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object attitude in sim

N

noob24

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unitedkingdom
I have noticed that my fuselage appears to be sloping upwards from front to rear in fsx yet in 3dsmax it is 100% horizontal and aligned exactly with the grid.

How does the sim place objects? im assuming it used world coordinates but not sea level rather local ground level or runway level. As far as im aware all runways in fsx are level(no slopes) so are a good indication of angle of object.

anyone seen this before?
 
Are you using the flight dynamics of another aircraft to test yours in sim?

If so, that could potentially have a huge effect on the attitude/positioning of the plane.
 
yes i am. i did not realise it would change the placement i thought how ever the object is compiled and exported is how it will be placed in sim. height above ground. left/right direction for example im simply adjusting in 3dsmax and are the same in fsx.

do you know where in the aircraft.cfg you can adjust such parameters and what ones are adjustable in sim? or is it just attitude/pitch or roll too


thanks
 
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And to get you ready for the tutorial - an aircraft is placed into FS sitting on its contact points. The first three lines of this section are usually the landing gear, and specify how high off the ground each of those three should be. All numbers are based on distances from the 0,0,0 point.

It is assumed that the plane is drawn level in GMAX, and then "settles down" onto its landing gear using the contact point values.

Hope this helps,
 
great thanks

so basically fs does not care about the geometary or where it is its all to do with what is written in the cfg. but the geometary needs to be placed correctly so it matches the cfg. for example getting the center piont of the fuse in exactly the same place as it is in the cfg so that spot view will rotate and centre correctly on it. i will look atthe tut and hopefully it will answer all these question

thanks all most helpful
 
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Yes, this is correct. The center point in GMAX is used for all further calculations in the cfg file.
 
tgibson said:
Yes, this is correct. The center point in GMAX is used for all further calculations in the cfg file.
Click to expand...

Well, for most purposes folks do assume that's true, but that isn't really the case at all.

All calculations are based on the reference_datum_position, which admittedly is almost always congruent with the "center point" 0,0,0...

...mainly because not too many people know better! :teacher:
 
Well, perhaps it is a good idea to take a step back as you can't very well be expected to do the suspension setup without doing the basic u/c elements first. For this I have found Chris File's Landing Gear tutorials most helpful, to be found at

http://www.freeflightdesign.org/tutorials.html

They are in the Gmax section, but you may find the whole site a beginner's treasure trove.
 
Uh Bill,

Isn't that reference_datum_position line in the cfg file? If so, then the 0,0,0 point in GMAX *is* being used as the basis for all further calculations. I never said it was used directly by the Contact Points section. :D
 
Strictly speaking, the gmax centre and reference_datum_position are used together for all further calculations, but we're hairsplitting here. If contact points are measured purely from the gmax centre, sooner or later I'll come to grief, as a number of models in CFS3 (which I model for) have the model centre at the front of the propeller spinner. :eek:
 
Yes, MS likes to put it there for some reason (I'm guessing they use station numbers, which are measured from the nose to the back of the plane).
 
tgibson said:
Yes, MS likes to put it there for some reason (I'm guessing they use station numbers, which are measured from the nose to the back of the plane).
Click to expand...

Well, I did state that very few do understand the purpose of the reference_datum_position, did I not?

Most developers simply ignore it entirely, or set it to 0,0,0 which pretty much makes it useless. And, I never once mentioned contact points...

But, be that as it may, the reference_datum_positon's x,y,z coordinates ARE the point from which ALL other x,y,z entries are referenced, not the absolute 0,0,0 (world coordinates) which are typically set at 1/4 chord, waterline, centerline.

In real diagrams, all measurements are made from an arbitrary point specified by the manufacturer, which is typically some few feet forward of the furthest point of the aircraft (usually the nose).

The major advantage of using this specifically defined reference datum is that all numbers aft, up, and right of it are positive. This makes the math much simpler, especially when dealing with fore/aft measurements.

From the modeler's perspective, it makes everything much simpler as well, because if -for any reason- the model's origin has to be moved in Max/GMax, only ONE ENTRY needs to be changed in the aircraft.cfg!

You guessed it; the reference_datum_position! :wizard:
 
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n4gix said:
not the absolute 0,0,0 (world coordinates) which are typically set at 1/4 chord, waterline, centerline.
Click to expand...

I think you are confusing the "absolute 0,0,0 position" with the empty weight CG position.
 
Ahhhhhh......

Vilkomm to zeh dark veldt of zeh AERO FILEZKI's!!!!!! A dark dungeon of errie findingkz zat are ZOOOOO SKEEEEERY, you might vett your zelfski's!


It is a dark art. I call this zone (airfiles tuning) the game of 'pick up sticks'. You mess with one setting, and you effect at least 3 others. NOT 1. NOT 2.... But THREE!!!!! or more. Sometimes 5. Sometimes 7. But never 2 or 1. Neh... Not one...

(NOTE: IF you can play the game Pick Up Sticks, where you pull out sticks from a stack and the mess doesnt collapse, then you TOO can tune airfiles).

Contact points have 3 settings in them for suspension. Milton gives a tutorial on how to adjust the suspension. A 737 for instance will have a different suspension reaction to a Cessna 172. Now, you 'could' copy/paste the 3 suspension settings into a similar size bird and see what happens. Adjusting the tire contact points is easy. Just keep changing the settings, reloading the plane in FS, and checking until the points are perfect. Milton uses a nav light at each contact point to find them, then sets them up from there.

You can drive through dirt to see if the tire patches are aligned right. (Yes, it helps).

If you are working in FS9, use Aircraft Container Manager (or ACM as we call it) to adjust the planes aero surfaces, even tire contact points, etc, etc, etc. Balances can be tuned in it also.

If you are using FSX, then use Aircraft Airfile Manager (AAM) to adjust the wing area, contact points, and other 'mysterious' settings (sticks in stacks).


Und, you muzt sprinklez Holy Wasser on zeh monitor before you vill verk on zeez aero fileski's!


:D
 
Golf-HotelDelta said:
I think you are confusing the "absolute 0,0,0 position" with the empty weight CG position.
Click to expand...

They are (unless specified differently) synonymous, George.

visual center (0,0,0)
...reference_datum_position
......empty_weight_CG_position
......all other defined x,y,z positions

Since nearly everyone places the "visual center" congruent with the a/c's empty_weight_cg_position, it's rare that you'll see any offset entered. However, even if there is one present, it's relative to the defined reference_datum_position.
 
We also need to consider that the config file gives the wing_root_chord and wing_pos_apex_lon only. So you would think the FlightSim Reference position (1/4 chord, centerline, waterline) applies to 1/4 of the root chord behind wing_pos_apex_lon. The CG will only be positioned there on an airplane with unswept and untapered wings.

The 1/4 chord position is actually 1/4 Mean Aerodynamic Chord (MAC). FS gives CG percent relative to MAC. For a swept and tapered wing MAC is less than root chord.

If you look at the B737 config file the empty weight CG is 9 feet behind the FlightSim Reference. The root chord is 21 feet, but MAC is actually 14. Depends on things like taper ratio and 1/4 chord sweep angle. Airwrench calculates all that stuff.

Roy
 
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