Rudder trim - strange behavior

The following is true for the Spit as well:

Start your engine on the ground, set your brakes. Apply full right rudder trim, and full left rudder. As you slowly apply power ,the plane will turn RIGHT, even though the rudder is fully deflected to the left as you look back.

This makes me think that trim is not modeled as it should be, but perhaps it's just a simplified "nudge" in the selected direction. Can anyone from ED chip in and confirm or deny?

The above mentioned behavior is strange indeed. :joystick:

Thanks!

Hmm, just did the test and it worked as expected. Watched it externally. Full right rudder trim, full left rudder, advanced the throttle and the mustang turned to the left as expected.

Then, from a stop again, full right rudder trim, 0 rudder input, advanced the throttle and could see the airflow move the rudder from the trim deflection and end up turning to the right minimally as expected.

You are definitely experiencing something different. Check to see if a brake is dragging or something?

For me it works correctly too, no problem here.

Rudder goes in direction where trim is set with increased airflow from propeller.

Only if i set trim to left, slight tendency to turn plane left is here with increased power, but it is OK, it goes with propeller torque.

Were the brakes set? I definitely can’t stop the trim induced right pivot turn with left rudder

Ah, didnt set the brakes, missed that part. But why would that be an issue, it is working correctly when the a/c is moving. Will test again with brakes on anyways just to see.

With the AC moving it is working correctly for me too. But I can’t do engine runups with the brakes set and the rudder trim adjusted, because the plane starts turning regardless the rudder input.

Parking brakes were set, when i tested it.

I checked it on "controls indicator".

edit: Spitfire`s moving rudder will change "brake power distribution", then plane can turn because one brake is not fully set

ok, with brakes set, right rudder trim, full left rudder and advance the throttle, I can see the a/c start to twist to the left correctly as it is bringing up the tail. With full right trim and no rudder input, the tail starts to deflect properly and a barely discernable twist to the right, but the tail come up and no turn is made under either circumstance.

With this information, its my guess that you are not getting full brake on the left, allowing the a/c to turn under power.

BINGO!

I thought I was going crazy, because on some occasions the plane stayed straight, on others it started to pivot. Then I figured it out:

With the brakes set the plane starts pivoting into a 10-14 kts crosswind as power is applied, but ONLY IF YOU START COLD. If you start with the engine running, it's stable and won't turn. I attached 2 missions, one cold, one hot, same conditions.

Get your engine running, set your brakes and apply 2000 RPM, and see what happens.

rudder.miz

rudder2.miz

Indeed something very strange is going on, I am unable to even SEE those missions from DCS. I have opened them up externally and all seems fine with the format, double checked folder locations, but my Beta version does not want to acknowledge that they exist.

What version is that mission from? (Though still should not be an issue, have opened plenty of missions from age old versions. Have never seen this happen before.

EDIT: Just found out why, you made the mission on the Normandy map right? I do not have that. Does the same thing happen on the Caucaus map?

I haven't tried, but it should be easy to test: spawn at a parking spot perpendicular to a 14 kts wind, try with hot and cold starts. I'm unable to create those missions at the moment.

Tried the same test on Causcaus map, similar winds (copied from your mission - A/C was at heading 220 with 14 knot winds at 128 ) with either hot or cold start. A/C is reacting as expected in regards to trim and or rudder input.

My a/c does not move at all other than a slight nudge forward that would only equate to only a few inches before stopping and with enough throttle, will dump it on its nose, no matter the trim and or rudder input in either direction, hot or cold start.

This leads me to believe that it is a brake issue that you are having that is allowing it to turn at all rather than just twist in place due to the forces applied.

Even with the Parking brake set, if there is fluctuation in the brake axis, that releases the parking brake on that wheel. Maybe it is this that is happening?

Or maybe it's the perforated steel parkings vs concrete?

Then why does it behave normally with hot start, and why does it happen when I have to start the plane? It's really strange.

Thanks for checking though. I wonder if my missions yield the same results to someone who owns the Normandy map.

Just to answer the question, have you checked your brakes via the Axis Tune while both still and under rudder deflection to make sure they are working properly?

Standard troubleshooting dictates eliminate the easy, common things first.

You might be experiencing a spike on the left brake when you move the rudder, or the brake is not actually getting to 100 percent.

Other things to test is what happens if you reverse the wind direction or no wind at all?

I just flew 2 missions on the Burning Skies server, cold start, P-51, wind from the right. The first time I experienced the pivot thing, the second time I cold started I didn't. It has nothing to do with the rudder trim, nor the brake axis settings.

Has anyone else experienced this issue? Because with only 2000 rpm, the a/c should not move at all with brakes properly set.

EDIT: Also, about the brakes, not saying it is a setting, but that it may possibly be an issue with your brakes. If they are older, have a short, they could be activating when they are not supposed to. Shorting conditions can come and go randomly untill is is bad enough and the component fails altogether. It is something worth checking, but either you are just not telling me or refuse to do a simple check of your equipment. Failing to do this, you may be chasing a bug that does not exist. That your a/c is turning instead of nosing over it linked directly to a brake issue not holding the a/c in place, especially at those power settings.

When the parking brakes are set in the P-51, the HOTAS brakes are not involved. They should be at 100%.

By the way I checked their calibration, they work properly.

Thank you for checking, that eliminates that as a possible issue. How do you release the parking brake? I use my toe brakes which releases the parking brake.

Me too. But this occurs before releasing them ,while they are set. I set them before I start the engine.

Have you found anyone else experiencing the same issue yet?

Here's another test mission in the Caucasus. No wind, no rudder trim, cold start. If I set the brakes after engine start, they work. If I set them the first thing I do, then sometimes the plane pivots to the right. Sometimes it doesn't . I'm trying to replicate it, but don't always succeed. That's the weird part. I think once you hit "Refly" it works fine, so try to set your brakes as the first step on your first try.

rudder.miz

I performed this test 10 times, each time shutting down the mission completely to get a fresh start. My a/c does not move in any of the tests.

Also tested, with the parking brake set, it IS possible to release the PB on one side by activating that brake without it affecting the other brake, and the parking brake handle is still pulled out. Works that way for either side, thus causing a turn in the appropriate direction.

 

Also tested, with the parking brake set, it IS possible to release the PB on one side by activating that brake without it affecting the other brake, and the parking brake handle is still pulled out. Works that way for either side, thus causing a turn in the appropriate direction.

So maybe that's why? Shouldn't the brakes remain set as long as the handle is pulled out?

Not really, it is set PER brake, that is why you have to push both brakes to set it, not just one. And you release the PB by using that brake, so, you have to activate each one to fully release it.

EDIT: From the Manual:

Figure 62: Parking Brake

To engage the parking brake, pull the parking brake handle out, depress the brake pedals, release the brake pedals, and then release the parking brake handle. To release the parking brake, simply depress the pedals.

Notice, it says Pedals, plural...

That's exactly it! I tried and I can also set half the brakes, and then the plane turns. I'm still not sure it's a realistic feature though. Never heard of this before, and the manual doesn't say explicitly that you can set half the brakes.