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Question: F-16 rolls slightly to the left when pulling the stick during takeoff (not a bug report at the moment)


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On 9/4/2021 at 2:15 AM, TobiasA said:

The slight roll to the left is most probably related to the heavy gun on the left side- 377kg on the left side with full ammo. The other side probably carries electronics which do not weight nothing but are most probably considerably lighter than the massive amount of hard steel near the left wing.
 

But why does the jet only slightly roll to the left when pulling the stick but without any stick input it flies perfectly level and balanced? 

On 9/5/2021 at 2:11 PM, Nealius said:

To rule out an internal fuel imbalance causing the rolling behavior, have there been tests with a full centerline tank or full wing tanks?

The rolling slightly to the left behavior when pulling the stick also happens with either a centerline tank or two full wing tanks. 

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7 hours ago, Mikaa said:

I've done some rough testing, and as far as I can tell, it seems like the CG shift from varying fuel levels and rolling tendency at higher aoa are separate issues. Even with the pylons and gun ammo removed, and regardless of fuel level there's an appreciable roll to the left at high aoa (worsening at higher fuel levels). It seems like the fuel imbalance at low fuel (favoring right roll) seems to offset this inherent left roll tendency, but does not appear to be the cause of it. You can even see this if you set fuel to 10%. During 1G level flight, the aircraft wants to roll right, however once you apply aft stick and increase aoa, this right roll is counteracted and overcome, causing roll to the left. I tested 100%, 50%, 20% 10% 5%, and 1% (with unlimited fuel on for the last one). Worst left rolling tendency is at 100%, and it decreases to minimal at 10-20%, and then worsens again at 1% fuel load, because there's nothing to offset it. 

 

The yawing tendencies during the TO Roll is far easier to explain. With an imbalance in loading (in this case due to fuel level changes and seemingly inherent rolling moment of the jet) the roll tendency is translated into yaw due to the increased friction created by more force acting through that side's landing gear tire - roll turns into yaw. At low fuel levels, the friction increase is felt on the right side of the aircraft. I'm also speculating that at higher fuel levels, whatever is causing this left roll tendency is the reason for the left yaw during TO as well. 

 

Something feels off here. I have tracks saved for the testing I did if needed, but I'm going to wait on the FM and FLCS changes before submitting a bug report. 

Yeah I guess it would be better to see if this behavior still occurs after the FM update. IDK if ED has said if they are going to make any changes on the FLCS tho. 

@Mover

Hi Mover, since you have flown the real F-16, could you please comment on this slightly rolling to the left behavior when pulling the stick? Does it also occur on the real jet? IDK if you are allowed to say anything regarding this, but if you could, your input as a former Viper pilot would be really helpful. Thanks in advance! 


Edited by SCPanda
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What we know is: The F16 is not built perfectly balanced laterally, it wont compensate for assymmetric loadout or assymmetric fuel states, the tanks have different volumes and are emptied sequentially, leading to a shift of the center of gravity depending on fuel state.

The FM update needs to come with a FLCS update if ED works on the g onset.

So the only thing that makes sense at this time is asking someone like Mover who has flown the F16 or an F16 engineer who knows about that stuff.... Or just trust ED they know what they're doing.

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35 minutes ago, darkman222 said:

What we know is: The F16 is not built perfectly balanced laterally, it wont compensate for assymmetric loadout or assymmetric fuel states, the tanks have different volumes and are emptied sequentially, leading to a shift of the center of gravity depending on fuel state.

The FM update needs to come with a FLCS update if ED works on the g onset.

So the only thing that makes sense at this time is asking someone like Mover who has flown the F16 or an F16 engineer who knows about that stuff.... Or just trust ED they know what they're doing.

Yep. It would be great to hear from a former or current Viper pilot regarding this issue. I'm sure ED has someone like Mover in their dev team as well, or at least they consulted pilots regarding the handling of the jet. But we are just not sure if this rolling to the left when pulling the stick behavior is a thing that is accurately modeled like the real jet or it could be a bug related to the FM or FLCS. We are indeed curious. 

 

Also, another thing I am personally curious about is the imbalance of the jet and how it affects the jet during flying. Because as long as the loadout is symmetric and with no wind, our DCS F-16 won't roll to the left or right by itself until the fuel drops below the critical level (when you see warning in cockpit that one of the fuel tank is either low or empty), and when that happens, it actually rolls to the right. So I guess despite the jet itself is not perfectly balance laterally, FLCS does indeed balance the jet itself in the air. However, once you pull the stick, it does roll to the left slightly. If this behavior is due to the imbalance of the jet by nature, it would be interesting to know why this only occurs when you pull but not when without pulling. Anyway, just curious. 


Edited by SCPanda
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I forgot to mention. If you want to test what assymmetrical loadouts do you can really push it to the limit, loading one wing with bombs and keep the other clean.

You can trim that out,but even a change in airspeed will mess up the trim and the F16 will start to roll. I think the F16 wont compensate for that is correct too, but nobody would fly with such an assymmetrcial loadout so the effect is barely noticable with a "realistic" loadout.

With that configuration pull some Gs and create a high alpha situation the jet will roll and is very hard to control maybe due to high alpha or because pulling Gs changes airspeed or both. I dont know.

I guess, like I said, that must be the limitations of a late 70ies jet FLCS.

 


Edited by darkman222
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15 hours ago, SCPanda said:

If this behavior is due to the imbalance of the jet by nature, it would be interesting to know why this only occurs when you pull but not when without pulling.

The products of inertia can be nonzero even if the cg is on the centerline, which would not cause a roll bias in steady flight, but would cause a roll rate induced by angular acceleration in the pitch axis, called inertia coupling.

 

There are also possible aerodynamic effects that don't show up in steady flight but appear during dynamic flight conditions such as pitch rate or AoA rate.

 

Gyroscopic coupling effects also don't occur in steady flight, but appear during rotation about any axis perpendicular to the engine.  So a pitch rate induces a yaw rate and vice versa. And yaw might couple into roll for the above reasons, as is generally true for conventional aircraft.

 

This is not to say the model is correct, just to give possible explanations for unintuitive motion. 


Edited by Machalot
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5 hours ago, Machalot said:

The products of inertia can be nonzero even if the cg is on the centerline, which would not cause a roll bias in steady flight, but would cause a roll rate induced by angular acceleration in the pitch axis, called inertia coupling.

 

There are also possible aerodynamic effects that don't show up in steady flight but appear during dynamic flight conditions such as pitch rate or AoA rate.

 

Gyroscopic coupling effects also don't occur in steady flight, but appear during rotation about any axis perpendicular to the engine.  So a pitch rate induces a yaw rate and vice versa. And yaw might couple into roll for the above reasons, as is generally true for conventional aircraft.

 

This is not to say the model is correct, just to give possible explanations for unintuitive motion. 

 

Thx for the explanation! 

5 hours ago, Glide said:

Is this a bug?  It won't hold trim anymore.  Constantly trimming back and forth.  It lists like a leaky tugboat. 🙂

We don't know. ED hasn't said anything yet. 

 

Regarding the trim, it's another issue. The trim hat on the stick is quite sensitive, and it only gives you a rough trim. To precisely trim the jet, I remember ED said you need to do it via the trim panel. 

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I know that the F-16 SSC is mounted with a 12 degree outboard offset, to counteract pilots unintentionally inputting left-roll while pitching aft (due to ergonomics). Maybe there's something weird coded in relation to that? Still, aft input is aft input, regardless of how the stick is mounted, so it shouldn't be a factor - I'm just spitballing for reasons why we're seeing this behavior.

 

This definitely could be a characteristic of the jet, however I have a hard time believing that the Air Force would accept that at 100% internal fuel, you have 20-30% less right roll authority in high aoa flight. Seems like a pretty significant flaw. Could also be a case of the characteristics being overmodeled, and need to be tuned back to something more representative of the real a/c. 

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Could it be a wrong calculation in the fuel balance?,

 

I know there is a certain tolerance in the Forward/Right and Aft/Left fuel split before there is a change in CG. The way the fuel is laid out in the F16 could be something to do with balance. There is an engine feed switch in the cockpit that allows you to manually draw from different tanks in order to correct for CG change if required. 

 

Might be a calculation error in fuel balance and feed, could explain the left/right tendency if there is no reaction with large throttle changes.

 

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On 9/7/2021 at 6:48 PM, darkman222 said:

What we know is: The F16 is not built perfectly balanced laterally, it wont compensate for assymmetric loadout or assymmetric fuel states, the tanks have different volumes and are emptied sequentially, leading to a shift of the center of gravity depending on fuel state.

 

That's true for every aircraft we have, but few (if any) of the other aircraft exhibit this behavior. 

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