Sign in to follow this  
vadupleix

Questions about the trim

Recommended Posts

Hello,

 

When I fly the 777, unlike the 707 I flew, the trim seems a bit sensitive and anti-intuitive to me. Since I'm new to aviation those behaviors that feels strange to me might be perfectly normal and basic but I still want to know why that happened. So here are the scenarios:

 

1.  Once I made a departure, climbed and then reduced the thrust, leveled the plane at 250kt, then the aircraft started to have a tendency to slightly pitch down, accelerate and sink. To prevent this from happening I need to constantly put back pressure on the yoke. Now if I add the throttle a little bit, it won't stop the plane from dropping and I will also begin to build up more speed. So naturally speaking, it's because my trim speed was too high right? But since I haven't moved the trim that much after take off, I decided to confirm this first before I do anything. So I turned on the "show FBW trim speed" and it shows: 160kt while my plane has already accelerated to 270 at that time and still wanted to descend. So well, I did what I had to do, I tried to trim up my plane further, like 4s after the trim speed reached its minimum (the stab trim angle did increase even after trim speed reaches minimum). Then after a short delay, the aircraft started to pitch up tremendously, finally, then I managed to adjust the trim speed back to where my speed was, everything normal after that.

 

2. When I turn the 777, with less than 30 degrees bank angle (15-25 normally), there should be no need to put back pressure on the yoke as the FBW will trim that for me right? However in the simulator if I don't pull back the yoke in a turn the pitch angle will start to fall after about one second. Actually if I want to hold a leveled-30-degree-turn, I would need to trim up the nose for about 4-6s (which equals to 40-60kt of speed) along with a slight back pressure on the yoke, and then trim down and push the nose when about to exit the turn. 

 

Thanks!

Zhan Shi

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

Hi there.

 

your 1st scenario is a strange one.

It allmost sound to me that your trim effords are not doing anything at all.

Do you have a little deadzone defined for your hardware yoke (I am assuming you use one)?

You should!

Either set up a small deadzone at the windows driver level (that is what I did) or in FSUIPC or in the FSX controller interface.

Elevator trim only works if you are trimming in the same direction as you are pushing/pulling on your yoke.

So if you have no dead zone, then it is possible that when you are trying to trim down with the yoke in neutral, that the yoke is actually giving a small pitch up signal....and those two opposite signals result in no trimming taking place.

 

Also make sure you have the latest 777 version installed.....a lot was done in the PMDG Service Packs to improve on the original trim behavior.

 

Scenario 2 is normal actually.

The FBW system will try to help stabilize the turn but this does not mean you can just bank without having to take care of pitch also.

So bank and simultanuously control pitch to not loose altitude (for a level turn) and once you are established in the turn (which is after 3-5seconds or so) then let go of the yoke.

If you are were trimmed correctly before the turn (use the FBW trim speed indication to check), then you should see a very stable turn that only needs occasional correction from you while the turn lasts.

When you roll out of the turn, you must again control pitch to prevent the airplane from pitching up. When level flight is established (again, this takes a few seconds) and if the airplane was trimmed correctly to begin with, you should be able to let go again and see a pretty stable airplane that only needs occasional corrctions to maintain altitude.

 

Note that occasional corrections are still required (as in every 5-10 seconds) even when perfectly trimmed.....the 777 FBW is not stable like an airbus!

Share this post


Link to post
Share on other sites

Hi there.

 

your 1st scenario is a strange one.

It allmost sound to me that your trim effords are not doing anything at all.

Do you have a little deadzone defined for your hardware yoke (I am assuming you use one)?

You should!

Either set up a small deadzone at the windows driver level (that is what I did) or in FSUIPC or in the FSX controller interface.

Elevator trim only works if you are trimming in the same direction as you are pushing/pulling on your yoke.

So if you have no dead zone, then it is possible that when you are trying to trim down with the yoke in neutral, that the yoke is actually giving a small pitch up signal....and those two opposite signals result in no trimming taking place.

 

Also make sure you have the latest 777 version installed.....a lot was done in the PMDG Service Packs to improve on the original trim behavior.

 

Scenario 2 is normal actually.

The FBW system will try to help stabilize the turn but this does not mean you can just bank without having to take care of pitch also.

So bank and simultanuously control pitch to not loose altitude (for a level turn) and once you are established in the turn (which is after 3-5seconds or so) then let go of the yoke.

If you are were trimmed correctly before the turn (use the FBW trim speed indication to check), then you should see a very stable turn that only needs occasional correction from you while the turn lasts.

When you roll out of the turn, you must again control pitch to prevent the airplane from pitching up. When level flight is established (again, this takes a few seconds) and if the airplane was trimmed correctly to begin with, you should be able to let go again and see a pretty stable airplane that only needs occasional corrctions to maintain altitude.

 

Note that occasional corrections are still required (as in every 5-10 seconds) even when perfectly trimmed.....the 777 FBW is not stable like an airbus!

Thank you for the reply! The second point is really helpful and it's good to hear that this is realistic. I was worried about this being a software problem before and reinstalled several times as no one seemed to have this problem.

About the first one, sorry I didn't describe it clearly, so let me put it shorter here:

In the case when your IAS and trim speed vary by a big amount (in the case I described above, the trim speed was almost 100kt lower than IAS).The airplane may behave not as it's supposed to, in my case, it pitches down instead of pitches up and accelerates instead of decelerates. So I'm wondering if it's caused by some aerodynamic factors that I don't know about?

 

ps. About the deadzone, I do have it set for 11 in P3D settings menu.

 

Regards

Share this post


Link to post
Share on other sites

 

 


In the case when your IAS and trim speed vary by a big amount (in the case I described above, the trim speed was almost 100kt lower than IAS).The airplane may behave not as it's supposed to, in my case, it pitches down instead of pitches up and accelerates instead of decelerates.

 

I am trying to digest this.  Indicated trim speed much lower than IAS... I think you are making this too complicated.  The indicated trim speed is just a reference and will loose all meaning in this situation.  Back to basics, she flies like a big Cessna and trim is designed to be intuitively simple.  You have a pitch down moment because you have trim set with too much nose down, trim to reduce your control forces.

 

If you are holding nose up add nose up trim until you can remove pressure on the control.  Ignore the trim speed for the most part.  Once you get the feel for the aircraft the indicated trim speed will start to make more sense for ya.  Don't forget, your performance  is based on attitude and power and the trim is just a tool to help with attitude... reducing power will also play a major role in trim flight at 250 IAS but I limited this to just trim.

Share this post


Link to post
Share on other sites

ps. About the deadzone, I do have it set for 11 in P3D settings menu.

 

Regards

I dont have P3D.....so I dont know what 11 means.

 

Like I said, I have a deadzone set at the driver level.

See this post:

http://forum.avsim.net/topic/451171-cessna-yoke-settings-for-777/

 

Maybe P3D behaves differntly from FSX once you are way way out of trim.

In any case, if your 777 is behaving correctly when only slightly out of trim then I would not worry about it.

It makes no sence to be 100kt out of trim (other than to see what happens).

When accelerating or decelerating, I retrim with a short trim burst each time I am 10kt...max 20kt..... away from current speed.

Share this post


Link to post
Share on other sites

 

 


In the case when your IAS and trim speed vary by a big amount (in the case I described above, the trim speed was almost 100kt lower than IAS).The airplane may behave not as it's supposed to, in my case, it pitches down instead of pitches up and accelerates instead of decelerates. So I'm wondering if it's caused by some aerodynamic factors that I don't know about?

This is a limitation of the simulation, not a feature of the real thing. The Introduction Manual contains a description of the fly by wire which hints at how PMDG simulated the 777 trim reference speed system. PMDG reduce elevator effectiveness if you are out of trim to increase the force you feel when out of trim. This can lead to strange things happening when trim speed is significantly different to current speed (such as a complete loss of control authority). As Rob said above, when hand flying it's best to re-trim whenever airspeed is changed.

 

I tried your takeoff scenario in the 777-200 but didn't get the same nose down trim you report. My 777 wanted to pitch up as soon as I released the column at 250 knots. The simulation model is the same for both of us so the only difference is in control set up. PMDG provide a means to tune that for FBW in the FMC CDU options. The deadbands set there do not affect your normal control calibration, it's purely for the FBW simulation.

Share this post


Link to post
Share on other sites
 

I dont have P3D.....so I dont know what 11 means.

Like I said, I have a deadzone set at the driver level.
See this post:
http://forum.avsim.net/topic/451171-cessna-yoke-settings-for-777/

Maybe P3D behaves differntly from FSX once you are way way out of trim.
In any case, if your 777 is behaving correctly when only slightly out of trim then I would not worry about it.
It makes no sence to be 100kt out of trim (other than to see what happens).
When accelerating or decelerating, I retrim with a short trim burst each time I am 10kt...max 20kt..... away from current speed.

You are absolutely right and I'm trying to keep my aircraft trimmed all the time as best as I can. Also thank you for the tip on yoke settings, that's a very useful link! 

btw, the settings in P3D, except for those graphics related ones, are exactly the same as in FSX.

This is a limitation of the simulation, not a feature of the real thing. The Introduction Manual contains a description of the fly by wire which hints at how PMDG simulated the 777 trim reference speed system. PMDG reduce elevator effectiveness if you are out of trim to increase the force you feel when out of trim. This can lead to strange things happening when trim speed is significantly different to current speed (such as a complete loss of control authority). As Rob said above, when hand flying it's best to re-trim whenever airspeed is changed.

 

I tried your takeoff scenario in the 777-200 but didn't get the same nose down trim you report. My 777 wanted to pitch up as soon as I released the column at 250 knots. The simulation model is the same for both of us so the only difference is in control set up. PMDG provide a means to tune that for FBW in the FMC CDU options. The deadbands set there do not affect your normal control calibration, it's purely for the FBW simulation.

That's interesting, so since you brought up this topic, the "trim by speed" logic is also something that makes me confused. In my understanding, stabilizer trim angle is directly related to speed (in a fixed configuration) in any aircraft and if you don't touch the trim and elevator, the plane will fly at a certain speed when it finally stabilizes itself after some oscillations. So what's special about the 777 system? I looked up in the FCOM but didn't find an answer.

 

Thanks!

Share this post


Link to post
Share on other sites

We have had many trim threads in the past going deep into how the 777 FBW system works.

Maybe Google will find them if you try.

 

But in short, what is special about the 777 is that it autotrims for config changes and thrust changes.....and with that, the stabiliser is not at a fixed angle anymore all the time.

 

When you add thrust on the 737, the moment caused by the engines mounted UNDER the wing will make the 737 pitch up.

If you dont want this, then you have to trim down.

 

When you add thrust on the 777, the moment is the same but autotrim will trim to counter the nose up moment.....and so you now have a different stab angle without a config change and without you having ever touched the trim switches.

 

Autotrim also counters the nose drop in a turn.

 

And it will counter moments caused by extending flaps/gear/speedbrakes.

 

But it will only counter all these moments somewhat.....not 100% as Boeing wanted to give the pilot the feel of a conventional airplane....rather than trim it all away till you feel nothing. So you still have to work for your money....just less

Share this post


Link to post
Share on other sites

That's why I said fly it like a big Cessna... if that doesn't work then you need to work on your piloting skills.

Share this post


Link to post
Share on other sites

 

That's interesting, so since you brought up this topic, the "trim by speed" logic is also something that makes me confused. In my understanding, stabilizer trim angle is directly related to speed (in a fixed configuration) in any aircraft and if you don't touch the trim and elevator, the plane will fly at a certain speed when it finally stabilizes itself after some oscillations. So what's special about the 777 system? I looked up in the FCOM but didn't find an answer.

 

Thanks!

 

The difference with the 777 is that it trims out configuration and thrust changes for you. A conventional aircraft won't do that. The only trimming you have to do is when you change airspeed. Operating the trim switches doesn't directly move the stabiliser in flight (it does do so on ground). It changes the trim reference speed. If you aren't flying at that speed the FBW computer puts in an offset to the pitch computation proportional to how far off the trim speed you are. So if you release the column it will tend to pitch to change speed to match the trim reference speed.

 

That's how the aircraft works anyway.

Share this post


Link to post
Share on other sites

That's why I said fly it like a big Cessna... if that doesn't work then you need to work on your piloting skills.

I know, and I have said that often as well....but sometimes people just want more info/proof I guess.

Share this post


Link to post
Share on other sites

Thanks very much for your clear explanation 777simmer and kevinh. I guess that takes out my confusion in theory and I just need to practice more. 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this