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Takeoff-roll at TOGA Engine Issues

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spain
Hi everyone,

After some long months of design, tweaking and a lot of input from this community, I am facing an issue I’m bot sure how to solve.

All engine tables have been set accordingly, specially tables 1503-1504, 1502 and 1506 (for net thrust). Everything works fine and is on spot more or less during Cruise, but during takeoff the engine seems to have issues.

Starting at static, with full power (parking brakes on), engine primary parameters (N1 and N2) are right on spot and maintain their values (N1 at 84% and N2 at 104%). Now, when I turn off parking brakes and initiate the roll, N1 and N2 start coming down to around mid-70s N1 at VR. This is not what should happen.

I understand once we get rolling, thrust should come down slightly with speed due to ram drag, but N1 should be maintained (even increase slightly) to compensate. 1503 and 1504 should not be a problem; they are set such that at full throttle, with increase Mach N2 (uncorrected) is maintained. 1502 is set the same for low and high Mach, so that for that given CN2, N1 (uncorrected) is maintained constant. But then we come to 1506 ... since drag is degrading thrust, I understand CN1 degrades as well ... but shouldn’t it be compensated some how? I always thought the flow was 1503-04->1502->1506 ... not the other way around.

any help is much appreciated!
 
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france
Hi,
Which plane are you talking about ? a turbo propeller?
If I remember correctly, turbo propellers have never been perfect in FSX and this could still be the case in MSFS.
 
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spain
Nope, an airliner with turbofans. I think part of the problem is with tables 1503-1504 ... for 1503, at maximum throttle I have 104% CN2. For 1504 (@Mach 0.9), at maximum throttle I have 96.5% (the equivalent CN2 due to Mach). Don't tell me why, but if instead of 96.5% I place 104%, then N1 and N2 maintain their expected values and behavior. Still investigating why this is so ...

EDIT: In real life, at SL, CN2 looks something like this at 100% throttle:

MACH
0​
0.1​
0.2​
0.3​
0.4​
0.5​
0.6​
0.7​
0.8​
CN2
98​
99​
99​
98​
98​
97​
96​
96​
95​

So, although LO and HI seem to follow the "rule", up to MACH 0.4 CN2 is increases a little bit and maintains itself.
 
Last edited:

Roy Holmes

Resource contributor
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I have a strong suspicion that the engine code has been changed. For example, with a single shaft turbine, ie N1=N2 and 1502 (n2_to_n1_table) set accordingly, N1 is different from N2.
On the ground after start you will get the idle N1 you have set, but once you start to move it goes up to between 60 and 70% and you will have to use speedbrake to slow down in the air. I have a particular airplane in P3D and now in MSFS and the P3D version behaves very much to the performance manual. The MSFS one is quite different but I have not yet gathered enough flight test data to determine what is going on. For sure the throttle seems to be out of the loop some of the time, ie no N1 change with throttle change.
I hope to start testing this weekend.
I'm pretty sure they have abandoned the "correct to sea level and then calculate for current flight conditions " method that is an industry standard and, more importantly, works as long as your tables are calculated properly.
Roy
 
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spain
Precisely, it feels like 1504 is not corrected. Still working on it and testing. Will get back to you all if I find a reasonable answer.
 
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spain
Ok, maybe I’m missing something here. I’ve developed tables 1503 and 1504 as per Roy’s paper; this means that at full throttle and SL, CN2 = N2 = 104% @M0 and thus CN2 = 96.5% @M0.9.
At takeoff (TOGA), we start accelerating and building up speed. Since LO/HI tables go from Mach 0 to 0.9, the Sim interpolates at Machs in between, which means that at Mach 0.1 (during takeoff), we have CN2 = 103.16% (N2 = 103.27%). So mathematically speaking, in the Sim, it makes sense that following Roy’s paper we would be experiencing N2 reduction (and thus N1) during takeoff roll.

Does my assumption make sense?
 
Last edited:

Roy Holmes

Resource contributor
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Not exactly,
CN2=N2/Sqrt(theta2)
N2=CN2*Sqrt(theta2)
At 0.1M and SL, Sqrt(Theta2) = 1.0002, so for 104% N2, CN2 would be 104/1.0002=103.8962%
N2 would be 103.8962*1.002 =104.
Remember the uncorrected value should vary with throttle only, altitude and Mach should not change it much except a tad because of interpreting results between table values
So N2 should sensibly remain constant on take off. N1 would also remain constant and it is N1 that dictates thrust.
The corrected values are just intermediate values in the calculations
Roy
 
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spain
Thanks Roy.

Yes, the theory is exactly as you say; I don't think I have a problem with that ;) So I do agree that at SL and Mach 0.1, CN2 should be 103.8962% and thus N2 is 104%. This is clear.

But correct me if I'm wrong; what the Sim does is interpolate (linearly) between tables 1503 (Mach 0 in my case) and 1504 (Mach 0.9). So at full TLA, I've got (at SL):

Tbl1503: CN2 = 104%
Tbl1504: CN2 = 96.5%

Interpolating both for Mach 0.1, gives me CN2 = 103.167% which if we "uncorrect", gives us a N2 = 103.27%. So the only way I can probably get a realistic TOGA performance, is by making tables 1503 and 1504 equal at TLA 1.0 (full throttle). Is the only way I've found I can avoid linear interpolation.
 

Roy Holmes

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You are wrong
At 0.1M and SL, Sqrt(Theta2) = 1.0002, so for 104% N2, CN2 would be 104/1.0002=103.8962%
N2 would be 103.8962*1.002 =104.
So N2 stays at 104% when at full throttle at all Mach numbers and altitudes.
Once you have created the tables, forget about corrected values.
If it does not work in MSFS, it is because they have changed the engine model. They said they would look at changing it.
Have you got thrust data and what did you do about the intake area given that it is a turbo fan you are considering?
Roy
 
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spain
You are wrong
At 0.1M and SL, Sqrt(Theta2) = 1.0002, so for 104% N2, CN2 would be 104/1.0002=103.8962%
N2 would be 103.8962*1.002 =104.
So N2 stays at 104% when at full throttle at all Mach numbers and altitudes.
Once you have created the tables, forget about corrected values.
If it does not work in MSFS, it is because they have changed the engine model. They said they would look at changing it.
Have you got thrust data and what did you do about the intake area given that it is a turbo fan you are considering?
Roy
I think we are misunderstanding each other here. Let me try to be more "graphical" on the issue at hand, and forget we are talking about MSFS (I am experiencing the same issue in P3D v5).

Here is an excerpt of tables 1503 and 1504 as per your document and common practice within the community:

1613981707209.png


Nothing new here. We obtain our CN2 in Table 1504 multiplying by 1/SQRT(theta).

Now the Sim works with these two tables and interpolates everything in between, right? But just so we understand each other, lets take for a moment that the Sim allows more tables between 1503 and 1504, and I have a similar table at Mach 0.1:

1613982285996.png


So up to this point, we are in the same page. As you can see, CN2 at TLA 1.0 and Mach 0.1 is 103.9 (with more significant figures we would get exactly your value).

But now lets move to what the Sim does. As far as I know, for Machs between LO (0.0) and HI (0.9) the Sim interpolates linearly. A simple linear interpolation between 104.0% (Mach 0.0) and 96.5% (Mach 0.9) gives CN2 = 103.1643% at Mach 0.1. So as you can see, and as we expect from linear interpolation, the value is off by a decent amount.

My solution: the only way I found I could have constant N2 and thus N1 values during takeoff roll at TOGA (maximum power) is to have the same values in both tables at row TLA 1 so that not interpolation is being done.

Not trying to be stubborn here, just don't want to leave the topic open with discrepancies so others might benefit from it.

Thanks for your support Roy (as always!).
 

Roy Holmes

Resource contributor
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I see your point, but is it relevant?
First, what is the difference for N2, not CN2? I think it is 103.2
Second, If you want N1 to be exactly 104 at 0.1 Mach so that thrust is more accurate, change CN1 in 1502 rather than messing up the whole thing by changing 1503/1504.
It is an interesting subject
Roy
 
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spain
I see your point, but is it relevant?
First, what is the difference for N2, not CN2? I think it is 103.2
Second, If you want N1 to be exactly 104 at 0.1 Mach so that thrust is more accurate, change CN1 in 1502 rather than messing up the whole thing by changing 1503/1504.
It is an interesting subject
Roy
Its probably not that relevant ... I've found several posts regarding FADEC vs. non-FADEC designs and this is where the main development differences are. Also, take into account this is my first experience with engine design for a flight sim; although most of my issues and doubts have been solved by searching and reading this forum (I try to only post after an extensive due diligence), there are some gaps that sometimes need answer.

Always a pleasure learning something new and always thankful for all your support.

Thanks!
 
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