Table 1505


First off, this is not the common "how do I prevent spikes", "change response time" etc. I'm not working on a flight model for any aircraft, I'm just looking for knowledge.

I know that 1505 is involved in determining new N2 values from present ones. In other words, delta_CN2 is somehow calculated from dThr/dt and the current CN2. (dThr is the difference in throttle position)

Also, since dCN2/dt changes smoothly, I also assume there's a d2CN2/d2t involved.

Does anyone know the equation for calculating the next CN2 given dThr/dt and 1505?
Having read through that thread I have a question regarding this statement:
Because the SIM is trying to simulate the engine's reaction to a fuel sprayer controlled by an electronic fuel system, you get 3 phases: accelerate, steady rate spool, and decelerate.
Has the exact way MSFS does that been determined yet?
not that I know of.

The angle of each slice determines the rate of N2 change; or the length of each slice determines the time it will take to go from N2 beginning the segment to N2 ending the segment, (assuming no spoolup or spooldown - accelerate and decelerate). A horizontal line will be constant forever regardless of throttle position or any other table value. A vertical line will jump instantly to the highest N2 at the end of the vertical segment and stay there until a different segment is reached.

The accelerate and decelerate are hard coded unknowns that are used to lag and smooth the transitions between segments. They cannot be edited.

The steady state is derived from 1505. There is also a feedback controller in play (if you are unfamiliar with PID, feedback is the 'error distance' from the target. If the target is further, the control input will increase the correction value). When you advance the throttle, 1505 rate + the feedback correction = the final output steady state rate of change.

There are posts in this forum with rough values of the timings in math form.
On the P3d feature request forum, WarpD asked for a controllable spool rate.

Mike Schroeter responded with

A previously undocumented capability is to configure your fuel_flow_gain in the aircraft.cfg to be a function of engine speed (N2 for straight turbines, N1 for turboprops). It might look something like this...

fuel_flow_gain.0 = 00.0, 0.011
fuel_flow_gain.1 = 25.0, 0.011
fuel_flow_gain.2 = 60.0, 0.05

You can configure up to 5 data points.

Hope this is helpful.

Does this change anything here? I am playing around with this on my GE90 airfile now

thx Keith
It only works with Prepar3D and I'm not even certain what versions. Changing the value of the gain will indeed impact spool rates.


Resource contributor
If it's set up like 1505, in is CN2 and out is a fuel flow constant. I never found out what exactly that "fuel flow constant" is supposed to do. All I know is that you can't make the slope of a segment too flat or your engine won't spool. If the initial segment is too flat, it won't even ignite.
With some trial & error, you can get the spool times of engines well replicated though.
never said I use it. Not sure what it is intended for. I'm sure there's a purpose that I haven't found. (I haven't done a turbo prop in ages)
Turboprop FDE ignores any 1505 entry into the .air file. To get the same behavior that 1505 would define if it were supported, you have to use the .cfg entries. They only work in Prepar3D.

Roy Holmes

Resource contributor
Turboprops are quite different than turbojets/fans in the sim.
First, N2 is not controlled by the throttle, it is controlled by the prop constant speed mechanism
N1 is not a function of N2, it is controlled by the throttle or PLA.
On start up the big overshoot of power is mainly a function of the prop constant speed mechanism and can be reduced only by restricting fuel flow, either directly or by controlling mixture.
Hence 1505 has no part to play and will do nothing if you include it.
I'm contemplating writing an article for the Wiki on the subject as a follow up to my one on turbojets.
Spent a painful 3 months working on the subject while doing the FDE for Dino's C2 and learnt a lot which I can share.