Question about trim on final? 777

[Dyos787 5]

I am unable to adjust trim at all and use the Logitech G940 rudder, stIck and throttle system. When I apply an input I can see the trim button on the aircraft control column actually moving but nothing happens in response and it is impossible to adjust the overall trim status. Do I need to instal FSUIPC as mentioned earlier in this thread?

 

 

Steve D

London UK

[StewCal 12]

I've had some strange control issues too, where as the NGX seems fine also. Something else I noticed was the yoke doesn't move in the VC. This makes me wonder if something' not installed quite right?

 

Do you have this same issue? I'll do some more research tonight.

The yoke not moving is due to having the axis in FSUIPC set to "aileron set" instead of "aileron axis set".

[lbarber 20]

I got myself a copy of FRAPS and made a video showing some of the things that we've been discussing in this thread. Apologies in advance for the sloppy flying, terrible narration, and the fact that you can hear me breathing in the video.

 

 

http://www.youtube.com/watch?v=GCp4AO_Ktf4

[Flamin_Squirrel 71]

I got myself a copy of FRAPS and made a video showing some of the things that we've been discussing in this thread. Apologies in advance for the sloppy flying, terrible narration, and the fact that you can hear me breathing in the video.

 

 

You've made the video private so no-ones going to be able watch it.

[lbarber 20]

You've made the video private so no-ones going to be able watch it.

It's link only and the forum parsed it into an embedded video, standby.

 

edit: uploaded it as link only and then got changed to private, should be working now.

[Flamin_Squirrel 71]

I didn't have a chance to watch the whole thing. If you don't have any luck, I suggest trying again with the flight controls EICAS display up and see what it's actually doing.

 

If you see the controls thrashing around that'll be a clue!

[lbarber 20]

I didn't have a chance to watch the whole thing. If you don't have any luck, I suggest trying again with the flight controls EICAS display up and see what it's actually doing.

 

If you see the controls thrashing around that'll be a clue!

I realized I should've done that about 15 seconds after I finished the video. Oh well.

[martin-w 3,235]

It's link only and the forum parsed it into an embedded video, standby.

 

edit: uploaded it as link only and then got changed to private, should be working now.

 

Nice video, explains the issues well.

 

Only thing I didn't see you mention was the unrealistic [we think] automatic trimming with elevator input.

 

Regarding unstable trim... I think I'm right in saying that the issue is related to the way a one second trim input equates to 10 knots of speed. Trouble is anything less than one second results in zero trim adjustment, so fine tuning trim is impossible. Also, when it does respond, it's an exaggerated response.

 

I hope PMDG are reading these threads on trim, because this is a significant issue.

[E6BAV8R 88]

You can try it yourself. Fly straight and level at 250 knots or so. A/T on. Increase speed by 30 knots. The nose will rise as speed increases, which is normal. Now push your controller forward to maintain 0 vertical speed until the speed is stabilised, you will find the T7 in trim.

 

Would be nice if someone tested this in case it's just my set up. It's not a huge issue of course, but certainly not as real as it gets, unless I'm ignorant of how the T7 works.

 

Are you saying that, without using any trim after accelerating, that the T7 will automatically be in trim to maintain the altitude at the new airspeed without any input from you?

 

From what I understand, the only time to be using manual trim is for speed and configuration (flap and landing gear) changes.  However, if there is a specified amount of pressure being applied on the elevator than it will also apply trim to keep the elevator control pressure within specified parameters (remembering that the aircraft is FBW).  Therefore, if you are pushing more than this limit manually to maintain the altitude while you are accelerating 30 knots than it could be rolling in trim automatically; just to relieve control pressures.  Otherwise, the aircraft should require slight manual trim changes to maintain the new, higher airspeed of 280 knots from 250 knots.

 

The T7 AutoTrim handles, for most practical purposes, like aircraft trim that most people are accustomed to.  However, AutoTrim makes the aircraft more stable in varying conditions.  As Jordan said, I believe this is the easiest, most basic way to describe it:

 

The 777 will maintain the trimmed speed, and as you say, thrust won't change that. If you change thrust, that will cause the plane to speed up or down at its current attitude. Because the plane is trimmed to maintain a speed that it's now moving away from, it will either pitch up or down to get back to that speed, our you'll have to re trim for the new speed.

 

Also, someone linked the document earlier but it offers a pretty good explanation - (http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_11.pdf) Section 11.8, Page 9 - Control Laws and System Functionality.

I hope PMDG are reading these threads on trim, because this is a significant issue.

 

I'd refrain from saying there are any issues with the aircraft when we are all still trying to figure out how the system is supposed to work.

[martin-w 3,235]

I'd refrain from saying there are any issues with the aircraft when we are all still trying to figure out how the system is supposed to work.

 

 

Those of us with issues are very aware of that. Precisely why myself and others have used tems like "we think" unless we are misinterpreting" etc.

 

Nothing definitive hear, we are hypothesising. 

 

From what I understand, the only time to be using manual trim is for speed and configuration (flap and landing gear) changes. 

 

 

Well no, configuration changes are trimmed out automatically, as are thrust increases. Speed changes require trim.

 

Therefore, if you are pushing more than this limit manually to maintain the altitude while you are accelerating 30 knots than it could be rolling in trim automatically; just to relieve control pressures.

 

 

If you are correct, that's a good explanation for effect we have noticed. However, this is a secondary matter of less importance to those of us with issues.

 

Otherwise, the aircraft should require slight manual trim changes to maintain the new, higher airspeed of 280 knots from 250 knots.

 

 

And this is the primary issue, "slight" trim changes are impossible. Resulting in over trimming. As you can see form the video kindly posted above.

 

The T7 AutoTrim handles, for most practical purposes, like aircraft trim that most people are accustomed to.

 

 

Only it doesn't for many of us. Trimming is hard, not easy. For me, [and do realize I've been simming since Clive Sinclair's spectrum] It's the hardest add-on to trim I have. Something tells me Boeing didn't design the aircraft to be hard to fly.

 

I welcome any more of your suggestions.

[E6BAV8R 88]

Well no, configuration changes are trimmed out automatically, as are thrust increases. Speed changes require trim.

 

Reference the linked document.  I've the specific part here:

 

http://www.davi.ws/avionics/TheAvionicsHandbook_Cap_11.pdf

 

The ‘‘U” term in C*U refers to the feature in the control law which will, for any change in the airspeed away from a referenced trim speed, cause a pitch change to return to that referenced airspeed. For an increase in airspeed, the control law will command the airplane nose up, which tends to slow the airplane down. For a decrease in airspeed, the control law causes a corresponding speed increase by commanding the airplane nose down. This introduces an element of speed stability into the airplane pitch control. However, airplane configuration changes, such as a change in the trailing edge flap setting or lowering the landing gear, will NOT result in airplane pitch changes, which would require the pilot to re-trim the airplane to the new configuration. Thus, the major advantage of this type of control law is that the nuisance-handling characteristics found in a conventional, mechanically controlled flight control system which increase the pilot workload are minimized or eliminated, while the desirable characteristics are maintained.

 

I think there are 2 different ways to read the bold part:

1) Either the AutoTrim system does NOT roll in any trim when making configuration changes, and therefore the trim must be done manually when making configuration changes; or,

2) The AutoTrim system DOES roll in trim automatically when making configuration changes, and therefore the airplane makes no pitch changes (because the AutoTrim system counter-balanced the pitching moments).

 

I'm more inclined to believe it is #2 because putting in flaps or extending landing gear will have an indirect effect on the aircrafts' pitch, as there will be increased drag in relation to the previous configuration, and the pitch attitude will inevitably change to reflect that.

Those of us with issues are very aware of that. Precisely why myself and others have used tems like "we think" unless we are misinterpreting" etc.

 

My point being that people are claiming they are having issues when the root cause could be that we are misunderstanding the AutoTrim system and are therefore using the trim incorrectly - leading to these secondary issues people are experiencing.

[martin-w 3,235]

Reference the linked document.  I've the specific part here:

 

 

That's' contrary to what you said Greg which was why I corrected you..

 

You said... "the only time to be using manual trim is for speed and configuration (flap and landing gear) changes. "

 

The bit you highlighted in bold, clearly states that...  changes in the trailing edge flap setting or lowering the landing gear, will NOT result in airplane pitch changes, which would require the pilot to re-trim the airplane to the new configuration.

 

You were wrong about manual trim being required for config changes.

 

 

 

 

I think there are 2 different ways to read the bold part:

 

No, there's only one according to correct grammar.

 

I'm more inclined to believe it is #2 because putting in flaps or extending landing gear will have an indirect effect on the aircrafts' pitch, as there will be increased drag in relation to the previous configuration, and the pitch attitude will inevitably change to reflect that

 

 

Yes it is number 2. The T7 auto trims for config changes, as I said originally. You said it didn't, which was why I corrected you.

 

The other point of course, is that increased thrust, from the engines beneath the wings, below the centre of gravity cause a pitch up. This too is automatically trimmed out in the T7.

 

As we have said before, config changes are trimmed out automatically it's only speed increase that needs to be trimmed manualy.

 

My point being that people are claiming they are having issues when the root cause could be that we are misunderstanding the AutoTrim system and are therefore using the trim incorrectly - leading to these secondary issues people are experiencing.

 

 

 

Yes I know what you were implying. Which isn't the case for me or the others debating this.

 

Trimming automatically with elevator input is a secondary issue we were discussing, those of us discussing it commented how we haven't been able to find anything definitive on this so remain ignorant. Even Kyle who was on the beta team is unsure.

 

The primary issue though, regarding real world trimming for speed  is well understood.

[E6BAV8R 88]

That's' contrary to what you said Greg which was why I corrected you..

 

You said... "the only time to be using manual trim is for speed and configuration (flap and landing gear) changes. "

 

The bit you highlighted in bold, clearly states that...  changes in the trailing edge flap setting or lowering the landing gear, will NOT result in airplane pitch changes, which would require the pilot to re-trim the airplane to the new configuration.

 

No, there's only one according to correct grammar.

 

Yes, I said that in reference to the document. 

 

Riddle me this:  If the auto-pilot and auto-throttles are off (manual flight) and we are maintaining 230 knots at 5,000 feet, and you extend the first notch of flaps, than the aircraft is going to begin to slow down from the additional drag.  The AutoTrim system is going to want to pitch down to counteract the increase drag and indirect change in airspeed, since the aircraft is trimmed for 230 knots and it has been trimmed to this airspeed, but it can't since we are maintaining an altitude of 5,000 feet.  The clear answer is going to be to increase thrust to maintain our airspeed of 230 knots (which will also have the by-product of increasing pitch attitude), but how does this relate to AutoTrim?  AutoTrim will counter the pitch attitude increase as a result of increasing thrust but the aircraft is going to require an even higher angle of attack with the slats out to maintain that same airspeed and altitude.  However, and back to the point, what is the AutoTrim system going to do?  Are you saying that this aircraft configuration change will not result in a pitch or trim setting change?

[scandinavian13 5,192]

 

 

Even Kyle who was on the beta team is unsure.

 

Just to clarify:

 

That only means that I don't have specific knowledge of how it's supposed to work, specifically enough to speak to the arguments here.  Personal thing.

 

I've stated my own interpretation of what I thought was the case before and rather publicly humiliated myself.  I'd like to avoid that where possible.

[Flamin_Squirrel 71]

Yes, I said that in reference to the document. 

 

Riddle me this:  If the auto-pilot and auto-throttles are off (manual flight) and we are maintaining 230 knots at 5,000 feet, and you extend the first notch of flaps, than the aircraft is going to begin to slow down from the additional drag.  The AutoTrim system is going to want to pitch down to counteract the increase drag and indirect change in airspeed, since the aircraft is trimmed for 230 knots and it has been trimmed to this airspeed, but it can't since we are maintaining an altitude of 5,000 feet.  The clear answer is going to be to increase thrust to maintain our airspeed of 230 knots (which will also have the by-product of increasing pitch attitude), but how does this relate to AutoTrim?  AutoTrim will counter the pitch attitude increase as a result of increasing thrust but the aircraft is going to require an even higher angle of attack with the slats out to maintain that same airspeed and altitude.  However, and back to the point, what is the AutoTrim system going to do?  Are you saying that this aircraft configuration change will not result in a pitch or trim setting change?

 

You're over thinking this, probably in part because "auto-trim" is probably a bit of a misnomer.

 

If you're flying level trimmed at a certain speed then deviate from this speed, any conventional aircraft will pitch up (or down), assuming no pilot input, with a tendency to maintain (approximately - there are so many forces in effect it won't be exact) the previous speed. If you want to change speed AND maintain level flight, you need to apply pressure to the controls or trim them out. This is true of most aircraft, including the 777.

 

The 777 moves the control surfaces (I'll avoid saying trim here) to definitively pitch the plane such that it settles back at the previous speed, thereby making the whole process described above more predictable. That's it.

 

... well not entirely, because the 777 will also tend to maintain speed despite configuration changes too. But the end result should be that the pilot doesn't really notice the plane doing anything special at all.