Balance point and calculated index

[blumenmann]

Hi everyone,

thanks for the great CRJ-200. I love this plane and it is so much fun to fly it.

But I have one small question. If my plane is fully loaded not only the weight changes also the balance point, but how do I calculate the correct value for my settings? Is it the index which I calculate on page 4 in the Pilot HandBook or is there another way to calculate the balance point? Or is this done just with adjusting the weight?

Thanks for all answers.

Greetings

Andreas

[cessna729]

If my plane is fully loaded not only the weight changes also the balance point, but how do I calculate the correct value for my settings? Is it the index which I calculate on page 4 in the Pilot HandBook or is there another way to calculate the balance point? Or is this done just with adjusting the weight?

Hi Andreas, If your using Windows OS, look in the CJR-200\Manual\CRJ Performance Planning Worksheet folder and you should find a MS Office spreadsheet that you can use, also check this previous thread.

cessna729.

[blumenmann]

Hi Andreas, If your using Windows OS, look in the CJR-200\Manual\CRJ Performance Planning Worksheet folder and you should find a MS Office spreadsheet that you can use, also check this previous thread.

cessna729.

Thank you for your answer. Yes I know the spreadsheet and I am using it. But my question is different. I try to make an example:

If you fill out that Planning Worksheet there is an index calculated for taxi, take off, cruise and landing. This index changes during the flight and that is also the thing I would expect. This index is used by the pilot to calculate the stab trim. This is clear.

So the question is do I need to adjust the balance point inside my plane settings (same menue like the fuel settings so I do not mean the stab trim settings)? If yes, there would be the next question: Is the calculated index the same value for the blance point settings? Or index * (-1) = balance point index or something different...

The next question would be: When I fly (as real as it gets) do I have to adjust the balance point setting after take off, during cruise and before landing?

I ask this, because if I use the correkt stab trim for take off and landing, the balance point must be set correct, too.

[philipp]

We have no direct mapping of the load stations to the index. That is, we assume you, as a good load manager in real life also would, will fill the plane in a way that it LEAST affects the index. Which means, if you start loading passengers, you first fill up the group that has least impact on the index. If those seats are full, you begin filling the next section with the second-minimal impact and so on. And when you then add the baggage, you begin loading it to counter the effects of the passengers, or at least not add to the effect. That is, if you end with a negative index after loading the passengers, you begin loading the cargo where you get positive index.

In real life, you have your dispatcher who calculates all this for you, and you have an experienced purser and ramp agents who know how to load your plane.

Ever travelled in a MD-83 and you were asked to change your seat? It sometimes happens that at the gate they assign passengers seats that in the end lead to the aircraft being unbalanced, and then the purser will ask passengers to relocate. That is a common experience in aircraft like the MD-85 which is very sensitive to the cg location, and it generally holds true for all aircraft with the engines at rear and cantilever-type stabilizer. Less for the CRJ than for the MD-83 though.

Okay, what am I building up to? In real life, you have people who take care of this for you and load your airplane so you don't get adverse effects on the balance. Therefore, you can treat the aircraft as perfectly balanced when you added the payload (don't touch the explane slider for the cg), and then add fuel and enter the calculation taking only the fuel's effect on CG, and assume the payload effect to be practically zero.

If you really want to load your airplane in an insane way, you can totally work through the whole Excel sheet and then manipulate the cg slider in X-Plane, just to experience how it feels when your dispatchers are monkeys, but I wouldn't recommend this for your everyday flight experience.

Philipp

Edited July 23, 2012 by philipp

[blumenmann]

Hi Philipp!

Thank you very much for that long explanation. That also explains some effects from the Planning Worksheet. I think it is also very interesting how strong the effect of the fuel and the cargo is.

Andreas

[jagipson]

I need to bump this, because I've been researching, and think I'm close to addressing this issue.

The MAC on the CRJ200 is 99.43"

The distance from the tip of the nose to the leading edge of the MAC is 493.8"

Using the charts in the POH to calculate the %MAC on 39,000 Lbs (dry weight), which is 15.3%; that makes the dry weight center of gravity 508.7075" from the nose.

It would seem that we could use Plane Maker to edit the CRJ CG to be 508.71" back. Then when flying the sim, calculate the fore/aft CG adjustment thusly:

1% MAC (on the CRJ) =

1% * 99.43 = 0.9943 inches

Subtract your load-calculated MAC based on fuel, cargo, PAX from the dry baseline MAC (15.3). Multiply that by .9943 and shift the CG on the plane by that much. Shift CG fore if your flight's MAC is less than 15.3, or shift the CG aft, if your flight's MAC is greater than 15.3.

All these numbers subject to verification, but anyone want to double check this?

Edited September 26, 2012 by jagipson

[jagipson]

It looks like plane maker is in feet, so you'll need to translate the inches above into feet for setting the CG. Incidentally it's a very minor change, except setting the aftward maximum (which was about 53').

Also, X-Plane takes into account for the changes in CG as a result of fuel burn, so, you want to use the index value LIZFW from the W/B form for determining where to set the CG Offset, however you still want to use the LITOW and LILW indexes to determine where to set the stab trim.

Edited September 26, 2012 by jagipson

[jagipson]

For whatever its worth, I made a very modified version of the included performance/fuel/weight spreadsheet which calculates how to set the CG offset in X-Plane and also calculates MAC values.

Http://drive.google.com/templates?q=crj200

Unfortunately it's not very well documented. Only edit cells with the cream colored background.

First fill out the PAX Distribution worksheet, then fill in the Main Calculations.

[cessna729]

For whatever its worth, I made a very modified version of the included performance/fuel/weight spreadsheet which calculates how to set the CG offset in X-Plane and also calculates MAC values.

Http://drive.google.com/templates?q=crj200

Unfortunately it's not very well documented. Only edit cells with the cream colored background.

First fill out the PAX Distribution worksheet, then fill in the Main Calculations.

Hi jagipson

Thanks alot, just had a quick look and your spreadsheet looks very interesting .

cessna729.

[cessna729]

For whatever its worth, I made a very modified version of the included performance/fuel/weight spreadsheet which calculates how to set the CG offset in X-Plane and also calculates MAC values.Http://drive.google.com/templates?q=crj200 Unfortunately it's not very well documented. Only edit cells with the cream colored background. First fill out the PAX Distribution worksheet, then fill in the Main Calculations.

Hi jagipson, been having a play with your spreadsheet, it's quite good

I have a few comments if your interested, I havn't gone searching for the original spreadsheet, so don't know how much you copied or which bits are you original work but you may have copied a rather large error .

My appolgies if i have miss read your work (just put it down to me getting too old and being half asleep)

1. The Density Altitude calculation given in the spreadsheet is IMHO only valid for an airfield at MSL (Mean Sea Level)

So for some high altitude airports, eg KLXV (elevation 9934ft), during the summer (temp 25°C), your calculation could be off by some 2300ft.

Temp graph for KLXV

From your sheet: Density Alt = Pressure Alt + (120 x (OAT - 15))

Which appears to be derived from the formula DA = PA + (120 Vt)

where DA = density altitude,

PA = pressure altitude at the level you desire density altitude,

120 = a temperature constant (120 feet per 1°C), and

Vt = actual temperature minus standard temperature at the level of the pressure altitude.

You appear to have just included the standard temp at MSL i.e. 15°C and thus missed off this bit:

ISA temp = (15-((1.98/1000) x level of the pressure altitude))

So your Density Altitude calculation should IMHO read:

Density Alt = Pressure Alt + (120 x (OAT - (15 - ((1.98/1000) x pressure alt))))

As a check you could use this table for an approximation:

2. I think you may be underestimating the PAX weights a bit . The FAA have increased the Standard PAX Weights form 185 lbs for male, and 145 lbs female (that includes 10 lbs carry on), to a "scale busting" 200 male and 179 female (that now includes 16 lbs carry on items). :o

3. I can't see why you need a separate sheet for PAX Distribution, when the figures are just copied to the Main Calculation Sheet B29:B33

4. What would be great is if you could get your spreadsheet to calculate Stab Trim, instad of the user having to use this graph (unless I've miss read the instructions again )

5. I like the LANDING DISTANCE Calc and the TOD Calc, any chance you could include TakeOff distance calc ect.?

Keep up the good work.

cessna729.

Edited October 21, 2012 by cessna729

[jagipson]

Hi jagipson, been having a play with your spreadsheet, it's quite good

I have a few comments if your interested, I havn't gone searching for the original spreadsheet, so don't know how much you copied or which bits are you original work but you may have copied a rather large error .

The original Performance Calculation spreadsheet is in:

X-Plane/Aircraft/X-Aviation/CRJ-200/Manual/CRJ Performance Planning Worksheet

My appolgies if i have miss read your work (just put it down to me getting too old and being half asleep)

1. The Density Altitude calculation given in the spreadsheet is IMHO only valid for an airfield at MSL (Mean Sea Level)

So for some high altitude airports, eg KLXV (elevation 9934ft), during the summer (temp 25°C), your calculation could be off by some 2300ft.

Temp graph for KLXV

From your sheet: Density Alt = Pressure Alt + (120 x (OAT - 15))

Which appears to be derived from the formula DA = PA + (120 Vt)

where DA = density altitude,

PA = pressure altitude at the level you desire density altitude,

120 = a temperature constant (120 feet per 1°C), and

Vt = actual temperature minus standard temperature at the level of the pressure altitude.

You appear to have just included the standard temp at MSL i.e. 15°C and thus missed off this bit:

ISA temp = (15-((1.98/1000) x level of the pressure altitude))

So your Density Altitude calculation should IMHO read:

Density Alt = Pressure Alt + (120 x (OAT - (15 - ((1.98/1000) x pressure alt))))

As a check you could use this table for an approximation:

I'll check in to this, and update the worksheet soon. I'll report back here once I have completed this.

2. I think you may be underestimating the PAX weights a bit . The FAA have increased the Standard PAX Weights form 185 lbs for male, and 145 lbs female (that includes 10 lbs carry on), to a "scale busting" 200 male and 179 female (that now includes 16 lbs carry on items). :o

I agree that the assumed passenger weights are dubious. They are from the original worksheet which is based on the Pilot Handbook, so the numbers match it. The way trim calculations are done in the Pilot Handbook makes no distinction between sexes. Essentially, the Handbook just requires a headcount in each section to get the "Index" value component.

I'd love it if a RW CRJ pilot would share how his/her company calculates W/B. I suspect that each airline has its own method that deviates from the Handbook.

3. I can't see why you need a separate sheet for PAX Distribution, when the figures are just copied to the Main Calculation Sheet B29:B33

The separate PAX worksheet was done because I wanted to minimize the edits to the Main Calculation sheet. Actually the PAX worksheet was meant to allow you to enter the number of souls, then it suggests the most balanced way to distribute the people, per section. Then, you (the pilot) enters "actual" values, which will deviate from the suggested seating arrangement because:

• you can't have fractions of people, so you have to round up/down in each section so the totals match
  
• it's possible that passengers might be arranged a little differently, due to cargo weights or maybe to keep families seated together
  

4. What would be great is if you could get your spreadsheet to calculate Stab Trim, instad of the user having to use this graph (unless I've miss read the instructions again )

Done.

Thanks for the suggestion!

5. I like the LANDING DISTANCE Calc and the TOD Calc, any chance you could include TakeOff distance calc ect.?

Keep up the good work.

cessna729.

The landing distance calculation is also from the original worksheet.

I will look into adding t/o distance, too.

[cessna729]

I'd love it if a RW CRJ pilot would share how his/her company calculates W/B. I suspect that each airline has its own method that deviates from the Handbook.

My guess (not having flow anything bigger than an F20 like this one IRL for a short period a long, long time ago), is that your spot on

For example: Taken from Determination Of Dispatch Takeoff Weight doc mentioned below.

Determining Weight of Checked Baggage

FAA AC 120-27E and JAR-OPS 1.620 are similar in offering three main methods to operators of large aircraft:

1) Use standard, average baggage weights as allowed by the regulations

2) Use standard, average baggage weights based on airline survey results

3) Use actual baggage weights

I suspect you allready know about the site SmartCockpit and the FlightOps sections, loats of good info. A couple in particular :

• Determination Of Dispatch Takeoff Weight
  
• The determination of Dispatch Takeoff Weight seems to be quite easy: start with the empty weight of the airplane and add the weight. But is it always that straightforward? Is everything actually wihed prior to the flight? How is the airplane empty weight determined? This Boeing document is a very well done brochure.
  

• Getting to Grips With Aircraft Performance
  
• This Airbus document provides reminders on aerodynamics, flight mechanics, altimetry, influence of external parameters on aircraft performance, flight optimization concepts, etc. Great information...
  

• Getting To Grips With Weight and Balance
  
• This material describes the cargo loading areas on Airbus aircraft and the systems related to cargo holds.
  

cessna729.

Edited October 23, 2012 by cessna729

[jagipson]

Okay

Special thanks to Cessna729 -- I have corrected the Density Altitude equation and modified the spreadsheet to support custom weights for men and women.

I noticed from the index tables, that (with the exception of the fuel index) they are linear in nature, so I have removed the look-up tables and now use the corresponding math formulas.

The "Main Calculations" page has been simplified, many of the less useful (but necessary) calculations have been move off this page onto the datarefs page.

I'm calling this version 2.0 at this point. The version number is on the disclaimer page.

[iGoApp]

Great table, jagipson! Very helpful when planning the flights. Especially when flying on VATSIM and every minute counts otherwise your arrival ATC will be closed by the time you get there.

I wanted to modify the table a little bit to be able to actually select the seats and choose who sits there (man, woman or child), so that the Passenger Index would be calculated based on that. I can use the pax index formula that you use, however it is not perfectly linear and sometimes the index gets skewed (by .1 or so -- not really significant but still). So, if I understood the literature correctly, I should find arms for the fuselage stations, multiply them by the weight in that station = moment. Then I can find CG by dividing total moment by total weight. And so on. The problem is that I simply cannot find arm values or arm centroids for passenger zones for CRJ-200 on Internet. Unbelievable, spent four days googling them -- just not there.

I would appreciate if someone can clarify or help me locate the arms for CRJ-200. Thank you!

Edited November 3, 2012 by igorland

[cessna729]

I will look into adding t/o distance, too.

Hi jagipson, the mods you've done look great!

I can't find the FCOM Part 2 (Performance Planning) for the CRJ anyware , and have only managed to find parts of the CRJ 100/200 AIRPORT PLANNING MANUAL. But attached is Section 3 which deals with FAR Takeoff Runway Length Requirements for the CRJ-100/200, I hope it will be of some use if you go ahead with adding T/O distance calculations to your spreadsheet.

cessna729.

[jagipson]

Great table, jagipson! Very helpful when planning the flights. Especially when flying on VATSIM and every minute counts otherwise your arrival ATC will be closed by the time you get there.

I wanted to modify the table a little bit to be able to actually select the seats and choose who sits there (man, woman or child), so that the Passenger Index would be calculated based on that. I can use the pax index formula that you use, however it is not perfectly linear and sometimes the index gets skewed (by .1 or so -- not really significant but still). So, if I understood the literature correctly, I should find arms for the fuselage stations, multiply them by the weight in that station = moment. Then I can find CG by dividing total moment by total weight. And so on. The problem is that I simply cannot find arm values or arm centroids for passenger zones for CRJ-200 on Internet. Unbelievable, spent four days googling them -- just not there.

I would appreciate if someone can clarify or help me locate the arms for CRJ-200. Thank you!

The spreadsheet calculates things the same way that the manual does, except that it allows for differentiating between male and female PAX and allows for setting the standard weight for them. Actually what the spreadsheet does is recalculate these into "standard people" the typical PAX used in the manual. The index values only have meaning when done this way (within the context in the manual).

It is likely that these PAX zones are used in RL, but very unlikely that in RL the airline weighs all passengers, and calculates their arm values based on their seats.

That having been said, the arm is the shortest distance from the center of the seat to a tangent of the tip of the nose (usually). This is more commonly expressed as "how far back the seat is from the nose." You could open the plane in planemaker or use a planview of the plane for this information.

[jagipson]

Hi jagipson, the mods you've done look great!

I can't find the FCOM Part 2 (Performance Planning) for the CRJ anyware , and have only managed to find parts of the CRJ 100/200 AIRPORT PLANNING MANUAL. But attached is Section 3 which deals with FAR Takeoff Runway Length Requirements for the CRJ-100/200, I hope it will be of some use if you go ahead with adding T/O distance calculations to your spreadsheet.

cessna729.

I'm at a loss as to how to translate those charts to a mathematical formula

[iGoApp]

The spreadsheet calculates things the same way that the manual does, except that it allows for differentiating between male and female PAX and allows for setting the standard weight for them. Actually what the spreadsheet does is recalculate these into "standard people" the typical PAX used in the manual. The index values only have meaning when done this way (within the context in the manual).

Thanks, jagipson. So if I understand it correctly, for example, Index for 3 passengers in Zone A is 4.9 only for an average passenger weighting 176 lb (in other words, index 4.9 is for total weight 528 lb). So, if I want to get the index for one man, one woman, and one child sitting in zone A (based on the revised standards), the index will be:

(200+179+79 [forgot what the exact weight for a child acccording to FAA is])*0.009[index point per 1 lb based on the manual] = 4.1.

Is this correct?

Thank you if you find time to clarify. Best.

Edited November 5, 2012 by igorland

[iGoApp]

OK. Something like that...

http://www.igorland....p_igorland.xlsx

jagipson, I used some of your formulas. I hope you won't mind -- they are just too good to be ignored.

I just got to the point of calculating the indexes. I modified the original jagipson's table:

1) Inserted an option to select seats using a plane chart;

2) Modified the formula to calculate the Passengers' Index. As I mentioned before, the relationship is not perfectly linear, so in my table the index is calculated using a ratio (index point per lb) based on the values in the CRJ-200 manual;

3) The Fuel index is more complex. I inserted three median points between every two values, and then used an interpolation formula. The calculated index is more or less exact now.

Please let me know if you notice a bug.

Cheers,

Edited November 7, 2012 by igorland

[jagipson]

Thanks, jagipson. So if I understand it correctly, for example, Index for 3 passengers in Zone A is 4.9 only for an average passenger weighting 176 lb (in other words, index 4.9 is for total weight 528 lb). So, if I want to get the index for one man, one woman, and one child sitting in zone A (based on the revised standards), the index will be:

(200+179+79 [forgot what the exact weight for a child acccording to FAA is])*0.009[index point per 1 lb based on the manual] = 4.1.

Is this correct?

Thank you if you find time to clarify. Best.

I get 4.2 but that's very likely a rounding issue in the spreadsheet. An index difference of 0.1 isn't going to effect the trim settings noticeably.

Your calculations look correct to me!

[jagipson]

OK. Something like that...

http://www.igorland....p_igorland.xlsx

jagipson, I used some of your formulas. I hope you won't mind -- they are just too good to be ignored.

I certainly don't mind. Use/modify it to your heart's content.

I just got to the point of calculating the indexes. I modified the original jagipson's table:

1) Inserted an option to select seats using a plane chart;

2) Modified the formula to calculate the Passengers' Index. As I mentioned before, the relationship is not perfectly linear, so in my table the index is calculated using a ratio (index point per lb) based on the values in the CRJ-200 manual;

3) The Fuel index is more complex. I inserted three median points between every two values, and then used an interpolation formula. The calculated index is more or less exact now.

Please let me know if you notice a bug.

Cheers,

The ability to adjust PAX per-seat is probably only helpful for getting an exact FORE/AFT CG Offset for setting in the plane's Weight/Balance window. I RL the crew doesn't know the actual weights of people in the seats, in fact I don't think they pay attention to their sex, either. I think they just rough-out an estimate using the per-zone PAX head-count and a standard generic person weight. When I buy commercial tickets, they never ask me my sex. I suppose it's possible they could "guess" based on my first name, but I doubt they even do that, since the effort of calculating *exact* trim settings based on PAX weight/seat arm values hits a point of diminishing returns quickly after a plain per-zone head-count.

So I guess if you want to be ultra realistic, use the exact PAX weight/seat arm value calculation to setup the aircraft weight/balance, but then calculate your trims using the generic head-count per zones as in the POH (since that's what a RL pilot would most likely do).

Any RL pilots want to chime in on this? Please do. I've never flown anything.

Thanks for sharing your spreadsheet, igorland -- but could you maybe make a Google Docs template out of it and share it? Then people who do not have Excel can benefit from it.

[iGoApp]

Thanks a lot, jagipson! This is exactly why I was looking for the arm values! I know the gender does not make difference for the pilots. I just use it for fun to load the plane and then calculate the total weights by zone. By the way, I went to PlaneMaker and still did not find the arm values there. Can you help locate it please?

I will try to save it in Google docs when it is finished. I am like 60 per cent through it. Need to figure out the best way to make speed corrections based on T and altitude. I think I have the idea though...

I will send it for a peer review. Cheers.

[iGoApp]

Hello, everyone. Can you please peer review the Flight Plan table? Unfortunately, I cannot save it as a Google document (the first page gets corrupted and some formulas do not work), so it is only available in Excel. This is the link:

www.igorland.com/flight/CRJ200_FP_v1.xlsx

Thanks for your time. If you find it correct and useful, I could upload it to the x-plane.org site as well.

Cheers.

[jagipson]

Announcement:

I have just released version 3.0 of my CRJ-200 Weight and Balance Spreadsheet Template. It is a Google Doc, so it should work in any modern browser. You need no special software, but you do need a Google account.

Http://drive.google.com/templates?q=crj200

New features:

Aircraft Menu (Appears on the menu bar 10 seconds after spreadsheet loads)

Automatically fills out the spreadsheet using randomized loads on PAX/Baggage based on a Male/Female PAX ratio you specify (typically 0.7) and a minimum and maximum number of PAX. Access this feature by using Load Aircraft on the Aircraft Menu.

Automatically fetches METAR, Field Elevation, Pressure, and Temperature on the Main Calculations page when you use Fetch WX on the Aircraft Menu. Before selecting this menu item, be sure to fill out your Origin, Destination, and Alternate airports in A4, A9 and A14 cells. This feature only works for U.S. airports. If someone can get me some info for Aus or EU airport METAR web services, then I might be able to expand coverage.

Edited November 15, 2012 by jagipson

[birdy.dma]

Hi Jagipson.

Can't load it. As the previous version, can you edit an xls one ?

Anyway, nice job.

Edited November 15, 2012 by birdy.dma