N1 difference

[kevinh 409]

Will it get programmed for the future update? Or who knows lol so what is the alternative way...

I doubt it, but you never know. It's a fairly minor issue that most people simply won't notice. I reported it a long time ago in a support ticket and provided some supporting information. Usually when something you report is going to be fixed PMDG let you know.

 

I already mentioned a possible way round it earlier in the thread.

[Driver170 42]

Packs off takeoff? I'll see how that works tonight then

[badderjet 181]

There is a difference [...] will vary slightly between engines.

 

Thanks a whole lot for that detailed information. Very insightful and a good read. Sure enough I noticed variations in all but the N1 parameters IRL and sure enough attributed them to slight variations between the engines, but I didn't know about the internal adjustments to the EEC.

 

Still I'm not 100% sure if I understood that part: In your second last post you say N1 (you mean actual RPM here?) equals thrust, but then you say a new engine might need 97% N1 to produce 24K, and an older one might need 99.5%. So in fact these two are running at two different speeds but yet produce the same thrust (and in that case N1 would not equal thrust), right? I understand a worn engine might need a higher fan speed to produce the same amount of thrust, I'm just not getting it together with the N1 value.

 

If I understand the essence correctly, you are saying the engines (or their indications) are 'matched' in a way so that if you have the same N1 displayed on the flight deck, they produce the same amount of thrust even if the actual RPMs (hence the actual N1) might differ slightly, is that correct? Sorry for the confusion.

 

[pcubine 143]

Packs off takeoff? I'll see how that works tonight then

 

So how did that takeoff and subsequent flight last night  go?

[Driver170 42]

So how did that takeoff and subsequent flight last night  go?

 

I haven't done any flying yet

[JRBarrett 3,034]

Thanks a whole lot for that detailed information. Very insightful and a good read. Sure enough I noticed variations in all but the N1 parameters IRL and sure enough attributed them to slight variations between the engines, but I didn't know about the internal adjustments to the EEC.

 

Still I'm not 100% sure if I understood that part: In your second last post you say N1 (you mean actual RPM here?) equals thrust, but then you say a new engine might need 97% N1 to produce 24K, and an older one might need 99.5%. So in fact these two are running at two different speeds but yet produce the same thrust (and in that case N1 would not equal thrust), right? I understand a worn engine might need a higher fan speed to produce the same amount of thrust, I'm just not getting it together with the N1 value.

 

If I understand the essence correctly, you are saying the engines (or their indications) are 'matched' in a way so that if you have the same N1 displayed on the flight deck, they produce the same amount of thrust even if the actual RPMs (hence the actual N1) might differ slightly, is that correct? Sorry for the confusion.

 

N1 is an indirect reading of thrust. It is giving you the speed of the fan (as a percentage of maximum rotational speed), but the only direct correlation to thrust is in the amount of air flowing through the bypass duct. Assuming that the fan blades and stators are perfectly efficient per their design, a given N1 speed (at a given air density and temperature) should give "x" amount of mass airflow. There is an indirect relationship between N1 speed and the airflow through the core, since the N1 rotor(s) are being driven by the power of the expanding gas from the core which in turn is rotating the N1 turbine(s).

 

The only way to directly relate N1 to thrust would be in an engine test cell.

 

EPR, unlike N1, IS a direct measurement of total thrust.

 

No two engines will produce exactly the same thrust at a given power setting, but in practical applications, a slight difference side-to-side is not going to matter all that much.

 

The "split" seen on the cockpit indicators or FMS at maximum calculated "N1 of the day" (which comes from the stored engine trim settings) would typically only be only one or two tenths of a percent between two engines. If you see a two or three percent split, that would be due to the bleed air configuration being different side to side.

 

Typically most takeoffs will be using some kind of derate, rather than at max permitted N1 in any case.

 

On an aircraft which uses N1 as a primary power reference, at all times other than a max power takeoff, the goal of the pilot, (or auto throttle system) is to typically match N1 speeds as closely as possible. This is not a hard and fast rule - old time pilots flying Boeing 727s equipped with JT8D engines, found through experience that they often got better results (when manually controlling thrust), by matching fuel flow readings between engines, (rather than N1 or EPR).

 

They discovered that there happened to be a coincidental correlation between fuel flow and airspeed.... i.e. setting a fuel flow of 2500 pounds per hour in level flight would give an indicated airspeed of 250 knots - 3200 PPH would give 320 knots IAS etc.

 

Not a deliberate design - it just worked out that way.

[Spam3d 54]

Dont have much else to add! Jim has nailed it, i have thrown you off slightly by giving a very split percentage in my example sorry about that!  but as stated the difference is very small. Unlike Jim i work only engine overhaul so am accustom to test cell data, not air side installation and ground testing. Always a school day...

[badderjet 181]

Thanks, again great read and well explained.

 

[kevinh 409]

 

 

The "split" seen on the cockpit indicators or FMS at maximum calculated "N1 of the day" (which comes from the stored engine trim settings) would typically only be only one or two tenths of a percent between two engines. If you see a two or three percent split, that would be due to the bleed air configuration being different side to side.

The split between the FMC calculated N1 rating is not related to any trimming of the engine control unit surely? The FMC uses data stored in its database to calculate rated N1 for the ambient conditions and the selected bleeds.

[JRBarrett 3,034]

[kevinh 409]

I can't speak for the CFM56 engines on a 737, as I have not worked on those, but on Honeywell TFE-731 engines ( which use EEC) and P&W 305/306/308 series engines which use FADEC, the answer is "yes". The trim information is stored in the engine computer, and the MAXIMUM permissible N1 displayed on the FMS perf screen comes from there - (the engine computer) - it is not calculated by the FMS.

 

I admit that the 737/CFM56 may be different, but in many years of working on multiple jet aircraft, I have not yet seen one that uses N1 as a primary thrust reference that does NOT do it this way.

 

The engine computer is the only piece of hardware on the aircraft that "knows" what the maximum allowable power setting is for the particular engine it is attached to for the ambient temperature and pressure at a given moment.

 

If you were to turn the EEC off, you would most likely lose the FMS N1 perf display, as well as the N1 bug on the EICAS. The EEC (on the engine) and FMS PERF computer communicate with each other continuously.

 

Again, this only comes into play for MAXIMUM non-de rated power settings - i.e. Power levers full forward. For any power setting less than maximum, then yes, the FMS alone can do the calculation of the appropriate N1. An N1 reduction on one particular engine caused by a non-standard bleed air configuration will also come from the FMS.

 

And don't call me Shirley...

Boeings generally use the FMC to calculate N1/EPR ratings. The FCOM mentions that the FMC produces the figures. So the N1 figures displayed in the FMC TAKEOFF REF page aren't affected by any kind of trimming. 757 and 767 are a bit different as they have a separate Thrust Management System rather have it as part of the FMS. As you say, the FADEC also calculates full thrust ratings and uses that data to display on the EPR/N1 indicator as well as control the engine. No doubt those calculations are affected by trimming, because they will determine the maximum thrust that can be set should the FMC not be producing thrust commands.

[Spam3d 54]

well the EEC is connected to the auto throttle computer and the FMC, so while the FMC may do the calculations and so fourth, I would have assumed it still takes the data from the ID plug for its calculations? it is the only place that the specific redline RPM and correct trim settings for all configurations of thrust settings are stored for that specific engine, i cant see how the EICAS would be able to show the redline N1 and so fourth? Interestingly the N1 modifier adjustments are effective up to an altitude of 15500ft or mach .45,

[Houghton11 12]

Totally unnecessary dump of useless knowledge incoming. Hopefully just answers all the questions.

 

Good discussion though regarding engines and trimming etc etc.

 

The EEC does send information to the Display Electronics Unit (and conversely, the FMC) about engine thrust rating (22,000 lbs) vs 24 vs whatever. It's not talking specially to the ID plug.

 

The engine's going to produce given amount of thrust. The FMC knows (for example) to 22,000 lbs equals 95.6% N1 in this environment (all the Air Data).

 

The EEC uses fan trim balance to adjust the engine fan speed for engine variations. After engine assembly, engine tests are done to make sure it meets engine performance requirements.
One of these tests is to measure engine thrust and fan speed. To make sure all engines have the same takeoff and early climb thrust for the same indicated N1 speed, the fan trim is used to
adjust indicated and commanded N1 speeds. This adjustment is only used to decrease the commanded N1 speed and increase the indicated N1 speed (think about it - it makes sense).

There are eight N1 fan trim levels. These levels 0 through 7 with 0 as no trim and 7 the maximum trim of 2.36 percent. The N1 trim level for the engine is stored in the engine identification plug.

 

I'm pretty sure the ID plug (or Characterization Plug as some call it) is not some fancy silicon chip or anything, but through the aviation grape-vine I hear it's just a bunch of small resistors laid out in a specific way and certain resistance.  I'm not sure though - the guidance of the ID plug that I have access to is pretty broad. It doesn't 'store' massive amounts of information, but just engine type, thrust rating, N1 trim, combustor configuration, and if it's an 'on condition' engine. There's no unique maximum RPMs or fancy variables.

[JRBarrett 3,034]

Totally unnecessary dump of useless knowledge incoming. Hopefully just answers all the questions.

 

Good discussion though regarding engines and trimming etc etc.

 

The EEC does send information to the Display Electronics Unit (and conversely, the FMC) about engine thrust rating (22,000 lbs) vs 24 vs whatever. It's not talking specially to the ID plug.

 

The engine's going to produce given amount of thrust. The FMC knows (for example) to 22,000 lbs equals 95.6% N1 in this environment (all the Air Data).

 

The EEC uses fan trim balance to adjust the engine fan speed for engine variations. After engine assembly, engine tests are done to make sure it meets engine performance requirements.

One of these tests is to measure engine thrust and fan speed. To make sure all engines have the same takeoff and early climb thrust for the same indicated N1 speed, the fan trim is used to

adjust indicated and commanded N1 speeds. This adjustment is only used to decrease the commanded N1 speed and increase the indicated N1 speed (think about it - it makes sense).

There are eight N1 fan trim levels. These levels 0 through 7 with 0 as no trim and 7 the maximum trim of 2.36 percent. The N1 trim level for the engine is stored in the engine identification plug.

 

I'm pretty sure the ID plug (or Characterization Plug as some call it) is not some fancy silicon chip or anything, but through the aviation grape-vine I hear it's just a bunch of small resistors laid out in a specific way and certain resistance. I'm not sure though - the guidance of the ID plug that I have access to is pretty broad. It doesn't 'store' massive amounts of information, but just engine type, thrust rating, N1 trim, combustor configuration, and if it's an 'on condition' engine. There's no unique maximum RPMs or fancy variables.

Excellent info, and CFM-specific! My own experience is in smaller biz jet engines, and the larger RR Tay 611-8 used on the Gulfstream IV. Interestingly, the Tay does not use ANY on-engine computer whatsoever. The engine fuel control is entirely hydro mechanical, and though engine test equipment is used for post-installation trimming, the actual trim adjustments are mechanical. The GIV does have an FMS-linked performance computer though, which sets calculated thrust for the active flight regime via the electromechanical autothrottle system.

 

Appreciate the info re: the 737 - though this topic has strayed a bit from the OPs original questions!

[Houghton11 12]

 

 

...My own experience is in smaller biz jet engines, and the larger RR Tay...

Those bloody Tays! Man are they loud. Once heard (more like felt) one compressor stall during some engine running/adjustments from across the field. I thought the flipping hangar roof was going to fall down.