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

[SCPanda]

Condition: No wind

 

Loadout: Clean (with or without pylons) or any symmetrical loadout, 100% fuel load, 100% gun ammo, 60 chaff and 60 flares

 

Hardware: Thrustmaster T16000M, Thrustmaster Warthog stick, or keyboard

 

Behavior: During takeoff, after I reach the desired takeoff speed and pull my stick to raise the nose, the jet leaves the ground like it should, but on the HUD, the roll indicator moves slightly to the left showing the jet rolls slightly to the left after it leaves to the ground. First I suspect it could be my input issue, such as I rolled a little to the left when I pulled my stick or I added some rudder input. So I disconnected my stick and rudder and used the default keyboard binding (i.e. the down arrow key) to pull the jet during takeoff, and I also used ctrl+enter to display my input. However, the jet still rolls slightly to the left even only with the Y axis of stick input (pulling) during takeoff. Ctrl+enter shows no other control input added except pulling the stick. 

 

Question: Is this a intended behavior that the real jet also tends to roll left a little bit after you pull the stick and the jet leaves the ground? I have also noticed this behavior when I watched some of Wag's F-16 tutorial videos. His jet also slightly rolls to the left after it leaves the ground during takeoff. I know this is a very small thing but I have been noticing this behavior pretty much since the release of the F-16 and I haven't seen anyone asking about this. So I finally decided to start a topic about this. If anyone can also confirm this behavior of the jet in the sim, I will upload a track and make a proper bug report in the Bugs section. Also, anyone who knows or have flown the F-16, please share if this behavior is realistic or not. Thanks! 

Edited August 31, 2021 by SCPanda

[Furiz]

I can only think of a gun being on the left side, that would be that extra weight on the left.

[SCPanda]

50 minutes ago, Furiz said:

I can only think of a gun being on the left side, that would be that extra weight on the left.

Could be. But shouldn't FCS already balanced out the weight of the gun when the jet came out of the factory? Or maybe gun ammo? IDK where the gun is loaded on the F-16 but if it's on the left side, it could be the case. Because all my takeoff attempts so far in DCS F-16 are all with 100% gun ammo. 

 

Edit: And I thought about this again, the jet is however perfectly balanced in the air (even with gear down after takeoff). It doesn't roll to either the left or right by itself. It only happens when you pull the stick during takeoff. 

Edited August 31, 2021 by SCPanda

[darkman222]

Weird. I noticed that too, but thought my stick could be not perfectly centered any more because I use it for quite a long time now and might be worn out. But I unplugged everthing and did a take off with keyboard only. So no roll is involved. The F16 seems to be left wing heavy. It gets less the faster you get.

I did a non AB takeoff and when you pitch the F16 up and down after that in slow speeds the left roll adds more and more.

Who knows something about it?

Gun weight? Turbine rotation, although its not a propeller engine, is it something the FLCS considers?

Track file:

https://www.dropbox.com/s/ew5zux27m1ylibk/left_wing_heavy.trk?dl=0

[Frederf]

Maybe some kind of engine rotation effect? That's the only obvious source of asymmetry I can think of. Real airplane are not 100% mass balanced laterally and maybe the DCS one isn't either. The SSC isn't mounted purely straight but rotated inward a bit. It's possible that "pitch" DCS axis is pulling straight aft on the stick which would be a combination aft and left roll. That doesn't seem terribly likely though.

[darkman222]

Did the same thing again. Just a keyboard take off. Which again, should eliminate any possible roll input. No ammo for additional left side weight. No flares no chaff. As clean as an F16 can be, same result.

https://www.dropbox.com/s/lgbuyol5npqp923/left_wing_heavy_no_ammo.trk?dl=0

Edited September 1, 2021 by darkman222

[SCPanda]

20 hours ago, darkman222 said:

Weird. I noticed that too, but thought my stick could be not perfectly centered any more because I use it for quite a long time now and might be worn out. But I unplugged everthing and did a take off with keyboard only. So no roll is involved. The F16 seems to be left wing heavy. It gets less the faster you get.

I did a non AB takeoff and when you pitch the F16 up and down after that in slow speeds the left roll adds more and more.

Who knows something about it?

Gun weight? Turbine rotation, although its not a propeller engine, is it something the FLCS considers?

Track file:

https://www.dropbox.com/s/ew5zux27m1ylibk/left_wing_heavy.trk?dl=0

Glad I am not the only one noticing it. 

 

3 hours ago, darkman222 said:

Did the same thing again. Just a keyboard take off. Which again, should eliminate any possible roll input. No ammo for additional left side weight. No flares no chaff. As cleas as an F16 can be, same result.

https://www.dropbox.com/s/lgbuyol5npqp923/left_wing_heavy_no_ammo.trk?dl=0

 

Okay so we can be clear that it's not the gun ammo doing it. 

[Mikaa]

There was post a while back for the jet rolling to the left at high alpha that was never confirmed by ED. I tested it myself in game in various configs, including clean with no gun ammo, and got consistent results. You need about 20-30% right stick to maintain bank angle during high alpha maneuvering. I'm tempted to make a new bug report for it, but the FLCS is WIP so that might be a contributing factor. 

[darkman222]

Nice, another mystery to solve about the DCS F16.

Take off with just 700 lbs of fuel and the F16 will roll to the right after take off, also yawing right a good amount on the runway.

Take off full with 7000 lbs of fuel and she will do as already mentioned, and roll to the left.

 

Seems to be a fuel related thing. Maybe in what order the fuel tanks are emptied and how the FLCS compensates for it. Could be correct or a bug though. I dont know. I am just playing the detective here. Whatever it is, I am sure @BIGNEWYcan explain or will move it in the right section hopefully.

 

Here are both track files for 700 lbs and 7000 lbs take off:

https://www.dropbox.com/s/7ay6a63u30botab/700_lbs_fuel.trk?dl=0

https://www.dropbox.com/s/m1i4n6z3p3k6rhy/7000_lbs_fuel.trk?dl=0

 

 

 

Edited September 1, 2021 by darkman222

[ARM505]

The ammunition feed drum is on the centerline anyway in the F16 (off the top of my head).

[SCPanda]

On 9/2/2021 at 12:22 AM, Mikaa said:

There was post a while back for the jet rolling to the left at high alpha that was never confirmed by ED. I tested it myself in game in various configs, including clean with no gun ammo, and got consistent results. You need about 20-30% right stick to maintain bank angle during high alpha maneuvering. I'm tempted to make a new bug report for it, but the FLCS is WIP so that might be a contributing factor. 

Interesting. I tested this with my jet in the air. By pulling vertically up, like I am trying to do a loop, using only the keyboard command so I don't accidentally add any rolling input, the jet does indeed tend to roll to the left, especially at lower speeds. 

 

So I guess this behavior of rolling to the left doesn't just occur during takeoff. 

[darkman222]

Same did I. Just keyboard input. @Mikaa said it is due to high alpha situations. So of course during take off you end up in a high alpha situation.

Its two things. The jet rolling either left or right depending on fuel state while in the air under high alpha.

But why is it also yawing when rolling on the runway? Could be one issue or two separate one. We need someone who knows about the F16s fuel management system if thats an issue in the real jet or if it is something just in the sim and if the FM update already has that on the list or if it needs to be reported.

 

[TobiasA]

Am 1.9.2021 um 19:13 schrieb darkman222:

Nice, another mystery to solve about the DCS F16.

Take off with just 700 lbs of fuel and the F16 will roll to the right after take off, also yawing right a good amount on the runway.

Take off full with 7000 lbs of fuel and she will do as already mentioned, and roll to the left.

 

Seems to be a fuel related thing. Maybe in what order the fuel tanks are emptied and how the FLCS compensates for it. Could be correct or a bug though. I dont know. I am just playing the detective here. Whatever it is, I am sure @BIGNEWYcan explain or will move it in the right section hopefully.

 

Here are both track files for 700 lbs and 7000 lbs take off:

https://www.dropbox.com/s/7ay6a63u30botab/700_lbs_fuel.trk?dl=0

https://www.dropbox.com/s/m1i4n6z3p3k6rhy/7000_lbs_fuel.trk?dl=0

 

 

 

 

 

Anything below  about 900lbs means that the reservoir is running dry, which might indeed shift CG to a side or front / back.
The aft / left is smaller in quantity as the front / right tank group (on the C but not on the D), so it might and probably will empty first which should explain what you are seeing there.
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.

The development of the 16 is slow, but the detail of simulation is amazing in some points of the flight model.

[Paladin1cd]

Shouldn’t the electric jet compensate for that? 

[Dee-Jay]

33 minutes ago, Paladin1cd said:

Shouldn’t the electric jet compensate for that? 

Hi! Nope. There is no auto compensation in roll axis on F-16 (nor on any other jet I know including M2000 & Rafale out of any AP mode or sub-modes).

[Paladin1cd]

So the hornet is wrong in how it does asymmetrical loads?

[Tholozor]

The Hornet does not automatically correct for asymmetrical loading either. There's specific differential trim requirements regarding catapult shots depending on the difference in ft-lbs.

Edited September 4, 2021 by Tholozor

[Machalot]

On 8/31/2021 at 5:16 PM, Frederf said:

Maybe some kind of engine rotation effect?

Do you know which direction the engine spools rotate? 

[darkman222]

Would be nice to know in general. But seems somehow irrelevant for this topic now. Because it depends on fuel state for sure. See my video. Either left or right roll depeding on if the internal tanks are full or empty. My feeling is that this behavior, if it depends on the F16s order of emptying the fuel tanks, should be correct because the F16 does not compensate for assymmetric loadouts. So why should she do fo assymmetric fuel loading either. That thing is from the late 70ies and not flying cell phone like the F35.

Edited September 4, 2021 by darkman222

[Nealius]

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

[Lace]

Gyroscopic precession effect of the rotating engine parts?  As you pitch up you are essentially applying a force to the 12 o'clock (looking aft) position of the turbine blades, which will translate 90deg left or right (depending on the direction of rotation of the engine) and become a yawing moment at the 9 or 3 o'clock positions.  The secondary effect of yaw is roll so this could be manifest as a slight left roll tendency on pitch up, assuming a clockwise (looking aft) engine rotation.

 

I'm a prop guy, so maybe the effects are negligible on a small diameter, low mass turbine. 

[Dee-Jay]

1 hour ago, Lace said:

Gyroscopic precession effect of the rotating engine parts?  As you pitch up you are essentially applying a force to the 12 o'clock (looking aft) position of the turbine blades, which will translate 90deg left or right (depending on the direction of rotation of the engine) and become a yawing moment at the 9 or 3 o'clock positions.  The secondary effect of yaw is roll so this could be manifest as a slight left roll tendency on pitch up, assuming a clockwise (looking aft) engine rotation.

 

I'm a prop guy, so maybe the effects are negligible on a small diameter, low mass turbine. 

 

Negligible. And otherwise, would would have noticeable effects anytime you are changing engine power (wide and rapid RMP variations).

Edited September 5, 2021 by Dee-Jay

[Lace]

5 hours ago, Dee-Jay said:

 

Negligible. And otherwise, would would have noticeable effects anytime you are changing engine power (wide and rapid RMP variations).

 

 

S&L power changes do not have a gyroscopic precession effect.  They would result in a torque moment (roll) about the longitudinal axis.  I think you might be confusing the two.

[chichowalker]

Hi! Nope. There is no auto compensation in roll axis on F-16 (nor on any other jet I know including M2000 & Rafale out of any AP mode or sub-modes).

In case of Eurofighter this functionality exists. Is auto trimmed in Roll, pitch and yaw

Enviado desde mi SM-G981B mediante Tapatalk

[Mikaa]

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.