trisho0

Trim question for PMDG 737NGX

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16 hours ago, Stearmandriver said:

This is just basic aerodynamics/airmanship.  Basically, you use elevator trim to remove the need to maintain constant pressure on the yoke; you trim so the plane will fly hands-off.  This is true and good airmanship in any aircraft, but even more important in a larger aircraft where the control forces can be significant.  

http://www.boldmethod.com/learn-to-fly/systems/how-pilots-should-use-trim-in-flight/

I will take look at that site. I guess the trim adjustment is optional after take-off?

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Posted (edited)
1 hour ago, trisho0 said:

I will take look at that site. I guess the trim adjustment is optional after take-off?

Trimming an airplane at the simulator is hard to explain. It would be interesting what kind of controls do you use? Before I learnt to fly I had no idea what exactly „trimming“ means and as I had a joystick with a defined center position all I noticed was that when I touched the trim, the nose went up or down. So often simulator pilots tend to „stear“ the plane using the trim instead of moving the elevator.

In reality there is no such center position, at least at simple airplanes like a Cessna. The airstream moves the control surface to a position and your stick or yoke is directly connected to that surface. But this position doesn‘t mean that te airplane keeps it pitch. To do that you will have to push or pull your controls and hold them. This requires a lot of force on some airplanes. To neutralize this force, there are little trim tabs with wires attached to them that are moved by a little lever or trim wheel. In bigger airplanes, such as an airliner, there isn‘t a trim tab anymore but instead the whole stabilizer (on which te elevators are mounted) rotates up or down. This also neutralized the forces on your controls as the stab lines up with the airstream. That‘s more or less the sense of an elevator or stabilizer trim. 

 

So yes, using the trim is optional, so to say. It‘s a comfort thing but every instructor will beat the hell out of you if you have to battle your airplane to keep its attitude instead of having time to manage other important systems.

Edited by Ephedrin

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1 hour ago, trisho0 said:

I guess the trim adjustment is optional after take-off?

Use of trim while hand flying is how you should fly; however, you do not touch trim if the A/P is engaged...in fact, it is trim that the A/P generally uses to maintain pitch.  The Airman's Information Manual (AIM) is here: https://www.faa.gov/air_traffic/publications/

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2 hours ago, downscc said:

Use of trim while hand flying is how you should fly; however, you do not touch trim if the A/P is engaged...in fact, it is trim that the A/P generally uses to maintain pitch.  The Airman's Information Manual (AIM) is here: https://www.faa.gov/air_traffic/publications/

Dan, thanks again. I just flew KJFK-PHDL and after take-off I adjusted the trim and yes the nose went up and down trying to follow the horizon pink line from PFD but it won't do ILS so when I got about 1K below the programmed Altitude I had to arm AP. Otherwise, the plane won't follow ILS. So, even I learned something about trim procedures I found that really I don't have to do more trim after AP engaged. From the AIM link that info is so technical. I will check on that link again to take my reading time. I am sure I will found more to learn.

Do you agree the trim after take-off is optional?

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2 hours ago, Ephedrin said:

Trimming an airplane at the simulator is hard to explain. It would be interesting what kind of controls do you use? Before I learnt to fly I had no idea what exactly „trimming“ means and as I had a joystick with a defined center position all I noticed was that when I touched the trim, the nose went up or down. So often simulator pilots tend to „stear“ the plane using the trim instead of moving the elevator.

In reality there is no such center position, at least at simple airplanes like a Cessna. The airstream moves the control surface to a position and your stick or yoke is directly connected to that surface. But this position doesn‘t mean that te airplane keeps it pitch. To do that you will have to push or pull your controls and hold them. This requires a lot of force on some airplanes. To neutralize this force, there are little trim tabs with wires attached to them that are moved by a little lever or trim wheel. In bigger airplanes, such as an airliner, there isn‘t a trim tab anymore but instead the whole stabilizer (on which te elevators are mounted) rotates up or down. This also neutralized the forces on your controls as the stab lines up with the airstream. That‘s more or less the sense of an elevator or stabilizer trim. 

 

So yes, using the trim is optional, so to say. It‘s a comfort thing but every instructor will beat the hell out of you if you have to battle your airplane to keep its attitude instead of having time to manage other important systems.

I use Logitech Extreme 3D PRO Joystick for flying my birds. I am learning about trimming thanks to Dan in this thread.

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Posted (edited)
3 hours ago, trisho0 said:

I am learning about trimming

 

There is actually not much to learn. The theoretical description isn't worth anything as long as you don't accept a simple fact: using the elevator trim is as normal and natural on an airplane as breathing. And hey, as long as you ask if it's optional you haven't accepted it... There is no option. Sure, you can fight with your airplane, strongly grab the stick/yoke and push it to the front to make the nose stay down. But there's a point your arm will become tired. As you accelerate your airplane will pitch up. To counter this movement, you stear down (push). Release the stick and the airplane will pitch up and climb again. So you push again. To make the stick stay forward at the correct position without having to force it you trim your nose down... elevator/stab trim down. No value. No theoretical setting... just enough that you don't FEEL any force on the stick anymore. So you can take your hands off it and the plane's nose will stay were it is. This will need some practise, sure, but I doubt that there is a lot to read about. It's a very simple thing actually, as long as you don't dive into its aerodynamical background. As simple as possible: When you have to force your airplane to keep its attitude then use the available trim. That's all.

 

Why you have to set a specific trim setting on a 737 for the takeoff is a little bit different. The stabilizer actually has the aerodynamical effect of an inversed (upside-down) wing. If you closely look at its profile, it's convex on the down side in contrary to a wing... (picture below) so its "lift" is directed downwards... it will hold the back down so the plane can keep its pitch. Now on take off you have to use a trim setting that neutralizes this lift for the given CG until - at Vr - you pull back the yoke and lift up the elevator. this elevator has the same effect as a flap, but in contrary direction and the tail "flies" down.. that's actually all about the trim mistery. There isn't too much. it's just an "angle of attack" setting for the stabilizer.

 

57Tmo.jpg

Edited by Ephedrin

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7 hours ago, Ephedrin said:

Why you have to set a specific trim setting on a 737 for the takeoff 

An alternative way of thinking about this is to say the takeoff trim setting is made so that the aircraft is already in trim before flight. Once in flight you can use the control forces to trim the aircraft. Before takeoff that can’t be done. So the trim setting is calculated. If the aircraft wasn’t correctly trimmed for takeoff the result could be catastrophic. 

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Posted (edited)

Some important things to remember about trim:

1) We trim for a pitch attitude, not an airspeed or altitude

2) The trim is not used to change the pitch attitude of the aeroplane (a major fault for many simmers)

3) The trim is there to make your life easier!

Keeping the aeroplane in trim whilst hand-flying is good practice for many reasons -- not least 1) it is less tiring on the arms, 2) it will allow you to fly more accurately and 3) give you more capacity to deal with other stuff that might require you to take your hand off the control column for a second or two.

So how do we trim the aeroplane?

The usual mantra is Select (the pitch attitude required, with the elevator) - Hold (again, using the elevator to hold the nose in the required pitch attitude) - Trim (to relieve the force you are having to exert on the controls to hold the nose in the selected position). Select, hold, trim.

This is possibly more easily demonstrated in a more simple aircraft because the pitch adjustments in a large jet like the 737 are much smaller and more difficult to see -- a small pitch change results in a big difference in performance. However, if you are going to practice this in the NGX, set yourself up in level flight at, say, 7000 feet and 230 knots with nice clear skies and autopilot/autothrottle engaged. 

Look outside and observe where the horizon line is relative to the top of the glareshield. You can also look at the ADI and observe the indicated pitch attitude (look at where the aeroplane symbol -- the small box in the centre of the wing bars - sits on the pitch ladder, which is graduated in 2.5 degree steps).

Now disengage both autothrottle and autopilot so that you are in fully manual flight (thrust changes will affect the aircraft's trim). Leave the throttle alone though -- you just want to remain at that fixed thrust setting throughout. The aircraft shouldn't really move much if at all from where it is -- the autopilot should have left it nicely in trim!

Next, apply a small amount of backpressure to the yoke and raise the nose, say, 5 degrees compared to its current value (two steps up on the pitch ladder). Observe where the horizon is relative to the glareshield and try and hold that new attitude as steadily as you can. You'll find you need to hold some backpressure, increasing as the speed reduces (again - don't touch the throttle). Once the airspeed stabilises at a new lower value, the amount of backpressure you require will also stabilise. Quite uncomfortable, isn't it?

What we can do to solve this is to trim the aeroplane -- and it's best to have the trim assigned to buttons on your joystick or yoke to do this because if you have to take your hands off the stick you won't be able to feel the change in pressure and if you have to change your viewpoint you won't be able to observe the movement of the horizon.

As we are holding backpressure, we need to apply nose up trim, which in the real thing is achieved by using your thumb to click the trim switches on the yoke 'down'/towards you (i.e. it is arranged to work in the same sense that the flight controls work -- down/towards you to go up, up/away from you/forward to go down). Slowly apply some nose-up trim (the easiest way to modulate this is to hold the trim switch down for a second or so, then release, reassess and go again). As you do so, hold the attitude absolutely steady -- keep that aeroplane symbol exactly where you placed it on the ADI/the horizon line exactly where you placed it relative to the glareshield a few moments ago, whilst you trim the aircraft.

As you apply nose-up trim, of course, the amount of backpressure you need to hold to do this will gradually reduce. Eventually you will reach a point where you are not holding any backpressure but the nose is still sitting in the new attitude. Voila -- the aeroplane is now in trim! Obviously selecting a lower nose attitude works in exactly the same way except with forward pressure and nose down trim.

Of course, this is not entirely a one-time set and forget thing - atmospheric disturbances, people moving about down the back, different weights etc will all require different trim and changes in trim, and so will adjustments to airspeed, thrust and configuration. However, the principle remains the same -- select the attitude required with the elevator, hold it there until you know how much pressure is required, then trim to relieve that pressure.

As you become more proficient with practice you will get a sense of approximately how much trim is required given the amount of backpressure you are holding and/or the size of the change in pitch attitude and so will get much faster at nailing it. But as I say, remember to hold that pitch attitude rock steady whilst you are trimming -- don't select the attitude then let go of the stick whilst you trim.

Hope that helps!

Edited by skelsey

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21 hours ago, Ephedrin said:

There is actually not much to learn. The theoretical description isn't worth anything as long as you don't accept a simple fact: using the elevator trim is as normal and natural on an airplane as breathing. And hey, as long as you ask if it's optional you haven't accepted it... There is no option. Sure, you can fight with your airplane, strongly grab the stick/yoke and push it to the front to make the nose stay down. But there's a point your arm will become tired. As you accelerate your airplane will pitch up. To counter this movement, you stear down (push). Release the stick and the airplane will pitch up and climb again. So you push again. To make the stick stay forward at the correct position without having to force it you trim your nose down... elevator/stab trim down. No value. No theoretical setting... just enough that you don't FEEL any force on the stick anymore. So you can take your hands off it and the plane's nose will stay were it is. This will need some practise, sure, but I doubt that there is a lot to read about. It's a very simple thing actually, as long as you don't dive into its aerodynamical background. As simple as possible: When you have to force your airplane to keep its attitude then use the available trim. That's all.

 

Why you have to set a specific trim setting on a 737 for the takeoff is a little bit different. The stabilizer actually has the aerodynamical effect of an inversed (upside-down) wing. If you closely look at its profile, it's convex on the down side in contrary to a wing... (picture below) so its "lift" is directed downwards... it will hold the back down so the plane can keep its pitch. Now on take off you have to use a trim setting that neutralizes this lift for the given CG until - at Vr - you pull back the yoke and lift up the elevator. this elevator has the same effect as a flap, but in contrary direction and the tail "flies" down.. that's actually all about the trim mistery. There isn't too much. it's just an "angle of attack" setting for the stabilizer.

This really explained much better than the manual, well detailed. Understood about trimming. Let me say if I am right:

1. Trim setting follow CG from FMC

2. Takeoff, push back the Joystick (Yoke) to climb

3. Keep hands off and start trimming accordingly to stabilize the plane

4. Once I am 1K feet near the programmed Altitude stop trimming and arm Autopilot, no more Trim to take car of.

Am I wrong?

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13 hours ago, kevinh said:

An alternative way of thinking about this is to say the takeoff trim setting is made so that the aircraft is already in trim before flight. Once in flight you can use the control forces to trim the aircraft. Before takeoff that can’t be done. So the trim setting is calculated. If the aircraft wasn’t correctly trimmed for takeoff the result could be catastrophic. 

That is what ephedrine said

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Posted (edited)
1 hour ago, trisho0 said:

3. Keep hands off and start trimming accordingly to stabilize the plan

Am I wrong?

Sounds like it.  Unless I'm misunderstanding what you are saying you do.

After you pull back on the yoke/joystick to raise the nose and begin your takeoff climb, you don't "let go" of the yoke and "start trimming" to stabilize the plane.  That is exactly the WRONG thing to do.  The autopilot is still disengaged, so YOU need to keep your hands on the controls and manually keep flying the airplane.  You NEVER, NEVER use the trim wheel/button to try to get the airplane in a particular pitch attitude.  NEVER.  You HAND FLY the airplane to the pitch attitude you want to be in.  Then, IF THERE IS ANY PRESSURE YOU HAVE TO HOLD AGAINST THE YOKE to maintain that pitch attitude, you use the TRIM to RELIEVE THE PRESSURE you have to hold on the yoke.  Never, NEVER try to "fly the airplane" by using trim adjustments to do it.  You will only end up chasing the nose up and down that way and your attitude achievement would just be "guessing what trim setting to use".  Get into the attitude you want FIRST by hand-flying the airplane, THEN only use the trim to relieve the pressure you have to hold against the yoke to maintain the attitude.  THEN you can "let go of the controls" and turn on the autopilot, etc.  You would NEVER do what you said in your #3 above.  Never let go fo the yoke first, then try to "fly the airplane" using trim adjustments.  That is totally backwards.

And to answer one other question you asked about WHY you would even want to use the correct trim setting.....

In some aircraft the autopilot will NOT engage if the aircraft is "out of trim balance" to begin with.  This is even correctly modeled in some of the PMDG aircraft (I think the 777...not sure about the 747).  So if you are fighting the yoke by having to hold excessive pressure on it to maintain the nose attitude you want, you might not be able to even ENGAGE the autopilot until you set the trim correctly to relieve the pressure on the yoke.  This is a SAFETY FEATURE built into the airplane's autopilot to prevent a lazy or distracted pilot from engaging the autopilot in a way out of trimmed aircraft configuration the autopilot would have to "fight" as soon as it was engaged.

Hand flying an aircraft while using trim correctly is a BASIC requirement every pilot should be thoroughly familiar with.  There is nothing that would indicate the LACK of knowledge and piloting skills of a pilot more than if they used TRIM to try to FLY the airplane, instead of the way it is supposed to be used.

EDIT/ADD:  Having said all of the above, the issue with our flight sims is that if we are not using a force feedback joystick or yoke, we may never feel the "out of trim" back pressures holding the joystick/yoke (it can depend on the centering qualities of the joystick/yoke, or how big of a "null zone" you may have set for your joystick/yoke, etc).  And even some force feedback joysticks don't simulate it realistically.  So in our sims we need to be more "visually aware" of an out of trim situation.  If I "let go of my joystick" for a second, does the airplane change pitch attitude?  Then the trim is not set correctly and needs to be adjusted.  If the airplane maintains the attitude, then the trim is already set correctly.  But if the airplane's attitude DOES change when I let go of the joystick, I don't then try to "make things right" using trim adjustments alone.  I re-hold the joystick/yoke to manually fly/readjust the pitch attitude back to where I want it, then re-adjust the trim in the proper direction while still holding the joystick/yoke.  You'll get the hang of it after some practice, but it is actually MORE DIFFICULT doing it in the sim without having the "pressure on the yoke/joystick feedback" you would feel in a real airplane.

 

Edited by FalconAF

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14 hours ago, trisho0 said:

That is what ephedrine said

Not exactly. I was trying to explain in the simplest way possible why it is necessary to set trim correctly for takeoff. Marc went into aerodynamic detail which is useful knowledge but isn’t the central point. That’s more about trimming in general, not takeoff in particular.

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14 hours ago, FalconAF said:

Never, NEVER try to "fly the airplane" by using trim adjustments to do it. 

Never shouldn't be used very often.  A good example is hand flying straight and level minor pitch adjustments are routinely made with pitch trim only.  I literally fly  with my thumb on the pitch switch or wheel... I prefer the wheel.  You can also use trim for a smooth transition from level flight to descent... in fact that's the only way I'd be doing it... cannot imagine pushing on the yoke to start descent unless I was in a hurry to get down.

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Posted (edited)

We're arguing semantics.  It's the same as the old discussion about "elevator controls altitude and throttle controls airspeed".  That will always confuse some student pilots until they grasp the "WHY" it is that way.  And even then, some experienced pilots will still say, "Well, not ALL the time...".  But that doesn't mean a flight instructor will ever teach it the other way around while maintaining any credibility. 

Trim is used to relieve pressure the pilot may need to use on the yoke or stick to maintain a given attitude.  You never use trim to "fly" the airplane from one attitude to another.  It just doesn't work that way, and would never be taught that way by any credible instructor or flight school. 

Edited by FalconAF

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21 hours ago, FalconAF said:

Sounds like it.  Unless I'm misunderstanding what you are saying you do.

After you pull back on the yoke/joystick to raise the nose and begin your takeoff climb, you don't "let go" of the yoke and "start trimming" to stabilize the plane.  That is exactly the WRONG thing to do.  The autopilot is still disengaged, so YOU need to keep your hands on the controls and manually keep flying the airplane.  You NEVER, NEVER use the trim wheel/button to try to get the airplane in a particular pitch attitude.  NEVER.  You HAND FLY the airplane to the pitch attitude you want to be in.  Then, IF THERE IS ANY PRESSURE YOU HAVE TO HOLD AGAINST THE YOKE to maintain that pitch attitude, you use the TRIM to RELIEVE THE PRESSURE you have to hold on the yoke.  Never, NEVER try to "fly the airplane" by using trim adjustments to do it.  You will only end up chasing the nose up and down that way and your attitude achievement would just be "guessing what trim setting to use".  Get into the attitude you want FIRST by hand-flying the airplane, THEN only use the trim to relieve the pressure you have to hold against the yoke to maintain the attitude.  THEN you can "let go of the controls" and turn on the autopilot, etc.  You would NEVER do what you said in your #3 above.  Never let go fo the yoke first, then try to "fly the airplane" using trim adjustments.  That is totally backwards.

And to answer one other question you asked about WHY you would even want to use the correct trim setting.....

In some aircraft the autopilot will NOT engage if the aircraft is "out of trim balance" to begin with.  This is even correctly modeled in some of the PMDG aircraft (I think the 777...not sure about the 747).  So if you are fighting the yoke by having to hold excessive pressure on it to maintain the nose attitude you want, you might not be able to even ENGAGE the autopilot until you set the trim correctly to relieve the pressure on the yoke.  This is a SAFETY FEATURE built into the airplane's autopilot to prevent a lazy or distracted pilot from engaging the autopilot in a way out of trimmed aircraft configuration the autopilot would have to "fight" as soon as it was engaged.

Hand flying an aircraft while using trim correctly is a BASIC requirement every pilot should be thoroughly familiar with.  There is nothing that would indicate the LACK of knowledge and piloting skills of a pilot more than if they used TRIM to try to FLY the airplane, instead of the way it is supposed to be used.

EDIT/ADD:  Having said all of the above, the issue with our flight sims is that if we are not using a force feedback joystick or yoke, we may never feel the "out of trim" back pressures holding the joystick/yoke (it can depend on the centering qualities of the joystick/yoke, or how big of a "null zone" you may have set for your joystick/yoke, etc).  And even some force feedback joysticks don't simulate it realistically.  So in our sims we need to be more "visually aware" of an out of trim situation.  If I "let go of my joystick" for a second, does the airplane change pitch attitude?  Then the trim is not set correctly and needs to be adjusted.  If the airplane maintains the attitude, then the trim is already set correctly.  But if the airplane's attitude DOES change when I let go of the joystick, I don't then try to "make things right" using trim adjustments alone.  I re-hold the joystick/yoke to manually fly/readjust the pitch attitude back to where I want it, then re-adjust the trim in the proper direction while still holding the joystick/yoke.  You'll get the hang of it after some practice, but it is actually MORE DIFFICULT doing it in the sim without having the "pressure on the yoke/joystick feedback" you would feel in a real airplane.

 

OK so,

1. Adjust Trim before takeoff

2. Hands on Yoke until near Altitude and arm Autopilot

This is what I was doing in the past months. After found Trim matters in AVSIM is when I started this thread to find out some info and answers.

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