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Smoky942

Reference datum Position

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Hi, I'm not a model builder, so hoping someone can tell me if I'm correct. Build four variations of the same aircraft, each with a different loadout of fuel tanks / weapons. Each will have a different CG to reflect the different weights and balance, and of course, alterations to the .air file to maintain correct flight performance, but, the Reference Datum Position MUST stay exactly the same for each variant. I don't want to say anything till I'm sure I'm right. Thanks.Russell.

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Hi RussSo far as I am aware, that's correct. However, be aware that the RDP isn't necessarily the the absolute middle of the CG; it could be a completely arbitrary point somewhere off the nose/tail of the aircraft. That's what makes creating airfiles so difficult; unless you know exactly where the RDP is any data you enter is meaningless.-Dai

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The Reference Datum Position is "the offset (in feet) of the aircraft's reference datum from the standard center point, which is on the centerline chord aft of the leading edge." Note that the RDP is variable: what is fixed is the Standard Centre Point which is effectively the centre point of the visual model - the point about which it rotates. CGs are referenced to the the RDP for variants of the same model.The sequence is:Standard Reference Point ------> Reference Datum Position ------> Centre of GravitySo, for example, the RDP is set by:reference_datum_position = 6.96, 0, 0where the three numbers are the x, y, and z coordinates.The empty CG by:empty_weight_CG_position = -6.06, 0, 0and the CG of individual loads bystation_load.0 = 170, -6.54, -1.20, 0.0where the first number is the weight in lbs.These are set in the .cfg file: not the .air file.

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As I said: the RDP could be a completely arbitrary point somewhere off the nose/tail of the aircraft. Only the manufacturer will know where the RDP is set.cfg file correction accepted.-Dai

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Many modelers choose to use the simple (aka: lazy) approach by either not specifying any RDP at all, or setting it to be identical with the empty weight COG position...

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Sorry for the delay, had to go out all day. Thanks for the informative answers. Much appreciated. Did'nt know about the Standard Center Point, I thought that was the RDP. Still not absolutely sure about the answer to my question though, so I'll try to explain. The aircraft in question is a private venture, and flys extremely well in all variants, in fact very very close to the real aircraft throughout the entire flight envelope, according to the lucky pilots who've flown the real one. Each one has the RDP set to 0.00, 0.00, 0.00. Someone then altered the first (longitude) figure to something like -0.980, to try to correct another small fault (it did'nt) but it did of course move all the nav light effects out of position, and caused problems with the VC. I imagine it also moved all the load points as well, is that correct ? So, is my understanding that once you have built a model, and set all the other coordinates from a certain RDP, that is then left alone, UNLESS you change every other coordinate to match...lights, load stations etc. I have nothing to do with actually building the model, just helped out with the effects, contact points etc. The actual building of the model (any model) is a very grey area for me, so just wanted to get a better understanding of what I was talking about, before shooting my mouth off, so to speak.Russell.

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That is precisely correct. IF specifed explicitly, all other positions are then stated as offsets from the RDP's x,y,z...reference_datum_position

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. By setting the Reference Datum Position, actual aircraft loading data can be used directly according to the aircraft's manufacturer. If not specified, the default is 0,0,0.
So, the sequence is this:model's centerpoint as defined in the model itself = 0,0,0 => RDP (if specified in aircraft.cfg) => empty weight CG position & all other positions defined in the aircraft.cfg file's [weight_and_balance] section, [contact_points] section, and any other section that defines x,y,z coordinate positions.The critical point to keep in mind is that the aircraft's Datum Reference Point is controlled by the presence or absence of an Reference Datum Position entry.It's truly unfortunate that MS/ACES chose to use two labels that are so similar sounding, yet so fundamentally different! It's been the proximate cause for confusion over the past decade... :(

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Brilliant, thats precisely the information I was after. Thanks for sharing your knowledge. I can now speak on the subject with (a little) more confidence. Thanks again. For some reason, my internet connection went down...hence the delay in replying.Russell.

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