Aircraft returning to level bank

[JoeHallenbeck]

Hello developers!

As I'm flying in turn for example 15 degrees level bank turn I have to keep ruder deflected in respect of turn direction to keep aircraft banked. Is this behavior normal? Shouldn't aircraft keep banked in level flight when I will bank it into certain angle?

As long as I will release rudder aircraft starts to return to level flight with no bank angle. This happens even in straight level flight.

Wrong moments of internia? Any thoughts would be appreciated, thanks.

Joe

 

[lionheart]

Hey Joe,

Some planes do this, and some dont. It sounds like your Template has that built into the airfiles.

You can adjust Roll and Yaw as well as Pitch in the AIR file in section 1101. There are inputs for amount of pitch and yaw that occur in a bank/turn.

Bill

 

[JoeHallenbeck]

Hello Bill

Thanks for your observation.

I would accept it in light type of aircraft with almost stable construction. In my case it is 6 tonnes of aluminum and steel as a fighter jet.

I forgot to mention that weather is clear no winds standard pressure.

I'll check your suggestions and try to find way to damp it to less value so it will be returning to level position but less visible.

Thanks

 

[Roy Holmes]

Joe,
What you are seeing is too much dihedral effect. First thing I would do is make sure you have the geometry correct in the config file. It is so easy to ignore the fact that the config file has the biggest effect on flight dynamics and miss some small entry error.

I have made that mistake many times.

Roy

 

[lionheart]

If you can get ahold of ACM (Aircraft Container Manager) it will show you your lines and wing/control surface geometry. Awesome tool for setting up your geometry.

 

[fanavion]

What you are seeing is too much dihedral effect. First thing I would do is make sure you have the geometry correct in the config file.

Roy,

dihedral effect (roll moment due to sidesilp) is determined by the coefficient Cl_beta which is stored in the .air file. Using Aircraft Aifile Manager (AAM) you can find it in section 1101 "Primary Aerodynamics" and/or in section 1543 "Roll Derivatives". Please be aware, that if the 15.. sections are present in the .air file the flight model will use data from these and not from the old records in section 1101.

Correcting wing_dihedral in the aircraft.cfg will have no effect, as well as wing_sweep, most htail and vtail data and some more geometrical data. They are all leftovers from earlier versions of MSFS which included FSEdit. This program computed aerodynamic coefficients out of these geometrical data and stored it in the .air files. The FSX SDK itself points to this fact:
[airplane_geometry]
This section has been added mainly for reference. Although you can edit these values by hand here in the aircraft.cfg file, modification of some of these variables will have little to no effect on airplane performance, as the flight model aerodynamic coefficients are all located in the .air file.

BTW the widely used expression dihedral effect is a somewhat misleading name for roll moment due to sideslip. It is the sum of individual effects caused by sideslip. Beside from the wing dihedral or anhedral proper, in a sideslip there is also a rolling effect from placing the wing high or low at the fuselage and from the the fin, to name the most important.

That is why transports with a high fin and shoulder wings often sport negative wing dihedral . Similarily fighters may have straight wings or even negative dihedral to minimise or cancel the roll due to sideslip from the other effects to increase maneuverability.

 

[fanavion]

The FS9 SDK says

[airplane_geometry] section
The airplane [airplane_geometry] section has been added mainly for use with the FSEDIT application and the ability to create a flight model from aircraft geometrical input. Although you can edit these values by hand here in the aircraft.cfg file, modification of SOME of these variables will have little to no effect on airplane performance, as the flight model aerodynamic coefficients are all still located in the [aircraft_title].air file. The FSEDIT program will change the aerodynamic coefficients in the .air file based on the values in this section (and others), but changing the values in the aircraft.cfg file WITHOUT running the FSEDIT application will result in little, if any, noticeable changes in aircraft behavior. Nonetheless, these values have been exposed for your convenience.

 

[fanavion]

As I'm flying in turn for example 15 degrees level bank turn I have to keep ruder deflected in respect of turn direction to keep aircraft banked. Is this behavior normal? Shouldn't aircraft keep banked in level flight when I will bank it into certain angle?
As long as I will release rudder aircraft starts to return to level flight with no bank angle. This happens even in straight level flight.

Some planes do this, and some dont.

Bill,
Real life or simulated aircraft ?

It has always intrigued me that most if no all addon aircraft, commercial or free, in a turn return to wings level vehemently after releasing aileron and rudder control. Much more than I experienced with MSFS default aircraft and with various real life GA Aircraft where it is barely perceptible among constant corrections against misrigging, crosswinds and gusts. A Catalina Pilot two years ago confirmed, that his aircraft keeps the bank of its wings in a turn.

In theory there is required a rudder and aileron deflection even during a turn, which is described in variuos sources.

This is a quote from Perkins / Hage : Airplane Performance, Stability and Control :
"In the perfect turn,... the aileron must always be held against
the turn to balance the rolling moment due to the yawing velocity,
while the rudder must always be held into the tum to balance out the
damping in yaw. These solutions are correct if the deflections of the
ailerons do not introduce yawing moments (adverse yaw). If the
ailerons do introduce yawing moments, the rudder angle required will
be altered. If the ailerons introduce adverse yaw, then the rudder
angle into the turn will be reduced."

Examples of other sources are:
U.S. NAVAL TEST PILOT SCHOOL FLIGHT TEST MANUAL

See How it Flies

Once the aircraft has achieved a steady turn the aileron and rudder angles required are usually quite small.
The requirement for rudder and aileron may differ with the design of an aircraft. It may be stronger in sailplanes with long wings and tails, turning in tight circles in a thermal and almost non-existent in modern jet fighters having short span, short-coupled tail, turning in wide circles. Fuselages with large side aeras cause much side forces (sidewards drag) with a requirement for more rudder into the turn.

This is how it works in real life. There will follow a post discussing the application of these facts to MSFS and JoeHallenbeck´s fighter jet.

 

[WarpD]

The FS9 SDK says

[airplane_geometry] section
The airplane [airplane_geometry] section has been added mainly for use with the FSEDIT application and the ability to create a flight model from aircraft geometrical input. Although you can edit these values by hand here in the aircraft.cfg file, modification of SOME of these variables will have little to no effect on airplane performance, as the flight model aerodynamic coefficients are all still located in the [aircraft_title].air file. The FSEDIT program will change the aerodynamic coefficients in the .air file based on the values in this section (and others), but changing the values in the aircraft.cfg file WITHOUT running the FSEDIT application will result in little, if any, noticeable changes in aircraft behavior. Nonetheless, these values have been exposed for your convenience.

This text is not in the FSX SDK. The FSX SDK entry is:

This section has been added mainly for reference. Although you can edit these values by hand here in the aircraft.cfg file, modification of some of these variables will have little to no effect on airplane performance, as the flight model aerodynamic coefficients are all located in the .air file.

Note FSEdit is no longer discussed, nor is it provided with the FSX SDK. Also note they use the word 'some'. In FS9 they tried to include more support for the aircraft.cfg file and in FSX they included even more. I would be careful about making claims that .cfg file entries don't do anything as in FSX things are completely different. So... something true for FS9 may not be true for FSX.

 

[fanavion]

This text is not in the FSX SDK.

I did not claim this, I wrote The FS9 SDK says. The text from the FSX SDK you quote I have quoted myself in the post above, introduced by The FSX SDK itself points to this fact.

Note FSEdit is no longer discussed, nor is it provided with the FSX SDK.

Neither did I claim this . But former use by FSEdit is an explanation for the unused records in the FS9/FSX aircraft.cfg.

I would be careful about making claims that .cfg file entries don't do anything as in FSX things are completely different.

Nor did I claim this globally. I speak solely of certain geometrical data and gave examples of these. They cannot be processed by the flight physics engine as these properties are reflected in the stabillity and control coefficients (derivatives) recorded in the .air-file.

 

[Roy Holmes]

Carl,
Thank you for your response, got me thinking.

I took a Mooney Bravo (which I never fly so will not miss) got it in the air and worked through its [airplane geometry] data. Very interesting exercise.

Here is the list of what actually does have an effect, by which I mean it will not fly normally if these values are very wrong or zero. Nothing else in that section has an effect on the ability to stay in the air or act in a normal fashion, so they are indeed for reference purposes only

wing_area = 175.0 Big effect, obviously
wing_span = 40 Put it zero and it stays in the air but at a strange attitude.
oswald_efficiency_factor= 0.6 zero efficiency gives zero lift
htail_incidence = 1 affects pitch angle appropriately
elevator_up_limit = 10 all control movements are needed for control
elevator_down_limit =10 0
aileron_up_limit =20.0
aileron_down_limit = 20
rudder_limit = 24.0
elevator_trim_limit = 19.5

You can put all control surface areas to zero and it has no effect, It is the amount of deflection that does something.

So you are quite right in what you say and I stand corrected. Thanks for the insight, I was not expecting it to be like that!

Roy

 

[lionheart]

Carl,
Thank you for your response, got me thinking.

I took a Mooney Bravo (which I never fly so will not miss) got it in the air and worked through its [airplane geometry] data. Very interesting exercise.

Here is the list of what actually does have an effect, by which I mean it will not fly normally if these values are very wrong or zero. Nothing else in that section has an effect on the ability to stay in the air or act in a normal fashion, so they are indeed for reference purposes only

wing_area = 175.0 Big effect, obviously
wing_span = 40 Put it zero and it stays in the air but at a strange attitude.
oswald_efficiency_factor= 0.6 zero efficiency gives zero lift
htail_incidence = 1 affects pitch angle appropriately
elevator_up_limit = 10 all control movements are needed for control
elevator_down_limit =10 0
aileron_up_limit =20.0
aileron_down_limit = 20
rudder_limit = 24.0
elevator_trim_limit = 19.5

You can put all control surface areas to zero and it has no effect, It is the amount of deflection that does something.

So you are quite right in what you say and I stand corrected. Thanks for the insight, I was not expecting it to be like that!

Roy

Dang! That is some very interesting findings. That kind of lets me down per the detail of the reading of the config file. I wonder what all in the config 'is' fake and only for reference.... It would be nice to know for the sake of tuning.

Its funny how when we release a plane, how people will write in asking why such and such a setting is 'something' instead of 'another something' and the email is very 'agitated' or angry and serious. Getting these birds to behave properly and using the exact 'numbers' sometimes just doesnt work...

 

[JoeHallenbeck]

Hello Developers,

It seems that Roy is approaching problem similar as I am. Changing value and than testing testing testing. There is theory and there is practice.

Thats why I was repairing that bloody spin whole summer. I felt like being construction designer employed at Mikojan Gurevich design bureau in 1950s

Concerning what I'm reading here, my long development is right. Some values which are calculated by simulator engine are giving me correct values thus my aircraft is behaving right. So slight returning back from bank angle at turn is correct. As I'm flying in level turn I have to keep rudder really slightly on and aileron too. Concerning other "outside" factors I'm testing always every change in "super conditions" meaning no weather at all standard pressure, no winds, no gusts not a single bird making fuss around.

aircraft.cfg values in airplane geometry gave me also some values right some I had to change in air file, some had to be added to geometry in cfg and there was no other way for example to get wing aspect ratio right than let it be calculated by simulator itself. Documentation of SDK is in my words something like "student notes" on lecture of aircraft design in FSX. It remembered me when I missed class at university and I had to learn from someone else notes, it was sometimes impossible to do it.

Thanks for your clarification Carl, certainly there is pure logic in returning to level bank in turn.

Joe

 

[fanavion]

Roy,

thanks for doing these tests. I´m always reliefed if findings are confirmed independently. Doing seriuos, unbiased tests is most important to understand the science of the MSFS flight model and the meaning of the parameters stored in the .cfg and .air-files.

It would be great if this thread leads to a sort of group reseach into the meaning of the records stored in the .cfg and .air-files. My guideline is not to tweak but to understand. Let us continue the invaluable work that once has begun in the long defunct Airfile Decode Forum. My greatest wish is that an aeronautical engineer with experience in real world flight dynamics could support us here.

Joe,

please give me some time to present my comments from the simulator point of view to your original subject Aircraft returning to level bank and to give recomendations how to reduce the excessive roll back to wings level in the simulator deliberately.

 

[JoeHallenbeck]

suggestion for new thread

Hello Developers,

Carl is right, we should create something like guide to understanding aircraft.cfg and air file structures.

What do you think about this:

I will create new thread where I will construct standard aircraft.cfg (something like dummy aircraft.cfg) file describing each section and its behavior in simulator. Than in next reply will be each section described line by line on each value. Third thread or reply could be focused on air file tables, their basic understanding and usage.

So three step project, basic structure, each section description and new chapter of air file editing as separate thread.

To change this dummy preview and its descriptions you can send me on private messages entries which I will be editing back into the first post so the dummy cfg will be updated correctly and will be in hand as first post in this new thread.

This way everybody who has anything to add to the dummy description will give helping hand with his or her experience in this field.

 

[WarpD]

Carl,
Thank you for your response, got me thinking.

I took a Mooney Bravo (which I never fly so will not miss) got it in the air and worked through its [airplane geometry] data. Very interesting exercise.

Here is the list of what actually does have an effect, by which I mean it will not fly normally if these values are very wrong or zero. Nothing else in that section has an effect on the ability to stay in the air or act in a normal fashion, so they are indeed for reference purposes only

wing_area = 175.0 Big effect, obviously
wing_span = 40 Put it zero and it stays in the air but at a strange attitude.
oswald_efficiency_factor= 0.6 zero efficiency gives zero lift
htail_incidence = 1 affects pitch angle appropriately
elevator_up_limit = 10 all control movements are needed for control
elevator_down_limit =10 0
aileron_up_limit =20.0
aileron_down_limit = 20
rudder_limit = 24.0
elevator_trim_limit = 19.5

You can put all control surface areas to zero and it has no effect, It is the amount of deflection that does something.

So you are quite right in what you say and I stand corrected. Thanks for the insight, I was not expecting it to be like that!

Roy

And with what .air file was this test performed? What records were contained within this .air file? I find it hard to believe positional information contained within the .cfg file have zero effect. I have also found that setting correct surface areas for controls has a significant impact as to their effectivity.

As I've told someone else who did a 'test'... setting values to zero isn't a valid test as not a single one of us knows if there is or is not some 'default' value pre-defined in FS so that a zero value can't have a detrimental effect (crash).

 

[Roy Holmes]

I think the idea of a thread to determine what parts of the config file are real is excellent.

I would like to add that when I first read Carl's input, I started to write a response that would dispute what he said, because it seemed so counter intuitive. In all 25 or so FS airplanes I have developed I used the sort of data in the airplane geometry section to establish the aerodynamic coefficients. This was done using Jerry Beckwith's Workbook.

I then thought about the coefficients and the aerodynamic effects they have and what they are applied against. I already knew that the tail surface areas had no effect, because their areas and distance from the wing center of lift just gives a "volume" number which generates the appropriate coefficients. If the coefficient is already established in the air file, changing the config file values does nothing.

Moving the wing or changing its sweep angle affects the CG position displayed in the fuel and payload drop down in the sim, but had no apparant effect on how the Mooney handled. Since I had the impression that dihedral angle affected behaviour in a turn, I input four or five values with no effect.

90° dihedral or wing sweep made no difference.

To keep the previous response simple I just listed my conclusions. In fact I went through every line, and input much smaller or much larger values, flying the airplane to see what difference there was, before trying a zero.

Remember I was being skeptical, my starting point was I did not believe what Carl said, but I was going to find out the facts before responding.

I even put the tail surfaces way in front of the wing, no effect, because the numbers that would change when developing the air file using Workbook already existed in the air file.

It basically comes down to what entries in the geometry are acted on by the coefficients in the air file, hence the list. Wing area is part of the basic lift and drag equations so its area is used. Oswald efficiency is a scalar on lift, put it to a smaller number you get less lift, put it to zero, you get no lift. Wing incidence has no effect because FS operates on fuselage AOA.

It was an eye opener for me, but it makes total sense.

In Workbook I can start over and check out what is done with the airplane geometry values, whether they just generate coefficients or whether they influence flight characteristics. I may find something that I missed during my torture of the Mooney!

So, although I presented my results in a simple fashion, the tests were methodical. Values were altered, handling was checked for changes and when none were seen I tried zero. I had the Mooney spinning violently when I put the H-tail incidence to 40°, barely able to fly with a small wing area, hanging in space with zero wingspan. Out of control with zero control ranges. Really interesting exercise.

Roy

 

[WarpD]

Yet you still haven't stated what the .air file's contents were. You say Mooney... FS9 Mooney or FSX Mooney? Default or modified?

In FSX there are a great deal of .air file entries that are no longer required as they are now supported in the .cfg file. So... what you use for testing, as a combined layout, I believe will affect the results of changes in the aircraft.cfg file a great deal.

 

[n4gix]

Given that the Prepar3D SDK is freely available, and that L-M has included the previously "super secret" asm2air.exe compiler, it might be more fruitful to begin the examination of "what works and what doesn't" from the beginning...

 

[fanavion]

In FSX there are a great deal of .air file entries that are no longer required as they are now supported in the .cfg file

We speak here about entries in the .cfg file which are not used, resp. never have been used by the sim engine of FSX and earlier versions. Only FSEdit , Workbook and AiRwrech need them to compute coefficients and write them into the .air file.

..and not about .air file entries in FSX which are no longer required as they are now supported in the .cfg file. Notheless this is another interesting and important subject.