Trim - asymmetric payload

So there are a few threads on trim already but I've failed to find a definitive answer on this from any of them...

Is the Hornet meant to autotrim in roll after dropping ordinance which results in an asymmetric payload? I've found that I have to trim manually to compensate but this I find is tricky and often results in the aircraft trim becoming "unstable" and not being able to fly straight and level. Roll stick input is needed to keep her wings level, if I hold her straight and then trim to no stick force, she bounces off in the other direction... now this kind of thing would be expected in an aircraft with no fly by wire system but I was expecting it to be easier in the F18, perhaps I'm being finicky but something doesn't feel right...

autotrim means trim for 1G and it's for pitch trim.

For roll trim you have to trim manually

Is the Hornet meant to autotrim in roll after dropping ordinance which results in an asymmetric payload?

Nope... Good old fashioned piloting skill involved I'm afraid ;) Although I know what you mean about the aileron trim a bit too sensitive, I have the same issue with most modules tbh.

I never understood why the plane doesn’t auto trim in the roll axis. The f16 in falcon bms has the same “problem”. Does anyone know the reason?

I never understood why the plane doesn’t auto trim in the roll axis. The f16 in falcon bms has the same “problem”. Does anyone know the reason?

Because that's how these aircraft behave in reality.

Because that's how these aircraft behave in reality.

I know this. That was not my question.

I know this. That was not my question.

Wasn't trying to be a dick, what is your question then? Why do the aircraft in reality behave that way?

I'm not 100% sure but I suspect it has something to do with the fact that auto trimming to 1g in the pitch axis still allows the aircraft to maneuver normally. How would you implement auto trimming in the roll axis? If the trim laws tell the aircraft to auto trim the wings level with the horizon then you wouldn't be able to bank the aircraft and keep it banked without constantly pushing the stick farther to the side, basically it would work fine as long as you want to fly straight and level but you'd be fighting the trim constantly while banking and turning, which is pretty important in a fighter. ie: you'd bank the aircraft, let off pressure on the stick to maintain a specific bank angle and then the FCS would start to trim it out so that it returns to wings level.

To auto trim roll you'd need something like a 'roll-rate' or angular motion sensor so that the system could trim to 0*/s roll, and there is no roll-rate sensor in the hornet or other fighter aircraft that I''m aware of currently. I'm sure it could be done, but given how easy it is to manually trim the roll axis, and how real life pilots don't seem to complain about it at all, means it's apparently not much of an issue for real pilots therefor isn't a priority for the engineers to implement.

Wasn't trying to be a dick, what is your question then? Why do the aircraft in reality behave that way?

 

I'm not 100% sure but I suspect it has something to do with the fact that auto trimming to 1g in the pitch axis still allows the aircraft to maneuver normally. How would you implement auto trimming in the roll axis? If the trim laws tell the aircraft to auto trim the wings level with the horizon then you wouldn't be able to bank the aircraft and keep it banked without constantly pushing the stick farther to the side, basically it would work fine as long as you want to fly straight and level but you'd be fighting the trim constantly while banking and turning, which is pretty important in a fighter. ie: you'd bank the aircraft, let off pressure on the stick to maintain a specific bank angle and then the FCS would start to trim it out so that it returns to wings level.

 

To auto trim roll you'd need something like a 'roll-rate' or angular motion sensor so that the system could trim to 0*/s roll, and there is no roll-rate sensor in the hornet or other fighter aircraft that I''m aware of currently. I'm sure it could be done, but given how easy it is to manually trim the roll axis, and how real life pilots don't seem to complain about it at all, means it's apparently not much of an issue for real pilots therefor isn't a priority for the engineers to implement.

I would be extremely surprised if the airplane does not have the ability to measure roll rate.

But in any case. I was thinking that 0 stick input = 0 roll rate.

I mean, it’s what we try to achieve when we trim right? Except that we have to keep trimming to compensate for airspeed etc.

I’m just wondering why they choose not to implement it. I’m sure there is a reason. Especially since the f16 is the same.

I guess the fact that current modern front line fighters *don't* auto trim the roll axis tells me there must be a good reason for it (even if my guess in my previous post is wrong). I'd be interested in hearing from a real pilot/engineer regarding the reason behind it.

Wasn't trying to be a dick, what is your question then? Why do the aircraft in reality behave that way?

 

I'm not 100% sure but I suspect it has something to do with the fact that auto trimming to 1g in the pitch axis still allows the aircraft to maneuver normally. How would you implement auto trimming in the roll axis? If the trim laws tell the aircraft to auto trim the wings level with the horizon then you wouldn't be able to bank the aircraft and keep it banked without constantly pushing the stick farther to the side, basically it would work fine as long as you want to fly straight and level but you'd be fighting the trim constantly while banking and turning, which is pretty important in a fighter. ie: you'd bank the aircraft, let off pressure on the stick to maintain a specific bank angle and then the FCS would start to trim it out so that it returns to wings level.

 

To auto trim roll you'd need something like a 'roll-rate' or angular motion sensor so that the system could trim to 0*/s roll, and there is no roll-rate sensor in the hornet or other fighter aircraft that I''m aware of currently. I'm sure it could be done, but given how easy it is to manually trim the roll axis, and how real life pilots don't seem to complain about it at all, means it's apparently not much of an issue for real pilots therefor isn't a priority for the engineers to implement.

This is not logical. If this were the case it would apply to pitch as well.

And the only reason Im not 100% equivocally saying that the Hornet has a roll rate sensor is because I dont have the actually manufacturing data in front of me, but how on earth could it not have one? There are so many systems that depend on it, attitude hold, radar antenna (always stabilised in the horizontal), inertial navigation etc etc...I mean, you stare through a roll indicator the entire time you are in the plane (pitch ladder in the HUD). How can these systems work without intimate digital knowledge of roll angle and roll rate?

That being said, I cant explain why there' no auto-roll-trim either.

This from an F16 driver:

"The roll rate trim still seemed sensitive to asymmetric loads more than I would have thought, but the Viper was less sensitive to uncommanded roll when dropping a big egg off of one side than anything I ever flew and dropped bombs from.

OTOH, when my LEF folded up, the FLCS tried to help me and I did not go into an uncontrolled roll or yaw. I still had to trim all the way and hold fifteen or sixteen pounds of pressure, but I am fairly sure the FLCS control laws were not designed to work properly with such a drastic "modification" to the aero properties the jet and I were experiencing."

I believe a lot of more modern jets than featured in DCS (think, F-35, Rafale, Typhoon etc) do auto trim in roll, and certainly will do more to automatically account for asymmetries when pulling G. They know what stores they have hung where and so when in an asymmetric loadout they can calculate how many lb/ft of torque that asymmetry will create and from that predict how much roll input is required to maintain zero roll rate per-G of pull at any specific point in the flight envelope. This kind of stuff is beyond what the early FBW jets like the F-16 and F-18 can do.

I believe a lot of more modern jets than featured in DCS (think, F-35, Rafale, Typhoon etc) do auto trim in roll, and certainly will do more to automatically account for asymmetries when pulling G. They know what stores they have hung where and so when in an asymmetric loadout they can calculate how many lb/ft of torque that asymmetry will create and from that predict how much roll input is required to maintain zero roll rate per-G of pull at any specific point in the flight envelope. This kind of stuff is beyond what the early FBW jets like the F-16 and F-18 can do.

Any source on that? Don't mean that in a confrontational way, just curious to read more about it if possible.

I believe a lot of more modern jets than featured in DCS (think, F-35, Rafale, Typhoon etc) do auto trim in roll, and certainly will do more to automatically account for asymmetries when pulling G. They know what stores they have hung where and so when in an asymmetric loadout they can calculate how many lb/ft of torque that asymmetry will create and from that predict how much roll input is required to maintain zero roll rate per-G of pull at any specific point in the flight envelope. This kind of stuff is beyond what the early FBW jets like the F-16 and F-18 can do.

Hmm. I can’t say you are wrong since I don’t really know but the plane has no trouble stabilizing itself when using autopilot. So it seems illogical to me that the is plane incapable of roll trimming. I’m more inclined to believe that they choose not to do it for some reason.

But this is maybe something only a Boeing engineer can answer.

Hmm. I can’t say you are wrong since I don’t really know but the plane has no trouble stabilizing itself when using autopilot. So it seems illogical to me that the is plane incapable of roll trimming. I’m more inclined to believe that they choose not to do it for some reason.

But this is maybe something only a Boeing engineer can answer.

Bear in mind that there is a difference between a ~1G autopilot dealing with an asymmetric load and an FCS system dealing with an asymmetric load while manoeuvring, thats more what I was getting at.

Any source on that? Don't mean that in a confrontational way, just curious to read more about it if possible.

I have nothing to hand that's public domain I'm afraid. Although if you want more in-depth reading about FCS systems I suggesting heading over to F-16.net as there are some extremely informative posts there from the actual guys involved in designing and testings the systems back in the day on that jet.

Half way down this page theres a post by Raptor_Claw thats worth a read. As are lots of his other posts...

http://www.f-16.net/forum/viewtopic.php?f=21&t=11691

Turned into an interesting thread, I too was thinking of an FCS system in which all three axes are fly by wire (Roll, Pitch Yaw) This usually negates the need for excess trimming in straight and level flight as the system senses all movement (ADC) and moves the control services to achieve the required level of pitch, roll and yaw inputted by the pilot via the stick, stick stays in the middle, aircraft flies straight and level regardless of balance.