trisho0

Trim question for PMDG 737NGX

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29 minutes ago, FalconAF said:

Trim is used to relieve pressure the pilot may need to use on the yoke or stick to maintain a given attitude.

In my flights I don't see/feel trim pressure so, am I lucky? I think is because I use Logitech Joystick and it is well adjusted via control panel from Windows 10.

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Think about this, too: If you're applying pressure to the yoke for pitch, you're not flying the tail as cleanly as you could. More pressure = more drag.

 

Interesting aside regarding trim:

I was in the sim (full-motion) a few days ago doing my recurrent and the instructor had us doing stall recoveries. The scenario was an auto-throttle malfunction on an ILS approach on final. The auto-throttle idled the engines and we're supposed to ignore the warnings until buffet/shaker.

I get the shaker and pushed the nose over and then added power. The nose came up to a good attitude, but wouldn't stop raising even with the yoke full forward!

So, I pulled the power back to maybe 80% while I was working on getting the trim set. My officer, doing his job of calling out any omissions, asked if I would like more power. NO! (I may have said it a little too harshly. Sorry, JP.)

The underslung engines were providing more of a pitch moment than the elevator could control, so initially I had to reduce power so we didn't end up in an upset. Once I got the trim set, all was okay and we cleaned up and it was my FO's turn.

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On ‎6‎/‎9‎/‎2018 at 3:38 PM, FalconAF said:

It's the same as the old discussion about "elevator controls altitude and throttle controls airspeed".   

Amazing.  The above is why I DO make "information correction" type posts in forums.

It's been over 24 hours since I posted the above.  And not ONE PERSON has questioned it.

The quoted part is "erroneous information" (that's what an Instructor (teacher) would call information that is FALSE).

The correct information concerning flying an aircraft is "elevator controls AIRSPEED and throttle controls ALTITUDE".  And that confuses the heck out of most new student pilots because it is totally backwards from what you use the "throttle" (gas pedal) in a car to do...increase or decrease speed.

And nobody here questioned it.

Amazing

 

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1.  A previous explanation said we trim for pitch attitude, not airspeed.  This isn't really true.  An airplane trimmed for a particular airspeed will pitch up or down as necessary to maintain that airspeed, so I've always found it easier as a teaching method to say that we DO trim for airspeed.  Try this: in any airplane from a Cessna to a 737, trim for level flight at cruise power, then pull the power to idle.  What you'll see is the plane pitch down and start to descend, roughly at the same airspeed you trimmed it for (I say roughly because with underslung wing mounted engines like the 737, every power change somewhat changes the speed you're "trimmed out" for unless you adjust the trim again.)

2.  In real life, elevator trim is definitely not "optional".  It's impossible to fly smoothly and accurately while needing to hold constant pressure on the controls.  In large aircraft (this includes the 737), if you're far enough out of trim it's almost physically impossible to use enough force to counter the situation.  In the sense that there's nothing realistic about how we control a computer simulation / game, sure it's optional.  We're all here for enjoyment so do what makes you happy (I mean that sincerely, not sarcastically).  But if we're interested in how things work for real, elevator trim is practically a primary flight control.  No pilot advances in even primary flight training until they can trim an airplane up.  Definitely not optional in reality. 

 

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5 hours ago, FalconAF said:

Amazing.  The above is why I DO make "information correction" type posts in forums.

It's been over 24 hours since I posted the above.  And not ONE PERSON has questioned it.

The quoted part is "erroneous information" (that's what an Instructor (teacher) would call information that is FALSE).

The correct information concerning flying an aircraft is "elevator controls AIRSPEED and throttle controls ALTITUDE".  And that confuses the heck out of most new student pilots because it is totally backwards from what you use the "throttle" (gas pedal) in a car to do...increase or decrease speed.

And nobody here questioned it.

Amazing

I'll question it if it makes you feel better... Because your definitive statement about power for altitude and pitch for airspeed isn't really true either ;-). 

That's pounded into the heads of primary students regarding slow flight and approaches (when you're flying at a higher AoA and towards the backside of the power curve), but isn't applicable in all or even most situations even in light airplanes.  For instance, if you're at cruise power and speed and find you're 50ft low, you aren't going to add power, you're simply going to raise the nose a bit and get that 50ft back.  Sure, you will have traded a bit of airspeed for it, but when you level you'll get it back. 

 In large airplanes like the 737, it's not even true on approach.  You use pitch to stay on glidepath and power to control airspeed.  There's not really a circumstance in the entire normal envelope of an airliner where power is for altitude and pitch is for airspeed, possibly excepting a balloon or bounce landing recovery. 

All of this requires understanding that, in any airplane or circumstance, a long - term change to one parameter will require a corresponding change to the other. They're pretty inextricably linked.  But for small, short term adjustments, this is how large aircraft are flown - power for speed, pitch for altitude. 

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Posted (edited)

Hi Andrew,

13 hours ago, Stearmandriver said:

1.  A previous explanation said we trim for pitch attitude, not airspeed.  This isn't really true.  An airplane trimmed for a particular airspeed will pitch up or down as necessary to maintain that airspeed, so I've always found it easier as a teaching method to say that we DO trim for airspeed.  Try this: in any airplane from a Cessna to a 737, trim for level flight at cruise power, then pull the power to idle.  What you'll see is the plane pitch down and start to descend, roughly at the same airspeed you trimmed it for (I say roughly because with underslung wing mounted engines like the 737, every power change somewhat changes the speed you're "trimmed out" for unless you adjust the trim again.)

You are indeed aerodynamically correct, and if it works for you I'm certainly not going to contradict you :). However, I find it is usually easier for students (FS) to get their head around the idea that they are looking for a particular "picture" out of the windscreen in any given situation.

Thus, when we level off from a climb we select and hold the level flight attitude -- reduce power to the normal cruising setting -- and trim to hold that picture. Power + attitude = performance, thus if we have set the correct power and the correct attitude the airspeed must by definition come along to join the party.

Likewise any other manoeuvre (powered descent/glide descent/climb etc) and or/airspeed will have an associated pitch attitude and power setting. Again, P-A-T rules -- select the power required, select the desired pitch attitude and trim out the control forces.

As I say, I personally find that an easier concept to get across and it gets the stude's eyes outside rather than overly staring at the ASI. However, as I say you are absolutely right from an aerodynamic point of view and this is why I always say people over-think the B777 trim system -- all Boeing did was program a computer to replicate how a conventional aircraft feels.

13 hours ago, FalconAF said:

Amazing.  The above is why I DO make "information correction" type posts in forums.

It's been over 24 hours since I posted the above.  And not ONE PERSON has questioned it.

The quoted part is "erroneous information" (that's what an Instructor (teacher) would call information that is FALSE).

The correct information concerning flying an aircraft is "elevator controls AIRSPEED and throttle controls ALTITUDE".  And that confuses the heck out of most new student pilots because it is totally backwards from what you use the "throttle" (gas pedal) in a car to do...increase or decrease speed.

Andrew has covered this -- but as you (I am sure) know, power for height/pitch for airspeed is drummed in at PPL level because you really, really don't want Bloggs in his small, draggy, low-inertia spam-can to respond to getting low on approach by yanking back on the stick. My general rule of thumb in the cruise is that for altitude changes of less than about 100-150 ft leave the power alone -- any more than that is likely to require a power change.

However, as Andrew says, when it comes to instrument flying, and particularly large slippery jets with slow thrust response, big thrust-pitch couples and lots of inertia, it is both much more effective and a lot easier to pitch for the glidepath and use thrust for airspeed - with the caveat, as already mentioned, that in the long term an adjustment to the other will likely be required also.

Edited by skelsey

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13 hours ago, FalconAF said:

And nobody here questioned it.

Amazing

 

THAT particular battle is a very old one and many (most) of us know of it.

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Posted (edited)
15 hours ago, FalconAF said:

Amazing.  The above is why I DO make "information correction" type posts in forums.

It's been over 24 hours since I posted the above.  And not ONE PERSON has questioned it.

The quoted part is "erroneous information" (that's what an Instructor (teacher) would call information that is FALSE).

The correct information concerning flying an aircraft is "elevator controls AIRSPEED and throttle controls ALTITUDE".  And that confuses the heck out of most new student pilots because it is totally backwards from what you use the "throttle" (gas pedal) in a car to do...increase or decrease speed.

And nobody here questioned it.

Amazing

 

Rick,

What is amazing is that you deliberately posted misleading information to see who noticed it. It might be a useful rhetorical trick in a classroom situation, but it's not at all helpful in a forum. Some might read it and take it as definitive. Some might have read what they expected to see, not what you actually wrote. I only noticed the post when you quoted yourself to prove an empty point.

Edited by kevinh

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Posted (edited)

It's not an empty point, Kevin.  And yes...I did it on purpose.  Because when someone asks a question in a forum like this, if ANYBODY is going to try to answer the question, the forum DOES become "a classroom" where learning can hopefully take place.

It sure did expand the number and type of replies to the original question...didn't it?  And addressed it in the many different ways it might be answered, which was one of the things I mentioned in my original post about different pilots having different opinions about it.

NOW, the original OP has a much more in-depth understanding (or capability to understand) the answer(s) to their question.

Press on.

Edited by FalconAF

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Posted (edited)

K. Zip it back up guys...before I ship you all copies of Stick and Rudder and Nerf bats...

...and lock this thread.

Edited by scandinavian13
  • Like 1

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Sick and Rudder, meh.  Let's all get "Conquest of Lines and Symmetry" and debate the relative merits of leading pulls, bottom rudder on roll entries etc ;-). 

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Posted (edited)
6 hours ago, kevinh said:

dRick,

What is amazing is that you deliberately posted misleading information to see who noticed it. It might be a useful rhetorical trick in a classroom situation, but it's not at all helpful in a forum. Some might read it and take it as definitive. Some might have read what they expected to see, not what you actually wrote. I only noticed the post when you quoted yourself to prove an empty point.

Easy to be confused as I am still learning about Trim. At the moment I figured the Elevator is the stick from my Joystick and it controls the airspeed and the Throttle controls the Altitude. I think if I am not wrong, trimming is very much similar as Throttle applying power for altitude. Any corrections for me?

Edited by trisho0
corrections added

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6 hours ago, scandinavian13 said:

K. Zip it back up guys...before I ship you all copies of Stick and Rudder and Nerf bats...

...and lock this thread.

Please, don't lock this thread is very interesting and sure somebody else is learning as I am doing here. I don't see Trimming lessons from the PMDG manual.

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5 minutes ago, trisho0 said:

Easy to be confused as I am still learning about Trim. At the moment I figure the Elevator is the stick from my Joystick and controls the airspeed and the Throttle controls the Altitude. I think if I am not wrong, trimming is very much similar as Throttle applying power for altitude. Any corrections for me?

There are many good sources of aviation basics available.  If you find the AIM too technical then I'm not sure where you are in terms of education and background.  The AIM is a very basic document orientated towards the student pilot studying for a private pilots license.  Give it a look.

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6 minutes ago, trisho0 said:

trimming is very much similar as Throttle applying power for altitude. Any corrections for me?

No. I think you're getting very confused :).

Did you try the exercise I suggested in my earlier post?

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