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Guest SAS449

747-400 Brake cooling

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Guest SAS449

The brake cooling period seems to be badly modelled in the 747. The time period for the temperature to come down does not seem dependent on ambient temperature, but on time alone, i.e. it takes just as long to cool the brakes to a zero value (as displayed in EICAS) regardless of whether OAT is +10 or -60. Can this really be correct? Doesn't everything cool down faster at low ambient temperatures. Is there an exception to carbon discs?Even if the gear is retracted, the wheel-wells are to my knowledge not pressurized or heated, and the temperature inside the wells should therefore be the same as outside the a/c. But it doesn't speed up even if lowering the gear at -58 Celsius...

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The brake cooling schedule is modelled using a time dependent heat conduction model supplemented by tabular data straight out of Boeing technical manuals (heat capacities, temp rise under braking etc). The results as tested repeatedly against measured values are accurate to the minute. Gear position and kinetic heating (that means TAT not OAT) is also taken into account as appropriate.Given the amount of time and effort to model what seems to be a detail's detail for the average user and thus something that could be largely overlooked (as in 99.99% of any add on out there) I really flip my effing lid (to put it in post-Victorian academic English) at such assertive statements as> The brake cooling period seems to be badly modelled in the 747Vangelis===================================== EMV Precision Manuals Development Group www.precisionmanuals.com=====================================


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E M V

Precision Manuals Development Group

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Which brings up another issue.I don't believe that the thickness of paint used on some of your liveries is correct. They seem too thick in some areas and not thick enought in others. I'm sure the thickness has an affect on fuel economies and range. Plus, I don't like the smell coming out of the toilet. Not rank enough.

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What does this have to do with anything ?Ohhhh....just piling on, I see :-roll


Regards,

Brian Doney

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Well, so since you guys looked at the brake cooling tables of boeing (or w/e that was) there was no diff in time to cool if the brakes are at a TAT of +30 and -60 celcius?

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Guest SAS449

OOooh, pleeeease forgive me for asking. As many others have done, I was merely asking a question on one of the many details on the 747 performance. Since PMDG prides itself with the attention to details, I didn't suspect asking a simple question would awake a sleeping tiger. But now I see that it's not the 747 that is badly modelled or ill behaved!

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OK,Cooling it. (a) We were fortunate to have rather extensive tables regarding temperature rise and cooling. (:( Detailed data. © A lot of it. (d) A nauseating lot of it.(e) From official sources.(f) And certified people to fly or fix the aircraft to check models out. (g) Not withstanding numerous tests, calibrations and such.Everything and eveyone seems to be in reasonable agreement with each other. Suddenly I see a person of unknown qualifications making a rather bold statement, not a question, regarding model quality in the middle of our forum. No documentation or undisputable technical evidence involved. Sorry not good ! Creates wrong impressions.HOWEVER :If that was a plain question I am terribly sorry and all I can say as a pointer is that going from an index of (high) 4 to an index of 1 needs about 60 minutes on the ground. About 8 minutes in the air with gear extended. Note that gear bays are heated up from conductive and mainly radiative heat loss. Carbon (in particular) made materials have this unfortunate effect. This is why overheating might become a major concern for airline pilots for fear of fire.From an index of 7 to 9, fuse plug melt down zone temperature as it is called, cooling takes a lot longer. These are not linear problems. Do you think that 30 or 40 centigrade difference on the "cold side" will have any significant effect on a temp rise that spans 2 to 3 orders of magnitude ? A temp difference of 1000 - 30 is practically identical to 1000 - 60. Boeing advises that in this scenario "...DO NOT SET PARKING BRAKE, DO NOT TAXI, DO NOT APPROACH GEAR FOR AT LEAST AN HOUR". This gives an idea of the temperature rise involved.VangelisPS_1. Sadly yes, you are right, this EMV model is well beyond repair, extremely ill behaved and a general curse to humanity. Call the shooting squadrons.PS_2 No Brian not offended at all.===================================== EMV Precision Manuals Development Group www.precisionmanuals.com=====================================


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E M V

Precision Manuals Development Group

====================================

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Guest SAS449

It was a simple plain question, no ulterior motives.You write that "all I can say as a pointer is that going from an index of (high) 4 to an index of 1 needs about 60 minutes on the ground". Is this without regard to the ambient temperature? To me it would be logical that the decrease of temperature would be relative to this?That it is not linear is understood, but I would have beleived that cooling would go considerably faster with at -60 Celsius compared to +20 Celsius?In the gear bays there is no windchill factor, but the surrounding surfaces would be close to the OAT and conduct the heat from the gear away, keeping the temperature of the bay down, thereby assisting the speed with which the brake temperature drop?However, I have not seen that the OAT or TAT has any significant impact on the timelapse regardless of whether extended or retracted.

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Guest SAS449

Only happens if you use an Apple computer with native programs when painting.If you don't think the smell from the loo is sufficient, suggest throwing content in a fan.Hope this helps you sort out your issues.

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>That it is not linear is understood, but I would have beleived>that cooling would go considerably faster with at -60 Celsius>compared to +20 Celsius?I am sorry but your lack of physics in this area is quite obvious.If you ever look up equations that show heat transfer between two materials of different temperature the first thing you notice that heat transfer rate is proportional to temperature diff expressed in Kelvin. Therefore most of the transfer occurs when heat diff is large (at the beginning), later the transfer is slow - behaves like an exponential function. Since temperature of 747 brakes can be substantial you can easily see that the transfer of heat will be slower in the +20 OAT case as compared to -60 by about 5% (in the first seconds). If you really crave for it I suggest you grab some book (or look up equations on the internet) and plug in some numbers and convince yourself about the magnitude of the difference. Frankly I think this would have been a complete waste of time to account for such things in this particular simulation. Michael J.


Michael J.

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"In the gear bays there is no windchill factor, but the surrounding surfaces would be close to the OAT and conduct the heat from the gear away, keeping the temperature of the bay down, thereby assisting the speed with which the brake temperature drop?"It's a pity we can't ask one of those unfortunate souls who thinks the wheel wells are an ideal place to stowaway. I've been told that if the gear retraction and lack of oxygen doesn't kill them, the heat from the gear cooks them alive. Less significantly, unless the APU is still running after takeoff, the very hot APU bleed duct also runs through two of the wheel wells.I've recently been involved in quick-turnaround gear-cooling exercises, using portable fans to cool the gear. It's surprising how slow this process is... and if you remove the fans prematurely, the temperatures start to rise again (go figure!). Also, the gear is in direct line with the outlet of the packs, and is a great (but noisy) place to stand on cold mornings. " That it is not linear is understood, but I would have beleived that cooling would go considerably faster with at -60 Celsius"As Vangelis stated, when talking about 1000degrees, 60~80 degrees is not going to make a significant impact (unless there is something to carry the heat away).If PMDG's brake cooling modelling isn't the best in Flight Sim history, I would be very, very surprised.Well, as they say.. if you're not happy, try and do better yourself ;)Cheers.Q>

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Guest SAS449

I know. I'm not an engineer. And I do underrstand the basics of linearity, the temperature dropping faster in the beginning than in the end. But I have timed the drop from level 5 in the EICAS, and the drop doesn't differ much - if any - between 5 and 4 or between 1 and 0. And that was the initial reason for my question, since it basically means you could take of with your brakes at level 4, and land with them pre-heated at level 2 on a shorter flight - if there is one where the 747 would be used. And IMHO that's not an ideal state.

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To put a practicle point to this the boeing QRH recommended brake cooling schedule doesn`t require any adjustments for OAT when using ground speed indications for the brakes on speed.It only requires oat,wind and altitude corrections if KIAS is used to enter the table,which is only to adjust to get the correct ground speed.You`ve got more chance of winning an arm wrestle with superman than winning an argument about physics and aerodynamics with Vangelis:-)I`ve had dinner with him,- I just had to nod in agreamemt:-)Jon

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