Trim question for PMDG 737NGX

[trisho0 23]

48 minutes ago, downscc said:

Trim can be way off, a little off or right on.  Anything close to right on could be called well trimmed.

How far can be trim way off? Visible from somewhere? From the trim wheel? Or, just looking outside of the plane thru cockpit cabin watching the nose position?

Edited June 21, 2018 by trisho0
details

[skelsey 765]

Just now, trisho0 said:

How far can be trim way off? Visible from somewhere? From the trim wheel? Or, just looking outside of the plane thru cockpit cabin?

I'm still not sure you're getting this (and in fairness it is difficult doing this in text!).

It is about how the aeroplane feels. Let's say you want the nose to be pitched 5 degrees above the horizon. Are you having to hold the stick way back to keep it there? If you release the stick even a little bit does the nose immediately want to drop quickly?

Then the aeroplane is not in trim (and indeed is a long way out of trim). Lots of movement of the trim wheel is necessary to put the aeroplane back in trim.

However, if you only have to hold the stick very gently back to maintain the pitch attitude you want then the aeroplane is very nearly in trim and only a small adjustment may be necessary.

(Of course, you could equally be having to hold the stick forward - the same principle applies).

If you would like to, I would be happy to spend an hour or so with you using the free JoinFS shared cockpit software we use for our training courses at BAV to teach you practically how to trim, which is probably a lot quicker and easier than trying to describe things in text  (on both sides of the conversation!) - just drop me a private message via the forum and we can get something set up (probably next week).

Best,

Simon

[Bobsk8 12,668]

I can't believe the length of this thread to explain trim which any student pilot learns in the first hour of dual instruction. You get the aircraft in the attitude you want, and at that  point you are most likely pulling or pushing on the elevator control . Then you move the trim, so that the amount of pushing or pulling goes to zero, and you can take your hand off the elevator, with no change in the pitch. That's it.. Nothing more. 

[trisho0 23]

On ‎6‎/‎21‎/‎2018 at 8:02 PM, Bobsk8 said:

I can't believe the length of this thread to explain trim which any student pilot learns in the first hour of dual instruction. You get the aircraft in the attitude you want, and at that  point you are most likely pulling or pushing on the elevator control . Then you move the trim, so that the amount of pushing or pulling goes to zero, and you can take your hand off the elevator, with no change in the pitch. That's it.. Nothing more. 

Yes, this is clear to me definitely. I meant, finally is clear for me and thanks for all of you Pilots and Support.

[trisho0 23]

On ‎6‎/‎21‎/‎2018 at 5:27 PM, skelsey said:

It is about how the aeroplane feels. Let's say you want the nose to be pitched 5 degrees above the horizon. Are you having to hold the stick way back to keep it there? If you release the stick even a little bit does the nose immediately want to drop quickly?

I don't have to hold the joystick on this case. If I release the stick the plane nose goes down quickly. I do trimming to pitch attitude and at the same time I can release gear hydraulics, Autobrake system Off and moving flaps up accordingly following the PFD. Now, no more trimming and arm AP.

On ‎6‎/‎21‎/‎2018 at 5:27 PM, skelsey said:

If you would like to, I would be happy to spend an hour or so with you using the free JoinFS shared cockpit software we use for our training courses at BAV to teach you practically how to trim, which is probably a lot quicker and easier than trying to describe things in text  (on both sides of the conversation!) - just drop me a private message via the forum and we can get something set up (probably next week).

I visited JoinFS site and I am new on that. I am not ready for JoinFS until I can afford it. Thanks so much for your great help as well. I think I understood about trimming anyway. Just to practice trimming with my birds. I am flying several times same route from MHLM rwy04 to MHTG rwy20 and manual landings. (No ILS available). Since the flight is very short time I am getting more experience with trimming and manual landings.

 

[Spin737 102]

15 hours ago, trisho0 said:

I don't have to hold the joystick on this case. If I release the stick the plane nose goes down quickly. I do trimming to pitch attitude and at the same time I ...

Hmm. The nose shouldn’t go down quickly. You should have the plane trimmed so that you can release the stick and the nose stays where it was. 

[trisho0 23]

22 hours ago, trisho0 said:

Since the flight is very short time I am getting more experience with trimming and manual landings.

The trimming is explained even more from 

and easy to understand.

[trisho0 23]

6 hours ago, Spin737 said:

Hmm. The nose shouldn’t go down quickly. You should have the plane trimmed so that you can release the stick and the nose stays where it was. 

Yes, not too quickly I know but it goes down if not trimming until the plane becomes balanced on airborne.

[kevinh 409]

On 6/20/2018 at 10:29 PM, skelsey said:

The 'correct' technique is to select the desired attitude with the elevator and hold said attitude steady whilst trimming until no remaining pressure is required.

You can trim just as correctly by holding airspeed with elevator. Pitch and airspeed are inextricably linked for a given flight path and the chances are the pitch you selected to trim at was governed by the airspeed you want to fly at. 

Generally speaking small changes in attitude cause larger changes in airspeed and vice versa. So holding airspeed is likely to be more precise than holding attitude. Especially with a PFD and a speed trend vector. Even more so at higher speeds, where pitch hardly varies.

 

Edited June 28, 2018 by kevinh

[skelsey 765]

8 hours ago, kevinh said:

You can trim just as correctly by holding airspeed with elevator. Pitch and airspeed are inextricably linked for a given flight path and the chances are the pitch you selected to trim at was governed by the airspeed you want to fly at. 

Generally speaking small changes in attitude cause larger changes in airspeed and vice versa. So holding airspeed is likely to be more precise than holding attitude. Especially with a PFD and a speed trend vector. Even more so at higher speeds, where pitch hardly varies.

As I said to Andrew -- if it works for you then that's fine, however:

• The primary effect of elevator is pitch
• We trim to relieve pressure on the controls
• We select a pitch attitude (and power setting) in order to produce a desired airspeed/performance (climb/descent at a given airspeed or straight and level etc).

Thus we are trimming for a pitch attitude, which we have selected using the elevator in combination with a power setting in order to achieve a particular airspeed/and or performance. One does not 'hold airspeed with elevator' -- one holds a pitch attitude, which in combination with a particular power setting will result in a particular airspeed.

If the aircraft is not trimmed then the pitch attitude will deviate from that which we had selected and the airspeed will also change. The airspeed changes because the pitch attitude has changed. To regain the desired airspeed we then use the elevator to re-select the associated attitude, and trim to relieve the control pressure.

If we subsequently change the power setting then the nose will indeed rise or fall, but I can't think of any practical situation where one would be altering power without also wanting to also positively change the attitude -- either to commence a climb or descent, or to maintain straight and level flight at a different airspeed.

As an experienced pilot you probably know the approximate pitch attitudes/change in pitch attitude required for given airspeeds etc for a particular type and so the process is very natural. As I mentioned earlier, however, there are a couple of issues with this approach, especially when applied to inexperienced pilots.

The airspeed indicator (as with all the instruments) is subject to a certain amount of lag; the inertia of even a small aeroplane means that there will naturally be a delay between the attitude being selected and the response on the airspeed indicator. If an inexperienced pilot is being told to pitch with primary reference to the ASI or other instruments (VSI/altimeter) the almost universal result is significant overcontrolling and quite unstable flight as they chase the needles around -- I've seen it! Once the student starts looking either out the window at the visible horizon at a basic level, or the artificial horizon at a more advanced level, and starts focusing primarily on the pitch attitude their flying invariably becomes much smoother and more accurate. They also get to know the pitch attitudes and power settings associated with different airspeeds.

On a similar note the lag associated with the pressure-operated instruments means that by the time the pitch attitude has changed sufficiently for a change in airspeed to occur, chances are that quite a large change in attitude has occurred. Again, the result of this is invariably overcontrolling. Indeed, this is why limited panel instrument flying is such a challenging skill and why when one starts to fly on instruments the teaching tends to shift to 'throttle controls airspeed/pitch controls rate of descent' -- it is much more practical when flying an ILS, for instance, to point the aeroplane toward the needles and maintain speed with power!

Changing airspeed does require a trim change, but (if we are talking about maintaining steady-state flight, whether straight and level or a steady climb or descent) this is because an attitude change is required to maintain the steady state (Lift = AoA x Airspeed). If you increase airspeed in level flight, you will have to use the elevator to lower the nose if you are to maintain altitude. This lower nose attitude requires, clearly, a change in trim to relieve the control pressure that would otherwise be required to hold the nose in the new attitude. The change in trim, however, is to maintain a different nose attitude, not simply because the airspeed has increased. To suggest that the trim needs to be changed because the airspeed has changed is rather the tail wagging the dog: it skates over the very important point that a new pitch attitude is required at the higher airspeed.

[Ephedrin 418]

This over-usage of the elevator trim for too many purposes (managing climb/decent, airspeed and so on comes directly linked to this trim button on the yoke. In the ASK13 I‘ve had to reach for a little handle that needed some force to adjust just a little and I used it basically for just few phases: winch launch, thermals, straight flight, landing. The nose came up and down with the stick. 

C172: little trim wheel below the throttle. You had to reach for it. No nasty buttons on the yoke... there you learn a proper and intelligent trim setting. You don‘t want to crab it all the time... therefore you have a yoke.

[trisho0 23]

We do trimming with AP off, correct? That's how I do trimming after takeoff and when the plane is well trimmed I arm AP. After AP armed then no more trimming.

[downscc 1,327]

7 hours ago, Ephedrin said:

C172: little trim wheel below the throttle. You had to reach for it. No nasty buttons on the yoke... there you learn a proper and intelligent trim setting. You don‘t want to crab it all the time... therefore you have a yoke.

Not my experience at all... I learned to fly C-150/172/182 and the C414 with a hand on the trim wheel... I still prefer to use the trim wheel in lieu of electric trim when available.  You can actually fly the trim unless you are doing maneuvers.

[Ephedrin 418]

2 minutes ago, downscc said:

Not my experience at all... I learned to fly C-150/172/182 and the C414 with a hand on the trim wheel... I still prefer to use the trim wheel in lieu of electric trim when available.  You can actually fly the trim unless you are doing maneuvers.

Yes, when you do a cross country flight, want to fly at 5000 ft and the fuel gets burned, the temperature changes and so on you will always have to do slight corrections to the nose. It would be pointless to actually push the yoke half a mm and then trim any forces off it... I would do the little "fuel corrections" with the wheel only, too. 

But when you overfly a big rail station with its thermals, then a cool forest and later on a large field you don't want to do this with your trim, although I would probably be quite fast to use my thumb button if I had one. And that's definitely not the purpose of the trim, to actually fly the plane manually.