Some doubts on Table 1506

[Taz5150]

I have come to a point where I need some expert and experienced opinions here. Just to clarify, I have read and searched extensively this forum and, of course, read Roy’s paper on thrust vs altitude.

The thing is I have some real N1 vs Thrust data at different altitudes and Machs. As the aircraft weight drops, you need less thrust to maintain Mach at constant altitude. So how do I deal with this? At same Mach, Altitude and cN1, I have different net thrusts do to changes in weight. Should I work with maximum weight for table 1506?

Enlighten me please.

thank you.

 

[Roy Holmes]

For a constant flight condition, same Mach, altitude and CN1 or N1 thrust is equal to drag. Drag is made up of form drag and induced drag. Form drag is constant because speed is constant, but induced drag varies with angle of attack which is a function of lift. As weight reduces so does the required lift and so angle of attack reduces as does induced drag. What you are seeing is what should happen. Net thrust = Thrust - Drag. As weight reduces, induced drag reduces so net thrust increases. That is why fuel consumption rate reduces as fuel is burned.
Weight is not an input/factor in 1506, so leave it as it is.
Also remember that I simplified my paper to make it easier to understand. It considers the engine to be a single stage where all the thrust comes from burning fuel in the jetpipe and by-pass N1 thrust is not considered. That will give you more variation than my paper assumes. Also it was written around the type of engine where thrust is not linear with throttle position as is largely the case with modern digital engine controls.
Roy

 

[Taz5150]

Thank you Roy. Yes, I think I understand the physics and the reason why N1 and Thrust change with time at same Mach and Altitude.

The problem and my question is more specific to Flight Sims and how to build table 1506 when you have so many data points. To give you an example:

Altitude: 29,000 ft
Mach: 0.7
CN1: 82.5%
CFN: from 8,500lbf to 11,000lbf (over 20 data points within this range)

Now, if you were to build 1506 given this data, with what CFN value would you work? Average? Maximum weight value of CFN?

Thank you!

Edit: Going over my data, I do have Alpha (AoA) for every measurement condition. As weight drops (same Mach and Altitude), also does Alpha until you get to almost 0 AoA. Should I take this measurement as the one to include in 1506?

 

[Roy Holmes]

There is something odd about that data. It suggests that for a constant 82.5% CN1 you have a wide range of corrected fuel flow. If that is the case it is dealing with a very large range of atmospheric variation, not a standard atmosphere. The whole point about corrected values is that you get them by doing math such that your data is converted to what it would be in a standard atmosphere. the 1500 series tables assume standard atmosphere, so I believe you get nothing from the non-standard data. I suggest you look at the 20 data points to see what they are covering and when you find one for the standard atmosphere use that.
Since you are using standards you do NOT have to do specific values for 1505, the trick is finding the best match for 1506 and that has nothing to do with AOA or drag just one curve for ISO conditions
Roy

 

[Taz5150]

Data is taken from the output of an aircraft manufacturer at ISA conditions, but I will re-check.

Still, going through the FCOM of a similar aircraft, when looking at Cruise Performance tables, I see the same behavior: same Mach and Altitude you get different N1 and FF for different weights.

Just need to understand what goes into 1506 ... I will try with maximum thrust (maximum weight) and see where that takes me.

 

[bstolle]

I'm adjusting 1506 to match performance for all weights. You know N1 for a specific weight, e.g. MTOW and the resulting TAS.

Next step is to reduce the weight, reduce N1 according to the performance tables and check if the resulting speed matches the performance tables. If not you have to adjust the curve.

You have to do that for all weight and altitude combinations. That takes a lot of time and as Roy has mentioned, getting things right for ISA conditions is time consuming enough.

 

[Taz5150]

Thank you bstolle. So the process is more or less what I was going to try out: calculate N1 vs Thrust for different Machs at MTOW, and then tweak curves at different weights.

 

[Roy Holmes]

1506 input is CN1 and Mach number
1506 output is thrust scalar versus CN1 and Mach
There is no real value of thrust and no value of weight involved in constructing 1506, the table outputs are scalars that apply to any engine irrespective of its actual thrust. A figure of 1.0 as a scalar in the Mach =0 column means the thrust is as it is written for sea level in the aircraft.cfg. for the same CN1 and Mach =1 the scalar will be higher usually. You could have several Mach columns, but that is all the tweaking needed. 1506 has no fuel consumption outputs. You might want to tweak curves following experiments but what would vary would be thrust scalar versus Mach and CN1
If you consistently find that actual thrust is lower than the specs state for a given CN1 and Mach, then you would increase the scalar for those conditions. However there are no gauges that measure thrust other than in ground rigs. If you have differences in speed versus CN1 then look at the drag values involved, in particular induced drag varies with the weights. Induced drag has nothing to do with engine parameters so none of the 1502 - 1507 values are involved.

"You have to do that for all weight and altitude combinations. "
Strictly not. The whole point of the "Corrected" method is that the airplane and sim will correct N1 based on altitude so N1 should always be correct. The correction comes from equations in the sim and as long as you have the 1501 to 1507 tables constructed like in my paper you need do nothing else engine-wise.
Roy

 

[Taz5150]

Thank you Roy. It seems once again the problem is understanding and decoding Asobo’s way of dealing with the engine model.
I will keep on investigating.

 

[jcomm]

I honestly wouldn't rely much on the current "jet engine" implementation n MFS.

Yesterday there was some post from an Aerosoft dev regarding their commuter CRJ release being on hold for a fix for a "strong" bug in the jet engine code of MFS. ASOBO is dynamically taking care of some of the feedback, and I guess the major players like Aerosoft, but also for sure PMDG and A2A are behind the scenes trying o push them to add and fix much stuff.

MFS is not similar to it's predecessors.. It's an ongoing project, for the good and for the bad it also appears to have borrowed this characteristic from X-Plane