Some questions about autopilot

As a new ka-50 simmer I've studied a lot of posts about autopilot and trimming and I'm managing to understand the philosophy of the system. But still I have some doubts:

1) does or does not the flight director affect altitude hold channel too?

2) what is the "rudder trim" option under "special" settings? By default it is unchecked, so the trim button does or does not trim the rudder? And if I keep it unchecked, when flight director is off, does or does not the autopilot act on heading channel (i.e. actively moves the rudder with its 20% authority)?

3) I prefer using central position trimmer mode instead of default; in the latter the time the sim leaves you for recenter your cyclic is always too short and I find myself bumping towards the direction of my trimming when I release the trim button. So I use central position trimmer: I stabilize the heli, then release the trimmer button and move the stick back to center with no hurry; the flight is much more stable. But here comes the rudder back again: have I to return to center also the rudder pedals? Even if "rudder trim" is unchecked? And what does the autopilot do on the rudder with "rudder trim" unchecked when flight director is off? And if I check "rudder trim"?

4) To trim without flight director I press the button, move the cyclic and then release the button. I know real pilots quickly tap on the button while moving, and by doing this way the autopilot does not "fight" with them because is somewhat forced to follow the pilot at every tap. But I can't do that, since not having a ffb joystick I shoud return the stick back to center at every tap, and this is physically impossible. And if I move without pressing, then the ap fights me because it tries to keep his initial position, pushing against my move up to his 20% authority, and then releasing all together when I tap the trim. So I keep the button pressed, disabling the ap, then make my moves, and then release the button when stabilized on my new attitude, resetting the ap. While I fly with flight director on I could simply move then tap, but I try to keep pressed in order to get used to do this way. Is it correct?

5) Collective brake: I need to keep it pressed, stabilize on a new altitude and then release when the altitude channel is on; when the altitude channel is off I can or can not press and nothing changes; if I've understood it well, in the real heli you cannot move the collective without pressing the brake, but in the sim if alt channel is off it is only a brake (not implemented, since the collective moves anyhow), but if alt channel is on then it becomes part of the ap and must be used like in the real heli. Does flight director on or off affect this behaviour in any way?

6) What is the function of the central position of DH/DT switch?

7) What is the function of the central position of baro/radar altitude switch?

I know it's a lot of questions, but I'm interested :D

Thanks

A lot of good info here on AP and trimming.

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

https://forums.eagle.ru/showthread.php?t=39015

1.

2. Rudder trim checkbox only determines if 0 position joystick input is changed to equal the current pedal position on trim or not.

3. The "tau" time value of default may be changed. Central trimmer, yes 3 axis must be inside the window to resume control. If rudder checkbox is not set, then it is always inside window.

4. Tap or press and hold, pilot technique varies. Good technique is not to release final trim until helicopter is steady in new flight condition.

5. Brake lever button does the same function in FD or normal mode. The difference is in FD mode the altitude hold displays HUD director instead of making control inputs.

6. The center of the DH-DT switch corresponds to the last captured heading.

7. https://forums.eagle.ru/showthread.php?t=187035

Sorry but english is not my native language, maybe it's me but something is still not clear (and I've read all your links, some I had already read before):

1. Ok, it does. But it does by providing you flight director cues on the hud to help you keep the altitude you set with collective brake, not by moving the collective with the ap as when flight director is off: when flight director is off I have to keep the brake pushed (in my hotas throttle it's a button) like in the real heli, in order not to fight the ap, then move the collective to stabilize my new altitude by keeping vvi arrow stable on 0 vertical speed, and then release the button to set the new altitude I want the ap to keep. But if flight director is on this is not necessary since no ap is acting, the hud shows me the corrections I should do to keep the altitude set, and when I tap the button a new cues position related to the new setted altitude is set, but then I have to move the collective to keep those altitude by following the cues. Since no ap is actively flying, if I move the collective without pressing the brake I don't have to fight anyone, simply I'll see the altitude cues become bigger cause I'm leaving the setted altitude, and then return to steady when I tap the brake, setting a new altitude. If for example if I'd like to do aerobatics, I can ignore those cues flying as I want, watching the vvi and regulating the collective accordingly, having at least only stability augmentation on collective. Does a stability augmentation on collective exist? I suppose not, and if not, with flight director enabled the only difference in having altitude channel on is the cues on the hud, but the behaviour of collective lever is the same both with altitude channel on or off (if flight director is enabled, of course). Is this correct?

2. I'm sorry but I can't understand, maybe because of my english. Can you explain me in other words what is the difference between rudder trim checkbox on and off? Is it better to keep it off as in the standard setting or to make it on?

3. Ok I can change the tau time but I've read that this can lead to problems in replays and I'd prefer not to modify .lua files. I'm fine with central trimmer option. So if I set rudder checkbox I'd have to recenter joystick AND rudder to resume control after trim button release? While if I leave rudder trim unchecked the rudder is considered always recentered after trim (and so doing the bump if then I really center it?)

4. "Good technique is not to release final trim until helicopter is steady in new flight condition". Ok! This is maybe the only thing that is clear to me. ;)

5. already discussed at 1.

6. Ok. But if route mode is off the ap always refers to the last captured heading! What is the difference between: 1) route mode on and DH-DT to center; 2) route mode off and DH-DT in any position?

7. ok, I got it. But then what is the usefulness of that position?

does or does not the flight director affect altitude hold channel too?

It does not. The altitude hold channel is the only channel that the Flight Director does not disable. You can verify this by checking the AOA indicator. Normally, with the altitude hold channel off, the AOA changes only when you move the collective. When you enable the altitude hold channel, the AOA will move even without any collective changes. This is the altitude hold AP at work. You'll notice that this happens when the altitude hold channel is on, regardless if FD is on or not.

what is the "rudder trim" option under "special" settings?

Normally, when you trim the aircraft, only the cyclic's position is 'saved'. If you check the rudder trim option, the rudder trim position will be saved as well, and the center position of the rudder will move to that trimmed position. You can check this by pressing RCtrl+Enter and looking at the input visualizer.

But here comes the rudder back again: have I to return to center also the rudder pedals? Even if "rudder trim" is unchecked? And what does the autopilot do on the rudder with "rudder trim" unchecked when flight director is off? And if I check "rudder trim"?

The rudder input is part of the heading hold channel. This is because the rudder input actually controls two things: the rotation of the blades, and the rudder that moves on the tail. Yaw is primarily controlled by adjusting the speed of one of the coaxial rotors, and the tail rudder is only there to help it out. This is why you still have a significant amount of yaw control even with your tail shot off.

When the heading hold channel is on and FD is off, the autopilot will adjust the rotation of the blades to hold the heading, but it will not move the tail rudder. In this sense, it 'affects' your rudder input, but it's more accurate to think of it as holding your heading.

If you have the rudder trim option checked, you should release pressure on the rudder pedals after trimming. This is because you trimmed the aircraft assuming a certain amount of rudder input. Because in this mode the rudder acts as a relative input, any additional input you provide after trimming will get added to the trimmed position, causing you to deviate from the trimmed position.

With the rudder trim option unchecked, you must keep the pedals in the same position they were in when you trimmed the aircraft. If you applied any rudder input while trimming, you must keep the rudder in that position if you want to stay in the trimmed position.

In practice, I would leave the rudder trim option unchecked and trim the aircraft with the rudder centered. It's very unintuitive to have your rudder behave as a relative input; imagine trimming the rudder to a 45% right position, where moving the rudder 50% to the left returns it to the center position in the aircraft.

To trim without flight director I press the button, move the cyclic and then release the button. I know real pilots quickly tap on the button while moving, and by doing this way the autopilot does not "fight" with them because is somewhat forced to follow the pilot at every tap.

When the trim button is pressed, it 'redefines' the center point for the force trim (a.k.a. force feedback). If you crank the cyclic hard to one side and press trim, all the force you're applying to keep it in that position is instantly removed, which will make you jerk the cyclic. This is why real pilots have to press the trim button every so often, in order to avoid this jerking. Also, they don't hold down the trim button because it disables the force trim, and thus making it impossible to tell how much you're deflecting from the center point.

Holding down the trim button will disable the autopilot hold channels, but it will remove the force trim effect. Enabling the Flight Director will also disable the autopilot hold channels, but it will keep the force trim effect. If you have a non-FFB joystick, feel free to use either, as it doesn't make a difference. For non-FFB joysticks, it's probably easier to just hold down the trim button, adjust, and release. For FFB joysticks, if I'm just trying to fly in a straight line, I leave FD off and tap trim constantly until I get the aircraft flying in the position I want. If I'm trying to maneuver a lot, I'll turn FD on, tapping trim occasionally to re-define my center position.

in the real heli you cannot move the collective without pressing the brake, but in the sim if alt channel is off it is only a brake (not implemented, since the collective moves anyhow), but if alt channel is on then it becomes part of the ap and must be used like in the real heli. Does flight director on or off affect this behaviour in any way?

As mentioned previously, FD does not affect the altitude hold channel. It does, however, change your HUD to indicate your deviation from your held altitude if the altitude channel is on with the FD on. When you press the collective brake in this mode, the HUD will update to indicate your new altitude hold position. As you've correctly surmised, if the altitude channel is off, the collective brake doesn't do anything in the sim.

What is the function of the central position of DH/DT switch?

It is considered a 'manual' position that allows you to set a manual heading with route mode. If you have it on DH and enable route mode, the helicopter will fly the shortest route to the next waypoint (in a straight line). If you have it on DT and enable route mode, the helicopter will fly to the next waypoint following the track as displayed on the ABRIS. When it's in the center position, neither will be used and the helicopter will use your heading hold position instead. In practice, this is rather pointless since it effectively disables route mode, so you should always have it in DH or DT mode.

What is the function of the central position of baro/radar altitude switch?

It uses the last altitude mode that you set with the altitude hold channel on, but in practice, it doesn't really do anything. There are two ways to 'capture' your desired altitude hold position; either by turning your altitude hold channel on (which sets the altitude when you turned it on as your desired position), or by releasing the collective brake. If you turn on the altitude hold channel when in BARO mode, then flip the switch to RALT mode, the helicopter will still use BARO mode at your last-set altitude until you do a new capture. If you do a capture in BARO mode, flip the switch to RALT then back to the center position and perform a capture again, it will use BARO mode because that was the last mode used to do a capture. In practice, this is really confusing, so you'll never want to use this position and just keep it in one of the two modes.

Rudder trimmer is basically a cheat to make things easier for you. When checked your rudder position will be locked just like your cyclic gets locked to a new "hands off position" when trim is set.

When trimming don't expect it to work perfectly and give you a perfect stable level flight with one press. I often do my initial trimming then readjust and re-trim 2 or 3 times after to fine-tune and stabilize flight.

Thank you Ranma, now it is more clear!

1. So altitude channel is independent from flight director, i.e. the ap tries to keep the altutude (actively moving the collective) both with fd on or off if altitude channel is on. With flight director on the hud shows me the altiude cues, but the ap is still moving the collective as in fd off (i.e. if I fight the ap and move from the altitude I set, I see the cues, but the ap still tries to keep the altitude). If I do not want the ap to help me with the collective, I have to disable altitude channel.

To summarize for the collective:

a) Altitude channel off, fd off: "If the altitude channel is off, the collective brake doesn't do anything in the sim", as you stated. I must manually move the collective to keep / modify altitude, no ap help.

b) Altitude channel off, fd on: as in a).

c) Altitude channel on, fd off: I must use the collective brake as in the real heli: keep pressed, move the collective to obtain a new altitude, and then release the brake in order to say to the ap "keep this altitude". The ap obviously is active and helps me.

d) Altitude channel on, fd on: same behaviuor as c), the only difference is the appearance of the altitude cues on the hud, the ap still continues to help me.

Is it correct?

2. got it! So it is better to leave it unchecked, having the mind to release trim button when the attitude is stabilized and the rudder centered.

3. clarified in 2.

4. and 5. ok.

6. So there is no difference between: 1) route mode on and DH-DT to center; 2) route mode off and DH-DT in any position. Is it correct?

7. Got it.

P.S. if I'd have a ffb joystick I'd try to repeatedly tap as the real pilots do, but I have a thrustmaster t16000m flight pack (same sensors and mechanics as the hotas warthog in a cheaper plastic device), so I have a non ffb joystick with spring, and it is easier to press-move-release.

With all written above stated, the best setting to try aerobatics should be pitch, bank and heading channels on, altitude channel off, flight director on. Is it correct?

(regarding altitude hold channel) Is it correct?

Yep, that's all correct. You don't have to keep the collective brake pressed while moving it though, you can move it and then just tap the collective brake button to set the new altitude. You'll be fighting against the 20% AP authority doing it this way, but sometimes this is easier than holding down a button while moving the throttle/collective in real life. What I do is bind the ALT HOLD button instead and tap it once to turn it off, adjust my altitude, then tap it once to turn it on again. I find this significantly easier than holding down a button on my throttle while moving it.

So it is better to leave it unchecked, having the mind to release trim button when the attitude is stabilized and the rudder centered.

Yes, this is correct. It may not be as realistic, but without force feedback rudder pedals, it's significantly easier to control the helicopter when your rudder position is absolute instead of relative.

So there is no difference between: 1) route mode on and DH-DT to center; 2) route mode off and DH-DT in any position. Is it correct?

I tested this and I was slightly incorrect. The DH-DT switch determines how route mode performs, so if route mode is off, it doesn't matter which position the switch is in. When route mode is on, it will use the 20% control authority it has to steer the helicopter towards the flight plan route. For example, if you trim to a steady hover and enable route mode, the helicopter will turn itself towards the next steerpoint and pitch forward to give you about a 20km/h airspeed. When you set the DH-DT switch to the center position, it will act as if your steerpoint is your last-trimmed heading, extending infinitely into the distance. In other words, if you trim to a steady hover and enable route mode with the DH-DT switch in the center position, your helicopter will simply pitch forward to give you about a 20km/h airspeed, but won't turn towards any steerpoint.

With all written above stated, the best setting to try aerobatics should be pitch, bank and heading channels on, altitude channel off, flight director on. Is it correct?

Yes, this is correct. If you don't want to keep holding down the trim button, turn on FD. As a side note, never turn off your pitch, bank, heading channels. The autopilot channels does two things: one is to hold the helicopter at the last-trimmed position, and the other is what the West would call a stability augmentation system (it dampens the pilot's inputs to prevent against pilot-induced oscillations). The trim button and the FD only disable the position hold portion; the SAS is always on as long as the channel is enabled and the light is not flashing.

I tested this and I was slightly incorrect. The DH-DT switch determines how route mode performs, so if route mode is off, it doesn't matter which position the switch is in. When route mode is on, it will use the 20% control authority it has to steer the helicopter towards the flight plan route. For example, if you trim to a steady hover and enable route mode, the helicopter will turn itself towards the next steerpoint and pitch forward to give you about a 20km/h airspeed. When you set the DH-DT switch to the center position, it will act as if your steerpoint is your last-trimmed heading, extending infinitely into the distance. In other words, if you trim to a steady hover and enable route mode with the DH-DT switch in the center position, your helicopter will simply pitch forward to give you about a 20km/h airspeed, but won't turn towards any steerpoint.

I tested too and found that having deactivated route mode, I could set a new heading by trimming only with DH-DT switch in the central position. If I moved it in DH or DT position the heading cue was locked on the heading of the waypoint set with PVI-800, even if route mode was off. I did not try to shut off PVI-800 to see if something changed. So with PVI-800 active and set on a waypoint I have to use central position to set desired heading with trim button even if route mode is off.