26-03-2011, 01:02 AM
Today I spent half-day understanding payload modeling (thanks God, it was a successful and quite an entertaining experience) and recently stumbled
upon a very interesting and useful post re: payload modeling by CowlFlapsOpen: http://goo.gl/D5FMo
My question will be based on this particular post, so if you're keen to answer, please read it first. If not, read it anyway 'cuz it goes in far
more detail on payload modeling and discusses some interesting approaches the author uses himself.
Question:
There are two values in aircraft.cfg which are important if you want to make a good payload model:
reference_datum_position=
empty_weight_CG_position=
In this particular example for default Cessna 182, they are shown as follows:
reference_datum_position = 3.6, 0, 0 // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)
empty_weight_CG_position = -3.0, 0, 0 // (feet) longitudinal, lateral, vertical distance from specified datum
station_load.0 = "160, -3.0, -1.5, 0.0, Pilot"
station_load.1 = "160, -3.0, 1.5, 0.0, Copilot"
station_load.2 = "120, -4.4, -1.5, 0.0, Passenger"
station_load.3 = "120, -4.4, 1.5, 0.0, Passenger"
station_load.4 = "30, 1.3, 0.0, 0.0, Baggage"
As also indicated in comments (lines after //), the longitudinal distances of both empty weight CG (EWCG) position and stations are in feet from specified
Reference Datum (RD) position. However, Dan's Guide http://www.fspassengers.com/forum/showth...p?tid=9687 specifically mentions that:
Now comes the funny part... and I'm getting confused! Please see attached image for visual representation of what I mean. E.g., if I take RD, PosZ for FSPassengers' stations will be calculated same as in aircraft.cfg, namely, PosZ for stations 2 & 3 will be -4.4 because, as I stated above, the longitudinal distances in aircraft.cfg are based on RD position which is equal to "0" being the datum point, as the name suggests. Don't care about its value (3.6 feet) in aircraft.cfg since it's relative to "FlightSim Reference position: (1/4 chord, centerline, waterline)"! In fact, its value is ZERO! So, all calculations and deduced values refer to this datum point (RD) with their relevant signs (+ if fore of RD; - if aft of RD). In this case -4.4 means "-4.4 feet aft of RD {zero} position". The same applies to CowlFlapsOpen's post where he also takes RD position as basis of coordinates.
On the contrary, if I take EWCG as a datum point (or, centre of coordinates) as advised in Dan's Guide, PosZ value for these stations will be -4.4 + 3.0 = -1.4. Why? Because in this case EWCG becomes "zero" point (or, datum point) even though its relevant position is 3.0 feet aft / back (-) of RD in aircraft.cfg and now all calculations refer to EWCG as centre of coordinates. Taking this onto C-182 example, to calculate correct PosZ values for stations 2 & 3 we start counting 4.4 feet {back} from RD until reaching EWCG ("point zero"), i.e. 3.0 feet. Then continue on until we cover another 1.4 feet (4.4 - 3.0). So, in this case PosZ for these two stations will be... -1.4 (note the minus sign since we are aft of EWCG!). Am I right?
Now, the question is - which VALUE should I take into consideration: i) reference datum position (RD), or ii) empty weight CG position (EWCG)?!?
Thank you very much in advance!
Post Edited ( 01-15-12 06:09 )
upon a very interesting and useful post re: payload modeling by CowlFlapsOpen: http://goo.gl/D5FMo
My question will be based on this particular post, so if you're keen to answer, please read it first. If not, read it anyway 'cuz it goes in far
more detail on payload modeling and discusses some interesting approaches the author uses himself.
Question:
There are two values in aircraft.cfg which are important if you want to make a good payload model:
reference_datum_position=
empty_weight_CG_position=
In this particular example for default Cessna 182, they are shown as follows:
reference_datum_position = 3.6, 0, 0 // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)
empty_weight_CG_position = -3.0, 0, 0 // (feet) longitudinal, lateral, vertical distance from specified datum
station_load.0 = "160, -3.0, -1.5, 0.0, Pilot"
station_load.1 = "160, -3.0, 1.5, 0.0, Copilot"
station_load.2 = "120, -4.4, -1.5, 0.0, Passenger"
station_load.3 = "120, -4.4, 1.5, 0.0, Passenger"
station_load.4 = "30, 1.3, 0.0, 0.0, Baggage"
As also indicated in comments (lines after //), the longitudinal distances of both empty weight CG (EWCG) position and stations are in feet from specified
Reference Datum (RD) position. However, Dan's Guide http://www.fspassengers.com/forum/showth...p?tid=9687 specifically mentions that:
Quote:... PosZ position in the payload editor is always RELATIVE to this ZERO CG position on the EMPTY aircraft.
in brief -5 will be 5 feet back to the calculated CG position on the empty aircraft and + 10 will be 10 feet in front
of the zero CG position.
Now comes the funny part... and I'm getting confused! Please see attached image for visual representation of what I mean. E.g., if I take RD, PosZ for FSPassengers' stations will be calculated same as in aircraft.cfg, namely, PosZ for stations 2 & 3 will be -4.4 because, as I stated above, the longitudinal distances in aircraft.cfg are based on RD position which is equal to "0" being the datum point, as the name suggests. Don't care about its value (3.6 feet) in aircraft.cfg since it's relative to "FlightSim Reference position: (1/4 chord, centerline, waterline)"! In fact, its value is ZERO! So, all calculations and deduced values refer to this datum point (RD) with their relevant signs (+ if fore of RD; - if aft of RD). In this case -4.4 means "-4.4 feet aft of RD {zero} position". The same applies to CowlFlapsOpen's post where he also takes RD position as basis of coordinates.
On the contrary, if I take EWCG as a datum point (or, centre of coordinates) as advised in Dan's Guide, PosZ value for these stations will be -4.4 + 3.0 = -1.4. Why? Because in this case EWCG becomes "zero" point (or, datum point) even though its relevant position is 3.0 feet aft / back (-) of RD in aircraft.cfg and now all calculations refer to EWCG as centre of coordinates. Taking this onto C-182 example, to calculate correct PosZ values for stations 2 & 3 we start counting 4.4 feet {back} from RD until reaching EWCG ("point zero"), i.e. 3.0 feet. Then continue on until we cover another 1.4 feet (4.4 - 3.0). So, in this case PosZ for these two stations will be... -1.4 (note the minus sign since we are aft of EWCG!). Am I right?
Now, the question is - which VALUE should I take into consideration: i) reference datum position (RD), or ii) empty weight CG position (EWCG)?!?
Thank you very much in advance!
Post Edited ( 01-15-12 06:09 )