Archived

This topic is now archived and is closed to further replies.

dragonflightdesign

Reference datum position

Recommended Posts

I have to confess, the following short statement in the Aircraft Configuration Files section of the SDK has been giving me nightmares:-'Offset (in feet) of the aircraft's reference datum from the standard Flight Simulator center point, which is on the centerline chord aft of the leading edge.'According to a friend who used to design aircraft and now designs airfields for his sins, there is no such thing as a 'centerline chord' in the real world. The planform view of the Hornet in the SDK suggests that Microsoft's centerline chord runs from somewhere on the wingtip to somewhere on the opposite wingtip. I have tried using the 744 example given in the SDK to puzzle out the position of the CC but I end up somewhere forward of the wingtips - or back of them, depending on where I start....I'd be very grateful if someone could put me out of my misery... :( -Dai

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

Dai , the way i visualize that is the Root Cord would be where the aircraft in flight would " Gimbal " or float on its wings , the offset is the distance from there to where the 0, 0 , 0 point is in your model , if you place the models 0 , 0 , 0 point at the " root cord " there is no offset.I had to provide a grid overlaid on the render of the latest project to provide a graphic reference, the Lysander's high mounted wings and the models 0,0,0 being dead center of the fuse provided for a bit of sport with this model's offset.

Share this post


Link to post
Share on other sites

The 0,0,0 point in the model is the visual center when loaded into FS. The term "centerline chord" is the 1/4 root chord. This is usually the exact same point in the model. By default FS defines the center of lift as the model's center, which means the center of lift is (by default) defined at the 1/4 root chord position.It's important to understand that the reference datum position is a value used to define just that. In the real world all aircraft designed have a reference datum position. It is typically defined in front of the aircraft's nose at some arbitrary point by the designer.If you leave all values at 0,0,0 then by default your center of lift, your visual model center and your reference datum position are all in the exact same location. However, if you have the correct reference datum position for the real aircraft and know the offsets of such things as fuel tanks from that reference datum position... then you want to use the reference datum position value in the aircraft.cfg file to offset it from the 0,0,0 default model center and move it to the real world location. By doing that, you can now use real world measurement information that's based on the real world reference datum to position pretty much anything.There are those who have said, and those who will probably say I don't know what I'm talking about and I'm 100% wrong. However, I used this on the Eaglesoft Citation X... and that aircraft's weight/balance setup is about as real as it gets.

Share this post


Link to post
Share on other sites

Ed is (mostly) correct...The sole exception to what he stated is this:

By default FS defines the center of lift as the model's center, which means the center of lift is (by default) defined at the 1/4 root chord position.
and even then is absolutely correct as long as the modeler did -in fact- set the model's origin to the 1/4 root chord position. If the modeler chose for some insane reason to set the model's origin at the tip of the aircraft's nose, then all bets are off on the model ever being able to be properly configured!!! :( The key word in any description of "reference datum position" is this: arbitrary... :( The only critical thing to keep in mind is that in FS, aside from the model's center (0,0,0) which is defined in the model file itself, every other positional setting in the aircraft.cfg file is relative to the defined "reference datum position."Since most modelers don't have access to the "real world engineering data" for the aircraft they are building, they simply set the "reference datum position" to be coincident to the model's center origin, then use the "by guess and by God" method to set all the other positional entries... :( In short, the "reference datum position" is completely optional and arbitrary, and even in the absence of accurate hard engineering data from which to work, it may be set to whatever is most convenient for the modeler!The chief benefit in such case is that this can eliminate the necessity of using negative offsets when dealing with y axis positions. Obviously, z and x axis points are not affected...

Share this post


Link to post
Share on other sites

Thank you all very much for your responses - the Hornet planform now makes sense because there's an awful lot of heavy engine at the back end of that aircraft. I don't have the original reference datum but there's a possibility that I can get it (I can only hope!). I also have some severely-crossed fingers that the aircraft modeller did set his reference correctly because the current model flies like a complete drunken pig. No, I don't have access to him [sigh]. Or the pig. :(-Dai

Share this post


Link to post
Share on other sites