737NG tendancy to roll back to wings level

[herculesthepilot]

Hi Guys,

how stable does the 737NG hold a given bank angle?

Do i need to give inputs to hold the bank, or is the bank angle stable?

I wonder if there could be a tendancy to increase the bank (unstable).

It would be interesting if there are differences in bank stability between clean config and flap config.

Any infos would be much appreciated

[BARKINGMAD]

Lesson 1 Effects of Controls: Select, Hold, Adjust.

Why NG as opposed to any other non-FBW swept wing, low wing-mounted with dihedral airframe?

I don't think Mr Boeing would release to the great unwashed a 'frame which increased bank after the initial application and neutralising of roll control inputs?

Clean, in theory the roll control has better authority than with flap, due to the spanwise distribution of lift with flap/slats deployed, but the roll control deflections are varied according to speed with inboard spoilers assisting roll at higher speeds.

All this extracted from my ancient brain after a few hours on the type, but doubtless someone with a better brain and/or access to the manuals will give more detail?

Still wondering why the question.......................:confused:

[herculesthepilot]

thanks for the reply.

The Reason for the question is easy. We are developing an aerodynamic flight model for a semi professional simulator.

The current model does not hold the bank, and you need permanent small inputs in turn direction to avoid an unwanted angle decreasing.

We got informations that below 28 degrees bank, the NG has slightly tendency to roll back to wings level.

Above 28 degrees there should be a slightly tendency to increase the bank and you need small inputs to avoid the increasing.

[herculesthepilot]

Hey.... no further ideas?

Is it correct that the tendency to roll back über 25 degrees exist and above 2t degrres the the plane tends a little bit to increase the bank.

Any kind of info would be very helpful.

[Lord Spandex Masher]

Most swept wing aircraft have inherent roll stability.

Secondary effect of roll is sideslip. Sideslip increases the effective span of a swept wing and, therefore, it produces more lift and rolls back towards wings level.

So, yes, you will have to hold a small amount of roll input to maintain X angle of bank.

But bear in mind that in a climbing turn you will require out of turn aileron to maintain X angle of bank.

Welcome to the can of worms that is aerodynamics.

[Tee Emm]

But bear in mind that in a climbing turn you will require out of turn aileron to maintain X angle of bank.


Have never experienced this in a 737 simulator. If you select CWS on autopilot and make a climbing turn at say 30 degrees angle of bank the control wheel stays centred throughout the climbing turn.

[Lord Spandex Masher]

TM, if you roll to 30 degrees and then only pitch you'll eventually end up pointing the other way with 120 degrees of bank on.

The only way to hold the initial bank angle is to roll out of the turn as your heading changes.

[herculesthepilot]

Thanks For the reply.

I more interesting in the practical experiences.

In theory, the tendency to roll back to wings level is always there.

But i did a ride on a full flight sim, and i experienced the opposite. The aircraft remained relative stable in the bank condition without any input from the ailerons.

I tried to reproduce that with cockpit videos, abd i could see the same behavior. More or less stable bank angles

Often i observed in the videos that the pilots gave small aileron inputs to avoid higher bank angles.

[Jwscud]

On T/O at takeoff flap settings (1/5), there is a definite tendency to roll wings level requiring a small continuous into turn aileron input. This is climbing at V2+20 in takeoff config.

[AirRabbit]

While I’m not a “nitty-gritty” aerodynamicist I think its accurate to say that because airplanes operate in the real world, they are subject to nicks, dings, warps, stretches, wear ‘n’ tear, and the like … meaning that small things can, and often do, generate their own individual inputs that may not be as apparent as some other larger anomalies. Also, any cockpit video is at least somewhat incomplete unless it includes visuals on all 3 of the control “appliers” in the cockpit – column (pitch), wheel (bank), and rudder pedal (yaw) – including some sort of readable scale - and it’s a lot harder to get good comparative video of the rudder pedals simply due to their location.

Two things about the "data" upon which the "aero-model" is based…
1) airplane data should be the primary contributor to the aero-programming (and it is usually supplemented with a good understanding - or reference materials - as to what was intended to be the aerodynamic result) and…
2) a well-qualified and experienced pilot’s input regarding nuances can be invaluable.

And the final recognition should always be that a simulator (the best kind of simulator, included) is not the airplane. Any training conducted in that simulator should be conducted by someone with considerable knowledge of that airplane AND that (specific) simulator - with particular emphasis on what THAT simulator does, does well, and does not-so-well. Or, said differently, un-supervised "practice" or other use of the simulator, should be very heavily scrutinized with a full understanding of the influence that can be generated by that very powerful training tool (i.e., the simulator).

Getting someone to the point of being able to fly the simulator well is NOT and should NOT BE, the goal of training. We are teaching pilots to fly the AIRPLANE ... and the simulator is a tool to help do that ... it should be acknowledged as being a viable, incredibly capable tool, and much more so when used to its full capability, but, we MUST recognize that it can generate negative issues when it is depended upon to impart the knowledge instead of assisting the instructor in doing that job.

[Centaurus]

TM, if you roll to 30 degrees and then only pitch you'll eventually end up pointing the other way with 120 degrees of bank on.
]

You may have misunderstood the use of the specific CWS mode in the 737 autopilot system. With CWS engaged the pilot controls the aircraft manually through the AP system. If you roll to 30 degrees angle of bank in CWS mode and the pitch attitude is correct, then on releasing the wheel the aircraft maintains 30 degrees angle of bank.

Extract from the FCOM as follows: "With CWS engaged, the A/P maneuvers the airplane in response to control pressures applied by either pilot. The control pressure is similar to that required for manual flight. When control pressure is released the A/P holds existing attitude".

[Jwscud]

AirRabbit - in the 737, the control wheel position *should* be the only reference you need for flight crew inputs as use of the rudder in turns in a normal configuration is strongly advised against by Mr. Boeing as the Yaw Damper keeps the aircraft co-ordinated.

There is no way for flight crew to see or feel the Yaw Damper inputs, though I imagine it is recorded on the QAR and FDR somewhere.

[Lord Spandex Masher]

Yes, but I didn't mention CWS nor did the OP.

Notwithstanding that CWS will require a small amount of out of turn aileron to maintain that bank.

[herculesthepilot]

yes thats all interresting, but what would like or need to know is much more the esperience part from our Bobby riders.

If you are in levelflight with flaps up and you turn with 25 degrees, does the aircraft hold the bank angle, or do you need to you need to use the yoke?

And how is it with 200 knots clean, 15 degrees flaps and 30 degrees flaps?

Thats what need:-)

[AirRabbit]

AirRabbit - in the 737, the control wheel position *should* be the only reference you need for flight crew inputs as use of the rudder in turns in a normal configuration is strongly advised against by Mr. Boeing as the Yaw Damper keeps the aircraft co-ordinated.

There is no way for flight crew to see or feel the Yaw Damper inputs, though I imagine it is recorded on the QAR and FDR somewhere.

I think you may have been just slightly misinformed ... while I agree that there should be no need to use rudder (or very much rudder) during turns … but that is due to the effectiveness of the roll-control spoilers, not the yaw damper. The yaw damper is there to positively damp combined lateral-directional oscillations ... meaning that if a "Dutch Roll" occurs, the aircraft will gradually stop oscillating. It is not required for dispatch and is there primarily for passenger comfort. The yaw damper moves the rudder a maximum of 2 to 3 degrees - depending on the airplane series and whether the flaps are up or down. You are correct that yaw damper inputs are not fed back into the rudder pedals, but prior to 2010 a yaw damper indicator was installed and available for crew reference, and I know crew members who were and some who were not able to "feel" the yaw damper when it was involved - and I don't mean through the rudder pedals.

[Lord Spandex Masher]

If you are in levelflight with flaps up and you turn with 25 degrees, does the aircraft hold the bank angle,

No.

or do you need to you need to use the yoke?

Yes.

And how is it with 200 knots clean, 15 degrees flaps and 30 degrees flaps?

The same but you won't be doing 200kts with flap 30.

[herculesthepilot]

@Lord; thanks a lot... that's what i wanted to know.

Of course Flaps 30 with 200 knots is not a good idea :-)

Can you describe me whether the needed inputs to stabilise the bank angle are strong or just small short ones?

In other words, how fast tends the plane to go back to wings level from 25 degrees bank?

It would be a big present, if you are able to tell me that in degrees/ second.


Thanks again.

[underfire]

Will let LSM answer, but in the meantime....

I would also help if you gave the specific NG variant, they do have different airfoils. (and winglets may have some effect on rate at 200 kts)

Many, many variables associated with roll rate. Altitude, AP engaged/not engaged, winds, aileron trim, fuel load, flap settings, LE, speed brakes, etc...

In the sim, noted a much different world, yoke loading, and far too much input required for elevator and aileron trim than the real ac...

[John Farley]

Lateral stability and Spiral stability.

These two topics are easy to get muddled up.

So if I may a bit of non-type specific comment which I think may help this specific thread.

Lateral stability – kick Right rudder under any conditions of flight and if the aircraft rolls to the Right it has positive lateral stability and if it rolls Left it has negative lateral stability. This of course is the aircraft’s dihedral effect in action.

Spiral stability – put the aircraft in a bank and release the controls. If the aircraft slowly but surely increases its bank (it can take quite a while) then it is spirally unstable. If it tends to roll out then of course it is spirally stable.

But (and it is a very big but) with many aircraft one can demonstrate that the aircraft is both spirally stable and spirally unstable. How come? Well the spiral stability term is usually very weak, very weak indeed. This means that the slightest bit of residual aileron left when the controls are released will determine whether the aircraft rolls out or in to the spiral. So in the absence of a cockpit indicator that shows aileron angle allowing one to acurately set zero aileron one can be misled by the aircraft response.

Therefore the aileron control run sticktion, friction and breakout characteristics are very important in this context. This means that if you want to confuse your know-all mates just roll into a bank Left but finish with a tiny Right input as you release the controls and chances are the small residual Right aileron will stick on and cause the aircraft to roll out (spirally stable) then repeat the demo but release the controls without the touch of right aileron at the end as before and the opposite will happen.

Good for a beer.

[Aluminium shuffler]

Without getting involved in the science behind it, I have long since noticed that the aircraft wants to increase its bank angle at higher AOB, needing a small corrective aileron input, but shows no distinct tendency at lower AOB. Specific thresholds I have no idea of, but circa 28deg seems about right. One thing I can say with absolute certainty is that at low AOB, it is more stable than the Classic, but only in zero-winds; as soon as you get the lightest winds or turbulence, it's much less stable than the older variants. I assume it has a much lighter wing loading, and the tail empennages are much bigger, so the larger sail areas are the likely causes. Winglets don't seem to affect stability, they just make x-wind landings a bit trickier because they increase float in the flare while also increasing side sail area. I wouldn't say the NG was especially stable - it needs a lot of corrective inputs when flown manually, especially in even slightly windy conditions.

[herculesthepilot]

@underfire: The specific type is a 738WL.

i am aware of the fact that i never will reach the realism of the real bird Thats simply impossible, because real is real and simulated is simulated.

The simulation platform from Lockhead Martin has also its limitations, so i try to get as close as possible.

The Project is privat and not Commercial, so the possibilities are limited


Lets say the aircraft has a gw of 60 tons and the weather is CAVOK, no wind.
No speedbrakes, no AP.

Intersting would be the following flight circumstances

250 knots FL 200
300 knots FL 300
220 knots FL 100
Minimum Clean Speed 4000 feet
FLAPS 1 with corespondenting maneuvering Speed
Flaps 15 Gear down
Flaps 30 Gear down

Its much apreciated :-) Sorry fot my english, i amware...that it is not the best..



Eveybody is invited to make a sim check::-)

[herculesthepilot]

@John
Thats very interesting indeed Its makes sense for me .

Will try your examples in the sim.

I would expect that the ailerons are quite precise and they goes perfect back to zero without active yoke input!?

If not, it makes the whole think much more complicated, because if the ailerons are not going to zero with Hands of the yoke, its clear that may the AOB increases slowly.

But much more interesting is the feel during turning. How long can i left the yoke in neutral position and look into the blue eyes of my stewardess without decreasing or increasing the AOB.

In a car i know very well that i have to rotate the wheel as Long as i want to turn the car. If i take the hand from the wheel the car drives forward.
I know....its a bad example....

[underfire]

blue eyed stewardess in the sim cockpit....no wonder you are spending so much time there! :D

[Aluminium shuffler]

Herc, while the controls will return to their trimmed position when "hands off", that doesn't necessarily mean neutral. The primary controls all have trim inputs, and the ailerons and elevator are trimmed by the autopilot when that's engaged, so your trim settings will be altered by the next time you manually control the aircraft. Add that each aircraft rolls of the assembly line with small variations in rigging angles, and how much their control balance is affected by wear and tear on the line and in maintenance, and you soon find that each individual airframe behaves differently. Some need tweaking of the ail/rudder trims for every change in thrust, speed or flap setting, while others need very little. They are not as predictable and uniform as a car model, even when new.

[Jwscud]

Aluminium shuffler - the NG autopilot does NOT trim the ailerons. In fact any use of aileron trim in conjunction with the AP is forbidden in the limitations section of FCOM 1.

[Aluminium shuffler]

Yep, don't know why I put that down - put it down to a brain fart. The ailerons are almost always slightly out of trim on AP disconnect due to variations in fuel tank levels and flap selection. The limitation on ail trim with the AP engaged is just because adjusting that trim will cause AP disconnection, just like elevator trim.

I'm not sure of the logic behind not having the AP trim the ailerons, but I suspect it's so that it doesn't hide things like fuel imbalance or high-lift device asymmetry. It did catch out a BA crew who had a big fuel imbalance, and when the AP had enough the aircraft rolled hard. Had the AP trimmed the ailerons, that wouldn't have happened, but maybe something worse would have resulted later.

[Jwscud]

I believe the limitation is to avoid excessive out of trim forces catching a pilot unaware on autopilot disconnection. Control wheel displacement is then a obvious clue that something is not right.

[Derfred]

Correct. Aileron trim does not cause AP disconnection.

You will get a very nasty shock when you disconnect the AP though.

[enicalyth]

I like visiting places, so where is it? Only asking... I was never any good at speeling which is why I moved from Sidney to Birsane, Bribsune, stuff it I come from Cairns.

Hat, coat...

[zzuf]

Can you get into the sim equations of motion?
I would be looking at derivative "rolling moment due to yaw rate".

[Virtus]

Most swept wing aircraft have inherent roll stability.

Secondary effect of roll is sideslip. Sideslip increases the effective span of a swept wing and, therefore, it produces more lift and rolls back towards wings level.

So, yes, you will have to hold a small amount of roll input to maintain X angle of bank.

But bear in mind that in a climbing turn you will require out of turn aileron to maintain X angle of bank.

Welcome to the can of worms that is aerodynamics.

LSM... the yaw damper keeps the airplane coordinated under normal flying maneuvers. The yaw damper prevents sideslip from developing. If any airplane is banked into a coordinated turn, the airplane will increase its bank angle due to the outside wing traveling faster. This is regardless of the airplane having anhedral or dihedral as those are only 'effective' if the airplane is in a slip.

John Farley pointed out a small factor that might make you think this is incorrect in real life, but it's because of the residual aileron if the airplane rolls to wings level.

[zzuf]

Yaw dampers are fitted to damp dutch roll oscillations which may be greater than that permitted by the certification standard.
If by "coordinated" you mean zero sideslip, then that is the job of the fin.
As John Farley wrote, the spiral mode is weak and may be either positive or negative. It may change sign with configuration or altitude.
A simple way to look at the spiral mode would be if directional stability is dominant, then the sideslip resulting from bank would cause the aircraft to "weathercock" into wind, the outside wing lift increases and the bank angle further increases - spiral dive.
If lateral static stabilty (dihederal effect) dominates then the sideslip caused by bank results in a roll away from the direction of sideslip, reducing the bank angle.
It is a fine tuning job to achieve the required levels of static and oscillatory stability, often poor oscillatory stability is the result and this is fixed with a yaw damper.
Irrespective of entering a coordinated turn, one has to release the controls to view the behavior under the forces resulting from the various stabilities

[Virtus]

If by "coordinated" you mean zero sideslip, then that is the job of the fin.

Coordination is the job of the rudder... not the fin.

[zzuf]

Fair enough....
To me rudder is:
1. A means of controlling yaw rate independent of bank angle, and
2. A means of selecting a sideslip angle which is different to current control free sideslip angle.