Vivian Meazza wrote:
> Here are some calculations on propeller rpm.
> [...]
> We can see that 2850 is unlikely to be the rpm of a 10.75 diameter
> propeller

Yeah, you're right.  This is a real bug.  I was playing with it this
morning, and we're hitting an edge case in the propeller solver.

The propeller as defined is actually fine.  It sinks the right amount
of power and generates appropriate thrust at the specified cruise RPM.
The problem is that (due to a deficiency in the model) the torque
required to turn the propeller are *lower* RPMs increases faster than
the engine torque does*.  So while the engine and propeller are
matched at cruise; the combination can't get there because it can't
accelerate the prop at low speeds.

The end result is that this breaks the "stabilize" step in the solver,
which tries to iteratively solve for the steady state RPM for an
engine/prop before running the aero FDM.  Mathematically, the current
propeller model has two minima, and it's picking the wrong one.

The spitfire is hitting the condition because of the high gear ratio,
recent changes in the engine code which reduce available power at low
speeds (to get idle speeds right), and a miscalbration quirk in the
manual pitch handling (setting "0.5" for manual pitch doesn't produce
the same results as a non-variable propeller).

I'm not quite sure what the right thing to do here is.  One trick
would be to jigger the stabilize routine so it starts from an RPM
within the right range, but that's going to be really hard to
maintain over time.  Let me think about it...


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