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Posted

But why does the pilot do not have to trim to keep his target vs of 1000ft/min? That's confusing me a little.

.

 

That is basic flying....aerodynamics.... and works the same for a C172 and a B777.

 

Ok, back to the basics.

Why do we have to trim in the first place?

Why does the aircraft not just stay where I point it to?

 

Because we have an elevator......well kind off...

Without an elevator the nose of the aircraft would allways point down and you would crash.

The reason for this is that the type of aircraft we fly are designed with their center of lift behind their center of gravity.

Huh?

Ok you want video.....I found this....just look at the first 2 1/2 minutes...the rest does not matter for today :-)

http://www.youtube.com/watch?v=P_0R0eWOKns

 

(ps....that guy is not me)

 

 

 

 

Ok so you now know the elevator is trying to oppose that momentum that is generated by the Cl being behind the Cg.

At every airspeed there is only one exact elevator position/angle that exactly opposes this momentum (in the 777 only one STAB position because the whole stabilizer can move unlike in your Cessna)

To much elevator in either direction and the nose goes up or down.

 

When you change speed the aerodynamic force on the elevator increases or decreases and now the balance is lost.

You need to change elevator position to again find that one and only perfect position for your new airspeed.

 

The elevator trim (or STAB) trim holds that perfect balanced elevator (or STAB) position for you because holding that for 10 hours with the yoke gets boring!

This is done mechanically in a C172 and by computers and hydraulic systems on the 777.

 

So to sum it up:

As airspeed changes, elevator (or STAB) tail down force changes and if we want to maintain our flight path, we have to adjust elevator. And then apply trim so you can let go of the yoke.

 

So now imagine you are in a climb of 1000ft/min.....

You have pulled the nose up to a pitch that resulted in those 1000ft/min.

Thrust has increased automatically for you in the 777 (in a C172 you must do it yourself but it is the same) so you are still at that constant 250kt.

Right?

The airplane does not care....level....or up.....or down.....250kt is 250kt.

250kt horizontally makes the same amount of air molecules go over your wing as 250kt in a climb or 250kt in a descend.

And thus the same elevator force and position (trim) is required to keep the nose balanced.....to oppose the momentum caused by Cl being behind the Cg.

So aerodynamically everything is the same, elevator/stabilizer position same same. Except that you have more thrust set.

 

All you are doing to climb at 1000ft/min is putting the aircaft in another attitude in space with more thrust:

- disturb the trim balance by pulling on the yoke. This causes a nose up momentum.

- As vertical speed increases, thrust must be added to keep the airspeed constant.

- once the correct pitch is established for 1000ft/min you can let go of the yoke.

- the yoke is now neutral again and the elevator is now back at the previous position.

- Your flight path is now a climb at the same airspeed and as long as no external factors mess things up you will not have to touch the yoke again. (but they allways do...wind/temperature/turbulence/the flight attendant pushing trollys from the back to the front, etc)

 

Yes?

Rob Robson

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Posted

Very good explanation Rob!

 

Problem is when we start looking at all of the variants, like aircraft with neutral pitch stability ( like many ww2 fighters and some modern high performance aircraft )....

 

 

But basically I think you have been able to nail the reasons why we need a horizontal satb in the first place, and the elevator too :-)

 

I also always tend to suggest a good reading of:

 

http://www.av8n.com/how/

 

and in particular in this case, this chapter. as well as this section

Main Simulation Rig:

Ryzen 5600x, 32GB RAM, Nvidia RTX 3060 Ti, 1 TB & 500 GB M.2 nvme drives, Win11.

Lenovo TB310FU 9,5" Tablet for Navigraph and some available external FMCs or AVITABs

Posted

Nice explanation Rob, i initally thouht that yoi trim to release force but for maintainning the desired Flight path too, but I learnt that you maintain flight path by adding more or less thrust for a given speed.

 

the thing I don't understand, is why boeing didn't implement a standard trim system lijke a cessna/B737, when you trim you only move the THS ( not the elevator), because the trim reference speed is not convenient I think but that's my opinion.

  • Commercial Member
Posted

 

 


the thing I don't understand, is why boeing didn't implement a standard trim system lijke a cessna/B737, when you trim you only move the THS ( not the elevator), because the trim reference speed is not convenient I think but that's my opinion.

 

 

...but it does!  You're just overthinking the issue.  It's still the same, exact, fundamental pitch and power discussion.  The very fact that there's a trim reference speed only serves to illustrate that point.

 

Why?  Every PPL student is taught what equation?

 

Pitch + Power = ____?

Airspeed.

 

What controls your pitch?

The flappy thingy in back (elevators, and therefore also trim).

 

What controls your power?

The thrust lever.

 

So, for trim - regardless of the aircraft - you're essentially setting a reference speed with the combination of your pitch device (elevators) and your power device (powerplant).  Excess power at that trim setting will result in you climbing, a lack of power will result in your descending.  This, all at the same trim setting, and therefore same speed.

 

Stick and Rudder should really be required reading for anyone who is serious about handling aircraft - be it simulated or real.

 

Exact.  Same.  Concept.

 

The only difference is that there are layers of convenience added on top (aircraft automatically trims for gear, flaps, etc).  While I'm glad it's helping to really expose a discussion on trim in general, I don't think it's truly doing anyone who needs said discussion a favor, since they're compartmentalizing it: they're thinking "this is a 777-specific discussion," when it's much, much, much broader.

 

 

 

 

Final point:

You're in a glider.  Since a glider doesn't have a power part to the pitch + power = airspeed, this situation is easier to visualize.  So, without power, you trim for ____?

  1. Altitude?
  2. Vertical speed?
  3. Airspeed?

Answer: 3. Airspeed.

 

Same thing in the 777.

Kyle Rodgers

Posted

yeah, thanks Kyle for your explanation

 

What I meant is that even if at the end , trimming the B777 is like trimming a B737, while trimming it won't behave the same :

 

for example, lets assume that we are flying level at 210 kts and I want to reduce it to 180 kts while still flying level :  (AT ON)

 

with the NGX :

 

i will just gently trim up as long as speed is decreasing in order to maintain V/S =0, so with the NGX it's easy to maintain LEVEL Flight in speed change without giving any input on the yoke

 

With the PMDG B777, it's a different story

 

again i will gently trim up, but because of the blid trim, the trim will go back to the current aircraft airspeed, so i have you trim more severly in order to avoid the blid trim to go into action, but the speed is not decreasing fast enough I will have to give input ( forward input as the trim speed reference will pitch up to target the trimmed speed, as I want to maintain level, need to push the yoke ). So i'm having a hardtime controlling FLIGHT PATH in speed changes. 

 

i don't how the blip trim enters into action in the real b777, but with PMDG, when your TRS is within 5 kts of your curent airpseed when you press the trim button, the blip trim enters into action, it will be better for example to give 2 trim tap within 300 ms to make the blip trim enters into action.

 

hope now you understand what i mean :)

Posted

I also always tend to suggest a good reading of:

 

http://www.av8n.com/how/

 

and in particular in this case, this chapter. as well as this section

 

 

Thanks for the links, 

 

 

 

  • Push/pull motion of the yoke can be viewed as an extension of the trim wheel — just another way of controlling angle of attack. It is very difficult to stall the airplane unless you pull back on the yoke and/or apply lots of nose-up trim. This idea could save your neck.

     

 

Funny, I have always been thinking the other way around, trim (wheel) beeing the extension of the elevators / yoke.

  • Commercial Member
Posted

 

 


i don't how the blip trim enters into action in the real b777, but with PMDG, when your TRS is within 5 kts of your curent airpseed when you press the trim button, the blip trim enters into action, it will be better for example to give 2 trim tap within 300 ms to make the blip trim enters into action.

 

Blip trim was added from the real plane, using the real function.  Again, I think you're overthinking it.  The reason to use fine trim is to finely set it to the current speed.  With blip trim, it removes the guesswork in finding that sweet spot.

 

To me, it seems like you're just not used to it and therefore don't like it.

 

...then again, the FBW has noted issues in its current state, so that could be the issue.  This should be easier after SP1b is released.

Kyle Rodgers

Posted

...then again, the FBW has noted issues in its current state, so that could be the issue. This should be easier after SP1b is released.

Easier is good :-)

 

I just posted a thread in which I described how recalibrating my yoke in Windows seems to have helped me a lot in regard to trimming.

 

Here:

 

http://forum.avsim.net/topic/448466-fixed-unresponsive-fbw-trim/

 

So far nobody has looked at it, lol, but I really think it could be something.

 

I could swear that on my last two flights the FBW trim speed worked better than ever!

* Set the trim reference speed to 320 kts, but fly at 210 kts

* To fly straight and level will require a lot of back pressure (I think you will have sufficient elevator authority to do this)

* Holding straight and level with the control column only (stick back), hold it there and trim nose up

* As you trim, do ***** NOT ***** move the stick to center! Hold it where it was the entire time

 

Does the aircraft pitch up? For me, it does NOT.

.

Robin, can you give my calibration idea a go?

 

Just re-calibrate your stick with the basic Windows game controller calibration tool and then try your exercise again.

 

I could swear that after recalibration I could feel out of trim forces dissappear while trimming. Maybe not perfect but a lot better as what I had before.....I just need to do more flights to confirm (and I am not at home) so maybe you can give it a shot in the mean time?

Rob Robson

Posted

Hi Rob. I will give it a go.

 

Can you also share your calibration settings in FSX? I'm asking because I don't know which settings are realistic.

(e.g. My sensitivity axes are almost full right and my null zones are all full left.)

Posted

Hi Rob. I will give it a go.

 

Can you also share your calibration settings in FSX? I'm asking because I don't know which settings are realistic.

(e.g. My sensitivity axes are almost full right and my null zones are all full left.)

It will depend on your joystick, unless you have the same as Rob

Posted

It will depend on your joystick, unless you have the same as Rob

 

I have the T.Flight Hotas X from Thrustmaster. If I remember correctly, Luke used or is still using the same joystick.

Posted

@ 777simmer

 

How do you trim when changing speed :

 

let's say you want to reduce 210 kts to 180kts while flying level, will you , :

 

* 1 press the trim button for 3 sec ( trim reference speed is now set to 180kts )

 

* 2 press gradually the trim buttton until you get 180 sec.

 

because if i do 1, the plane will severly pitch up to reach the new trim reference speed, so I need to give lots of forward input to maintain the A/C level

 

with 2 it's hard because the blip trim enters in action each time so the trim reference speed barely moves and as the plane is decelerating, the A/C pitch down severly so i have to keep back pressure.

 

I found it to be more difficult to trim regarding with the NGX.

 

With the NGX when decelerating to 180ktsd from 210 kts, i just often tap the trim switch and I barely give input with the yoke to maintain level path....

 

CMB

 

EDIT :

 

By switching off the primary Flight Control ( PFC) thus in direct law, i found that trimming is easier jsut like the NGX


I have the T.Flight Hotas X from Thrustmaster. If I remember correctly, Luke used or is still using the same joystick.

 

What a coincidence, I have the same one :)

 

If you find a setting that works realistically please share with me :)

Posted

Hi Rob. I will give it a go.

 

Can you also share your calibration settings in FSX? I'm asking because I don't know which settings are realistic.

(e.g. My sensitivity axes are almost full right and my null zones are all full left.)

We are talking about the same I hope right?

 

I mean calibrate your stick in Windows.....with the windows game controller tool (under control panel) or with the software provided with your hardware.

I dont have any software for my 20 year old yoke so I use the Windows game controller tool to calibrate.

neutral - click a button

full left/right/up/down - click

that stuff.

 

In FSX I have my sensitiveity to max, nullzones min and thereafter I disable the FSX joystick setting (no checkmark in FSX for enable).

So I dont think these settings matter at all for me, but just in case, I have set them that way.

 

Ann then I use FSUIPC to detect my yoke axis.

Like "axis elevator set" or "axis aileron set" and click sent DIRECT to FSX.

 

So neither FSX nor FSUIPC is doing any calibrating.

Only the Windows tool calibrates the way I use things.

Rob Robson

  • Commercial Member
Posted

 

 

Problem is when we start looking at all of the variants, like aircraft with neutral pitch stability

 

Doesn't affect the basic principle. I'll bet you don't worry too much about positive/neutral/negative spiral stability do you?

 

The tailplane in a conventionally rigged aircraft is a lifting device like anything else. The "lift" produced by the tailplane pulls the tail down (the CoG on a conventional aircraft is ahead of the center of pressure and would result in nose drop if not corrected).

 

Think of the fuselage as a see-saw. Without the tail to balance the forces, the aircraft will simply pitch down. The tail provides a force in order to prevent the nose from dropping.

 

Regardless of whether we are talking about elevators, stabilators, canards, deltas, etc.., if you change a parameter (airspeed) then the lift generated will change, the forces required to maintain a given pitch attitude will change, and so you must re-trim the control forces for the new speed. This applies to any aircraft, regardless of how it is achieved.

 

Pitch stability (actually, longitudinal static stability) refers to how an aircraft will react if it encounters a pitch down disturbance.

 

Positive: it will pitch up with increasing airspeed

Neutral: it will neither pitch up nor down with increasing airspeed

Negative: it will pitch down even more with increasing airspeed.

 

Modern airliners are rigged to be neutral at worst, with most having some degree of positive longtitudinal stability. Negative stability is a feature of fighter aircraft, and require flight control computers to give the aircraft stability, otherwise it would be impossible for a human pilot to fly.

 

Best regards,

Robin.

  • Commercial Member
Posted

I found an issue with the FBW and trim. I'm going to try and bullet point this to make it as clear to understand as possible.

 

* Aircraft is OUT OF TRIM, and "nose heavy".

* I pull back on the control column to counter the out of trim condition

* Once I stabilized the pitch, I start trimming nose up, and returning the control column to center at the same time (same as on the NGX).

 

PROBLEM: the trim reference speed changes, but the trim is NOT applied until I return the control column to dead center, at which point ALL the trim change I made is applied at once, and if I got the trim wrong, the aircraft would then aggressively pitch relative to that out of trim condition.

 

The result is that I can't trim the aircraft by feel. If I don't have the trim reference speed displayed, it is virtually impossible to trim.

 

Seperately, the trim often either doesn't move, or even moves in the opposite direction to that selected.

 

Anyone else see this?

 

Best regards,

Robin.

 

I'm seeing exactly this, but thought it was common knowledge and addressed in SP1B..

 

The trim ref speed only is active when the joystick is in dead centre.. As soon as I apply any elevator, I feel no trim at all. (i can be 100knots out )

 

Alex

Alex Ridge

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