Trim problem

[Conure]

Hi all,

 

When I try to trim the shark on the pitch (using the input representation, red box thing..) if I pitch it down a lot it will ALWAYS pull back up a bit. What I think I've realised is, that (as I read a while ago) the trim on the heli is very different to that on a fixed wing. When I read it initially I thought 'yeah yeah, you'll tell me it's different but we both know it'll do exactly the same thing..'

 

Seems I was wrong! Is the trim in anyway connected to the autopilot? I'm not totally sure how it works, despite reading up on it, which is having quite a negative impact on my ability to control the heli!

 

Hope you guys can help!

 

Thanks

[miguez]

Hi Conure,

 

Plenty to read around here on trim, pour yourself some hot tea or beverage of choice and read on, but yes, the trim and the autopilot (AP) are not only connected, but the trim is but a feature of the AP.

 

Keep in mind that when we say autopilot we mean a damping system, something to make the helicopter easier to fly. This is unlike the AP on a typical airplane.

 

The trim is just a reference of pitch, or bank, or heading, or any combination of that (you turn them on or off individually) that the helicopter will try and maintain via this AP. So if I bank some and hit the trim, the bank at the moment I release the trim button is the new bank reference, and the helicopter will try to maintain that. Same for pitch and heading.

 

Notice I said try to maintain it. That's because the AP channels only have a little bit of authority, so if a control input outside of that authority is required to maintain the trim you chose, she simply won't.

 

That's it in a nutshell.

[Nate--IRL--]

Trim sets the centre-point for the cyclic, it ALSO tries to maintain the pitch and attitude when the trim button is re-released (Unless the flight director is on).

 

The new centre point and the new pitch and attitude, nearly always don't match, so the autopilot has to use some of its 20% control authority to move the cyclic from the new centre-point to maintain the new pitch and attitude.

 

Nate

[Conure]

Thanks guys, that has cleared most of it up! Why is that if (hypothetically, I know I shouldn't do this) I switch off the altitude AP, then pitch the chopper down to say 40o with max collective so the chopper moves at 280km/h, if I release the trim from the position I'm holding it at (the 40o angle) it will ALWAYS pull up (as such gaining alt and losing speed)? I'd presumed initially that the shark was trying (with extreme difficulty) to maintain angle of pitch, speed and altitude (which was impossible), however I thought that if I took the altitude hold off it would maintain that angle of pitch, it doesn't!

 

I will also go find some more articles on the trim,

 

Cheers!

Edited June 6, 2010 by Conure

[Nate--IRL--]

Start here (scroll down to the section on flying the Helicopter)

 

http://forums.eagle.ru/showthread.php?t=33527

 

Nate

[Conure]

Start here (scroll down to the section on flying the Helicopter)

 

http://forums.eagle.ru/showthread.php?t=33527

 

Nate

 

That link is excellent!! Thank you, I will work my way through it now.

[beers]

Conure; I suspect the behavior you describe, where your pitch pulls up after you trim, is linked to you having ROUTE mode on. maybe? Not 100% sure of how this is supposed to work, but when I trim with ROUTE turned on, the trim is factored into your attitude, but ROUTE mode forces the airframe into a rather horizontal, flat position. (what i mean is that trim is factored in, but ROUTE overrides much of the intention of your trim) With that in mind, (again, not knowing if this is standard-op-procedure or just the way I figured things out), I use ROUTE to point myself to the waypoint (unless I'm pretty much that direction anyway) and then disengage ROUTE, trim my desired attitude/direction, go kill bad guys.

 

Before I settled on that procedure I used to fly with ROUTE engaged almost the entire flight. Using ROUTE all the time, I used to peg the attitude at nearly 100% of the throw of the collective but the attitude would only be a slight bit forward and would give me about 210 km/h (where in that collective position without ROUTE mode engaged the Ka-50 would be well over the 300 km/h speed limit after a few seconds of flight).

 

Hope that helps.

[EtherealN]

One further note on the simulator-specific machinations of the trimmer:

 

In the real-life helicopter the trimmer works as a physical mechanism as well through a set of loaded springs. This is the one part of the trimmer that's slightly similar to a fixed-wing trimmer, but it is special in the case that the mechanism is not over by a control surface (like a small movable surface as part of an elevator, aileron or rudder) but rather at your control interface - the cyclic (stick) and rudder (pedals). The intended function in real life is that when you release the trimmer, two things will happen:

 

1) Your controls will remain in place. If we start with neutral cyclic, then deflect 5 degrees left and release trim there, the stick will physically stay in that position.

2) The AP channels will attempt to assist in maintaining the attitude (bank/pitch angle etc) you had at that time.

 

Now, #1 can be problematic since a simulator stick does not have a spring mechanism. This function only works with force feedback, but has the added difficulty that there's pretty much no such thing as force-feedback rudder pedals on the market. Indeed, among HOTAS sticks there's actually pretty much no Force Feedback at all anywhere - Logitech G940 is the one I know of but it is very expensive and it has driver problems last I looked at it.

 

For this reason, the simulator has to do a few tweaks to cyclic/rudder behaviour on non-FFB sticks/pedals. There are two ways it can do it which you can set up in the options screen:

 

1) Upon release of trimmer it will "halt" input from your stick and rudder for 0.5 seconds. This gives you time to center them (if you are quick). After the 0.5 seconds the center position on your stick will correspond to where it was at as you released the trimmer. So if you released at 5 degrees left, center-stick becomes 5 degrees left and 5 degrees left (physically on your joystick) becomes 10 degrees left (5 degrees from trim plus 5 degrees from your stick movement.)

 

2) Upon release of trimmer it will wait until all controls are centered before reading input. Only recommended for good sticks in my opinion - for example my previous stick, a Saitek Cyborg EVO, was not reliable with this mode because it had flaky potentiometers, so even though my stick was physically centered the stick was telling DirectInput (and thereby the simulator) that I had not yet centered my stick. At that point it becomes a rush to try to get the pots (all of them, at the same time, for all axes) to report center at the same time, preferably before I have spiraled to the ground or flown into a building.

 

Experiment a bit with those and find the one you like. If you prefer the second one but have issues with locked controls, adding a little bit of deadzone through axis tune is an option.

 

Finally, there is a file called "Producer.cfg". Within that there is a string reading ForceFeedbackEnable = true;

 

If you are having issues, it might be worth doublechecking that one to ensure that it says

ForceFeedbackEnable = false;

 

This because the simulator might be trying to use force feedback to hold your stick in position, which is kind of futile if the stick doesn't have force feedback (or is a saitek, I never got FFB to work with my saitek cyborg stick in DCS:BS).