Jet Engine Performance

[Roy Holmes]

Stock and unedited 1505
Roy

 

[WarpD]

Read through this thread at AVSIM:
http://forum.avsim.net/topic/74600-...page__p__521064__hl__engine spool#entry521064

 

[Roy Holmes]

Thanks for pointing that out. Very interesting. Need to study it more. Guess you are on the money with your conclusions.

Roy

 

[mgh]

That's an interesting link which I haven't yet fully understood. However, it refers to Table 1501 which isn't in the default FSX B747 .air file. Should the "spool up rate" parameter be in the.cfg file?

 

[Roy Holmes]

Should the "spool up rate" parameter be in the.cfg file?

I believe that 1501 has not been used in stock FS airplanes since FS2002.
I also believe it has been replaced by
fuel_flow_gain = 0.002 //Gain on fuel flow
in the [TurbineEngineData] section of the config file.

Increasing fuel flow gain does increase the speed of engine reaction to throttle movement. Much less lag.

But I have never thought what it does to start timing and results, which is why I am so interested in that post. I have a few jets that take too long to get started. If you look at Mixture settings they are zero until the engine gets combustion, so somehow that is factored into the start cycle. "Mixture" in FS jets acts like HP fuel flow, which you open at certain RPM's during engine start, by either opening a HO cock or moving the throttle to Idle.

My conclusion is that there is some relationship between fuel_flow_gain and when combustion begins during engine start. However I have no data at the moment.

Roy

 

[Roy Holmes]

Let me correct some of what I said in my last post.
1501 Engine Turbine contained the following records
Static Thrust
Inlet area
Spool up rate
Rated N2 RPM
Afterburner true/false
Bypass ratio?
Relative efficiency?
The one without the ? are now in the config file

Roy

 

[mgh]

It gets more intriguing. Without by-pass ratio how does FSX distinguish turbo jet and turbofan engines? and, come to think of it, between single and twin spool engines - by setting N1 = N2 in Table 1502?

 

[Roy Holmes]

by setting N1 = N2 in Table 1502?

That is what I did when modeling a Hunter.

Roy

 

[sergio]

Mgh
... The chart as plotted does not help understand it, but if you transpose the axes, it becomes clearer. ...

I think the axis of this table are swapped because "Input values in tables should be defined in ascending order." (from the ESP SDK).

In the old Aviation History Forum there was a long discussion about this table at the time of its introduction (FS2000). Here some hints by Ron Freimuth:

True, slope of TBL 1505 affects the current 'spool up rate'. However, it has definetly been shown to control the "Corrected Fuel Flow" parameter available to gauges.

... TBL 1505 maps CN2 to Corrected Fuel Flow. Actual fuel flow in PPH is
Thrust_Reference * 1505(CN2) * Delta_t/[Theta_t]^0.5
Any other effect is secondary.

Actually 1505 is related to CFF = SL_thrust_100%CN1 * [1505]. So, [1505] is CFF/Thrust(100% CN1) which kind of a 'corrected' SFC...

... fuel_flow_scalar DOES NOT change this new pph. However, moving the 'x' value in TBL 1505 does (based on current CN2).

Not sure if this findings for FS8 and FS9 are still valid for FS10.

 

[taguilo]

Read through this thread at AVSIM:
http://forum.avsim.net/topic/74600-...page__p__521064__hl__engine spool#entry521064

I remember that long time ago discussion...The guy was talking mostly on how to update Table 1505 to achieve a correct turbine spool rate. My point was the idea wasn't bad, but I thought he was too much generalizing and also there were other critical aspects -ie bleed air pressure available- that were not considered by FS, and therefore needed to "model" by gauge code.

On those days I was working on a 757-200 RR engine model, which I finally got to behave very close to the real, especially in the startup process, where FS has its weakest point on all the different types of engines simulated.
To obtain a proper functioning, I had to modify the shape of table 1505, which turned from being the typical sloped found in FS default jets into an almost straight line, as you can see in the attached image.

The problem with the sloped shape was in the spiking effect at acceleration, which made the needle an exaggerated overshoot and roll back to the N1 value selected by throttle position. I tried to overcome this first by altering fuel_flow_gain values but then, if I lowered the default value, spike was reduced but spool rate decreased too much; and when I increased the value, spool rate improved but spike effect increased as well. I found an acceptable solution by snooping on LEVELD 767 .air file, where I discovered they used a straight shape in 1505 and their engines spooled quite right. Besides 1505 I worked a lot on 1502 CN1-Cn2 (which I properly gave a CN2 52% idle), 1503-1504, and also designed an EPR-vs-CN2 table, but that's another story.

Now, I am rather frustrated these days cause I'm working on a C208 Caravan and unfortunately the turboprop model lacks of table 1505 and 1502, and tables 1503 and 1504 are useless. This means Gas Generator turbine (N1) idle value is hard coded at 62 % and cannot be modified by altering the table (in C208 and most PT6s is 52%) so the only way to get the right value is by gauge code (RA Turbine Duke uses this approach as well). But the worst part is the spooling rate of torque gauge. No matter the best FDE you fly with, torque will respond with an important lag, also giving the infamous spike effect I talked about. Again, if you increase fuel_flow_gain, spool increases but spike is so bad that, for example at take off it overshoots the torque limit. I still have to find the right polinomial to update the gauge with though...

Tom

 

[mgh]

I created a spreadsheet (S/S) to check the steady state values against those given by AFSD (“first things first”.) The values of thrust are in very close agreement. However there are anomalies with fuel flow.

Generally, fuel flow seems to be given by Net Thrust (= Gross Thrust – Ram Drag) * ThrustSpecificFuelConsumption (from the .cfg file). I’ve checked this at a number of speeds and altitudes and the fuel flows match to better than 0.5%.

However, there is a significant anomaly with throttle closed, stationary on ground:

AFSD gross thrust = 2598 lb, ram drag = 0 lb, net thrust =2598 lb, fuel flow = 1436 pph

S/S gross thrust = 2601 lb, ram drag = 0 lb, net thrust =2601 lb, fuel flow = 1040 pph

Also when the throttle is closed at speed the Net Thrust becomes negative so the previous simple fuel flow calculation obviously fails.

AFSD gross thrust = 5456 lb, ram drag = 6086 lb, net thrust = -630 lb, fuel flow = 1476 pph

S/S gross thrust = 5452 lb, ram drag = 6079 lb, net thrust = -627 lb, fuel flow = ???? pph

I haven't found a way to derive the FSX value. Any thoughts?

Also FSX seems only to interpolate within the .air file tables. If an input value is outside the range then the nearest output value is used.

 

[Roy Holmes]

Fuel flow and gross thrust are related. Ignoring engine efficiencies, double the gross thrust uses double the fuel.

Ram drag is airplane specific and in FS is a function of inlet_area in the config file.

Idle fuel flow can be affected by the individual throttle mechanism unless you set it to zero. Similarly full throttle movement does not necessarily give 100% throttle.

A:TURB ENG FUEL FLOW gives a correct value when the throttle is closed at speed and A:TURB ENG JET THRUST will go negative, so FS must be using Gross thrust for fuel flow.

Since this has obviously got your interest, you might want to look at the thrust reduction with increased altitude in FS. Thrust appears to reduce with air pressure ratio rather than air density ratio. The tables use Inverse Air Pressure because it allows input values in ascending order. The result of using pressure rather than density is that airplane max Mach decreases with altitude when it should generally peak somewhere around 30K. Ceilings are lower than the real airplane had for the same reason. There are a variety of ways to get around this.

Also FSX seems only to interpolate within the .air file tables. If an input value is outside the range then the nearest output value is used.

FSX and FSXA do this differently and you need to consider this fact if you expect to exceed the high Mach figures in the tables. For the same table values thrust performance differs between FSX and FSXA under these conditions.

Roy

 

[mgh]

I've included all the relevant .air file tables in my spreadsheet and replicated FSX's calculations, which is why I have something to compare AFSD with.

Table 1506 has maximum CN1 of 110. The largest CN1 returned from Table 1502 is 113.5. (The maximum in that table is 118). The result is that Table 1506 returns the thrust coefficient for CN1 =110, not for my actual CN1 = 113.5. This could be correct by an extra row at the bottom of Table 1506.

However, this wouldn't affect the fuel flow anomalies i reported.

 

[Roy Holmes]

@mgh
I believe that FSXA looks at the max CN1 of all the tables and limits CN1 accordingly. This is based on having up to 130% CN1 in all tables except 1505which had 105 as the max CN1 and that was exactly what I was getting although I was expecting around 115 in the test conditions.
Put 1505 up to 130 and got CN1 115.
FSX would extrapolate values when the table limits were met, but FSXA maintains values at the max table entry.

Roy

 

[mgh]

I haven't had much time to pursue investigations into how Microsoft models jet engines and haven't made any real progress. Everytime I think I've found somewthing it promptly slips away!

 

[sergio]

... Without by-pass ratio how does FSX distinguish turbo jet and turbofan engines...

It seems to be a matter of the shape of table 1507.

Some time ago Geoffrey Abbey (not sure about the spelling) published a spreadsheet to recalculate an existing table 1507 to match different inlet area and different bypass ratio.

Here a screenshot of his excel file:

I don't know if this spreadsheet is still available on the net.

 

[jx_]

I believe the title of table 1505 makes it unnecessarily confusing. Table 1505 is simply "time to N2" and has NOTHING to do with fuel flow, PPH, or SFC.

When the engine starts, N2 will IMMEDIATELY go to x=0's N2 value:


x=0, y=20 (in this case the engine jumps to 20%N2...unless of course the ignition occurred at 20)


then each x point after is time to reach the next N2 value. The closer to the left---the faster (steeper) and the farther right---the slower (shallow slope.) If you were to put in a declining slope N2 would decrease, at the time rate specified.

so if...

x=0, y=20
x=1.00, y=30
x=2.00, y=60

...the time it take to go from 20-30 N2 and 30-60 N2 would be identical. Table 1505 allows you to adjust spool rate in different N2 ranges.

Fuel_flow_gain is just a scalar of Table 1505.

fuel_flow_gain 1.0 = 2048 @ 100% x.

The chart is 60 seconds of 2048 points long from left to right.


To get an approximation of the timing you want....

Fuel_flow_gain x 34.133333 = number of x points per second.

0.002 x 34.133333 = 0.0682... points of x = one second (at full throttle commanded)


Helpful hints:

Most important: Don't change Y values to change the spool rate. Set your Y values based on where you know the engine should spool slower/faster then move x to make adjustments.

Second most: Never use fuel_flow_gain to tweak one problem. It is a SCALAR of the WHOLE chart.


To make one x point (1.00) = 1 sec use a fuel_flow_gain of 0.02929688. Then your highest x should = 60.00. I don't but maybe someone else would.


To accommodate FS's silly auto start (hard wired to fire at 20%) make x a negative value for y = 20. The value should be exactly the same (opposite) of your first x point after x=0.

Example:

x=-100 y=20 (x-100 & y-5 from x=0)
x=0 y=25
x=100 y=30 (x+100 & y+5 from x=0)


If you want to trick autostart to fire at the correct N2, make an XML gauge:

Autostart bool true if{ N2 less than X% if{ 0 (ENGn mixture) } els{ 100 (ENGn mixture) } }



Common Questions:

What if you don't use the whole 60 secs?

Don't worry about it... FS scales everything to fit 2048/60 sec. However, this is why the overshoot problem happens. If it is overshooting at all RPMs turn down fuel flow gain. If it is overshooting at one RPM range, the curve is to steep within that range. If the spool rate is good yet still overshooting, your whole chart is too short, make it longer (stretch it to the right.)


Focus on getting the curve drawn right then use fuel_flow_gain to tweak overall timing. Just remember the steeper the faster & the shallower the slower.




====================


@taguilo that was me... ATP Experience. If you explain your problem more in depth maybe I can help you find a solution.

 

[jx_]

It gets more intriguing. Without by-pass ratio how does FSX distinguish turbo jet and turbofan engines? and, come to think of it, between single and twin spool engines - by setting N1 = N2 in Table 1502?

FS doesn't need to know. The only difference in the sim world that matters is Specific Fuel Consumption. One pound of thrust is one pound whether it came from a jet or a fan or 1-2 spools. What is important is how much fuel will that pound will cost.

The other differences can be modeled through other means such as the thrust tables.

 

[jx_]

A:TURB ENG FUEL FLOW gives a correct value when the throttle is closed at speed and A:TURB ENG JET THRUST will go negative, so FS must be using Gross thrust for fuel flow.

Actually no. I thought the same thing at first but I am pretty sure it uses Net to output fuel flow, it's just a little bit silly. I think they meant to use gross but screwed up the calculations...can't be sure.

What you are seeing is the thrust to fuel ratio. If it were 1:1 both numbers would've been the same (well...Fuel Flow capped at 0.) If it is 2:1 as are most jets (SFC 0.5-0.6) then you can easily have +500 fuel flow and -500 thrust. FS doesn't use it's 'brain' sometimes. You can't produce negative thrust from the engine itself. If you go low enough the fuel flow will drop to 0, which is impossible on a running engine. I don't know if it will show less than 0 because I'm sure the gauge is limited to positive numbers.. I can try it if you like.

So this is why I believe it is using Net thrust for fuel flow. If it were using Gross the fuel flow would never drop below idle. It SHOULD be using gross!!!

you might want to look at the thrust reduction with increased altitude in FS. Thrust appears to reduce with air pressure ratio rather than air density ratio. The tables use Inverse Air Pressure because it allows input values in ascending order. The result of using pressure rather than density is that airplane max Mach decreases with altitude when it should generally peak somewhere around 30K. Ceilings are lower than the real airplane had for the same reason.

You are correct about IAP, but that table doesn't affect thrust, it only affects the throttle position relative to CN2. Thrust (Power and fuel flow) is set by CN1 to Mach.

In other words, IAP, affects where N2 spins at a given throttle setting. For example, idle RPM at low Mach, interpolated to IAP. (Does altitude affect your Aircraft's N2 position?) It has no affect on thrust whatsoever.

In an Airbus or any 'smart' aircraft with FADEC and EEC this is irrelevant. Set the N2 the same because the computer will interpret and adjust. But for let's say a CRJ200 that has no EEC, the pilots have to monitor the RPM in climb and descent because the N2/N1 changes. These tables (1503 and 1504) is where this is modeled. N2 changes due to altitude for each throttle position.


If you are seeing fall off at altitude, adjust the power curve in table 1506 by increasing by a percent or two within the problem area mach. I have flight test data on my aircraft that shows the real world thrust in pounds at altitudes and FS is outputting the thrust with no problem. The only problem I have found is in the sim, SFC never changes, thus fuel flow is a compromise between high/low altitude and high/low RPM.

My conclusion is that there is some relationship between fuel_flow_gain and when combustion begins during engine start. However I have no data at the moment.

Roy

I don't believe it is. I have yet to be able to determine what it is tied to. So far I have seen it as low as 16% N2 and no higher than 23% N2. I need 26% =(.

My fuel_flow_gain is currently set to 1.0. I have checked, fuel_flow_gain doesn't change it. Something in 1505 does, probably a ratio between something.

 

[jx_]

In what way would 0.54 of fuel flow equate to 100% CN2?

x tells FS the time it takes to reach y when a change is applied.

If all y are the same (horizontal line), the engine idles at whatever y is, regardless of throttle position and will never change.

If the interval to the next y (N2 AT next x position) is shorter than the previous, the spool rate will speed up.

So let's say it this way,

x = arbitrary time value
y = N2

on the vertical line you see y = 100, the time value is 0.54
on the vertical line where you see y = 80, the time value is 0.14

0.54 is obviously longer in time than 0.14

so from start ignition to 80% N2 would take 0.14 * fuel_flow_gain * 2048 = FS time variable used to simulate lag.

while from start ignition to 100% N2 would take 0.54 * fuel_flow_gain * 2048 = FS time variable used to simulate lag.

In the chart you posted, you can see at engine start the RPM will surge (almost vertical, then the rest of the RPM is a smooth slope uphill.)

x's value doesn't matter, it is all scaled by setting the fuel_flow_gain accordingly. What matters is getting the curve just right. Hope this helps. =)


Note: The 2048 is probably a 16384 or 32xxx or 65xxx but it really doesn't make a difference.