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PatSim



Joined: 05 Sep 2014
Posts: 67
Location: KSAN

PostPosted: Wed Feb 08, 2017 12:43 pm    Post subject: Reply with quote

Not sure if I understand my CDU vs Aircraft.cfg engine thrust values.
* My CDU shows a 24k lbs engine thrust for TO on the N1 Limits page.
* My P3D SimObject Airplanes Sim Avionics B737-800 .cfg [Tubine Engine Data] show static_thrust=27,000

Neither value is the 26k lbs of thrust I would prefer for "Reduced Temp Derate" N1 Value calculations.
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Pat
San Diego
DSTD-737
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PatSim



Joined: 05 Sep 2014
Posts: 67
Location: KSAN

PostPosted: Mon Mar 20, 2017 12:13 pm    Post subject: Reply with quote

I yet to understand the difference in the Sim-A B737-800W .cfg thrust of 27k lbs vs the CDU N1 Limit page, 24k lbs of thrust. The Function of Reduced Temp Temp. on Line 1L of the N1 Limit requires a value calculated in TopCat or Topper among other means. These calculations default to a 26k lb Thrust -800 Engine.
Using the "Derate" CDU Line L3 (22k) or L4 (20k) is not a preferred method to reduce thrust on takeoff as no margin of additional thrust is available at T/O for any reason. In the Assumed Temp Reduction method the engines can still output full power (Firewall'd) even if they don't "Think" they can. So to sum up my issue's
1. Is the plane 27klbs or 24klbs of Thrust
2. -800.cfg & CDU N1 Lim don't match
3. I'd really like to exchange my engines for 26k lb versions
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San Diego
DSTD-737
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omera60



Joined: 18 Nov 2014
Posts: 131
Location: Istanbul/Turkey

PostPosted: Mon Mar 20, 2017 1:38 pm    Post subject: Reply with quote

Hi Pat,

On the real aircraft with 26K N1 page de-rate options should be 26K, 24K, 22K and 27K BUMP respectively. Also the engine rating should be 26K on the IDENT Page of the CDU instead of CFM56. As far as I can see the aircraft has 27K engines (from the aircraft.cfg). I think Mark will sort these out as soon as he sees this post.

Best,
Omer
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kand



Joined: 03 Feb 2016
Posts: 85
Location: Buckinghamshire, UK

PostPosted: Mon Mar 20, 2017 1:59 pm    Post subject: Reply with quote

static thrust read from the aircraft.cfg is 27300lbs

The CFMs on the 737NG are all the same, just that the FMC limits their rated thrust by the max N1.

So you can use the airfile table 1506 to find where the static thrust correlates to the rated N1 you require. Once you have identified that, set your CDU cfg max N1 and voila, you have the engine you require
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PatSim



Joined: 05 Sep 2014
Posts: 67
Location: KSAN

PostPosted: Thu Mar 23, 2017 12:27 am    Post subject: Reply with quote

Mr Kand
My aircraft.cfg is also 27,300lbs. So why does my N1 Limit Page in the CDU show 24,000 lbs? What does your CDU N1 Page show?

I'm not sure I understand what you mean by the "airfile table 1506". But more importantly the N1 I require is not available for "Assumed Temp" Reduced Thrust from either TopCat or Topper. Both of these programs only work for a 26,000lbs Thrust CFM Engine on a B737-800.
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kand



Joined: 03 Feb 2016
Posts: 85
Location: Buckinghamshire, UK

PostPosted: Thu Mar 23, 2017 4:59 am    Post subject: Reply with quote

Im a 777 user so not familiar with the SimA NG! Fsim perf is a very complex subject, sometimes its better to accept one version of the truth, that being what SimA gives us rather than the following!


The CFM56-7 on the 737 is basically the same engine throughout the NG range with thrust output limited by max N1.

following figures are taken from the actual engine perf manual (may differ to simA, this is fsim after all), all sea level ISA and static thrust so no ram drag etc etc

CFM56-7B27 = max thrust 27,300lbs 100.9% N1
7B26 = max thrust 26,300lbs 98.7% N1
7B24 = max thrust 24,200lbs 96.25% N1
7B22 = max thrust 22,700lbs 91.8% N1
7B20 = max thrust 20,600lbs 90.2% N1
7B18 = max thrust 18,500lbs 87.3% N1

I dont know whether you can select these different engines in SimA but by limiting your N1 to the SimA thrust equivalent, you can model the correct engine in the fsim world. Table 1506 is one way of looking within the fsim internal airfile structure, another way would be to use AFSD to check the actual fsim thrust at ISA, note the N1 and then set this as your max N1 in simA.

http://www.aero.sors.fr/designer_pilot_utilities.html

Mark may well be doing stuff within simA server which makes the engines underperform just like they do in real life, but due to fsim voodoo, matching real perf tables is very difficult so there needs to be compromise.

Have you tried playing with the max n1 setting in simA?

You asked "My aircraft.cfg is also 27,300lbs. So why does my N1 Limit Page in the CDU show 24,000 lbs? What does your CDU N1 Page show? "

well, if you understand the above, you should now realise why! its the same CFM56 engine just limited by n1 like it is in reality. So technically you can simulate any of the CFM56-7 range because the max thrust is available to you.

Not sure how TOPCAT works, I am all for real as it gets but nowadays I know I can get out of most aiports at max TOW with runway to spare, unless its somewhere like Mexico city! I just take the CDU V speeds, works fine for me!

Check out fsdeveloper for more info, but be warned it is deep stuff!
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