Pavement Loads for FS Aircraft

Having failed to find any data on B727 CG limits I decided to work them out for each -100 and -200 version (at max gross weight) from published max static pavement loads using the formula for calculating an aircraft's CG position, namely:

distance Datum to CG position = distance Datum to Main Landing Gear position - (pavement load at the nosewheel * wheelbase / total aircraft weight for that nosewheel pavement load),

and converting the results to %mac.

This done I thought I'd try to rearrange and adapt the above formula to see if I could come up with a formula for calculating pavement loads for FS aircraft, the result being:

pavement load at the nosewheel = (distance Datum to Main Landing Gear position - distance Datum to CG position) / wheelbase * total aircraft weight for that CG position.

This formula can be confusing to use in FS, particularly when the Datum/FS_Reference is located between the CG position and Main Landing Gear position, so I simplified it to:

pavement load at the nosewheel = distance from CG position to Main Landing Gear position / wheelbase * total aircraft weight for that CG position.

This formula is accurate in theory only, given the inability to level and weigh aircraft in FS, but should be accurate enough for FS use.

Haven't yet worked out how to calculate pavement loads for aircraft having more than 2 main gear struts which are not all aligned in the same longitudinal position (eg: B747).

I've always been curious to know how the pavement loads of my FS 727 fleet, particularly the -200 loaded to 210,000 lb, would compare to published max static pavement loads. Now I know. And I now have my Load Sheets set up to calculate and adjust them for me automatically.

You might want to take a read of this: Flight Dynamics in MSFS V1.0.pdf, by Yves Guillaume. It is readily available, just Google it. I think it's the second link down it gives that you want for the DL.

It's everything you ever wanted to know about how FS Sim .air files work, including a great section about the CG and how to calculate it. I can't speak to the "Pavement Loads", but it might help you out. It has a lot of great mathematical formulae for you to use. Including reference datum to CG. At least in the FSims.

You might also take a glance at this site: https://msdn.microsoft.com/en-us/library/cc526949.aspx#mozTocId136011. LOTS of good info on aircraft.cfg files.

I can't speak to the real world formulae, but I can probably find them if you want me to. I'm pretty good at finding "stuff" on the net...

Hope this helps out a little bit...

Pat☺

Aircraft.cfg defines the EMPTY WEIGHT CG thus:

reference_datum_position = 0.0, 0, 0 // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)

empty_weight_CG_position = 0.0, 0, 0 // (feet) longitudinal, lateral, vertical distance from specified datum

CG is therefore not a calculable except where its origin is picked up from the .cfg. I don't see that factored into the calculations above, which is important where ACES couldn't be bothered to refine the CG to an actual level (hence the zeros) and chose to reference it to the Visual Model Origin (the reference datum) - If anyone ever wanted to know why the CG cross is always in the wrong place in FSX but in the right place in FS9 that's the reason - lazy programming.

If the aftermarket developer has used a more sophisticated model to define the EWCG, by actually not being lazy and leaving it on the 1/4 chord MAC then it is imperative that your spreadsheet reads the aircraft.cfg sections above to create an initial weight point as weight on wheels is meaningless without.

Remember you are dealing with a virtual world where truth is what it is programmed to be, without the constraints of constants like gravity or aerodynamics.

Nice effort, but reinventing the wheel as there are a number of freeware and payware tools in the market for many years that already do this.

It's actually a rather simple correlation of positional data.

Model origin is by default reported by the 'physical model' as 0,0,0.

Reference datum position is an arbitrary position used by the manufacturer on their blueprints. Where "laziness" comes into play is that many (most?) developers don't understand this, and simply define it as 0,0,0 as well.

Empty weight CG position is declared as an offset from the stated reference datum position, and typically would be adjusted fore/aft as needed to be at the 1/4th chord MAC position.

Really "lazy" developers set the model's origin at the 1/4 chord MAC position to begin with, so that's why the other two datum are also 0,0,0... ;)

Aircraft.cfg defines the EMPTY WEIGHT CG thus:

 

reference_datum_position = 0.0, 0, 0 // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)

empty_weight_CG_position = 0.0, 0, 0 // (feet) longitudinal, lateral, vertical distance from specified datum

 

CG is therefore not a calculable except where its origin is picked up from the .cfg. I don't see that factored into the calculations above, which is important where ACES couldn't be bothered to refine the CG to an actual level (hence the zeros) and chose to reference it to the Visual Model Origin (the reference datum)

Self evidently wrong.

The default FSX A321 has:

reference_datum_position = -4.5, 0, 0 // (feet) distance from FlightSim Reference position: (1/4 chord, centerline, waterline)

empty_weight_CG_position = -4.0, 0, 0 // (feet) longitudinal, lateral, vertical distance from specified datum

Empty weight CG position is declared as an offset from the stated reference datum position, and typically would be adjusted fore/aft as needed to be at the 1/4th chord MAC position.

Surely that's a tautology. The empty weight CG position is where it is - regardless of the 1/4th chord MAC position. Unless you define 1/4th chord MAC position = empty weight CG position?

Aircraft.cfg defines the EMPTY WEIGHT CG thus:

 

CG is therefore not a calculable except where its origin is picked up from the .cfg. I don't see that factored into the calculations above.

FS uses wing_area, wing_span, wing_sweep, wing_root_chord and wing_pos_apex_lon in the aircraft.cfg to calculate the position of the MAC Leading Edge relative to the Datum, then calculates the distance from that MAC Leading Edge to the empty_weight_CG_position shown in the aircraft.cfg, divides that distance by the applicable MAC length and shows the answer as a percent MAC on the Fuel and Payload Planner CG diagram and the CG gauge on the panel.

I don't know the formula FS uses to do these calculations but I have been able to come up with a mathematical formula of my own that provides exactly the same answers as the FS formula does.

The Load Sheet clearly shows the position of the MAC Leading Edge in both inches from the Boeing Datum and in feet from the FS_Ref/Datum of the relevant model and the Load Sheet calculates the relevant CG position and converts it to %mac in the same way FS does.

QUOTE "it is imperative that your spreadsheet reads the aircraft.cfg sections above to create an initial weight point as weight on wheels is meaningless without."

I used published max pavement loads to calculate the fwd and aft CG limits shown. I used actual CG positions to calculate the actual pavement loads. I did not use the pavement loads to calculate the actual CG positions.

QUOTE "If anyone ever wanted to know why the CG cross is always in the wrong place in FSX but in the right place in FS9 that's the reason - lazy programming."

I run the same 727s in both FS9 and FSX. The CG cross is in exactly the same position in both FS versions. The only way it could be different is if some wing parameters and/or some positions in the FS9 version of the aircraft.cfg are different to their counterparts in the FSX version.

One of these days I'll figure out how to use the QUOTE function properly.

The FS_Reference_position is the position at which the model's developer "fixes" the aircraft. Microsoft has been suggesting for as long as I can remember that aircraft should be "fixed" at ¼ chord (ie: 25% mac ?), centreline, waterline, but I have yet to find an add-on aircraft or even an FS default aircraft that has been "fixed" in that position (see below: A321 FS_Reference_Position).

The only reason Microsoft included a Reference_Datum_Position was, to quote Microsoft, "By setting the Reference Datum Position, actual aircraft loading data can be used directly according to the aircraft's manufacturer". Which means the developer/user can position the Datum in the same position the aircraft manufacturer uses [130 inches forward of the nose on 727 aircraft (which is the same position on the 727-200 as 1070 inches forward of the main gear as I have shown on the Load Sheet)] and then position everything in its real position relative to that Datum position (which I have also done on the Load Sheet). The Load Sheet then converts all these real positions (inches aft of Boeing Datum) to their equivalent distances (long ft, vert ft) from the FS_Ref/Datum (shown in inches next to "Current Setup" on the Load Sheet) of the model being used or worked on of which I have 3.

Most developers position the Reference_Datum_Position at the FS_Reference_Position for convenience, not out of laziness. Because all positions in the Aircraft.cfg are relative to the Datum and all positions in the .air file are relative to the FS_Reference_Position, separating the Ref_Datum_Pos from the FS_Ref_Pos means having to calculate one set of positions for the Aircraft.cfg and another set for the same positions in the .air file.

To my knowledge Microsoft has never suggested that the Empty_Weight_CG_Position should be at ¼ chord (25% mac). I do not have any published data regarding the specific or typical position for the Empty_Weight_CG_Position on the 727-200. I have reason to believe that a 727-200 with a Basic Index of 0 would have an Empty_Weight_CG at about 30.5% MAC and, as it appears the Adjusted Basic Index cannot be negative, 30.5% should be the forward limit of the Empty_Weight_CG. I positioned it at 32% MAC (which appears to be equivalent to an Adjusted Basic Index of about 38-40) because my Trim Sheet tells me that's about as far forward as it should be if I want to keep the aircraft in trim at ZFW (see attached Trim Sheet). And I'm thinking of moving it to 33.0% (which would be equivalent to an Adjusted Basic Index of about 58).

None of the panels I use has a CG gauge. I have just installed a Bendix-King/Honeywell FMC gauge and discovered it includes the CG% at RK4 on the Performance page. It reads 12.1% at AUW, 14.1% at ZFW and 32.0% at OEW. Compared to the corresponding CG figures on the Load Sheet that's as close to perfect as it gets.

Since the default A321 aircraft has been mentioned, here are a few figures for that aircraft that might be of interest to you.

MAC Leading Edge Position (as calculated by FS) = 0.000000% MAC = 1.624201 feet aft of the Datum (see: light positions 8 & 9 below).

MAC Length (as calculated by FS) = 13.692514 feet.

¼ Chord = 5.047330 feet aft of the Datum (where Microsoft suggests the aircraft should have been "fixed" ??). I think you guys need to dispense with what appears to be an obsession with ¼ chord. If there is anything meaningless in all of this, that thing is ¼ chord.

MAC Trailing Edge Position = 100.000000% MAC = 15.316715 feet aft of the Datum (see: light positions 10 & 11 below).

MAC Lateral Position (as calculated by FS) = 21.496968 feet either side of the longitudinal centreline (see: light positions 8,9,10,11 below).

As record 1515 is not included in the A321.air file and therefore does not provide a real or alternative MAC length (which would over-rule the one calculated by FS), the MAC length FS uses to calculate the CG position for this aircraft in the Fuel & Payload Planner, or a panel CG gauge, is the one calculated by FS.

FS_Reference_Position = -44.726634% MAC = 4.500000 feet forward of the Datum = 6.124201 feet forward of the MAC Leading Edge !!!!!

Empty_Weight_CG_Position = 17.351083% MAC = 4.000000 feet aft of the Datum.

CG at Zero Fuel Weight 108,002 lb = 15.826894% MAC = 3.791300 feet aft of the Datum.

CG at All-Up Weight 160,195 lb = 24.651326% MAC = 4.999587 feet aft of the Datum.

Main Landing Gear position (extended) = 69.934560% MAC = 11.200000 feet aft of the Datum.

And in case you're wondering where the MAC Leading and Trailing Edge positions are on the Model (as calculated by FS) I've added 4 lights to the aircraft to show you (see attached picture).

light.8 = 3, -1.624201, -21.496968, 2.75, fx_navwhih ,

light.9 = 3, -1.624201, 21.496968, 2.75, fx_navwhih ,

light.10= 3, -15.316715, -21.496968, 1.75, fx_navwhih ,

light.11= 3, -15.316715, 21.496968, 1.75, fx_navwhih ,

Thank you all for taking the time to respond to this post and for the links provided. Your interest and input are appreciated.

I think you guys need to dispense with what appears to be an obsession with ¼ chord.

I agree with that. The ¼ chord is a no more than an arbitary datum based on a very very simple wing theory for infinite spans which ignoied incompressible and inviscid flow. That theory states

C_L = 2π(α + β)

C_M = ¼Cl - ½β

where

α = attack of attack

β = 2 * mean camber

It's actually a rather simple correlation of positional data.

 

Empty weight CG position is declared as an offset from the stated reference datum position, and typically would be adjusted fore/aft as needed to be at the 1/4th chord MAC position.

 

Adjusting the Empty weight CG position in the aircraft.cfg so that the CG% in the Fuel & Payload Planner is at 25% mac would only give a realistic indication of the position of the aircraft's empty weight if you knew that that the MAC Leading Edge was at the leading edge of the model's wing (assuming, of course, that the model's wing is in the right place on the aircraft).

Before you move the EWCG position you have to know that the MACLE is in the correct position at the leading edge of the wing. If it isn't you have to move wing_pos_apex_lon until it is. Only then can you move the EWCG to the position you want it to be in, which doesn't have to be 25% mac.

The lights I added to the A321 indicate that the MAC Leading Edge is about where it should be at the wing leading edge on the model and moving the Empty weight CG position from -4.0 to -5.047330 would position the CG% in the F&P Planner at ¼ chord/25% mac and this would therefore be a realistic indication of the position of the empty weight actually being at 25% mac.

If the MACLE is not in the right place the CG% in the F&P Planner is a totally misleading and useless piece of information. How many developers position or even know if the MACLE is at the leading edge of the wing ? Not many it would seem.

I am aware that wing position and shape (as constructed by FS using the relevant wing parameters) and the position of the CG% in the F&P Planner have absolutely no effect on flight dynamics. Move wing_pos_apex_lon 30 feet fwd or aft of its present position and the aircraft flies exactly the same. It's the position of the weight that matters. If you want the CG% mac to give a realistic indication of the position of the weight, position the wing correctly.

I know that the CG% figures on my 727-200 load sheet, which have proven to agree with the CG% position calculated by FS via the FMC CG%, are a realistic indication of the position of the weight because I have the wing positioned so that the MACLE is in its exact real Boeing position (980.2 inches aft of the Boeing Datum) and the MAC length FS is using to calculate the CG% is the one in .air file Record 1515 (15.058333 ft which is equivalent to the real Boeing MAC length 180.7 inches) which over-rules the 17.281166 feet MAC length calculated by FS which FS would use for CG% calculations if record 1515 wasn't in the .air file.

Here are the CG Limits (at Maximum Design Taxi Weight) for various versions of the 727 calculated from published Max Static Pavement Loads as mentioned at the beginning of this post.

CG Limits below 170,000 lb (-200 series) and 153,000 lb (-100 series) remain unknown.