The reason I wanted to add this feature was for aircraft fitted with
variable pitch props. As it is, its a bit too easy to set a given rpm
and manifold pressure for cruise. In the real world as the rpm is
changed by changing the prop lever the manifold pressure should
change very slightly, needing a slight adjustment of the throttle
again to get the desired settings, and thus possibly another slight
adjustment of the prop lever if its not a constant speed prop. Thats
what I would think anyway, IANAP.
The easy way is to model the throttle lever as a variable size hole
and the engine as a volume flowrate that is proportional to RPM.
The input to the throttle and the output from the engine are both
the static pressure (as reported by the environmental module).
Solving tells you immediately what the instantaneous MP should be.
Given the MP, you have the energy required to crank the engine,
due to the volumetric flow being pressurized from MP up to static.
Combining the crank torque with friction and prop thrust
tells you whether the engine will speed up or slow down.
There is the same thing implemented in the steam for the vac pump.
Oh, and the prop control is a pure integrator, complete with all the
overshoot problems and the like you'd expect from control theory,
with the gain term more or less proportional to oil temperature
in fahrenheit (if you want a lazy way to model it) so that the
time constant is about one second in normal operating modes.
As a rule of thumb, changing altitude or any of the engine controls
on a controllable prop aircraft will usually affect both RPM and MP.
It is probably a good idea to detect overpressure-underspeed (on the
C310) and cause an engine failure ... to encourage careful usage...!
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