In essence C*U law (as in the 777 and 787) should work just like a C* law (as in the Airbus) except an offset is applied to the auto-trim feedback loop proportional to the difference between the selected trimmed speed and the actual airspeed. So if you are at a speed away from the trimmed speed it will only be stable if you have an elevator offset. Only at the trimmed speed is it actually trimmed out. Essentially it shouldn't be much more complicated than adding the speed offset term. Easier said than done of course. :wink:

 

http://cf.alpa.org/internet/alp/2000/febfbw.htm

 

C*U ("U" represents forward velocity in flight equations) is a modified C* control law used in the B-777 to provide apparent speed stability. The trim switches set a reference speed that is summed with the actual speed in the feedback loop in such a way that the pilot feels conventional control force cues as speed changes. You "trim a speed," not the stabilizer (weight off wheels). Because the max trim reference speed is 330 knots, you would have to push on the control wheel to further increase speed toward Vmo. This provides a tactile high-speed cue.

 

Sometimes when you simulate something by observing how the system works means you build in lots of logic and modelling to engineer what you see happening. The thing is, Boeing didn't design the control law to work like the video shows. They added artificial speed stability to the C* control laws, as describe in the linked article, and it flies in that way as a result.

 

According to the write up on FBW in the SP1 manual addendum PMDG have also put in an offset to the control authority. That part didn't make any sense to me but I suppose there is a reason why it was done. It may be the best way they found to produce the out of trim effect. It's possible this offset is causing problems if you trim in the conventional way.

 

I haven't had the chance to fly the 777 enough post SP1 to comment on what Robin and Rob said, but it is a bit worrying that another FBW development iteration might be necessary. Hopefully that isn't the case, but it might mean a slightly different trimming technique has to be adopted.

I've just made a flight in the 300ER specifically looking at trim. From my "non-pilot" perspective there doesn't seem to be too much wrong with it. Quite a few times I retrimmed manually and when I switched on the trimmed speed indication it was satisfyingly close to my speed bug. Sometimes it wasn't that great but the issue there is the spring feedback from my joystick is so light that it's hard to nail it exactly. If you have A/T engaged then if you deviate from the bugged speed too much the thrust changes feed into pitch and confuse things. Any thrust change will cause a trim position change. Using F/D (as in the video) helps a lot.

 

Probably the best thing to do is forget all about the FBW system, turn off the trimmed speed indication and trim as you would in the NGX.

 

Regarding the way the trim appears to jump suddenly, this seems to be to do with the trim indication only reading in quarter unit steps, although the actual trim moves steplessly. Is this really correct to the aircraft? Even the EICAS trim position indication only shows changes in 0.25 unit steps, yet is displayed top two decimal places.

S***. Last time Rob have said "something is not right! " we waited almost a full year for fix   

Lets just hope it is some problem on my end.....a setting....my 20 year old yoke.....etc.

I found an issue with the FBW and trim. I'm going to try and bullet point this to make it as clear to understand as possible.

 

* Aircraft is OUT OF TRIM, and "nose heavy".

* I pull back on the control column to counter the out of trim condition

* Once I stabilized the pitch, I start trimming nose up, and returning the control column to center at the same time (same as on the NGX).

 

PROBLEM: the trim reference speed changes, but the trim is NOT applied until I return the control column to dead center, at which point ALL the trim change I made is applied at once, and if I got the trim wrong, the aircraft would then aggressively pitch relative to that out of trim condition.

 

The result is that I can't trim the aircraft by feel. If I don't have the trim reference speed displayed, it is virtually impossible to trim.

 

Seperately, the trim often either doesn't move, or even moves in the opposite direction to that selected.

 

Anyone else see this?

 

Best regards,

Robin.

 

I'm not seeing this. (I'll try to make a video of my joystick proving it when I have some time) The idea of trimming by feel as you gradually release the deflection needed to hold the desired path was something I pushed for very hard in the SP1 development - it works for me and there's no way we'd have released if it didn't. If I speed up by say 50 knots, I'm pushing my X52 stick quite far forward to maintain level flight. As I increase the trim reference speed while doing this, I need less and less forward deflection to maintain level flight. Works exactly as designed for me.

 

There is an inherent problem here though in the FSX environment - in the real airplane, it's not extra deflection that you need to maintain level flight in this situation, it's extra pressure/force on the yoke, which maintains the same physical deflection the entire time regardless of how out of trim the airplane is. When trimming the real airplane, you're relieving that pressure, not changing an angle of deflection. I flew the real 777 full flight sim and it's much more intuitive than it is in FS due to this simple fact. That's why we put the -FBW marker in and tried to simulate control force loading by reducing elevator effectiveness when inputting against the out of trim state.

 

Regarding the trim moving backwards - if current ref speed is within 5 knots of the current airspeed, it will reset to the current speed first regardless of which direction you push the trim in. The real airplane actually does this (blip trim). I think you're probably overreacting to it just because you can see it happening due to the presence of the -FBW marker. 5 knots is a barely perceptible trim change in this airplane and having it jump back down or up for a second by 5 knots or less and then start trimming in the commanded direction. In the real airplane there is no -FBW marker, it's all done by the pressure feel on the yoke and there's not this super precision type thing going on with trying to match some marker on the PFD to an airspeed.

 

I have not seen the trim fail to move - if you have a reproducible case of that that I can try, let me know.

Well i did my first flight yesterday night with the SP1. 

 

I 'm really having a hard time trying to maintain the desired flight path:

 

because generally with the NGX, when I wanted to change the flight path, I pitch up or down change the thrust in order to maintain the desired speed and then trim in order to relieve the pressure on the yoke.

 

But if I do this on the b777, it doesn't work because when i trim the aircraft, I'm not relieving the pressure on the yoke but ordering a new speed trim reference so when I release the yoke, the plane will pitch up or down to reach the new trim speed instead of maintainning the new flight path, 

 

So now I'm lost I don't know how to approach, now without giving too much inputs.

 

maybe my nullzone is not well set so the auto trimming doesn't work when yoke is released even when the trim speed is close to the current AC speed;

 

CMB

I think the point here, is if Rob, a real world 777 pilot tells us that the 777 should be as easy to trim as a 737, and he's finding that not to be the case, and others among us are finding the same... then some fine tuning is required.

 

I don't have the issue Robin mentioned, whereby the trim doesn't apply till the yoke is centred, but it is harder to trim than a 737, and it shouldn't be. 

I am assuming this is what makes the difference between trimming the PMDG NGX and trimming the PMDG B777:

 

(...) tried to simulate control force loading by reducing elevator effectiveness when inputting against the out of trim state.

(...)

I am assuming this is what makes the difference between trimming the PMDG NGX and trimming the PMDG B777:

 

Exactly !  This makes quite a difference, and is actually the same option Hardy Heinlin made when he implemented "control column force" in PS1, and now in PSX too.

 

Deflection proportional to force.

 

For me this is a very acceptable way of porting to the typical controller some sort of "force feedback". We just have to get used to it.

 

I should point out that I do not have the 777 installed, but I have been following this thread which brings a lot of interesting view on the "Boeing FBW" and it's implementation in PMDG's 777

I'm not seeing this. (I'll try to make a video of my joystick proving it when I have some time) The idea of trimming by feel as you gradually release the deflection needed to hold the desired path was something I pushed for very hard in the SP1 development - it works for me and there's no way we'd have released if it didn't. If I speed up by say 50 knots, I'm pushing my X52 stick quite far forward to maintain level flight. As I increase the trim reference speed while doing this, I need less and less forward deflection to maintain level flight. Works exactly as designed for me.

 

There is an inherent problem here though in the FSX environment - in the real airplane, it's not extra deflection that you need to maintain level flight in this situation, it's extra pressure/force on the yoke, which maintains the same physical deflection the entire time regardless of how out of trim the airplane is. When trimming the real airplane, you're relieving that pressure, not changing an angle of deflection. I flew the real 777 full flight sim and it's much more intuitive than it is in FS due to this simple fact. That's why we put the -FBW marker in and tried to simulate control force loading by reducing elevator effectiveness when inputting against the out of trim state.

 

Regarding the trim moving backwards - if current ref speed is within 5 knots of the current airspeed, it will reset to the current speed first regardless of which direction you push the trim in. The real airplane actually does this (blip trim). I think you're probably overreacting to it just because you can see it happening due to the presence of the -FBW marker. 5 knots is a barely perceptible trim change in this airplane and having it jump back down or up for a second by 5 knots or less and then start trimming in the commanded direction. In the real airplane there is no -FBW marker, it's all done by the pressure feel on the yoke and there's not this super precision type thing going on with trying to match some marker on the PFD to an airspeed.

 

I have not seen the trim fail to move - if you have a reproducible case of that that I can try, let me know.

The trim apparently failing to move is I think due to the coarseness of the trim indication. 0.25 unit steps on the trim indicator and the EICAS page. Are you sure that's correct? I have no experience of the 777 but have not seen this in other Boeings. This makes it look like the trim isn't moving then suddenly it jumps a quarter unit.

 

Regarding the reduction in control effectiveness PMDG added, can this be made an option like the trimmed speed index. If you aren't fairly well stabilised the direction of trim can get confused and therefore the control effectiveness might increase/decrease unexpectedly. It might be making trimming harder not easier. The lack of force feedback is just the same for the NGX yet people can trim that ok.

Hi,

 

 

 

The idea of trimming by feel as you gradually release the deflection needed to hold the desired path was something I pushed for very hard in the SP1 development - it works for me and there's no way we'd have released if it didn't. If I speed up by say 50 knots, I'm pushing my X52 stick quite far forward to maintain level flight. As I increase the trim reference speed while doing this, I need less and less forward deflection to maintain level flight. Works exactly as designed for me.

 

I'm testing this now in the 300ER (just in case it makes any difference).

 

Best regards,

Robin.

I'm not seeing this. (I'll try to make a video of my joystick proving it when I have some time) The idea of trimming by feel as you gradually release the deflection needed to hold the desired path was something I pushed for very hard in the SP1 development - it works for me and there's no way we'd have released if it didn't. If I speed up by say 50 knots, I'm pushing my X52 stick quite far forward to maintain level flight. As I increase the trim reference speed while doing this, I need less and less forward deflection to maintain level flight. Works exactly as designed for me.

There is an inherent problem here though in the FSX environment - in the real airplane, it's not extra deflection that you need to maintain level flight in this situation, it's extra pressure/force on the yoke, which maintains the same physical deflection the entire time regardless of how out of trim the airplane is. When trimming the real airplane, you're relieving that pressure, not changing an angle of deflection. I flew the real 777 full flight sim and it's much more intuitive than it is in FS due to this simple fact. That's why we put the -FBW marker in and tried to simulate control force loading by reducing elevator effectiveness when inputting against the out of trim state.

Regarding the trim moving backwards - if current ref speed is within 5 knots of the current airspeed, it will reset to the current speed first regardless of which direction you push the trim in. The real airplane actually does this (blip trim). I think you're probably overreacting to it just because you can see it happening due to the presence of the -FBW marker. 5 knots is a barely perceptible trim change in this airplane and having it jump back down or up for a second by 5 knots or less and then start trimming in the commanded direction. In the real airplane there is no -FBW marker, it's all done by the pressure feel on the yoke and there's not this super precision type thing going on with trying to match some marker on the PFD to an airspeed.

I have not seen the trim fail to move - if you have a reproducible case of that that I can try, let me know.

Thx for that reply Ryan.

 

Makes quite perfect sence what you are saying.

Lookinf fwd to trying things out again tomorrow or the day after.

 

PS, if I may ask.......I was actually interested jn one of those force feedback yokes that are on the market.

With the way you simulated the FBW system, do you thing force feedback would still work or not?

 

thx

Thx for that reply Ryan.

 

Makes quite perfect sence what you are saying.

Lookinf fwd to trying things out again tomorrow or the day after.

 

PS, if I may ask.......I was actually interested jn one of those force feedback yokes that are on the market.

With the way you simulated the FBW system, do you thing force feedback would still work or not?

 

thx

FSX doesn't calculate control forces to feedback so there's no reason why a force feedback stick shouldn't work with the 777X. However it probably won't help much. They are mainly for providing special effects vibrations and pulses, mostly unrealistic. I bought one a while ago to try and get more centering force to replicate the heavy spring feel of an Airbus sidestick but the maximum force was no more than the spring in a normal stick. That won't help the out of trim feel. There is a stick available which is rigidly fixed and uses force sensors to generate control inputs. This might work better as it would be able to generate quite high forces so it would always be clear if you are not in trim and in which direction.

 

As you will know, but others may not, the force feedback Boeing provide in the 777 is not much different to the artificial feel found in most aircraft with powered flight controls.

 

As a 777 driver, can you comment on the way the PMDG trim indication moves in 0.25 steps. Is it correct to the aircraft? It seems to me this characteristic may be why some people see the trim moving all at once.

There is a stick available which is rigidly fixed and uses force sensors to generate control inputs. This might work better as it would be able to generate quite high forces so it would always be clear if you are not in trim and in which direction.

It's the Saitek X65F you are referring to.

 

I don't think that would help though would it Kevin. As it's the operator that's providing the force, not the stick.

Thx for the reply Kevin and Martin.

 

I have never had a force feedback yoke so I do not know how they work.

I can only assume....which is never good with FSX :-)

 

I would assume that the more out of trim you are with the default Cessna.....the more force a force feedback yoke would excert on you.

 

But you are right Kevin, that does not seem much different than a yoke with a spring. Because with a device like that (like my CHpro yoke) the more out of trim you are, the more elevator deflection is required and thus the more the spring will be compressed.

So same same but different, I guess.

 

I think you saved me a lot of money.......no need for force feedback.

 

The reason I doubt that a force feedback yoke will work on the 777 is because of the way PMDG describes how they had to program the FBW system.

Quote

You will notice reduced elevator authority in the direction that opposes the trim force as a stand-in for the physical force that exists in the real airplane. This was the best compromise we could make for something that is inherently difficult to simulate without a sophisticated force feedback hardware system.

unquote.

 

If you create something that works on the assumption that force feedback systems are not used.....then what happens if you DO use a force feedback system?

I can see the reduced elevator effectiveness and the force feedback system fighting each other, no?!

As a 777 driver, can you comment on the way the PMDG trim indication moves in 0.25 steps. Is it correct to the aircraft? It seems to me this characteristic may be why some people see the trim moving all at once.

If I remember correctly, before SP1 the trim indicator moved at 0.1 steps and PMDG mentioned they would correct that, so that it would be as in the real thing.

So I think 0.25 is correct.

 

I would have to take a look at the flight control page on the ND next time to confirm.

 

Today I looked at the trim indicator band (next to the throttle quadrant) during T.O. and acceleration/ clean up. One realy can not conclude much from that thing I tell you.

Most of the time during acceleration the trim indicator did not move at all.

It was at 4.0 pretty much the whole time (or at least each time I peeked at it) from T.O. till

we were clean. Still though, the out of trim force was gone (trimmed away by me).

 

When I looked again a few seconds later, the trim indicator had moved about 1.0 unit.

So maybe that is due to the fact that the FBW trim signal (trim ref speed change) first mives the elevator and thereafter it moved the stahiliser so things are flush again.

So it might take a while for the teim input to appear as a stabiliser position change.

 

anyway......I can look at that flight control page next time and see what happens but I am afraid I will not see much there either.

But I personally do not think 0.25 or 0.1 is the cause of the "all trim is being applied at once" discussion! (I still have to try and reproduce that finding by the way)

 

cheers.

 

 

So maybe that is due to the fact that the FBW trim signal (trim ref speed change) first mives the elevator and thereafter it moved the stahiliser so things are flush again.
So it might take a while for the teim input to appear as a stabiliser position change.

 

I think that in the FCOM or FCTM, it says that contrary like the NGX or other conventionnal aircraft, when you change the trim reference speed, the elevator will first make the pitch corection and then the THS will move accordingly.