Question about trim on final? 777

[martin-w 3,235]

Personally, I think this has been a great debate... interesting reading the different interpretations and understandings of the T7 FBW implementation.

 

I think Martin's dedication to finding out whether this is an 'issue' or whether we are all in fact dealing with a FBW systems that we know little of yet (aside from assumptions), has been useful and perhaps I am wrong, but it could be the case that a thread like this, could be of use to PMDG, rather than a hinderance or poor use of time.

 

I don't know if any issue exists or not with the PMDG T7s FBW but I do know that I love the hand flying characteristics and I love the 'assistance' from the autotrimming.    As long as you don't fight the trim, the system works beautifully and gracefully to get you stabilized in all phases of flight.

 

 

Thanks Dave, I appreciate you taking the time to say that. :smile:

[777simmer 117]

...never mind the fact that we had a bunch of 777 pilots on the tech team.

Which I did mention in my replies in that respective thread!

 

And I would be truely surprised if the PMDG trim does not work as in real life.

Most of all because to the pilot it should not feel any different than the NGX.

(other than the missing noise of those NGX trim wheels make :-) )

If you can simulate correct behavior in the NGX, you can simulate the 777.

[Alec 370]

The FCTM makes pretty clear that the 777 Flight Controls Handling Characteristics is of Speed Stability for the Pitch, so as Airspeed changes, the pilot should counter it with column and trim. What shouldn't change the pitch is thrust changes, but airspeed does.

 

In the 737NGX, if you increase your airspeed, hold the Yoke forward to keep the altitude, then let go of it, the nose will rise and you will gain altitude. With the PMDG 777 the system will trim your aircraft and you will hold the altitude, so it's not working like the 737.

 

I agree something is not right, or  we're not looking at the documentation the right way!

[martin-w 3,235]

Most of all because to the pilot it should not feel any different than the NGX.

Well it's certainly way different to the NGX.

 

As I've said a few times before, there's zero response for one second, and then when you do get a response it's too much.

 

Makes trimming very difficult.

[777simmer 117]

The FCTM makes pretty clear that the 777 Flight Controls Handling Characteristics is of Speed Stability for the Pitch, so as Airspeed changes, the pilot should counter it with column and trim. What shouldn't change the pitch is thrust changes, but airspeed does.

 !

Yes

In the 737NGX, if you increase your airspeed, hold the Yoke forward to keep the altitude, then let go of it, the nose will rise and you will gain altitude. With the PMDG 777 the system will trim your aircraft and you will hold the altitude, so it's not working like the 737.

 !

But it should.

 

Downloading 777 now.

[777simmer 117]

I just posted my initial flight experience here:

 

http://forum.avsim.net/topic/413705-fly-by-wire-of-the-777-and-its-simulation/page-2#entry2791730

 

But here is a copy of that.

I guess it would make sence to stay in on thread on this subject. rather than havingbto read at three different but similar ones.

 

 

Oh boy oh boy.....You guys are right.

This seems wrong (I am being carefull here...more testing to do)

 

VRMM tuturial (everything setup as in tutorial), FSUIPC removed, only a CH yoke and rudder connected (granted, twenty year old analog stuff connected to a digital USB converter).

Using keyboard F1 - F4 for thrust at the moment.

 

AP off, FD off, AT disconnected.

 

I can go in level flight in 5000ft from 300kt down to 180kt (you would not do this in real life cause thats 10kt above yellow bar and way below flaps up maneuvering speed) and back to 300kt again......without ever heaving to touch the trim!

My StB trim automatically changes from 4.1 to 5.4 while slowing down and back to 4.1 when at 300kt again as I am manually maintaining 5000ft.

Yes you do have to compensate with the yoke for the speed change initially (pulling for decelerating and pushing during acceleration) and you COULD trim this away.....but the thing is....if you dont trim it away it gets trimmed away automatically!

 

Now the only problem with this situation is.....and that is why I am now being carefull with saying this is all wrong......In real world flying, this not trimming for airspeed and just holding the yoke to compensate gets quite heavy real quick.

10kt out of trim and you would feel those forces (ok simulated feedback forces) immediately.

So in real life you dont sit there and wait what the aircraft does if you dont manually trim.

You instintively trim away those forces.

 

I have never tried what happens after a while if you dont trim away those forces.

 

I dont think that the FBW system will at some point automatically trim away those forces......but I wont know for sure untill my next rw flight ;-)

 

I must also say....that other than that, the smoothness of the plane, the rate at which it pitches and rolls, that is all extremely well simulated.

The real thing realy does fly like this (other then maybe the trim)

 

I

[martin-w 3,235]

And when you do trim, how do you find it? This is my issue.

 

There's zero response for one second, and then when you do get a response it's too much.

Makes trimming very difficult.          

[Alec 370]

My trimming doesn't work at all here. It sometimes work, sometimes it doesn't do anything. Really strange.

[Morganw487 2]

It seems if a 777 pilot doesn't agree with trim/FBW logic, it needs to be evaluated by the team or explained if we are erroneously operating the plane. I noticed right away trying to hand fly that my inputs were unusual; I was over controlling because too little is required to fly her. In 777 landing videos I have seen a good exercise of trim; with the PMDG 777, doing the same results in unstable pitch control. As a Cessna pilot, I wasn't sure if this was accurate, but I can see that it is not entirely if a RW 777 pilot doesn't approve. Still a great bird though.

[scandinavian13 5,192]

 

 

It seems if a 777 pilot doesn't agree with trim/FBW logic, it needs to be evaluated by the team or explained if we are erroneously operating the plane.

 

...again...despite the 777 pilots on the tech team...

[Alec 370]

I'll open a PMDG ticked asking for support, something is VERY wrong on my setup.

[martin-w 3,235]

...again...despite the 777 pilots on the tech team...

 

737 pilots were on the tech team for the NGX, but despite that the pitch on the approach was somewhat out. After quite a bit of arguing on the forum, with PMDG disagreeing... it was finally revealed that it was indeed wrong. Misinterpretation of the documentation I recall.

 

Just trying to point out that errors do creep in.

[777simmer 117]

...again...despite the 777 pilots on the tech team...

I am not sure exactly what you are trying to imply with this Kyle?

 

Have you spoken to those 777 pilots about our case/problem?

What did they say?

 

You could help here with contacts like that!

[Morganw487 2]

Yes there were 777 pilots on the team; invaluable no doubt. But when your average, ordinary customer who sees the problems 70% of us have is also a 777 pilot, it needs to be evaluated. I'm not saying anything is wrong 100%; I don't have the technical experience to make that call. But when a 777 pilot purchases it and has trouble out of the gate like most of us, we need to hear from the team. If it is correct, which is debatable, we need to know. If not we need to know also.

[Chri 0]

While reading through that boeing patent ( https://www.google.com/patents/US6158695 ) that someone posted earlier (I'm not sure which thread it was) I've found something interesting

 

Figure 1: https://patentimages.storage.googleapis.com/pages/US6158695-2.png

 

As will be observed from FIG. 1, the preferred embodiment of the present invention also uses the augmented feedback command AFB.sub.COM to produce a trim negation signal TNS via a stabilizer off-load processor 21. The stabilizer off-load processor 21 adjusts the stabilizer such that elevator authority is available throughout the flight. Once the augmentation feedback signal command AFB.sub.cmd exceeds a certain threshold position, the stabilizer is moved. As the stabilizer is moved, the stabilizer position sensor (not shown) senses the position of the stabilizer off-load processor 21 where it is multiplied by a gain and differentiated to form the trim negation signal TNS supplied to the integrator 28 to be added to the integral C*U command. In this manner, the stabilizer movement gives the elevator additional authority. Off-loading the elevator also helps to reduce drag penalty on the aircraft. Suitable stabilizer off-load processors 21 are known and may be used in the present invention. For example, trim negation commands are often provided by auto flight systems. The novelty of the present invention includes positioning the stabilizer off-load processor 21 in that portion of the pitch-axis stability and command augmentation system 19 that represents the augmentation portion of the elevator pitch command signal δ.sub.e,FILT only. This is important, since it is not desirable to move the stabilizers in short term maneuvers.

Might this be the answer to what we are experiencing regarding "autotrimming" while pushing or pulling the control column?