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Hello,In a previous message, I said I had discovered a small issue with the Trim Settings (Jet Tech confirmed that). He said that normally 4 is neutral, but it is showed 8 for the neutral.But I would like to know the following : Can we equal the center of gravity (CG in the menues) and the % MAC ?For me it's not the same, and it's inaccurate to set the trim according to the CG...So, I would like to know if it's possible to develop a small utility which can tell us the REAL % MAC as a function of the load (cargo, passengers, fuel) ?ThanksMarcVarlamoff

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Guest Roy

You know, it's possible to load/unload both passengers and cargo in addition to fuel.I guess you already knew that, but I thought I'd mention it anyway :-)Roy B. :-waveOslo, Norway

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Roy,Thanks for your advice, but I already knew that... ;-)The problem is that thre is no way to know the exact % MAC of the aircraft, so I'm unable to trim correctly the aircraft for TO...and it's a bit annoying because either the aicraft doesn't want to rotate at Vr, or rotate by itself before Vr...MarcVarlamoff

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Hi there !!!I agree that this is a point where it need clarification.CG in FLY!II menus IS NOT MAC%. Is something else...I think that positive values is more weight aft and negative values more weight fore (to the front). But I dont know where this number refers to. For example 0 it means CG equals Center of pressure or 50% of MAC ??If we can know, the we can guess what the 34 value that defaults the 757 means. Most probably it meens that the aircraft is tail heavy. With 207000 lbs default weight and trim setting 7.5 (almost double of 3.5 and assuming that we use the double value of that in the table ) we can guess that 34 of menus CG is 29 MAC%.Do not forget the even the platform itself (FLY!II) may pose problems to PMDG in order to help us about MAC% and trim.Enjoy the 757 is superb !!!!Samuel KalachorasAthensGreece

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The "CG" indication in the correspoding window in Fly! is calculated as follows :100 * (Current_CG - Empty_CG) / (Empty_CG - VisualModelCentre)The empty CG is placed at 24% MAC following the Boeing issued load-balance computer.Vangelis--------------------------------------E. M. VaosPrecision Manuals Development Group--------------------------------------

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Vangelis,thanks for the info.But is there a way to know the MAC % when the plane is NOT empty ?ThanksMarcVarlamoff

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George,I've been studying your post for me than 2 hours, and I don't understand how you calculate your % MAC...The first thing I don't understand is the Fly! 2 CG calculated value... What's the meaning of (Empty_CG - Visual Model Centre) ? Is a corrected value of the empty CG, modified to stick with the visual model ? If it's the case, why don't we use this corrected value in order to calculate if the CG is fore or aft of the real CG ?Moreover, I don't understand why sometimes you consider 23% as the default Empty CG, sometimes 24%You wrote :"Example:Say you see 36% CG in Fly!. Hence CG_loaded is placed at 6.08*36/100=2.188 ft behind CG_Empty or 2.188/16.641 = 0.13 MAC units. If follows that current CG is placed at 24%+13% = 37% MAC(ouch not very good loading)The absolute CG limits are 13% to 36% MACMy response:Default 757 model is 23% (graphically depicted as forward) so using the formula for the default loading of the aircraft:6.08*23/100=1.398 ft behind CG Empty or 1.398/16.641 = .084 MAC unitsHence: the CG is placed 23%+8% = 31% MAC. Within the limits.My example from my previous flight 41% forward in Fly2:6.08*41/100=2.492 ft being CG empty. .15 MAC units. 41%+15%=56% or well outside of the CG limits of the Aircraft. Ouch "In your first calculalus, you use the value showed by Fly2!, you make the conversion in MAC units, and you sum it with the Empty CG.In your last calculus, you use the value showed by Fly!, you make the conversion in MAC units, and you sum it with the value showed by Fly!2.Could you explain why you do that ? What's the difference ?Thanks for your repply !MarcVarlamoff

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Hi Marc,First let me state that Vangelis, from PMDG, has far more expertise in this than I do. Some background; there was a lengthy discussion concerning CG during the beta and Van was attempting to explain further how CG is calculated in Fly2. So, let me see if I can explain further by answering your questions:>What's the meaning of (Empty_CG - Visual Model Centre) ? Is a >corrected value of the empty CG, modified to stick with the visual >model ? If it's the case, why don't we use this corrected value in >order to calculate if the CG is fore or aft of the real CG ?I believe what Van was explaining is that Fly2 uses the "Visual Model Centre" as part of the calculation for the location of the CG. This is also how Fly2 generates the visual slider when one brings up the CG window. Van was using the calculation to demonstrate how to convert the Fly2 %CG to %MAC. I was testing based on Fly2's CG calculations, so this formula allowed me to quickly calculate the Fly2 CG limits for the 757 (based on %MAC). After customizing the loading of the 757, I could use the tools built into Fly2 without having to calculate %MAC (as an extra step).>In your first calculalus, you use the value showed by Fly2!, you >make the conversion in MAC units, and you sum it with the Empty CG.>In your last calculus, you use the value showed by Fly!, you make the >conversion in MAC units, and you sum it with the value showed by Fly!2.Yes, great catch! Looks like I made a big mistake by confusing the values!! The %MAC calculation should have been based on the Empty CG. Here are the corrected calculations:Default 757 loading:6.08*23/100=1.398 ft behind CG Empty or 1.398/16.641 = .084 MAC unitsHence: the CG is placed 24%+8% = 32% MAC. Within the limits.My aircraft on that faithful day of testing (ouch):6.08*41/100=2.492 ft being CG empty. .15 MAC units. 24%+15%=39% Outside the limits. >The absolute CG limits are 13% to 36% MACHope this has helped,George

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George,Thanks for your reply.You said that the absolute CG limits are 13% to 36% MAC.In the manual(page 1-9), you can find TO Stabilizer Settings which are in the 9 to 39 % MAC.Can you explain this difference ?For Vangelis,Could you validate the calculus from George (Falcon99) ?I sum up : to find the exact % MAC, you take the CG value calculated by Fly! (exemple 35%). and you do the following calculus(example) :6.08*(35/100)=2.128ft. It means that the CG is 2.128ft behind empty CG (if you find a negative value, I would interprete it as the CG is located x.xxx ft ahead of empty CG.)Moreover, we know that empty CG is located at 24% MAC and that MAC=16.641ft.So 2.128/16.641=0.128=12.8%MAC (we round off this value, and we take 13%)So, the MAC % is : 24% + 13%=37%Is this exact ?If it's right, I will try to code a small utility which will tell you the MAC %, in order to configure the 757 for TO.ThanksMarcVarlamoff

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>You said that the absolute CG limits are 13% to 36% MAC.>In the manual(page 1-9), you can find TO Stabilizer Settings which >are in the 9 to 39 % MAC.Vangelis provided the 13%-36% values.George

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>You said that the absolute CG limits are 13% to 36% MAC. >>In the manual(page 1-9), you can find TO Stabilizer Settings >which are in the 9 to 39 % MAC. >Can you explain this difference ? >>For Vangelis, >>Could you validate the calculus from George (Falcon99) ? >I sum up : to find the exact % MAC, you take the CG value >calculated by Fly! (exemple 35%). and you do the following >calculus(example) : >Actually George just copied an emai I sent to the beta frum some time ago (thanks George I would not want to re-write all this). Hence, yes I can validate the whole lot since I wrote it. The official balance computer [section 51-05] shows absolute limits between 9% and 39% MAC. Limit lines [A-B line on the chart] however are *also* shown within these absolute limits defining proper operation. For a nearly empty plane (125000 lbs) these restrict CG to 14%-32%. For heavier loads these rapidly increase to the quoted range of 13%-36%. In fact for loads above 180000 lbs the lower limit decreases further to 12%. I thought that these complexities are not of interest to everyone hence I just quoted the most common range (and of primary concern for landing) hence 13-36.The math is very simple (we are not integrating pde's here). If you intend to write a utility (thx and thx again) just drop me a private email.Cheers,Vangelis===========================================E. M. VaosPrecision Manuals Development Group===========================================

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Marc,I can send you (or anyone else who would like it) a small excel spreadsheet which performs the calculation if you would like?Thanks,George

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George,Yes, why not ;-)MarcVarlamoff

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