[NO BUG] Uncommanded left roll

I hesitated to add this to the bug forum because I wanted to eliminate any possibility of it being a hardware issue. And after spending the entire morning doing that, I am confident that it is not an issue with my hardware and it appears to indeed be a bug.

I have been able to consistently reproduce this issue in the sim:

1) Take off from airfield in MIL power

2) Climb to whatever altitude, 10,000-15,000 in my case, and trim for level flight

3) Maintain 400kts or so and make sure the bank angle is as close to 0 as possible, and go hands off. The aircraft maintains 0 bank indefinitely

4) With hands off the stick, advance throttle to full AB. Let the plane accelerate and make only nose down trim inputs to maintain altitude. Shortly after passing Mach 1, around 1.2, the aircraft begins a slow left roll. And this continues, even when decelerating below Mach 1. And when looking at the inclinometer, you can see the ball every so slightly deflected to the left. It is right up along the left center line. Normally it is perfectly between the two white lines. This explains why below a certain speed, the roll reverses to the right. Because at high speeds, the aircraft rolls opposite the rudder input, in this case with the ball to the left, the aircraft has a slight yaw to the right and thus rolls left. And when slowing down the aircraft rolls into the yaw.

And up until the last flight I just did, I have not been able to pinpoint how to eliminate the uncommanded yaw/roll. It would go away, but I wasn't sure why. And I just discovered that if I add rudder inputs and then neutralize the rudder, the ball will center and return to coordinated flight. But until the rudder is deflected, the aircraft will stay in a slight right yawed state.

Something else I noticed about AB engagement. When engaging AB and paying close attention to the yaw, you can see the aircraft slightly yaw when AB is engaged. And I remember reading that you guys modeled the AB engaging at slightly different times on both engines, thus explaining the yaw because of slightly differential thrust. However, it appears that engaging AB somehow throws in a permanent slight right yaw movement that stays until the rudders are used and the inclinometer returns to center.

Again, I eliminated all possibility of hardware issues. I have done flights where I trimmed for level flight and the aircraft flew perfectly with zero yaw/roll. I went into the control menu and cleared my rudder/pitch/roll axis and engaged AB, and sure enough the yaw to the right appeared.

I managed to find someone on discord to try it out on his PC and he said he saw the same behavior. He was flying hands off level flight and then engaged after burner and after a little bit the aircraft started slowly rolling left.

Engines generating the exact same power? Slight differential thrust will induce a roll over time.

Engines generating the exact same power? Slight differential thrust will induce a roll over time.

Yup, exact same power. RPMs are identical and the little control popup shows the throttles identical. Plus i only have 1 throttle axis for both throttles. If I dont engage afterburner the issue won't pop up during the flight. Going into AB is doing something to the controls.

I will say, my right engine always has a higher fuel flow than the left. But the RPM indicator has them exactly the same. And I can't have differential throttles due to my single axis throttle. And if it was a differential thrust issue, the problem would show up even without AB at high power settings like MIL. But I can be in MIL until the fuel runs out and take my hands off the stick and the plane will hold 0 bank perfectly.

For a given RPM, turbine engines can vary a fair bit in their power output, depending on their wear, age, etc,

I think this is modeled in the hornet, not sure about the tomcat.

Here is a video I made showcasing this. I just uploaded it so it may be a bit until it can be viewed in higher quality

The engines are modeled to have very slight differences in FF and EGT for a given power setting. This changes every time you spawn a new jet. I’m fairly sure the rolling is due to hardware, but I’ll keep looking for something in the FM.

The engines are modeled to have very slight differences in FF and EGT for a given power setting. This changes every time you spawn a new jet. I’m fairly sure the rolling is due to hardware, but I’ll keep looking for something in the FM.

Thanks for the reply. I notice the differences in FF and EGT however it is the right engine every time, I have never seen the left have higher FF or EGT.

As for hardware issue, if that was the case then it would roll left constantly. It holds perfect 0 bank flight until I go into afterburner. And it would happen with other aircraft. Doesn't happen with the Hornet or anything that I fly in XP11 or P3D. Here are the steps I have taken to eliminate hardware as a possibility

1) Entered massive deadzones in both the windows controller software and DCS controls menu

2) My controller software shows everything centered

3) I got into straight and level flight and cleared the roll/pitch/rudder axis in DCS, so my joystick and rudder pedals weren't mapped to the game

4) The ingame controller indicator shows everything centered

And you can easily test this for yourself. Just takeoff in MIL power and trim for level flight. Go into afterburner and see if the aircraft rolls on its own after a few seconds

So I tested it with joystick/rudder disconnected and used the keyboard to eliminate possibility of it being a hardware issue and attached a link to a youtube video I made of it

This possibly have anything to do with fuel imbalance in the left/right side of the jet as shown on the gauges sometimes?

Also this is my 1000th post. Woop.

This possibly have anything to do with fuel imbalance in the left/right side of the jet as shown on the gauges sometimes?

 

Also this is my 1000th post. Woop.

That crossed my mind at first and then I did some reading on the fuel system. The left indicator shows the left feed and aft tanks, the right indicator shows the right feed and fwd tanks. The right feed and left feed are only 100 pounds apart in capacity. The fwd and aft are 300. So you're looking at 100lbs difference laterally, I don't think that would be enough to really cause this roll. Plus, the roll doesn't occur until AB is engaged. Something with the AB is causing this it seems

I’m going to try to reproduce this on my machine tonight after work. Will report back with the result.

I’m going to try to reproduce this on my machine tonight after work. Will report back with the result.

Sounds good. Thanks for looking into it!

This possibly have anything to do with fuel imbalance in the left/right side of the jet as shown on the gauges sometimes?

 

Also this is my 1000th post. Woop.

those are for the feed tanks and are in front and behind each other on the jets centerline. So they wont induce a roll.

Any update from one of the devs on whether they were able to reproduce?

I can eliminate this issue by enabling unlimited fuel, so it might have something to do with fuel distribution.

I frequently notice a left roll and it's almost always directly after AB usage and/or heavy maneuvering. Every time I've noticed the roll I have a fuel imbalance with 1800 in the left feed and <1000 in the right feed (the L and R digital numbers, not the tapes). In my case the imbalance seems to always self correct. During subsequent straight and level flight I can watch the right feed number rise again and after a few minutes there's no more left roll tendency.

I checked HB's manual and it doesn't mention if AB is fed from only one system or not so I don't know if this is a feature or a bug, but every uncommanded left roll tendency I've experienced is almost certainly due to fuel imbalance.

A more ambitious fellow can probably find the answer in NATOPS.

I frequently notice a left roll and it's almost always directly after AB usage and/or heavy maneuvering. Every time I've noticed the roll I have a fuel imbalance with 1800 in the left feed and <1000 in the right feed (the L and R digital numbers, not the tapes). In my case the imbalance seems to always self correct. During subsequent straight and level flight I can watch the right feed number rise again and after a few minutes there's no more left roll tendency.

I checked HB's manual and it doesn't mention if AB is fed from only one system or not so I don't know if this is a feature or a bug, but every uncommanded left roll tendency I've experienced is almost certainly due to fuel imbalance.

A more ambitious fellow can probably find the answer in NATOPS.

+1 exactly my findings as well. Hence why I mentioned the fuel imbalance on the previous page :P

Yup, exact same power. RPMs are identical and the little control popup shows the throttles identical. Plus i only have 1 throttle axis for both throttles.

The boldened sentence can't be true in DCS, because of this:

For a given RPM, turbine engines can vary a fair bit in their power output, depending on their wear, age, etc,

 

I think this is modeled in the hornet, not sure about the tomcat.

And I made the experience in the Hornet, the Tiger and the Tomcat as well, and they all share one commonality, that in each of them, the left engine is always stronger and putting out a tad higher RPM than the left one, never ever the other way round or both of them just being even. Using a TMWH throttle, but I also checked the Ctrl-Enter display and mapped both throttles to a single slider instead just to make sure. This is fact guys. And probably the reason why all those planes constantly yaw to the right with inputs zeroed out, which again appeared in this very module just to stand in line with all the others. The only one, that doesn't really fit in is the Viggen. It has the same issue, but lacks one engine to have an actual reason for that. The only question that remains is, could it be engine torque just like in propeller driven planes causing this phenomenon? And as a side note, my testing missions are sans wind, standard DCS day, CAVOK to completely eliminate atmospherical inluences of any kind.