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# Question about Bleed air

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Hi, I have some questions about the bleed air system.I did the Boeing 737 CBT and played around with the switches of the bleed air system. I was surprised that the pressure in de system in the pmdg737 is different from what I saw in the CBT.http://home.planet.nl/~harto740/bleed1.jpgIn the first picture you see the APU bleed on, right pack off, and isolation valve auto. Pressure is 15In the second picture you see the same config in the Boeing CBT. The pressure is now almost 40 psihttp://home.planet.nl/~harto740/bleed1a.jpgWhat do we do next. We open the right pack, the right pack uses bleed air so the pressure should (??) decrease. But is does not. We have still a pressure of 15 psihttp://home.planet.nl/~harto740/bleed2.jpgIn the picture of the Boeing CBT the pressure decreases to 20 psi.http://home.planet.nl/~harto740/bleed2a.jpgSo the questions: Why is there a difference between the Boeing CBT and the system modelled in the PMDG? And why is the pressure much lower in the PMDG? How is it in real life?I hope somebody has an answerBest regards,

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Don't know if this has anything to do with it, but the CBT seems to be for another model, like -600 or -700 while the one you posted a shot looks like an -800 -900.

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I noticed also that the bleed air pressure is very low. Min start pressure for the CFM56 engine is 30 psi and the best I've got EVER in the PMDG is 20psi. Kris

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FlorisYour CBT seems to comply with the 737 Classic behaviour. That is the 3/4/500 series. All the bleed air in the NG is collected in a right and left pneumatic duct. In thess ducts you have pressure regulators that will try to keep the duct pressure at a constant level regardless on the source and on the demanded load i.e. packs etc.If the load increases the duct regulator will sense a trend of decreasing duct pressure and give a signal to the APU regulator to speed up a bit and by that regain the demanded duct pressure. Though the pressure seems a tad low.At least this is how I understand it. I'm sure Ian or some other more proficient person will chip in here if I'm totally lost. ;-)Hope it helps,

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Hmm it may have a difference between APU or ground supply. Maybe they are "using" some other facility than APU in CBT. Though PMDG has a bit too low yes.

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MatsI think the comment was that the pressure is just too low rather than the 'how it works'. The norm for an engine start is 40psi with the min being 30psi (for ALL variants of the CFM56). There are all sorts of valves and ducts etc, what we were trying to get across is that the PMDG model only ever reads around 20psi well below the norm. the pressure shown on the PMDG gauge is more normal for when a pack is being driven by the APU not when the APU is driving nothing! At lease I think that was what was going on, or did I just modify the entire argument in my own little world?Kris

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KrisYes, you are right. I made a hen out of a feather I guess. And yes the duct pressure in the PMDG NG seems to be too low.Sorry if I unintenionally confused anyone! ;-)Cheers,

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Thanks for all the responses!I hope (if this is a real bug??) that this will be fixed :)Regards

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Semi-official answer coming up....I believe it was agreed during beta testing that duct pressures on the demand (adjusting) bleed systems on the NG's were, at best, unpredictable (across the worlds' fleets in many different climates). The age of the APU was even a consideration... Sometimes demand type systems tend to lose (over time) their designed ability to cope with increasing load over time. I realise that there are lots of new -800's/-900's out there, but...As a result, I think it was agreed to leave the pressures static (at least with the APU supplying bleed air... unfortunately, as a beta tester, I didn't have time during beta testing to see what the pressures were with the engines running).BTW, here are the duct pressures I noted on a real NG recently.APU only with no load = 10psiAPU only with 1 pack = 40psiAPU only with 2 packs = 28psiAPU only with main engine start in progress (with no packs) = 35psiAPU and 1 engine with no packs = 20psi (fluctuating slightly)PLEASE NOTE: I'm not saying these values are typical... and if you can make sense of how the NG produces these values, please tell me!Here is another shot of the overhead panel on an NG with the packs in an odd configuration.http://members.ozemail.com.au/~b744er/737N...soValveOpen.jpgAny help?Cheers.Ian.

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I hope i would have known this topic before, when i posted mine about the fuel system and all i got was some shouting "you! lamer perfectionist! this is not a CBT it's a fs!" :(

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Claudio??Nobody called you a "lamer perfectionist" in that thread ;-)However, I think all these things come down to the fact that at a certain point during development a compromise will have to be made as to how perfectly realistic one can make an add-on to MSFS behave, Ian's post above seems to confirm this.If we want the full-blown air and fuel systems working behind the scenes (let alone the properly working and integrated IRS!), we'll be loosing CPU cycles at such a rate that our 737s will be unflyable.Just my 2 Eurocents worth...

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Yes but here is an example of polite discussion about the matter, with even someone trying to remember an old decision during development (why the PMDG team never answers such threads and always leave to other the responsibility to do so?), in my thread, i have only been accused of being a perfectionist.Noone saying that it was a known thing but that it has been decided to leave it that way....That is not the way to do i'm afraid :(Especially since this topic here appears to be the same type as mine, but far more polite.BRClaudio

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"Especially since this topic here appears to be the same type as mine, but far more polite."I would try reposting at a later date, Claudio. You may get some more positive responses next time.By the sounds of things, one of the respondants was under a lot of personal pressure and could not be held responsible for his answers ;-)I think the PMDG Team are enjoying a well-deserved break, with at least one key member on his honeymoon, so don't expect too many comments from them. And, by the way, go easy on the folks that are left holding the PMDG baton.Good luck. Cheers.Ian.

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