Andy Ross said:

> 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...
> 

Any ideas on this yet?  I caught a little bit of this thread before heading to
NY and have been sitting on the edge of my seat ever since :-)  It is an issue
with the p51-d as well, of course.  It'd really be nice to finally get that
one right.

Best,

Jim


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