# Reference Datum!

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Hi!Can you explain me where this point exactly is?? Quote from the SDK: "...which is on the centerline 1/4 chord aft of the leading edge..."It would be nice if you could explain me where this point is! (would be the best with e picture) greetzfluxim

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>Hi!>Can you explain me where this point exactly is??> >Quote from the SDK: "...which is on the centerline 1/4 chord>aft of the leading edge...">>It would be nice if you could explain me where this point is!>(would be the best with e picture) See the pic attached.Marcohttp://forums.avsim.net/user_files/144911.jpg

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ok next question:is this right?(don't laugh at the picture) fluxim

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As with most aspects of Flight Simulator, things are rarely a simple as they seem.The following picture shows my understanding of the relationships. I've chosen the default 747 values to illustrate them.http://forums.avsim.net/user_files/144998.jpgThe Visual Model Origin (VMO) should represent the point about which the visual model will rotate in pitch as determined by the .mdl file. It needs to be related to the aerodynamic model defined by the .cfg and .air files. Starting at the Visual Model Origin (VMO) the Reference Datum (RD) is defined relative to the VMO. In this case, +83.5 ft forward of the VMO. The position of all the other parts of the aircraft are defined relative to this. As an example, the picture shows that the Wing Apex Longitudinal Position is 58.2 ft aft of the RD, hence the need for the - sign. The Root Chord is 48.4 ft so that the position of the 1/4 chord point is 58.2 + 48.84 ft = 70.4 ft. This isn't the same as the VMO -it's some 13 ft forward - and, in fact, doesn't seem to be directly referenced.The mean aerodynamic chord (MAC) represets the effective average chord of a wing. On an un-swept, un-tapered wing it's the same as the wing's chord. The 1/4 chord point is a reasonable approximation to the centre of lift. This is the point about which the aircraft will rotate in pitch. However with a swept and tapered wing the the MAC can be taken as the average of the root and tip chords, located halfway along the semi-span as shown in the picture. Its 1/4 chord point can still be taken as an approximation to the centre of lift but is obviously further aft as is expected with a swept wing.Note Microsoft's careful wording in the FSEdit SDK - "Typically .... this point is ... 1/4 chord aft of the leading edge."

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Excellent presentation!The fact is, that many (most?) FS modelers don't understand these relationships well at all, and wind up setting the VMO and Reference Datum to be identical; both at 0,0,0... ;)I much prefer using the proper method, because it's a lot easier measuring from a known, visible point (Reference Datum) than it is an invisible point such as the VMO... :-hah :-cool

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ok! thx!now next question!i have given a MOI in kg! how can i calculate the unit i need?fluxim

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A very handy conversion tool everyone should have is "Convert".For the latest information regarding Convert, go to http://www.joshmadison.com/software.

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Presumably your MOI is in kg m^2. Microsoft, still being in the dark ages, requires slugs ft^2. 1 slug = 32.1740 lb = 14.5939 kg1 ft = 0.3038 m

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