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.

 

Keep in mind this two things here please:

 

Aerodynamically, there are actually two pitching up moments. (in general and to be understandable, I simplify the actual aerodynamic process a little)

 

One is the short term, produced by the increased thrust by the engine, wich ONLY lasts until the engines have accelerated to the new thrust setting (that its, seconds). Once the engines are at the new thrust, moment stops from increasing.This moment has to do with the engines line of thrust not being at the same line that the CG. In this case and on most if not all in all of the wing-mounted engine aircraft, the engine line of thurst is below the aircraft CG, wich produces an upward moment with thrust increases. This is the moment that the FWB tries to counteract automatically when thrust is changed.

 

The other one is the long term one, and that we're all talking about these days and wich adds to the former one. This is produced by the tail surfaces as the airspeed increases or decreases. Once the engines have accelrated to their new greater thrust (in seconds), the airplane keeps accereating because of the thrust-drag inbalance, and as it accelerates, this increases the the tail-down force wich is the one that tries to raise the nose in order to keep trim airspeed, and this is what we must fight with forward yoke and/or trim if we want the aircraft to speed up. The more the airspeed increases, the more forward yoke is needed if we, for example, are trying to maintain a constant flight path such as  level flight. Once the thurst/drag balances again, the airspeed does no longer increases and the airplane is in dynamic equilibrium at a greater speed, but at this point the tail downforce is greater and we are fighting it (to prevent the nose from going up wich is what the tails is trying to do) with the yoke, or we trim (if no done previoussly) for the stabilizer to exert the force for us....or with the PMDG 777, we wait a few seconds and the FWB trims the stabilizer for us, as we've been discussiing.  ^_^

 

 

Cheers.

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.

 

Agreed, but the fact remains that regardless of WHY, the FSX behavior is different (read: incorrect) from the real-world observed behavior.

 

You can ultimately interpret the manuals any way you like - the real thing requires manual trim input after a speed change. FSX version does not.

 

Best regards,

Robin.

"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."

 

You are only looking at the trim gauge though Kevin. The gauge may not be providing an accurate indication of what is actually transpiring in terms of this aspect of the flight model.

 

Who knows

Keep in mind this two things here please:

 

Aerodynamically, there are actually two pitching up moments. (in general and to be understandable, I simplify the actual aerodynamic process a little)

 

One is the short term, produced by the increased thrust by the engine, wich ONLY is present until the engines have accelerated to the new thrust setting (that its, seconds). Once the engines are at the new thrust, the moment disappears.This moment has to do with the engines line of thrust not being at the same line that the CG. In this case and on most if not all in all of the wing-mounted engine aircraft, the engine line of thurst is below the CG with produces an upward moment with thrust increases. This is the moment that the FWB tries to counteract automatically when thrust is changed and as I say, it only lasts the seconds the engine needs to develop the new thurst.

The thrust pitching moment does not disappear (the thrust is still there), the trim has corrected for it by providing a balancing aerodynamic pitching moment from the stabiliser. I realise the effect overall is as you say, but the way you wrote it makes it sound like the engine stops producing a pitching moment. Aerodynamics and thrust moments are interlinked and you can't separate them this way, even for the purposes of simplification.

 

The other one is the long term one, and that we're all talking about. This is produced by the tail surfaces as the airspeed increases or decreases. Once the engines have accelrated to their new greater thrust, in seconds, the airplane keeps accereating because of the thrust-drag inbalance, and as it accelerates, this increases the the tail-down force wich is the one that tries to raise the nose in order to keep trim airspeed, and this is what we must fight with forward yoke and trim if we want the aircraft to speed up. The more the airspeed increases, the more forward yoke is needed if we, for example, are trying to maintain a constant flight path such as level flight. Once the thurst/drag balances again, the airspeed does no longer increases and the airplane is in dynamic equilibrium, but at this point the tail downforce is greater and we are fighting it (to prevent the nose from going up wich is what the tails is trying to do) with the yoke, or we trim (if no done previoussly) for the stabilizer to exert the force for us....or with the PMDG 777, we wait a few seconds and the FWB trims the stabilizer for us, as we've been discussiing. ^_^

 

 

Cheers.

As it happens I'm an aeronautical engineer, so I understand perfectly what you are saying. However I'm not an expert on 777 FBW (I'm more familiar with Airbus FBW). If I understand you correctly, you are saying the FBW behaves differently between short term and long term trim changes. That makes sense, however in the kind of tests Martin and others have been describing both effects are happening over the same time frame so the FBW can't possibly distinguish the two. In the test Martin describes at the end of the acceleration the thrust is reduced, but at the same time the elevator input is being removed. The FBW cannot avoid trimming out the aircraft at this point because of these short term changes and this will include trimming for the new speed.

 

If the test is conducted hands free, and the aircaft was allowed to respond without a column input, we might see a more true picture of the trim behaviour. But as it is being described in this thread I think it's perfectly correct for the trim to behave as it does.

"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."

 

You are only looking at the trim gauge though Kevin. The gauge may not be providing an accurate indication of what is actually transpiring in terms of this aspect of the flight model.

 

Who knows

I would be very surprised if PMDG had to do anything special with the trim for FBW. In a conventional aircraft it goes where it's put, either by the pilot, the AP or the mach or speed trimmer (if the plane has one). It basically responds to nose up or nose down commands. In the 777 it can be driven by the FBW as well. It won't need to be put in a false position, so I don't think the indicated trim is likely to be misleading.

3-2-1-Now, on 18 Sept 2013 - 7:41 PM, said:

Agreed, but the fact remains that regardless of WHY, the FSX behavior is different (read: incorrect) from the real-world observed behavior.

 

You can ultimately interpret the manuals any way you like - the real thing requires manual trim input after a speed change. FSX version does not.

 

Best regards,

Robin.

Robin, please see my last my reply to Carlos above. The problem with the test being described by Martin in this thread is that as the new speed is reached the FBW is faced with trimming for the configuration change as thrust is reduced. It cannot avoid trimming for the speed as well. This gives the appearance of it trimming for speed. The only way to be sure it can't be doing this is to avoid any configuration change when the new speed is reached, and then see if it is in trim. However I think it will be more or less in trim as the trim change with speed is small at the moment.

 

 

The thrust pitching moment does not disappear (the thrust is still there), the trim has corrected for it by providing a balancing aerodynamic pitching moment from the stabiliser. I realise the effect overall is as you say, but the way you wrote it makes it sound like the engine stops producing a pitching moment. Aerodynamics and thrust moments are interlinked and you can't separate them this way, even for the purposes of simplification.

 

Hi Kevin

 

Yes, I rectified it with an EDIT two minutes after I wrote it because I realized I typed it wrong due to typing and thinking too fast For any reason it does not shows my edited test but the orginial one.

 

 

Yes, I did not want to say "disappears"...obviously, the moment is still there...I wanted to say, "it stops growing", but I perfectly know the moment is still there and is greater for at a greater thrust output.

 

I knew this was going to happen, that any one was going to catch my error before I had time to change it  :lol:

 

Nevertheless, the two effects can be perfectly separated. They both produce their own pitching moments, and the make their individual contribution to the net effect. 

 

Cheers.

Cowl, on 18 Sept 2013 - 8:42 PM, said:

Hi Kevin

 

Yes, I rectified it with an EDIT two minutes after I wrote it because I realized I typed it wrong due to typing and thinking too fast For any reason it does not shows my edited test but the orginial one.

 

 

Yes, I did not want to say "disappears"...obviously, the moment is still there...I wanted to say, "it stops growing", but I perfectly know the moment is still there and is greater for at a greater thrust output.

 

I knew this was going to happen, that any one was going to catch my error before I had time to change it :lol:

 

Nevertheless, the two effects can be perfectly separated. They both produce their own pitching moments, and the make their individual contribution to the net effect.

 

Cheers.

Never mind, it happens all the time with long posts.

 

I'm sure the two effects can be perfectly separated in the aircraft, though how it is done is important to be able to understand. If it is done as you say on rapid pitch changes then any time that happens the trim speed datum would also be reset. Normally such rapid pitch changes would occur before any speed change, but what if a rapid change happened after the speed change? However the main question is whether the sim can separate the two types of trim. I still go back to the observation that the steady state trim position is not very different in the sim between low and high speeds so it's very difficult to tell whether the FBW is working properly or the FDE is not correct.

 

It would be very interesting to know what the trim position is for high speed and low speed in the aircraft, or even in the FFS you use for training.

I think I understand what you are sayting regarding the auto-trimming due to speed change, but what I'm referring to is this:

 

If you start with a steady state (no pitch, speed or thrust changes) and move the control column in pitch (let's say you pitch up), the aircraft will decelerate. According to the flight controls laws, NO configuration change was made. The speed will bleed off due to insufficient thrust for the new, higher pitch attitude. If you hold this pitch attitude, there is a distinct change in trim that tries to slowly pitch the aircraft up *even further*, despite a CONSTANT stick pressure that HAD resulted in a stable pitch condition at the new higher pitch attitude (follow me so far?).

 

What happens next is that if you continue to hold a constant stick pressure, the aircraft pitches up *even more*. The result is that to hold the new pitch attitude, you MUST slowly release pressure as the aircraft trims with you ON ITS OWN. The result is that you eventually (within 30 seconds) end up with neutral stick input, and an aircraft that has now trimmed itself to a much lower airspeed (remember, the speed is bleeding off the entire time).

 

Once the stick reaches this neutral point, the aircraft is trimmed for a LOWER speed, and now the nose starts to drop as the result of the airspeed still continuing to bleed off.

 

If you add thrust to stabilize the speed, you will find it is auto-trimmed to a LOWER speed, and NOT the speed at which you started the maneuver!

 

It is critical that two things are true: you start at a high airspeed, and do NOT change thrust, in order to eliminate all other compensations built into the FBW system.

 

I hope that helps define the test conditions, and illustrates the exact problem.

 

Best regards,

Robin.

Hi Robin.

 

 

 

The speed will bleed off due to insufficient thrust for the new, higher pitch attitude. If you hold this pitch attitude, there is a distinct change in trim that tries to slowly pitch the aircraft up *even further*, despite a CONSTANT stick pressure that HAD resulted in a stable pitch condition at the new higher pitch attitude (follow me so far?).

 

Absolutely...I have also noticed exactly what you're saying from the very first test-flights.

 

 

 

Once the stick reaches this neutral point, the aircraft is trimmed for a LOWER speed, and now the nose starts to drop as the result of the airspeed still continuing to bleed off.
 
If you add thrust to stabilize the speed, you will find it is auto-trimmed to a LOWER speed, and NOT the speed at which you started the maneuver!

 

 

Correct. I foud just the same when I did my tests.

 

As we've been commenting these days, it seems extrange. We're in the same point of view. But we will have to wait til PMDG investigates the issue and finally finally expose their conclussions. Otherwise I think we (including me) are hypothesizing wether this is a bug or what.

 

I was astonished when Ryan said Boeing was involved in some way with the PMDG 777. 

Boeing is bigger words. If they are into this, apart from thinking it is marvelous, I'm very interested in what finally they, as well PMDG, conclude about this one.

 

I'm also expectating to what 777simmer has to say about after his flight today.

 

Regards Robin.

The problem with the test being described by Martin in this thread is that as the new speed is reached the FBW is faced with trimming for the configuration change as thrust is reduced. It cannot avoid trimming for the speed as well. This gives the appearance of it trimming for speed.

 

 

Not sure what you mean by "as thrust is reduced" in the test I describe we are increasing thrust to reach a higher target speed. But it works the same both ways.

 

What you may be missing, is the fact that thrust increase occurs for a matter of a few seconds. Automatic trimming for the thrust increase occurs first and is completed relatively quickly, as soon as the engines have spooled up. It's beyond that time frame that the auto speed trimming effect occurs. As evidenced by the fact that initially elevator input is required to counter the out of trim condition. It's not until later, when the new speed is established and stabilised that the magical auto trimming for speed occurs, and it's possible to cease elevator input.

 

So you ask how the system knows what it's trimming for... simply because auto trimming for config changes, and [we believe fallacious]  auto speed trimming, occur at different times. If they were occurring together, we wouldn't need to fight the out of trim condition with elevator input.

 

As an example... if we accelerate from 200 knots right up to 300 + knots. The engines have spooled up to the new thrust setting and auto trimmed to the new config relatively quickly. But right up to the point of reaching 300 knots, we are still applying elevator input to counter the out of trim condition. Once the new speed is stabilised, the magical auto trim for speed kicks in.

 

 

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.

 

OK, just to make sure I'm getting this thread;

 

Flying straight and level no AP at 250 knots. If I reduce thrust the aircraft should pitch down to maintain 250 knots at some new V/S (depending on how far back the thrust levers were pulled) without having to touch the trim. If I increase thrust the aircraft should pitch up to maintain 250 knots without having to touch the trim since that's the speed it's trimmed for...

 

Flying straight and level no AP at say 250 knots and drop in a notch of flaps, the aircraft should start descending because the drag will have incrased and the aircraft will nose down and re-trim itself (config change) to maintain the 250 knot speed...

 

Flying straight and level no AP at 250 knots and reduce thrust to slow down, I will need to manually trim the aircraft to be level at the new desired speed...

Flying straight and level no AP at 250 knots and reduce thrust to slow down, I will need to manually trim the aircraft to be level at the new desired speed...

 

 

That's how it should work, just like any other aircraft.

 

But we find in the PMDG 777, that with elevator input, and no manual trimming it auto trims just as the new speed stabilises. Two real world 777 pilots in this thread disagree with this behaviour. And it doesn't seem to comply with how we believe the 777 trim functions. 

 

PMDG are looking at these issues.

 

For me, I'm more bothered  by the way when we do trim, there's a significant delay and then it trims too quick and too much.

Hey martin, just as an other idea to not using manual trim forr the time being....have you tried to fly in secondary mode?

(CDU fail normal to secondary mode)

I found it works realy nice.

Just like flying a 737.

I know it is not what you want ultimately, but try it out and fly like that in the mean time if you like it better.

 

My hand flying results should be there tomorrow (not back home yet).

Hey martin, just as an other idea to not using manual trim forr the time being....have you tried to fly in secondary mode?

(CDU fail normal to secondary mode)

I found it works realy nice.

Just like flying a 737.

I know it is not what you want ultimately, but try it out and fly like that in the mean time if you like it better.

 

My hand flying results should be there tomorrow (not back home yet).

 

 

I haven't tried that yet Rob.

 

Will hopefully give it a go tomorrow.

 

Spent most of the day at hospital with my 24 year old son. He was demonstrating kung Fu to my daughter and gave himself a hernia.  :rolleyes:

 

Has a minor operation due in a few weeks.

I haven't tried that yet Rob.

 

Will hopefully give it a go tomorrow.

 

Spent most of the day at hospital with my 24 year old son. He was demonstrating kung Fu to my daughter and gave himself a hernia.  :rolleyes:

 

Has a minor operation due in a few weeks.

Oh no...how horrible :-(

I hope he gets better soon.....hopefully the fact that he is still young will help him heal quickly. Good luck!

Oh no...how horrible :-(

I hope he gets better soon.....hopefully the fact that he is still young will help him heal quickly. Good luck!

It's not really much of a problem.

 

Our family doctor rang the hospital and they advised he went in for clinical assessment. We spent a few hours at the hospital waiting for the senior doctor to be available, to decide if he needed an operation immediately or if it could wait.

 

Turned out it was fat that was poking through the abdominal wall rather than intestine, so it was okay to wait a few weeks for an operation.

 

He's not in any pain.

 

That's what happens when you do Bruce Lee impersonations to entertain your sister.