Let as know outcome.

I may be misunderstanding the intent of some of what is being said here but N1 is directly related to thrust. N1 is the % of the fan speed. The fan is directly connected via a shaft to the low pressure turbine which is the last stage of the engine. The more trust coming out of the burner section the faster the turbines spin and the faster the fan spins. In a high bypass turbofan engine the majority of the thrust is from the fan.

 

The OP is correct that you should see an increase in N1 when the bleeds are off. Bleed air is taken from the compressor section of the engine. If you don't take that air from the compressor there is more air to go into the burners and higher pressure air exiting through the turbines causing the fan to spin faster. We use N1 for our power settings in the 737 so if there was no increase in N1 by truing the bleeds off there would be no point in doing it. In training they tell us that when we are checking what our take off power is going to be and comparing it to our performance numbers if you see a big difference from what was planned check the packs and bleeds to make sure they are in the proper configuration.

Thanks for confirming this Tom, and thanks for explaining why the fan speed increases when the bleeds are off, its appreciated, so i guess this will be added to the list for SP2?

What the OP was asking about was not what happens to engine N1 with no bleed, but the rated N1 for a bleeds off takeoff. This doesn't seem to be working as it should.

Ralgh,read jeppessen's dual engine aircrafts manual.you'll get N1 isn't directly related to thrust,its simple:physics!thrust is related to amount-tempreture-speed of air,going in and out of engine,

Guys can we be clear on something here, are you making the distinction between 'Thrust' and Thrust lever angle'?

Ralgh,read jeppessen's dual engine aircrafts manual.you'll get N1 isn't directly related to thrust,its simple:physics!thrust is related to amount-tempreture-speed of air,going in and out of engine,

Maybe I'm misunderstanding what we are arguing about here. What I'm saying is N1 is the % of the fan RPM. The fan provides thrust just like a propeller on a prop driven airplane by spinning blades with an airfoil. Almost all of the air the fan section is pushing is not going through the compressor, burner and turbine sections of the engine. It goes around the outside of it. Yes the compression and then burning of air and fuel create thrust too but the vast majority of the thrust on a high bypass turbofan is created by the fan section which bypasses the core hence the name high bypass. The description you used it accurate to a pure turbojet engine though. We could use RPM to measure fan speed like in a prop plane but the standard in jet engines is N1. It's because N1/fan speed have a direct correlation to the amount of thrust the engine is making is why we use N1 as the means determining how much thrust the engine is putting out on the 737.

 

Edit: I just discussed this with my brother. I think maybe we're getting caught up on what we each mean when saying  "directly related".

Okay we are starting to get off topic, Tom, in my original post you agree that the behavior in the PMDG NGX is incorrect and is a bug, and will it be added to the list for SP2?

Guys can we be clear on something here, are you making the distinction between 'Thrust' and Thrust lever angle'?

But no one mentioned thrust lever angle.  We are talking about rated thrust and actual thrust.  How thrust relates to N1 is not the point but the thread deviated in that direction.

Just an addition: the engines have an specific amount of thrust (26,4k LBS on a 738 at least here at air berlin). How fast the engines need to spool (N1) to give this thrust varies with temperature of engine, outside air temperature, wear, vibrations, fuel used (here in germany you can get only jet A1) and pressure. The computer calculates a N1 setting to get this thrust (in reality it'll guess how much N1 is needed because wear of shaft or the oil used, or even fuel etc can not be calculated and there are no sensors for thrust generated). If you use less N1 you have not the full thrust (simple!). So N1 is a value for thrust used. You can say less N1 results in less thrust. But you can not say "I'll use xyz percent less N1 so I have thrust 0815 LBS". Also N1 (and therefore thrust produced in lbs) is limited by various indicators (mostly EGT but also N1 and Vib). So on old engines or hot temperatures you won't get your full 26,4k LBS of thrust.

 

As said the OP is correct. For takeoff calculations and the resulting EGT, N1 and thrust values it makes no difference if packs are off or engine bleeds are off with apu powering the packs; both are "packs off takeoff" procedures. Nowaday's the fuel is pretty expensive and you use the apu only for bleed to start the engines or in cold (<14°C) or hot (>30°C) weather for air conditioning. If so you would power up the apu 10 minutes before boarding and shut it off after most of the passengers are gone (let's say 5 minutes, just watch the cam which shows passengers deboarding in front of the cockpit door).

 

Also you can use 1 pack in high or 2 packs in normal mode if powered by apu. For a apu takeoff you'd use pack 2 (cabin) at high and 1 (mostly cockpit) is shut off. Also you will switch on recirc fans if you switch off the packs for starting engines. So passengers have a adequate airflow and they don't panic as it would become quiet. And if it's late in the evening you'll fly with recirc fan 1 off so it's a little bit more quiet inside cockpit at low speeds (approach). It's not simulated within pmdg 737 either.

 

Sorry for my bad english. Hope you can read and understand it.

Okay we are starting to get off topic, Tom, in my original post you agree that the behavior in the PMDG NGX is incorrect and is a bug, and will it be added to the list for SP2?

Sorry for going off topic but yes I agree. You would only do a bleeds off takeoff if you were already doing a 26K max power takeoff. For a given set of atmospheric conditions you would get a certain N1 with bleeds on. Since we use N1 as the power setting in the 737 the only reason to do a bleeds off takeoff would be to get more power which would be indicated by a higher N1.

N1 is what we use to set power, but it's not a "direct" indication of thrust. 90%N1 is not 10%more thrust than 80%N1. 90%N1 at 15C is not the same same thrust as 90% at 35C.

 

EPR is a more direct indication, but GE/SNECMA likes  N1, I guess so here we are.

 

Jet Transport Performance Methods has a pretty good discussion on this topic. I should be required reading for RW and simmers, alike. Google it.

I confirmed the operation of this in the real plane yesterday. With the packs and bleeds on we showed an N1 of 89.2 for both engines. I switched the #2 bleed off and the right N1 changed to 89.8 and the left stayed at 89.2.

Let as know outcome.

Ryan said he's passed the information on to the programmers, hopefully it will get corrected in future, but no promises given.  Right now they are concentrating on the 777 and 747v2, so no time for the NGX.

I confirmed the operation of this in the real plane yesterday. With the packs and bleeds on we showed an N1 of 89.2 for both engines. I switched the #2 bleed off and the right N1 changed to 89.8 and the left stayed at 89.2.

I just tried this today. Sitting at the gate, N1 was 91.1 with bleeds ON and packs OFF. I turned OFF a bleed, no change. I turned ON a pack, and the N1 dropped to 90.5.