight.  I read that as excessive stick at low AOA could result in significant adverse (opposite) yaw, but I don't know if adverse roll with excessive rudder would follow   Or it could just be a coincidence

 

This^

NATOPS has a lot to say about how the Hornet is particularly susceptible to departing due to nose-slice. It also has a lot to say about the systems that prevent nose-slice departures.

Is the described behavior correct? NATOPS isn't specific enough RE: what the FCS does to prevent nose-slice departures, but such departures have very hard positive roll associated with them, so for all we know, negative roll is the FCS fighting you to keep the aircraft out of nose-slice.

The FCS already coordinates the rudder with stick input to prevent adverse yaw. The stick already gives you a great way to roll the aircraft. Kicking rudder for fun seems like the sort of thing the FCS would frown on, since it has all kinds of ways to prevent sideslip.

Note: comparing the FBW system of the Hornet to that of the F-16 is a meaningless analogy, and there seem to be a lot of myths surrounding what FBW does. Read the NATOPS manual. FBW is not what most people seem to describe, and it certainly isn't the same thing in the Hornet as the Viper. Everything from performing a pirouette maneuver to rudder travel limits at various speeds or angles of attack are very specific and are also decidedly not simply "I'm telling the aircraft to yaw and now it has to figure out where to put the control surfaces to obey my will."

If that were true, the ATFLIR (or simply dropping ordnance) wouldn't cause roll asymmetry, etc. FBW isn't magic. The Hornet's FCS hates sideslip. Physics still matter.

 

2 hours ago, Akiazusa said:

Hi,

Just tested, if you try activating the MSRM at 40-50 AOA, you'll also get an opposite roll by the rudder input, while bypassing the flight computer.

NATOPS manual says if you yaw at 50-55 AOA, you should let go of the controls and wait for the aircraft to recover because life is about to get real interesting.

This is likely not the result of the same cause that's happening at low AOA, not least because your rudder deflection limits and toe-in/out are completely different.


 

Edited April 17 by Chaffee

[Akiazusa]

21 hours ago, Chaffee said:

NATOPS manual says if you yaw at 50-55 AOA, you should let go of the controls and wait for the aircraft to recover because life is about to get real interesting.

This is likely not the result of the same cause that's happening at low AOA, not least because your rudder deflection limits and toe-in/out are completely different.
 

All I see is unexciting things with super stable nose, and no sign of nose slice or sideslip excursions. That's why I filed a bug report about the absence of adverse yaw at high AOA.

Rudder deflection limits and toe-in/out are completely irrelevant in my testing, since MSRM is activated and all the feedbacks in the control loop are removed. You basically have direct and maximum control of the aileron and the rudder.

Those are called Departure Training Flight, and you can find the instruction cards from https://trace.tennessee.edu/utk_gradthes/2312/

 

NATOPS only have a shallow description of the FBW system. For a more thorough and deep reading I would suggest the following reading list:

https://ntrs.nasa.gov/api/citations/19920024293/downloads/19920024293.pdf from page 83

https://trace.tennessee.edu/utk_gradthes/2312/

https://trace.tennessee.edu/utk_gradthes/2372/

 

Quote

so for all we know, negative roll is the FCS fighting you to keep the aircraft out of nose-slice.

This is an example of over-comprehension. The FCS do have a Rudder Pedal to Rolling Surface interconnect function, or rudder to roll crossfeed, similar to the Rolling Surface to Rudder Interconnect (RSRI), only that the logic runs backwards. This is to deflect the aileron in the same direction as the rudder input, and is only active above 13 deg AOA (Roll Function 39). Since it has a positive gain (RK10), it will never cause a negative roll as you described.

The block diagram at page 101 of the NASA document listed above explains that in detail.

Edited April 18 by Akiazusa

[Ahmed]

Not a Hornet but: 

Regardless of being FBW or not, aircraft are usually designed to fly like..... aircraft.

 

 

[DummyCatz]

On 4/15/2024 at 3:19 PM, Tom Kazansky said:

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

 

Glad to help. Now that I can resume my posting, I'd like to share the said NASA wind tunnel data and the formula behind such opposite rolls.

Apart from the canted vertical stabilizers you explained earlier, a rolling moment is also created due to how the force is being applied at a higher position relative to the aerodynamic center, which acts as a pivot point. This rolling moment (coefficient denoted as Cl-δr) is opposite to the direction of yaw.

But then as sideslip builds-up, the rolling moment by the rudder can be cancelled and overwhelmed by another rolling moment created by sideslip (β) due to lateral static stability (Cl-β), which is in the same direction of yaw.

To simplify a bit, you can calculate the combined rolling moment coefficient (Cl) at a specific AOA, by (Cl-β * sideslip angle + Cl-δr * rudder deflection).

So at low AOA, it's close to -0.1 * sideslip + 0.02 * rudder deflection. As you can see it can be negative, and also can be positive, depending on how many sideslip and rudder deflection is combined.