The current ITT model in the simulator is quite limited. I will use turboprops
as my basis, but what I’m suggesting could be applied to jets too, to an
extent. Currently, ITT is based on fuel flow, a cooling rate, a min and a max
scalar. The three latter aren’t accessible for tweaking to turboprops at all,
so that’s the first problem. However, looking at data from the PT6A, you will
notice that ITT remains relatively stable at lower Ng settings and increases
more rapidly in the last 20% of Ng. The minimum and maximum values, though,
depend on more factors than just fuel flow. Therefore, my suggestion is to
introduce tables to handle ITT vs fuel. SUGGESTION 1 A table of ITT vs
fuel flow:
- Propeller is assumed to be at fine pitch
- ICAO standard day, sea level
- Value pairs of fuel_flow:ITT
The sim will then use a the current formula with the time constant, tuning
constant and scalar to calculate ITT values. The table method will allow us to
control the minimum and maximum ITT for standard engine operation and even
give us a bit more control about how/when it will overheat. It still won’t
account for things like hot starts, but it is an improvement over the current
version. SUGGESTION 2 - ADVANCED VERSION Two tables:
- fuel flow vs ITT: same as above (pph:rankine)
- residual fuel vs delta ITT (pounds:rankine)
Residual fuel: excess fuel left in the engine before combustion is achieved.
If no combustion is achieved and the fuel valve/mixture is closed:
residual_fuel = fuel_flow * time_valve_open If no combustion is achieved
and the fuel valve/mixture is closed: fuel_drain: fuel_drain_constant *
time_valve_closed The ITT calculation for when combustion is present will be
done using the regular formula. Then delta_ITT will be added on top of it to
give an “offset” for the extra fuel that’s been in the engine due to the pilot
letting the fuel accumulate, or not letting the engine drain. The final ITT
value will be the regular ITT calculated with the method above, plus the
delta_ITT which would be calculated from the residual fuel. Delta_ITT can
never be below 0. While this second method leaves a lot to be desired, it
could open the door to simulating some things such as hot starts, or
firewalling the throttles (which would cause a fuel flow spike that’s greater
than the engine can burn).