Bingo! In the sim it trims for itself, which is the point we are trying to make. From your comments it appears the real thing definitely does NOT do this.

It's more the case that in the sim there is no need to trim for speed, not that it trims itself.  The real thing clearly does require trim changes with speed.  It ought to be possible to make an FSX sim need trim for speed changes, the real weaknesses in FSX are in lateral and directional axes, not longitudinal.  I'm sure people would have noticed long ago that they don't need to trim much in FSX as speed changes. 

 

No doubt the majority of all beta test flights would have been using AP (just as most real flights would be) and so not experiencing hand flying apart from during takeoff and landing.

 

 

Cowl, I'm sorry if you think that there is a contradiction between what I and 777simmer are saying. I don't see it as a contradiction, but more a difference of opinion on how true to the "spirit" of the boeing FBW system the PMDG777 simulates.

 

Don't be sorry K. I very much appreciate your comments here

 

I'm confused because this issue is confusing, starting from the very FCOM itself....but the truth is out there, is looking for us, and  will  finally find us  :lol:

 

Let's see what Rob finds out tomorrow on the real one....Sure he will step-down the aircraft with bigger biceps...

 

 

Best regards

Cheers.

kevinh,

 

Well, this was just an idea I had, but clearly it has nothing to do with anything! This makes it even simpler then, if the 747 controls act the same way as the 777, it's just a matter of tweaking that FBW system.

kevinh,

 

Well, this was just an idea I had, but clearly it has nothing to do with anything! This makes it even simpler then, if the 747 controls act the same way as the 777, it's just a matter of tweaking that FBW system.

 

No worries, glad to explain. Ideas are all good. It's when you stop having them that you need to worry. :biggrin:

 

I don't think the problem with the 777 is the FBW though. I think it's a problem in the flight model itself. The trim does not need to be changed much as speed changes. This makes it look like the FBW has re-trimmed the aircraft, but no trimming was necessary.

 

That's my idea at least.  B)

[quote name="Cowl" post="2799292" timestamp="1379454024"Let's see what Rob finds out tomorrow on the real one....Sure he will step-down the aircraft with bigger biceps...

 

Bigger biceps.....pfffff I doubt that, lol

 

I don't think the problem with the 777 is the FBW though. I think it's a problem in the flight model itself. The trim does not need to be changed much as speed changes.

Whatever it is, the secondary mode works just fine.

Try it. Go into the CDU and fail the normal mode to secondary!

 

All they need to do (it seems) is implement the speed trim behavior of the secondary mode into the normal mode.

But I am no programmer so what do I know how hard that is.

 

Now that I know PMDG are taking a closer look at it with their pilots I feel better already :-)

What I am saying is that even though this is NOT TRUE to the aircraft, this is the way FBW is modelled in the FSX simulation platform.

 

Just FYI, FSX platform etc has nothing to do with the discussion here. The system we programmed is 100% custom (uses no FSX default stuff) and if something needs to be modified we can do that easily.

<grin>  Ryan, in following this discussion I am struck by the thought (and this is a very positive comment) that PMDG have made a rod for their own backs, so to speak. If you didn't produce such incredibly fine and detailed simulations, you wouldn't have such picky customers.  

 

So thank you all for keeping up the sky-high standards that we have come to associate with PMDG.  :clapping:

 

Cheers,

 

Brian

Ryan,thanks for clarify. That is what I thought after pilotresponse comment.

 

So let me understand:

 

If you reduce throttel and descelerate speed to the new speed, the real 777 will not auto trim at this point right? So then i must trim the plane or force the column or my hardware yoke.

 

If we are on stable speed and stable altitude then FBW will trim automatically a little bit.

 

Right?

 

If you reduce throttel and descelerate speed to the new speed, the real 777 will not auto trim at this point right? So then i must trim the plane or force the column or my hardware yoke.

 

Yes. This is correct behaviour.

 

If we are on stable speed and stable altitude then FBW will trim automatically a little bit.

 

Yes, if we apply elevator input. But NO, this we believe is not how the real aircraft functions. Auto trim only takes place [to a degree] with config changes like thrust increase and flap deployment. Speed changes should ALWAYS require the pilot to trim manually.

It's more the case that in the sim there is no need to trim for speed, not that it trims itself. 

 

I would say Robin was correct.

 

"The PMDG 777 system"  does seem to be "trimming itself"... when we apply elevator input.

 

An increase in velocity, unless we decide to defy the laws of physics, will cause an aircraft to climb due to increased speed and thus lift. If this isn't occurring as long as we apply elevator input then the 777 is trimming itself.

 

Elevator input is causing an auto trim effect. Whether it appears on the trim, indicator or not.

 

It has been suggested that this is related to a yoke pressure release auto trim effect that Boeing intended. But according to the real world 777 pilots in this thread, that does not occur in reality.

 

 

 

We really need to keep this simple gentlemen.

 

The issues are straightforward.

 

1. When we apply elevator input the PMDG 777 is auto trimming. We don't need to manually trim under these circumstances. This we believe is wrong. We believe speed changes should ALWAYS require manual trim. Two real world 777 pilots in this thread have confirmed this.

 

2. Trim adjustment when we do trim for speed is awkward in operation. There is a one second delay, and then it trims too fast and too much. This we believe is wrong. Trimming in a 777 should feel no different to trimming any aircraft. Boeing intended it this way.

 

 

 

 

 

 

And then one gentle tap of the trim and the nose goes flying up or flying down.

 

Yep, precisely. See issue "2" above. I've been saying this all along.

 

 

Elevator input is causing an auto trim effect. Whether it appears on the trim, indicator or not.

That would be even more worrying.  The only way the FBW can trim an elevator input is by using the stabiliser.  If the trim isn't moving nothing is doing any trimming.  Still it's good to know PMDG are looking into this.

 

I'll try what Robin suggested (FBW secondary mode) when I get a chance.  I tried similar tests in the 747X and in accelerating from 220 to 360 knots at 10,000' the stab trim moved forward by 1.5 units. Very different to how the 777X behaves, where the trim barely moves.

 

FSX certainly includes the capability to compute pitching moment change with Mach and this term is present in the 777X air file.

That would be even more worrying.  The only way the FBW can trim an elevator input is by using the stabiliser.  If the trim isn't moving nothing is doing any trimming.  Still it's good to know PMDG are looking into this.

 

 

 

 

 

Except that as Ryan said, the coding is 100% custom.

 

The trim gauge may not be moving, but that doesn't mean that PMDG's custom, [outside of FSX] programing isn't at work behind the scenes auto trimming in this scenario.

 

It may well be that an attempt has been made to simulate some kind of yoke pressure relief system, and that it may not have been interpreted correctly in regard to how the real aircraft functions. PMDG have already told us how the interpretation of the terms used are proprietary information, and thus confusion could have occurred.

 

I feel we are wasting out time speculating, only PMDG know how they programed their own custom fly-by-wire simulation to function.

 

 

It's very easy to demonstrate to yourself what is happening.

 

1. Fly straight and level

 

2. Increase speed

 

Use elevators to maintain zero vertical speed [You require forward yoke input to do so, thus trimming is required]

 

3. Do not trim manually.

 

4. As soon as the new speed is stabilised, magically the aircraft is in trim.

 

You can plainly see from "2" above, that your hypothesis that  "it doesn't trim itself", is incorrect.

 

If  elevator input is required to counter the climb, then the "need" to trim is present. Auto trimming is simultaneously occurring with elevator input. It must be, because if you don't counter with elevator input you will continue to climb. Post elevator input, the aircraft is magically in trim.

Except that as Ryan said, the coding is 100% custom.

 

The trim gauge may not be moving, but that doesn't mean that PMDG's custom, [outside of FSX] programing isn't at work behind the scenes auto trimming in this scenario.

 

It may well be that an attempt has been made to simulate some kind of yoke pressure relief system, and that it may not have been interpreted correctly in regard to how the real aircraft functions. PMDG have already told us how the interpretation of the terms used are proprietary information, and thus confusion could have occurred.

 

I feel we are wasting out time speculating, only PMDG know how they programed their own custom fly-by-wire simulation to function.

 

 

It's very easy to demonstrate to yourself what is happening.

 

1. Fly straight and level

 

2. Increase speed

 

Use elevators to maintain zero vertical speed [You require forward yoke input to do so, thus trimming is required]

 

3. Do not trim manually.

 

4. As soon as the new speed is stabilised, magically the aircraft is in trim.

 

You can plainly see from "2" above, that your hypothesis that "it doesn't trim itself", is incorrect.

 

If elevator input is required to counter the climb, then the "need" to trim is present. Auto trimming is simultaneously occurring with elevator input. It must be, because if you don't counter with elevator input you will continue to climb. Post elevator input, the aircraft is magically in trim.

Even though PMDG are using custom code they should be moving the indicated trim to match, even if they don't drive the FSX variable.

 

Additional the test you describe isn't as simple as you state. While accelerating you have an excess thrust, which also gives you a nose up pitching moment. You are countering this with elevator. As speed stabilises the excess thrust is washed out. Removing the need for the elevator.

 

I'm not saying your conclusion is wrong, but the reasoning you used is incomplete. As you say PMDG knows the full facts, I'm only speaking from an aerodynamic stability and control viewpoint not how FBW interacts with that.

Additional the test you describe isn't as simple as you state. While accelerating you have an excess thrust, which also gives you a nose up pitching moment. You are countering this with elevator

 

.

Excess thrust is automatically trimmed out in the T7. Not completely, but enough to invalidate your point. And guess what, you don't see this on the trim gauge. Somehow, given that this is PMDG we are dealing with, I feel confident that they have modelled auto trimming for thrust changes.

 

The T7 automatically trims for thrust increases and flap deployment.

 

It's "speed" increases that require manual trimming.

 

Even though PMDG are using custom code they should be moving the indicated trim to match, even if they don't drive the FSX variable.

 

 

Well, we have established that the PMDG T7 isn't functioning correctly. As said, it may be the yoke pressure relief system [previously mentioned by PMDG, in response to a support ticket] that is incorrectly coded. 

 

Actually, I've just done a few tests. Accelerating from 200 knots to 300 knots results in a trim change, registered on the gauge. It's not much of a change, only one degree, but it moves all the same. This may be the pressure relief system at work, correct for the gauge indication, but exaggerated in terms of the effect on the flight model.  

 

Who knows, pure speculation, but the good news is that PMDG are investigating.

 

Something else I've noticed. During an auto-land, auto thrust throttles back at only 15 feet. May be why some are having issues with floating. I seem to recall the engines should throttle back to idle at 25 feet.

 

 

Actually, I've just done a few tests. Accelerating from 200 knots to 300 knots results in a trim change, registered on the gauge. It's not much of a change, only one degree, but it moves all the same. This may be the pressure relief system at work, correct for the gauge indication, but exaggerated in terms of the effect on the flight model.

Yes and I did the same tests myself (see above in this thread).  The fact that in steady state the trim is barely different between high speed and low speed supports my impression that the problem is in the aero model.

 

The thing is the 777 FBW is a very complex subject and it's not possible to fully understand how it works in all circumstances from a few paragraphs in the FCOM.  For example, while it's trimming for configuration and speed is changing how does it differentiate between the two.  Maybe it doesn't. Also while you are forcing the nose down you are moving the flightpath vector it is essentially trying to maintain hands free.  Too many complex interactions to easily analyse in a forum such as this with no real information to go on.