investigating Roll trim resets/centers during takeoff (asymmetric stores)

[Col. Kernel]

When applying roll trim before takeoff (e.g. due to an asymmetric stores load), the roll trim resets/centers during acceleration with the wheels still on the ground. With an asymmetric stores load, this causes the aircraft to roll at lift-off. I've read in the manual, that PITCH trim centers when ground speed exceeds 60 knots, but this is not mentioned for ROLL trim. In fact, the manual says that roll trim should be applied before takeoff in case of asymmetric loads to prevent wing drop.

 

Usual caveats: I've never flown the Viper IRL, nor do I know the inner workings of the FLCS, but I suspect this behavior to be incorrect since it negates the use of roll trim at takeoff. I couldn't find this particular trim behavior reported in the forum, but please ignore, if it already has been reported.

 

1. Spawn on runway with an asymmetric loadout

2. Set roll trim away from center (using the trim hat on the stick)
3. Advance throttle
4. Takeoff (hopefully)

I've attached a short track file (pardon my messy takeoff). It's recorded in VR so the default camera views are a bit wobbly. Press [RAlt]+4 twice, [RAlt]+2 twice and move the camera left to taste in order to see the roll trim wheel reset by itself.

Roll trim.trk

[SCPanda]

+1

 

Also, it does not only reset during takeoff. It also resets when you put your gear down before landing. It's kinda annoying that you need to re-trim the jet after putting the gear down. After rearming and refueling, the trim also resets despite you trimmed it during landing. 

[Col. Kernel]

Another observation that I made during takeoff:

If you apply roll trim using the trim wheel (on the left panel) instead of the trim hat on the stick, the trim wheel does not reset during takeoff. However, the trim indicator (the gauge next to the trim wheel) resets to zero during the takeoff roll, so that there is a discrepancy between the wheel and the indicator. Once airborne (or perhaps when the gear is up), the trim indicator jumps back to the same non-zero value as before it reset.

[F16FLCS-SME]

ROLL trim should never reset by the FLCS.

There is no logic (hardware and software) what so ever to implement that function.

There is logic only for the PITCH trim.

[Frederf]

• Confirm, when at the 60 knots (111km/h) wheel speed the roll trim is being driven to center. This should only happen with pitch.
• Confirm, trim wheels are completely disconnected from needles when above the 60 knots wheel speed and they are neither driven as needle is driven nor is moving the wheels cause any control over needles. When the wheel speed is again slow the needles re-synchronize with the wheel positions wherever they are at that time.
• Cannot confirm trim changing when lowering landing gear or opening AAR door.

Note that because trim is overridden to not equal wheel position when wheel speed >60kt that during landing the trim will be driven to center in both axes and then resume equal to wheel position when airplane slows to <60kt.

 

As far as I'm aware needle position and wheel position shouldn't de synchronize like this. I don't know if needle and wheelare mechanically linked or not. What should happen is that the pitch wheel is electrically driven to the takeoff position. So if wheel speed goes below-above-below 60kts the needle doesn't resume where it was before. The needle moves to the middle position as a consequence of the wheel moving. Upon landing I am not sure if trim change occurs. According to -1 whenever the speed speed is >60kt pitch trim centers so that would be a yes.

 

The stick trim hat appears to not operate the wheels properly. If the wheel is quickly moved from one extreme to the other the needle takes a while to catch up. If the stick hat is operated while the needle is in motion the wheel snaps instantaneously to match the needle. I think correct behavior is that wheel would be motivated from its present position and the needle would chase it. The stick hat doesn't move the needle. The stick hat moves the wheel and the wheel moves the needle.

[F16FLCS-SME]

Just to clarify:

The Manual Trim Panel contains a PITCH trim unit and a completely seperate ROLL trim unit.

Each unit has an electric motor that drives (through a gearbox) a potensiometer for position feedback, a needle for visual feedback and a thumbwheel for manual pilot inputs.

Any movement of the PITCH or ROLL thumbwheels will mechanically move the corresponding needle, pot and the motor, regardless of electrical power status.

Holding the trim hat switch will move the corresponding motor and through the gearbox, the needle, pot and thumbwheel in a manner that will make it difficult to manually move the thumbwheel to another angle.

The gearbox has some inertia so using opposite trim hat commands will take up to 2 trim dots to stop and reverse direction.

There is no, and never was, a logic to auto-drive the ROLL trim unit.

Only the PITCH trim unit has logic to auto-drive it to center.

I was a shop manager and fixed myself more than 100 MTP's through my service so I have intimate knowledge about that box.

[Frederf]

That was my next question, are the thumb wheels and needles mechanically linked?

 

Is the needle/potentiometer truth for FLCS input or is it possible that FLCS's internal value may mismatch with needle display? Example, if you jam a screwdriver in the pitch trim wheel so it cannot rotate and attempt a takeoff with non-centered pitch trim will FLCS center the pitch trim value logically despite needle showing non-centered or does preventing wheel-needle-pot motion prevent this feature from operating entirely? If hat switch is held commanding pitch change will the automatic centering or stick hat have priority?

[SCPanda]

On 6/16/2021 at 7:49 AM, Frederf said:

• Cannot confirm trim changing when lowering landing gear or opening AAR door.

Take an asymmetric loadout, takeoff, trim the jet in the air, return to land, put the gear down, you will see. 

[Frederf]

Hmm, no such behavior seen. Here's the track.

 

Now if I actually land and spin up the tires above 60 knots then yes it will do it but not airborne with gear down.

F-16 trim gear down.trk

[F16FLCS-SME]

16 hours ago, Frederf said:

That was my next question, are the thumb wheels and needles mechanically linked?

 

Is the needle/potentiometer truth for FLCS input or is it possible that FLCS's internal value may mismatch with needle display? Example, if you jam a screwdriver in the pitch trim wheel so it cannot rotate and attempt a takeoff with non-centered pitch trim will FLCS center the pitch trim value logically despite needle showing non-centered or does preventing wheel-needle-pot motion prevent this feature from operating entirely? If hat switch is held commanding pitch change will the automatic centering or stick hat have priority?

The gearbox mechanically links the motor, pot, needle and thumbwheel.

If the motor moves, everything else moves, so the pot sends a new value to the FLCS.

If the thumbwheel is manually moved, everything else moves, so the pot sends a new value to the FLCS.

The motor is quite strong, but can be stopped by holding the thumbwheel in place. The gearbox has a clutch so the motor will keep spinning without damage.

Whenever I replaced a trim unit, I had to calibrate it so when the thumwheel is centered, the needle is centered and the pot signals 0 ohms. The motor is an AC motor so has no center position.

Regarding the priority, I don't have access to the schematic drawings (27FI/GS) and I don't remember...

The pilot (forward seat) has priority over the aft seat.

[SCPanda]

19 hours ago, Frederf said:

Hmm, no such behavior seen. Here's the track.

 

Now if I actually land and spin up the tires above 60 knots then yes it will do it but not airborne with gear down.

F-16 trim gear down.trk 1.06 MB · 1 download

Uh.... interesting.....Try firing some missiles or drop some bombs . I have had many missions where I fired some of my weapon and had some left (usually 1 aim-120 or aim-9), I trimmed the roll on the way back to the airport, once I put my gear down, the roll trim is messed up. I had to re-trim the roll so the jet don't automatically bank left or right due to asymmetrical loadout. 

Edited June 18, 2021 by SCPanda