Nose wheel suspension problem ?

[hjwalter]

Hi All,

 

Not a very big deal but with my recently installed Fairchild Warthog, when applying it's brakes on the ground while moving, the nose dips down as should be quite normal but in external view, the nose wheel sinks through the runway/taxiway surface and that's not very realistic to watch in replay mode.

Most probably some kind of suspension error in the first "contact point" entry in the plane's aircraft.cfg file. However, when parked and/or when taxiing, all three wheels are correctly on the ground and without issues.

 

Does anyone know how to fix this nose wheel issue ?

 

Thanks in advance and regards.

 

Hans

 

 

[jgf]

Maybe this will help-

 

//0 Class <0=none,1=wheel, 2=scrape, 3=float/skid>
//1 Longitudinal Position (feet)
//2 Lateral Position (feet)
//3 Vertical Position (feet) -4.70, -4.99
//4 Impact Damage Threshold (Feet Per Minute)
//5 Brake Map (0=None, 1=Left, 2=Right)
//6 Wheel Radius (feet)
//7 Steer Angle (degrees)
//8 Static Compression (feet) (0 if rigid)
//9 Max/Static Compression Ratio
//10 Damping Ratio (0=Undamped, 1=Critically Damped) 0.9100
//11 Extension Time (seconds)
//12 Retraction Time (seconds)
//13 Sound Type
point.0=1, -17.40, 0.00, -4.60, 1181.1, 0, 0.6349, 60.0, 0.4000, 1.5, 0.5100, 4.0, 4.0, 0, 0.0, 0.0

so “point.0” is just the sequential number of each individual contact point

“=1” is the class or type of contact point it is, meaning 0=none,1=wheel, 2=scrape, 3=float

The next three are just the position location format you are now familiar with. The fourth tells us how hard a landing this gear will survive, in this case 1181, whatever that actually means (PSI of pressure impact? Dunno).

The fifth one is the brake map, and I recommend that the nosegear is listed as “0” for none (just like this one is), or it will interfere with the differential braking ability, while the others will need to be listed as either left or right (1 or 2). You will know which one is the left wheel by seeing it has a minus – sign on the lateral position value.

The sixth 0.6349 is the diameter of the wheel, if you have wheels. The seventh value will only likely apply to the nosegear, as this determines the degree of freedom the nosewheel will turn for taxiing, here it’s 60 degrees.

Eighth is Static Compression, meaning how much will the gear compress 0.40 when the aircraft is parked on the ground. 9th, Max/Static Compression Ratio tells us how much difference there is between the static compression position, Vs the maximum compression position (like during a very hard landing that fully compresses the shock absorbers) here 1.5 (feet? Not sure if just a ratio though). Tenth is the Damping Ratio meaning how hard are the shocks (0=Undamped, 1=Critically Damped) here it’s about half ways at 0.51.

The last three only edit the speed at which they extend/retract, and whether a sound should be associated with the event.

 

Static Compression and CompressionRatio, items 8 and 9, are what you need to tweak.  But it's a royal PITA as there is some interaction between these settings and the fuselage angle and the damping ratio and....

 

Good luck

[hjwalter]

Jgf,

 

Thanks very much for your kind explanations, which only add to my personal experiences which are that "contact point" sections are extremely difficult to fully comprehend because different parts within each line, seem to influence other parts within the same line. So, after endlessly messing around with my Warthog's nose strut/wheel specifications without any form of success, I finally decided to CHEAT !!

 

I've had a navy Grumman fighter jet in my collection for many years and which I predominantly use for carrier operations (= hard landings/cable trappings/catapult launches, etc.). That aircraft did not display any of my Warthog's problems so, I copied/pasted the non nose wheel strut positioning part of it's first contact point line into the same Warthog's line and PRESTO !! That seems to have done the trick, with only a remaining acceptable part of my Warthog's nose wheel tire still visibly sinking below a runway/taxiway surface, while braking.

 

Only problem now left for me is to try to understand the why and how, including in this specific case, the possible influence of the Warthog's model (MDL) file.

 

Thanks again and regards

 

Hans

[jgf]

Here is some more data from ages ago;  I understand it but cannot figure out how to derive the data it wants to compute the landing gear parameters.contact_points.pdf

[Tom Gibson]

The maximum compression can be determined by using ModelConverterX, assuming you don't have the source code for the model. 

 

1.  Import the plane of interest.

2.  Open the Hierarchy Editor, check the Highlight Selected box, and look through the parts on the left for the landing gear.  I do this in wireframe view which is the easiest to see.

3.  When you find the landing gear (we'll use the right main gear as an example) look for a SceneGraphNode or ModelPart that uses the animation r_gear (in this case, other choices are l_gear and c_gear) and the animation type is Position.  Make sure the part/Node is highlighted as selected.

3. On the right side  right click the word Animation and choose Edit Animation.

4.  In the resulting dialog box look for keyframes 100 and 200 - between the two that is the maximum compression.

5.  Take the numbers in the Y box for the two keyframes and subtract one from the other.  This would give you the maximum compression (mine is in feet, I don't know if that depends on your MCX settings).

 

From this value and the Static Compression value in the aircraft.cfg file you can calculate the needed Compression Ratio.  If you change the Static Compression value you will also need to recalculate the Compression Ratio.

 

As an example my flyable DC-6B has a Maximum Compression of 1 foot.  If I would set the Static Compression at 0.8 feet, then the ratio would be 1/0.8 = 1.25.  I typically set the damping ratio to 0.9 or higher (max = 1).

[hjwalter]

Hi there Tom,

 

You are always way ahead of most of us armchair pilots with your deep technical knowhow on many FS/PC fronts and because of that, the saying remains true that flight simming, is far more complex for most of us than real world flying.

 

My nose wheel sinking through the runway/taxiway problem, was definitely not a serious issue but was far more born out of my inquisitiveness based on the notion that runway/taxiway surfaces should generally be as hard as concrete and in no way should let gear wheels sink through them. In my mind there were therefore other processes involved eminating from the aircraft in question and especially from it's "contact points", some parts of which will always remain a mystery, at least for me.

 

In the end I managed to solve my "issue" by what I called "cheating" but why and how this actually worked remains unclear, on top of which I have absolutely no experience and/or knowledge about the software you are evidently using but in the meantime I will certainly try to find the "ModelconverterX" program and will have a go at seeing what it does and/or can do.

 

Anyway Tom, thanks for your strongly professional reaction and .... Long Live FS2004 .... !!

 

Regards

 

Hans

[jgf]

22 hours ago, Tom Gibson said:

The maximum compression can be determined by using ModelConverterX

 

Thanks, I'll give that a shot.  I have an old Fs2002 airplane that I'm trying to tweak for FS2004, abandoned the project a couple of times, eventually ran it through Airwrench, which helped ...but i still cannot get the gear adjusted properly (which is how I found the above pdf).  It either:  bounces all the time it's on the ground, or bounces all the time it's taxiing, or bounces whenever above 40kts, or doesn't bounce but the nose gear buries in the runway as soon as you touch the brakes, or all works well but the main gear are a foot off the ground.  Not sure which is the current state, haven't cursed it since last summer.

[ViperPilot2]

It works exactly as it should in the Sim, but if it doesn't 'look' right Visually, then it can sour the entire experience of flying the Airplane.

 

Why is that? Isn't it weird... 

[Baron Fritz]

Have you guys considered Aircraft Container Manager? This was formerly a payware program, however it has been freeware for some time now. ACM presents the exact aircraft you intend to work on in a detailed schematic form, which eliminates much of the guesswork you are now experiencing. The area of concern which you are presently interested in is addressed under the "parameters-contact points" section. Here you will be able to examine and modify items such as gear location, compression, damping ratio, steering angle and many others. Even the wheel size can be adjusted. A word of caution is required at this point. As ACM permits a great variety of adjustments to your aircraft, it very likely that you will become sidetracked and find yourself spending much more time as, what started out as a simple gear adjustment, has morphed into a complete fine-tuning of the entire aircraft! Be sure that you allow extra time to accommodate this phenomenon.

I downloaded Aircraft Container Manager at Simviation, though it may be available elsewhere.

[jgf]

9 hours ago, Baron Fritz said:

you will be able to examine and modify items

 

But you still need the correct values to enter.  I have Aired, AAM, and Airwrench, all of which easily edit these parameters;  but there is so much interaction between them and also with the fuselage angle, and editing any one usually entails editing at least one other that it often seems like trial-and-error in hopes of getting something "close enough for government work", much less finding the truly correct values.  Airwrench implies it can recompute the gear data, but either that is an optimistic claim or I'm not using it correctly.

[Baron Fritz]

While I can't lay claim to expert status by any means, I can share my experience in "fixing" misbehaving aircraft. This is roughly what I would do:

The first and probably the most important thing on the list is to obtain an operating manual for your intended victim, if that is not possible, try looking up a copy of Jane's All The World's Aircraft for the year of your aircraft. These year books are very detailed, though you may have to hit the library.

 

Next on the list, obviously, is to make a backup file before you do any work.

 

I would highly recommend installing Aircraft Container Manager and Aircraft Airfile Manager. I have AirWrench, however, like you, I have never gotten acceptable results using it. It seems as though when one item is changed, this program adapts the entire flight envelope to accommodate the change you made, whether wanted or not. Usually you wind up chasing your tail trying to achieve one simple change, so I just gave up on it.

 

Also, you will need a sea level, standard day, no wind airfield as you will be using it when you conduct flight tests. The theory being, once you get your aircraft performing properly here, it will perform properly everywhere else.

 

Last, but no less important, put on a fresh pot of coffee, the aircraft rehabilitation process has a sneaky way of eating up an afternoon.

 

Using ACM, first check the basics of your plane, such as making sure that gear location and spacing matches up with the diagram, quite often it does not, and that in itself is one cause of weird behaviour. Check the contact points also, however you may have to fine tune these later on the basis of a taxi test. You mentioned that you were experiencing problems with fuselage angle. That can be corrected by adjusting the angle of attack in the Geometry section, many aircraft use an angle of 3 degrees, and that is a good setting to start with. It is also a good time to check the placement of the wing itself. Often the wing is incorrectly shifted fore or aft, resulting in mysterious behaviour issues. The horizontal tail is usually set at a zero degree incidence, though there may be exceptions.

 

As for wing and tail areas, that info can be found in Jane's, and checked against the data listed in the Geometry section of ACM. Now you are ready for your fist taxi test! While taxing, do some heavy braking and bouncing tests to observe suspension behaviour. The suspension can be adjusted in the Contact Point section. I recommend that you only adjust one parameter at a time such as nose gear or main gear. Only adjust one thing a few percentage points at a time, based on the original data, do not forget a taxi test after each change you make. It helps to have ACM running in the background in order to flip between it and the sim. It is a pain in the butt, however, don't forget to reload your victim after each change and before a test. Like all good test pilots, it is best to be slow and methodical, this approach will reward you in the end.

 

I do not believe in crunching through a massive column of numbers to achieve real world results, as quite often this is not the case. Adjust each section only a few percentage points at a time.

Remember, be patient..... coffee not brewskis!

 

 

 

 

[hjwalter]

Hey Guys,

 

Yes, this whole aircraft.cfg "Contact Point" section certainly seems to keep us on our toes, mainly because of their inter-actions with other entries within the same section and even with parts within the lines themselves.

 

It's in fact one of the very first things I always check after downloading any new plane (AI and/or flyable) and am often surprised by the amount of tweaking necessary to get the new plane to visibly and correctly stand "on it's three wheels". To check this I press my "Y" key with the new plane in external view and I just keep on tweaking and pressing my "Y" key until the aircraft no longer moves.

 

However, I've never even looked at issues like suspension, etc. and never really deemed those to be important untill by pure chance suddenly seeing an otherwise correctly tweaked nose wheel sinking into the concrete while braking. This had made me very inquisitive as to possible technical solutions, other than just making the nose wheel 100% "stiff" (= cheating).

 

Regards

 

Hans

 

 

[Baron Fritz]

Hello Hans!

You really should look into Aircraft Container Manager and Aircraft Airfile Manager, I find them virtually indispensable for precision tweaking! They are free and if you have any spare time, learning to reconfigure a wayward aircraft will certainly take care of that problem.

[jgf]

2 hours ago, Baron Fritz said:

I do not believe in crunching through a massive column of numbers to achieve real world results

 

Quite often this doesn't work anyway, probably the main reason so many parameters have a scalar adjustment - you plug in all the real values then adjust the scalar for the expected end result.  (A real world application of the old engineering term "Finagle's Finagling Constant" - that number which when divided by, multiplied by, add to, or subtracted from, the number you got gives you the number you want.)

[hjwalter]

Baron Fritz and jgf,

 

I found and downloaded all three of the mentioned programs and took a quick "sneak-peek" into each of them, with my nosewheel problem kept in the back of my mind.

 

However, my very first impression was that the sheer volume of complexities and choices, especially within the ModelConverterX program, made me initially agree with jgf and even more so because copying/pasting (= cheating) the necessary parameters from other FS9 aircraft's contact point lines, worked just as well .... and .... a lot quicker. Therefore my sincere compliments to those developers who actually went through all those complexities to get their contact point parameters so close to the real world actualities and also very much so for the pioneering specialists at MS, who made this all possible in the first place.

 

I will certainly take another good look at the ModelConverterX program as advised by Tom Gibson but now only as a possibility for improving gear suspensions ..... without ..... any wheels sinking through concrete runways/taxiways. On top of all that you never know what else I might run up against.

 

Hans

 

 

[Baron Fritz]

I have gone ahead and downloaded Model Converter X. My first impression is that the program was aimed at users with a far higher level of skill than I, people like Mr. Gibson for example, who have a Master’s degree in developing, (and there are many examples of his outstanding work out there!) While it is still very early in the examination process, I intend to continue monkeying around with MCX , though it seems to be a very daunting undertaking so far. At this point, ACM seems to be the program of choice for me, if only because it allows one to "see" what you are changing as you go on a schematic of the actual aircraft. Model Convert X may be capable of doing something similar, however I have yet to discover it

[hjwalter]

Baron Fritz,

 

Your first impressions about the Model Converter X complexities are evidently more or less the same as mine. On top of that I regard really diving into this program only for my external gear view issue, as a completely separate adventure and in my specific case, with a rather low priority.

 

In the meantime I will perform experiments with jgf's items 8 and 9.

 

Regards and good luck.

 

Hans

[DirkDP]

Hi,

I had the same problem with an aircraft. But it had nothing to do with the landing gear settings.

The problem seemed to be balance. I had to edit the wing_pos_apex_lon in aircraft.cfg to where it's supposed to be.

After that I needed to edit the empty_weight_CG_position to get the CoG back to it's original position.

 

Regards,

DDP.

 

 

 

[Baron Fritz]

Hello DDP,

As you are discovering and correcting errors in your aircraft's behaviour, which is a problem all too common in many FS aircraft, you may find this gauge of interest, rwyroll.zip.

This gauge allows you to easily compare the take-off. / landing performance of your aircraft against "book" figures. I install it as a pop-up in my aircraft as a matter of course. You may very well be in for some surprises! I downloaded the file here some time ago, before FlightSim got "improved", with luck it should still be available.

[DirkDP]

Hi Baron Fritz,

 

Thanks for the suggestion, but it seems the file got "improved away"...

 

Regards,

DDP.

 

[Baron Fritz]

Hi DDP

The file is still available at Avsim. Over there they identify it as runwayroll.zip, Runway Roll Gauge v 2. https://library.avsim.net/download.php?DLID=68781

Happy tweeking!

[DirkDP]

Yep, found it.

Thanks again.

 

Regards, DDP.

 

[Baron Fritz]

No charge....!